Using a Glycol Ground Loop to Condition Ventilation Air

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Using a Glycol Ground Loop to Condition Ventilation Air

How much energy can one of these $2,000 systems collect?

Posted on Apr 10 2015 by Martin Holladay

Most energy-efficient homes include a mechanical ventilation system — often an HRV(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air. or ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV. that brings in fresh outdoor air while simultaneously exhausting an equal volume of stale indoor air. The main problem with introducing outdoor air into a house is that the air is at the wrong temperature — too cold during the winter and too hot (and often too humid) during the summer.

HRVs and ERVs address this problem by passing the outdoor air through a heat-exchange core designed to take the edge off extreme temperatures. (For more information on this type of ventilation system, see HRV or ERV?) While the tempering function of the heat-exchange core helps, it isn’t a perfect solution. Unless the outdoor air is already at room temperature, the air delivered by an HRV or an ERV will always be cool in the winter and warm in the summer. Moreover, in cold conditions an HRV core starts accumulating ice. Manufacturers have developed a variety of solutions to the frost problem. For example, a cold HRV core can be warmed by temporarily closing the outdoor air damper and circulating indoor air through the core (that is, by putting the HRV into “recirculation” or “exhaust only” mode). Another way to address ice buildup is to include an electric resistance heater that raises the temperature of the incoming outdoor air.

Some HRV and ERV manufacturers (including Zehnder and Ultimate Air) offer a third option: a system which conditions incoming outdoor air by blowing it through copper heat-exchange coils connected to a buried ground loop. This buried ground loop consists of hundreds of feet of PEXCross-linked polyethylene. Specialized type of polyethylene plastic that is strengthened by chemical bonds formed in addition to the usual bonds in the polymerization process. PEX is used primarily as tubing for hot- and cold-water distribution and radiant-floor heating. tubing (usually between 3/4 inch and 1 1/4 inch in diameter) filled with a glycol solution; operation of the system requires a pump to circulate the glycol solution through the heat-exchange coils.

These ground loops resemble the ground loops installed for ground-source heat pumpHome heating and cooling system that relies on the mass of the earth as the heat source and heat sink. Temperatures underground are relatively constant. Using a ground-source heat pump, heat from fluid circulated through an underground loop is transferred to and/or from the home through a heat exchanger. The energy performance of ground-source heat pumps is usually better than that of air-source heat pumps; ground-source heat pumps also perform better over a wider range of above-ground temperatures. systems; however, ground loops used to condition ventilation air don't require a heat pump. The temperature of the fluid delivered to the water-to-air heat exchangerDevice that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank. is close to the temperature of the soil adjacent to the ground loop.

The ground loops are usually buried between 4 and 7 feet deep. Installing a buried ground loop is obviously easier during a new construction project than during a renovation project. If a backhoe is already on site to prepare a foundation hole or to dig a trench connecting the basement with a drilled well, a PEX ground loop can be installed in conjunction with other excavation work. The coils can be located in pipe trenches, around the sides of a basement excavation, or under an insulated basement slab.

During the winter, when the air temperature might be 10°F, the soil near a buried ground loop might be at 40°F or 45°F. And during the summer, when the air temperature is 90°F, the soil near the ground loop may be at 50°F or 55°F. These moderate soil temperatures can be used to take the edge off incoming ventilation air.

The glycol-filled ground loop approach has substantially replaced the use of earth tubes (buried ducts). Earth tube installations are now rare due to concerns about condensation and mold.

Do they work?

Glycol-filled ground loops work.

Jesper Kruse, a Passivhaus builder, had this to say about a house he built in Newry, Maine: “The data so far suggest that the [Zehnder ComfoFond] ground loop is amazingly effective. We didn’t get the temperature sensors installed in the ventilation system until February of this year, but when the outdoor temperature was 15°F, the incoming air was warmed to 38°F by the ground loop. And in early June, when the outdoor temperature was 94°F, the ground loop cooled the incoming air to below 60°F.”

Of course, just because a ground loop works, doesn't mean the system is cost-effective. Many energy experts have speculated that the pump needed to circulate the glycol solution uses almost as much energy as the system collects. The results of one monitoring study indicate that these experts may be right; data gathered in Vermont suggest that the simple payback period for this type of system may be as much as 4,400 years.

Before discussing the study, however, I'll take a closer look at the design and installation of ground loop systems.

What does Zehnder recommend?

In the U.S., most of the ground loops used to condition ventilation air are ComfoFond systems sold by Zehnder, a European manufacturer of HRVs and ERVs (see Image #2, below). ComfoFond hardware consists of a water-to-air heat exchanger designed to be mounted near the HRV or ERV; a pump; and a pump control connected to temperature sensors. (The required PEX tubing is supplied by the owner, not Zehnder.)

For one popular model, the ComfoFond-L Eco 350, Zehnder recommends the use of buried tubing with a diameter ranging from 3/4 inch to 1 1/4 inch, and a length of 200 to 360 feet. (See Image #4, below.) The Zehnder instructions note, “The recommended collector examples named here are recommended minimum requirements which may be increased depending on the location and the operating mode of the system. We recommend having the ground [soil] inspected by an expert. … The data assumes a minimum outside temperature of -4°F and a laying depth of 4 ft. to maximum 6.6 ft. The named pump stages are reference values, and may vary depending on the configuration of the pipework in the building. The ground collector pipes are not allowed to be spaced less than 2 ft. apart in all directions and are also not allowed to be less than 3.3 ft. in any direction from pipes carrying water. It is not permitted for the collector field to be built over or sealed.”

What does UltimateAir recommend?

UltimateAir is an ERV manufacturer headquartered in Athens, Ohio. While UltimateAir promotes the use of ground loops to condition ventilation air and sells equipment for the purpose (namely, the UltimateAir water-to-air coil), the manufacturer provides less published guidance than Zehnder. The message from UltimateAir is basically, “When it comes to ground loop design, consult an engineer.”

Instructions for the water-to-air heat exchanger note, “Suggested applications: Pre‐heat defrost for the RecoupAerator when the incoming outside temperature air is below 10°F (hot water or geothermal ground loop).” However, there instructions don’t provide any more hints on system design or performance.

Instructions for a controller sold by UltimateAir note, “UltimateAir 12" WTAC (water coil) module is located in the incoming fresh air stream before the ERV. … Set the Controller 1 set point to 12°F. The controller will close the contact when the temperature of the incoming air drops below 12°F, and energize the connected water pump to circulate water through the coil thereby warming the incoming air. The ground loop length, ground depth, and material can be of many forms per climate and application. In order to determine these parameters, a local mechanical engineer should be consulted.”

Jason Morosko, the vice president for engineering at UltimateAir, was able to provide me with a few rules of thumb for designing ground loops. “We can use a 55 to 45 degree water temperature for preheat,” Morosko told me. “Objective number one is making make sure that the incoming air doesn’t freeze the ERV core. If you can circulate 45 degree water through the coil, then you will keep the incoming airstream above 12 degrees. As long as the incoming air is above 12 degrees, you don’t have to worry about frost in the core — at least for our ERV. Others brands of ERVs will behave differently. If the incoming air is below 12 degrees, you’re susceptible to having frost in the wheel.”

I asked him how a designer would know whether the fluid in the ground loop will be above or below the desired temperature of 45 degrees. “Generally I go by what is recommended in the geothermal industry, where the usual guideline is 300 feet of loop per ton,” Morosko told me. “The application we are talking about is well under a ton, because the loop is going to deliver about 3,400 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /h. So I recommend that customers put in 300 feet of 3/4 inch PEX.”

Morosko provided one more piece of advice: “We recommend that these loops be filled with glycol. I got one callback from a customer who didn’t use any glycol — just water — and the fluid froze in the copper coil.”

How much does a ground loop cost?

I talked to four people who provided estimates of the typical cost of a ground loop and associated hardware (the water-to-air heat exchanger, circulator, and controls). These estimates ranged from $2,000 (the amount cited by Alex Wilson and Marc Rosenbaum) to $3,000 (the amount cited by Vermont homeowner Chris Pike, who evidently was quoted that figure by a contractor).

A report sent to me by Peter Amerongen, an energy expert in Edmonton, Alberta, estimated the cost at $2,772.

Energy performance estimates

One rough-and-ready way to estimate the benefit of this type of buried ground loop is to estimate the electricity savings associated with eliminating the need for Zehnder's electric-resistance heater. According to monitoring data provided by Alex Wilson, the electric resistance heater on his Zehnder HRV uses about 244 kWh during a cold winter, or 154 kWh in a milder winter.

I asked four energy experts to provide estimates of the net annual kWh savings that could be attributed to this type of ground loop. These estimates were made for a variety of purposes — for example, in spreadsheets that looked at whether a list of energy measures were cost-effective — and none of the consultants I spoke with made any particular claims for the accuracy of the estimates.

The estimated annual energy savings attributed to a ground loop ranged from 244 kWh (a figure based on Alex Wilson's monitoring of his Zehnder's electric-resistance heater), to 347 kWh (the high estimate of three scenarios in a spreadsheet created by Vermont energy consultant Andy Shapiro), to 424 kWh (an estimate by Stuart Fix, a mechanical engineer from Edmonton, Alberta), to 500 kWh (an estimate by Peter Amerongen), to 1,315 kWh (estimate by a group of mechanical engineering students working with Peter Amerongen).

Skepticism that the investment is worth it

Does this type of ground loop collect enough energy to justify an investment of $2,000 to $3,000? Although the answer depends on many factors, including the local cost for electricity and the severity of the climate, the answer is probably no.

According to a U.S. Department of Energy report on a project in California, “Sonoma House: Monitoring of the First U.S. Passive House Retrofit,” “The original HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. system design … called for ground loop thermal heating to temper outside air before entering the ERV system. The horizontal ground loop would have consisted of two lines of 150 ft. of ¾ in. PEX tubing located 5 ft. underground where the expected soil conductivities would be between 0.5 and 0.7 BTU/ft-°F-hr. A 50-watt pump would circulate water to a heat exchanger placed at the inlet side of the ERV. The team’s analysis indicated that the payback on this design would be minimal (i.e., pre-heating intake air with ground loop thermal heating is an expensive investment with little return) in such a mild climate [i.e., in Sonoma, California].”

At a Passive House conference in Portland, Maine, in September 2014, Marc Rosenbaum, the director of engineering at the South Mountain Company on Martha’s Vineyard in Massachusetts, gave a presentation on net-zero-energy homes. Rosenbaum said, “I refuse to put in a ground loop. It can break. It can cost $2,000.”

While the first owner of a ventilation system equipped with a ground loop may pay attention to the system's performance and make sure that all of its components are well maintained, it's likely that subsequent homeowners — the ones who buy the house when the first owner moves away in seven years — won't notice when the circulator conks out.

Back-of-the envelope time

Let’s look at the various estimates for annual energy savings by these ground loops and figure out what it would cost to purchase a PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. system that would generate the same amount of energy as a ground loop. We’ll make the calculations for Chicago; the output of a PV system will be somewhat different in different locations. These calculations assume that a PV system costs $3.50/watt.

To generate 244 kWh/year would require a PV system rated at 210 watts; cost = $735.
To generate 347 kWh/year would require a PV system rated at 300 watts; cost = $1,050.
To generate 424 kWh/year would require a PV system rated at 370 watts; cost = $1,295.
To generate 500 kWh/year would require a PV system rated at 430 watts; cost = $1,505.
To generate 1,315 kWh/year would require a PV system rated at 1,200 watts; cost =$4,200.

In short, unless you believe the most optimistic of these five estimates for the energy collected by a ground loop, an investment in a PV system usually yields more energy than an investment in a ground loop.

The comparison made here — between an investment in PV and an investment in a ground loop — is similar to comparisons made between an investment in PV and an investment in thick subslab foam. However, it's worth pointing out an important difference between these comparisons: while subslab foam can be expected to last much longer than a PV system, the components of a ground loop system may not last as long as a PV system.

Monitoring data

It's difficult to find good monitoring data to show how well this type of ground loop performs. Ideally, it would be useful to know how many BTUs of useful heat were collected by the ground loop in winter, and the extent to which a ground loop reduced the need for air conditioning in summer. To figure the net benefit of the system, the energy used by the circulator would have to be subtracted from the amount of energy collected by the system.

When I requested data on ground-loop performance from UltimateAir, Morosko wrote, "I wish I had data for you. I do not."

The best monitoring data I found were provided by Peter Schneider, a senior project manager at Vermont Energy Investment Corporation. Schneider has been monitoring the energy used by Zehnder HRVs in two identical Vermont houses. (Although he first described the houses as "identical," Schneider later amended that to "very similar.") Each house has a Zehnder ComfoAir 350. One house (House A) is equipped with an electric-resistance heater to raise the temperature of the incoming air; the other house (House B) has a ComfoFond-L ground loop.

Schneider's monitoring data provide evidence that most estimates of energy savings due to the use of a ground loop are wildly optimistic.

The results of one year of monitoring are surprising: The house with the electric resistance heater used 290 kwh (170 kWh to run the fan and 120 kWh to run the electric resistance heater) for one year, while the house with the ground loop used 311 kWh (186 kWh to run the fan and 124 kwh to run the ground loop circulator) for one year.

Schneider noted that these data are incomplete: "The electric pre-heater on House A stopped working during a portion of this analysis period, so the [recorded] energy usage is estimated to be between 10-20% lower than it should have been."

If we add 20% to the energy used by the electric resistance heater to account for the glitch reported by Schneider, the total for House A rises to 314 kWh (170 kWh to run the fan and 144 kWh to run the electric resistance heater) — very slightly more than the 311 kWh required to run the house with the ComfoFond. In other words, the ground loop appears to save about 45 cents of electricity per year. If this level of savings is typical, then the simple payback period for a ground loop is about 4,400 years.

Schneider also noted that the monitored ground loop system uses Zehnder’s "earlier generation" of circulator (a Grundfos pump) while "the new ComfoFond has a Wilo pump that uses fewer watts to pump the water/glycol mixture."

Energy savings aren't the only consideration

Even though energy savings are too small to justify the high cost of a ground loop, there may be other reasons to consider installing this type of system.

According to Schneider, “At the end of the day, if you install a ground loop, you're not doing it for the BTUs saved. It’s for the increased comfort — to raise the delivery temperature of the ventilation air delivered to remote spaces. With the ComfoFond, that air will be within 2 degrees of your setpoint. With Zehnder's electric preheater, the air will be cooler.”

Other options for the Zehnder HRV

Many Passivhaus builders specify Zehnder HRVs, which have a reputation for efficiency. Purchasers of a Zehnder HRV or ERV must choose between two defrost approaches: either installing a ground loop, which is a significant one-time expense, or using an electric resistance heater to warm incoming outdoor air in cold weather, which is a significant ongoing expense.

Alex Wilson installed a Zehnder HRV without a ground loop. He wrote, “Electricity consumption for the HRV this year was 112 kWh in January, 146 kWh in February, and 35 kWh so far in March (through the 26th of the month). By contrast, when it’s not as cold out, consumption is far lower — just 18 kWh last December, for example. … Last year (2014) wasn’t quite as cold, but the electricity consumption by the HRV was still significant: 65 kWh in January, 83 kWh in February, and 60 kWh in March.”

Chris Pike told me, “The Zehnder HRV has an electric heater that draws 700 or 800 watts, so that’s a fairly significant draw when it’s on.”

Some energy experts are critical of Zehnder's decision to use an electric resistance heater to raise the temperature of outdoor air during the winter, rather than the time-tested North American method used to defrost HRV cores (periodically recirculating indoor air through the core during cold weather).

If the house is equipped with an air-source heat pumpHeat pump that relies on outside air as the heat source and heat sink; not as effective in cold climates as ground-source heat pumps. with a COP between 2.0 and 3.0, some argue that it makes more sense to allow the ventilation air to enter the house at a lower temperature (assuming, of course, that a technical solution exists to prevent the HRV or ERV core from frosting up). Once the air is in the house, its temperature can be raised more efficiently by the heat pump than by Zehnder's electric resistance heater.

Rather than choosing between Zehnder's two unattractive options — an expensive ground loop or an 800-watt electric heater — some energy consultants are taking a second look as simpler solutions — for example, specifying a $1,000 ERV from Renewaire. (Renewaire ERVs have no condensate drain. Because moisture is transferred across the unit's permeable ERV core, these units don't need to be periodically defrosted.)

“The old-fashioned North American defrost strategy actually makes a lot of sense when you’re heating system has a COP better than 1 — or even (from a source energy point of view) when you heat with natural gas or wood,” Peter Amerongen recently wrote in an email. “The notion that it is unacceptable to interrupt the fresh air stream during the defrost cycle doesn’t make sense to me. Bringing in fresh air is a process of freshening by dilution. We’ve been putting HRVs in well-sealed houses for 35 years. We have never had a negative comment on the freshness of air unless the HRV failed completely for a few days, and we’ve had many positive comments on air freshness.”

Martin Holladay’s previous blog: “How to Install Rigid Foam On Top of Roof Sheathing.”

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Image Credits:

  1. Image #1: Charles Bado
  2. Image #2: Peter Schneider
  3. Image #3: Isover Multi-Comfort House
  4. Image #4: Zehnder

Apr 10, 2015 11:00 AM ET

by Charlie Sullivan

Thanks for the collection of data!

Did any of the analyses consider humidity removal in the summer? For my zone 6 central NH location, that looks to me like it will be the biggest benefit, and was the tipping point for deciding to install a system. I'm cheating though and using my existing geothermal wells, rather than running new tubing. Ideally that cuts the cost some, but it requires adding a heat exchanger, so I can use a higher-glycol mix in the air-to-water coil than in the ground loop, to prevent freezing.

Dehumidification in the summer and avoiding defrost energy in the winter seem to me to be the only good arguments for it. Preheating or pre-cooling air that is going to run through an HRV system anyway has little benefit. Your numbers show that avoiding defrost energy is a small effect, and would be even smaller if a better defrost cycle was used. So it seems like dehumidification might be the best argument.

Apr 10, 2015 12:14 PM ET

Edited Apr 10, 2015 12:14 PM ET.

a few thoughts
by Marc Rosenbaum

In the second paragraph, you wrote:
"For example, a cold HRV core can be warmed by temporarily closing the outdoor air damper and circulating indoor air through the core (that is, by putting the HRV into “recirculation” or “exhaust only” mode)."

Most North American HRV/ERVs use the recirculation method - the exhaust air is recirculated through the incoming side of the core. This keeps the house in a balanced pressure condition. The exhaust-only mode is simpler - it turns off the supply air fan, so the exhaust air melts any ice out of the core on its way outdoors. It puts the house into a negative pressure condition with respect to outdoors.

People can (and will, this IS the Internet) debate endlessly about the desirability and drawbacks of each of these methods - recirc, exhaust-only, preheater, ground exchange. The Zehnder has a back-up control feature that puts the unit into exhaust-only if the preheater doesn't work properly to implement defrost control (and presumably this is also true if the ComfoFond fails). Exhaust-only puts the house at a pressure lower than outdoors which has potential to backdraft a combustion device such as a wood stove, so that defrost method is my personal least favorite.

I wouldn't say that I "refuse" to put in a ground exchange loop - there's no sense in being that hard line - but every time I look at the cost I choose another approach. In budget-driven buildings, a choice such as the Renewaire EV90P, which has very good thermal performance, decent electrical efficiency, and no defrost requirements, is worth considering.

Finally - it's worth noting that the higher the thermal efficiency of the heat exchange core, the more time it is at risk of frost in the core, so these issues are exacerbated with the very high efficiency Paul cores in the Zehnder and other European products, Zehnder has announced an ERV core advance that they say gets closer to the HRV core's thermal efficiency, which may be a good choice, because overall cores that can transfer moisture spend fewer hours in a defrost mode.

Apr 10, 2015 12:35 PM ET

Edited Apr 10, 2015 12:37 PM ET.

Response to Charlie Sullivan
by Martin Holladay

You're right, of course, that a coil connected to a ground loop will cool ventilation air during the summer and provide a limited amount of dehumidification. It's hard to quantify the energy saved by these systems during the summer, however. I welcome any data or calculations on this issue from GBA readers.

In a dry climate, the amount of dehumidification may be minor or unneeded. In Vermont, where most homes aren't air conditioned, there may be a small improvement in comfort, but no energy savings. (In fact, there will be an energy penalty -- because the circulator energy use during summer months is an added load that isn't present in most homes.) In other words, it's complicated.

Because most ventilation systems move between 50 cfm and 100 cfm -- not much air flow -- I suspect that the summer energy savings attributable to a ground loop are minor. But I invite GBA readers to pull out their psychrometric charts, sharpen their pencils, and post their energy savings estimates here.

Apr 10, 2015 12:38 PM ET

Response to Marc Rosenbaum
by Martin Holladay

As usual, I am grateful for your comments. And thanks for clarifying the offhand remark you made during your presentation in Portland; it's good to correct the record.

Apr 10, 2015 1:43 PM ET

It's almost time to stop comparing system efficiency costs to PV
by Dana Dorsett

The cost of PV is crashing fast, and rarely are equipment & building efficiency upgrades purely an energy cost play. Nor are the full lifecycle costs typically included in the analysis.

For example: It makes sense to install an R65-75 attic/ roof in central NH no matter WHAT the energy savings are over a code-max U0.026 attic would be. The energy cost savings are truly tiny, but the difference in ice-dam potential in a location that gets that much snow is significant.

Another example: It makes sense to install U0.25 or lower windows in any US climate zone 7 or higher location instead of a code-max U0.32 window independent of the cost any cost of energy, for both window condensation and human comfort reasons.

At the decades long ~20-25% per doubling learning rate of PV , it will become the cheapest form of energy of ANY type by 2030. In Saudi Arabia it is beating $10/bbl oil fired power generation on full lifecycle costs right now, based on recent bidding into that market! When PV was still by fa the most expensive form of energy out there , the statement, "The savings of that feature is more expensive than PV" had some weight.

In 2015 the lifecycle cost of residential rooftop PV in New England is substantially less than the cost of purchased power from the utility in most of the region, but so what? It doesn't mean there aren't reasons to hook up to the grid (or to remain hooked up). Nobody (yet) is shaking their head and saying, "The cost of using grid power is more expensive than PV", with the implication that paying to be grid-tied it's a silly or a bad investment.

The comparison has lost it's punch- time to get rid of it.

The versatility of the product returned by an investment in rooftop PV is much higher than any Rube Goldberg style ventilation system's defrost/pre-conditioner, and that can't be ignored. Criticism of overly complex & expensive mechanical systems is apt, but it's independent of the cost of PV.

Given that the PV may last longer than the glycol loop ventilation defrost/pre-conditioner, is the lifecyle of a Zehnder or Ultimate Air unit as long as a PV system? And, no matter how (or even if) you defrost/pre-condition the air, is a Zehnder or Ultimate Air ever a financially rational decision, based on the net-present-value of future energy savings, or any other factor? If people want to go that route for the marginal comfort uptick or have their own $/ton pricing for the marginal carbon footprint offset that's their business, and it has nothing to do with the cost of tea (or PV) in China.

Apr 10, 2015 4:05 PM ET

Dana makes some good points
by Marc Rosenbaum

I will stipulate to the fact that PV cost comparison on energy saving strategies has declining utility.

On the topic of how long various items last -
We have good data PVs last 30 years or more. The interesting question is whether newer products of any type with digital electronics will last nearly so long as their simpler predecessors. Recently I went to a solar house I designed in VT that was built over 30 years ago. The simple HRV still runs. The simple solar fan system, using a Honeywell line voltage thermostat, still runs. No electronics. When I picked a heat pump water heater for my own house, one reason I picked the Stiebel Eltron was no electronics (the new smaller one has digital elements).
I have the Olympus rangefinder I bought in 1977, which has been with me on four continents and been dropped enough that it's dented. Still works. Meanwhile, I've been through three or four digital cameras since I bought my first about 10 years ago.
HRV/ERVs are good in superinsulated houses because they deliver air close enough to the comfort conditions that the occupant doesn't turn them off, and one can set the desired amount of air supplied to each space and verify it (at least until the owner neglects to change the filter for a few years)

Apr 10, 2015 6:32 PM ET

And, the residual value of PV past it's prime...
by Dana Dorsett

... is still likely to be WAY more than zero, as discussed in this very recent GTM blog piece:

The residual value of a complicated HRV system gone south is pretty much it's scrap value.

Electronics are not necessarily short-lived- the greater number of electromechanical parts of a system are a better predictor of mean time between failure than the number of microprocessors or power-semiconductors are involved.

Modern PV inverters & chargers have a lot more "smarts" in them and run at higher efficiency than goods sold back in the 1980s, (and they cost less too.) Consumer goods like cameras aren't really a great thing to compare with HVAC equipment or power handling equipment, most (but not all) of which GAIN reliability with smarter semi-conductor based controls.

In the middle are the nickel-cost engineered sorta-durable-goods items like hot water heaters & clothes washers, where a whiz-bang programmable user interfaces with cheap keypad switches can make them wretched things to own for the long haul. Simpler and more rugged really is better, but that can be had with electronics too. Automotive spark ignition and fuel injection systems are FAR more reliable than the electromechanical equivalents of yore, often (but not always) lasting the full lifecycle of the car (which has also nearly doubled since 1980.)

But then, I'm an electrical engineer, I have certain biases. :-)

Apr 10, 2015 7:40 PM ET

We need a "Pretty Good" ground loop system
by Kye Ford

It seems to me that the costs associated with the ground loops is the sticking point. We should be able to install these systems for less than a $1000....If your foundation hole is already open the biggest cost is already taken care of.

Alright all the mechanical engineers out there, can we build one of these with of the shelf parts? Grainger?

Let's see.
4.Cheaper water/air exchanger. Would a radiator work?

Pretty Good

Apr 11, 2015 5:17 AM ET

Edited Apr 11, 2015 5:19 AM ET.

Response to Dana Dorsett (Comment #5)
by Martin Holladay

You wrote, "It's almost time to stop comparing system efficiency costs to PV." I agree that we may have almost reached that point. But until we do, I imagine that I will probably still find the comparison useful, because it is a good stand-in for discussions of whether an energy-saving measure is cost-effective.

You provided a few good examples of features that are desirable even though they aren't cost-effective from an energy-savings standpoint: insulation that is thick enough to prevent ice dams and windows with a low U-factor that are chosen for reasons of comfort. Of course, homeowners choose features like that all the time -- and we can stretch the discussion to include granite countertops, whirlpool tubs, and a wet bar on the patio if we want. It's OK to choose house features for reasons other than energy savings.

A ground loop isn't like a whirlpool tub or a wet bar, however. There is no obvious reason why a homeowner would want one -- so if any green building guru is going to step forward and suggest the installation of a ground loop, or any other feature that isn't cost-effective from an energy standpoint, we need the guru to explain why anyone would want to buy one. You've provided examples of how that might be done: "This insulation will prevent ice dams," for example, or "These expensive windows will be comfortable to sit beside when the thermometer drops to -10°F."

So, here is a logical framework: (a) if any feature smells a little like an energy-saving feature, we assess the cost-effectiveness of the investment with the PV comparison, and (b) if the feature fails the PV comparison test, we ask our green guru why the feature is desirable.

Apr 11, 2015 5:27 AM ET

Response to Kye Ford (Comment #8)
by Martin Holladay

You wrote, "We should be able to install these systems for less than a $1000." This is a fairly common response whenever a discussion of cost-effectiveness comes up. The argument is often used when I discuss solar thermal systems: "Martin doesn't like solar thermal systems that cost $6,000. But these systems should only cost $3,000. In fact, I think I can install one myself for about $2,900."

My usual response to these discussions is: "The real price is what a contractor needs to charge to install it." And yes, contractors need to cover all of their overhead expenses and also make a profit.

Kye, if you can install a ground loop for $1,000, go ahead. If you do, you will have shortened the system's simple payback period from 4,400 years to 2,200 years.

Apr 11, 2015 12:24 PM ET

by Kye Ford

Why the idea of being able to install a cheap ground source heating loop is appealing to me is that it seems like a simple straightforward application that would do two things.

1. Preheat incoming cold air
2. Reduce or eliminate the defrost recirculation cycle for HRV

My biggest problem with my Venmar EKO 1.5 ERV is that when the unit goes into its recirculation defrost cycle, escpecially during the middle of the night, is that its really annoying. It kicks into high speed and makes the otherwise silent system loud.

We ended up turning the unit off many nights this past winter.

If I could avoid this, preheat the air, due this more efficiently than say a electric preheater, for say $750, why wouldn't you do this.

To compare a payback period of $1000 or $750 when you are spending $200,000 plus to build a house seems funny.

I'm all for PV but it isn't going to make my ERV quiet. You could argue that you could install an electric preheater with the off set savings but then you are making things much more complicated.

I'm thinking for a simple solution during the construction process that will get the highest and best use of the mechanical systems we are putting in. Pex, pump, controller, exchanger. Doesn't seem like much more work for what you are going to get out of it.

Apr 12, 2015 11:07 AM ET

On the cheap
by Charlie Sullivan

Martin is right that "possible in theory for $1k" and "can hire a contractor and get it done for $1k" are very different things. However, which of those to pay attention to depends partly on what we are trying to achieve. In the short term, if you can't get it done cheaply with what is available now with the skills available among contractors, it's not of interest. But 10 years ago that was the situation with PV. Stubborn idealists installed PV anyway, and now it's mainstream and cheap, partly because those stubborn idealists (including the German government) helped pay for the learning curve. So I would argue that if something makes sense in the long run, it is worth installing and promoting when you can, to help it get to the tipping point. On the other hand, if it's never going to make sense because of some fundamental problems, pushing it only diverts attention and investment from the approaches that have a better chance of success.

So I do think it's worth looking at the fundamentals of the cost. The first two are prices from "outdoor furnace supply"
300' 1" PEX: $155
12" x 12" water-to-air heat exchanger: $70
Circulator: $100. Doesn't need to have the high-temperature or high pressure operation capability of a hydronic circulator, but it would be good to have a high-efficiency motor.
Electronic controls $150 (could be $20 in volume production but I'm being conservative.)
Misc. plumbing parts $100
Insulated box and condensation pan. $100

That's $675 of parts. A contractor who installs these routinely should be able to do it for $250. So that leaves a thin profit margin to install a system for $1000, and perhaps a decent profit margin if it becomes higher volume operation. That's assuming we use a trench that is already open for other purposes.

Based on Martin's chart, above, that's a good deal relative to PV, even if it's not a clear win. But as others point out, electricity from PV is more versatile and probably more reliable--the circulator will fail eventually, for example. So we probably need another argument, such as comfort.

Kye's argument is silent defrost vs. the Venmar high-speed recirculation. But that could work at low speed too, and fixing the controls to do that at low speed should cost Venmar almost nothing. So I don't buy that argument as a reason in general, only for his particular case.

My argument above is that the summer dehumidification is the biggest benefit in a New England climate. I see this as a three-fold benefit:
1) Comfort, including avoiding a "muggy" feeling at moderately warm indoor temperatures, and avoiding a hospitable climate for dust mites, mold and mildew. Also, at moderately warm temperatures, running a dehumidifier leads to uncomfortable heat, whereas running an air conditioner leads to excessive cooling.
2) Energy savings from not running a dehumidifier or air conditioner, and
3) Cost savings from not needing to install an air conditioner, or as large an air conditioner (doesn't apply to minisplit owners), or a dehumidifier.

I think I'll use a large bucket for condensate from my system this summer, so I can collect data that could be used to compare to what it would cost to do that with a dehumidifier. I did run an analysis of how many hours a year the outdoor dew point exceeds 55 F and by how much, which might be the point at which I'll turn on the system for dehumidification purposes, but I didn't follow through to calculate how much water I'd expect to collect. The bucket will be a more definitive test anyway.

The other argument that will eventually be important is that electric pre-heaters only use electricity during the absolute worst peak hours of the year. So the electricity they use would be very expensive electricity if that consideration were properly reflected in consumer bills. A ground loop system can mitigate that peak, whereas PV can't. So in that sense a kWh of electricity saved by a ground loop is much more valuable than a kWh of electricity generated by a PV array.

Apr 12, 2015 11:38 AM ET

DIY and Costs
by Malcolm Taylor

Charlie wrote:
"A contractor who installs these routinely should be able to do it for $250. So that leaves a thin profit margin to install a system for $1000, and perhaps a decent profit margin if it becomes higher volume operation."

With all respect these numbers simply help illustrate the problem. No contractor can stay in business with margins like that. The higher the volume the quicker they would go feet up.

The larger problem with introducing DIY costs into discussions is that they are almost invariably used to make the case for something that otherwise doesn't make sense. Removing the labour component from a process to make the numbers work can be done with almost anything - including building the whole house - and we know how often those savings materialize when clients act as their own GCs.

Apr 12, 2015 3:09 PM ET

I agree about costs
by Charlie Sullivan

Malcolm, I agree. I didn't explain what I meant by higher volume. I did not mean that a contractor barely making a profit would do better by doing more such jobs. I meant that if the unit becomes a commodity product and costs much less (say $350 for a box with everything I listed except the PEX), that leaves more room for profit, perhaps $250 on a job with $250 of labor and $500 of materials. And I didn't say that would be a great business model. I said only that "perhaps" it would be "decent".

The other thing I didn't mention is the assumption that this would be part of a larger HRV installation job, which means the overall profit on the job would be larger.

I certainly did not at all mean to imply that DIY costs = what it should cost.

Apr 12, 2015 5:31 PM ET

by Malcolm Taylor

Sorry, my reply conflated the two issues. The DIY costs comment was not directed at your post. I guess there is a better case for your numbers if it was a small part of the larger HVAC budget, but I still find it hard to see why a contractor would tack on such an additional item and not include the same margin they make on the rest of their work.

Apr 13, 2015 6:07 AM ET

Edited Apr 13, 2015 6:13 AM ET.

Response to Charlie Sullivan (Comment #12)
by Martin Holladay

I know that you have conceded most points about the cost issue. But one aspect of your estimate hasn't been discussed: Because you "assumed we use a trench that is already open for other purposes," you calculated that the cost of installing the PEX ground loop is zero.

Let's see: unloading 300 feet of PEX; talking to the backhoe operator about widening the trench; supervising the widening of the trench; unrolling the PEX and connecting it to the plumbing fittings indoors; pressure-testing the ground loop for 24 hours; and supervising the backhoe operator during backfilling to be sure that no sharp rocks are used for the first 12 inches of backfill. You're right -- the cost of that work must be zero.

And the backhoe operator throws in the extra time for free.

And then you need to calculate some amount of time for callbacks...

Apr 13, 2015 8:04 AM ET

Good point
by Charlie Sullivan

Thanks Martin. Given my recent query arising from the poorly done backfilling on my own project, I should know that getting even a basic excavation job done right is not easy, let along one with a new wrinkle.

Apr 17, 2015 2:16 PM ET

Effectiveness for larger buildings, MURBs?
by Kyle Anders

Very interesting article Martin, thanks for the in-depth analysis on this topic. I wonder whether there might be a better case at the commercial scale for multi-family buildings or offices with higher ventilation loads? The heat transfer needed would be a higher rate for sure, and would increase the tube length/diameter requirements, but the corresponding energy savings for the building as a whole should certainly be higher as well. Ventilation is often the 'weak link' in a larger building's energy performance.

Apr 17, 2015 3:28 PM ET

Response to Kyle Anders
by Martin Holladay

You're right that some systems that aren't particularly cost-effective for single-family homes (for example, ground-source heat pumps) sometimes make sense for large commercial or institutional buildings. I invite GBA readers with data on this issue share what they know.

That said, my gut tells me that these systems require such expensive hardware, and collect so little energy, that they are unlikely to make much sense at any scale -- especially when compared with PV. But I'll keep an open mind on the issue until I see some data.

Aug 19, 2015 7:40 AM ET

what pump might work on such a loop?
by Bob N

say I have 300' 3/4" pipe what low wattage pump could do the job?

What flow rate

I have the area excavated just want to be sure a pump to do this is not too expensive and readily off the self.

the other choice I have are earth tubes

Aug 19, 2015 9:39 AM ET

Response to Bob N
by Martin Holladay

As my article shows, many of these systems use just as much electricity to operate the pump as would be required to operate an electric-resistance heater to heat up the air. That's what makes these systems a bad investment.

This type of system requires the best possible pump, so if you can't be dissuaded from installing this type of system, buy a Wilo pump. You can contact Wilo to get pump sizing help. Choose the smallest possible pump (with the lowest energy use) that will do the job.

Here is the the contact info:

Wilo USA
9550 West Higgins Road, Suite 300
Rosemont, IL

Aug 19, 2015 10:37 AM ET

thanks, I emailed them. to
by Bob N

thanks, I emailed them.

to make sense the pump needs to draw 50-100 watts be on demand, cost under $100

pipe $100

air to water ex-changer $75

I'll have 3,000 watts of PV to run it during the day,

I realize the payback, but just a nice toy perhaps.

If the pump doesn't work out cost wise the earth tubes can be tried and if they don't work seal them off use outside air for make up.

It's located in the Ga mountains so it's not a real cold climate, but does get 50"s of rain.

Aug 19, 2015 2:14 PM ET

Edited Aug 19, 2015 2:15 PM ET.

didn't hear back Wilo yet but
by Bob N

didn't hear back Wilo yet but found this pump using only 55-95 watts depending on speed Wilo-4090765-Star-S-21FX about $65

"the unit uses 98 watts on speed 3, 77 watts on speed 2, and 55 watts on speed 1. Also, the pump is rated for continual use"

So now the glycol loop make sense compared to earth tubes.

<$300 all in, I'm going to spread out the pex under a 1,000 sq ft passive solar slab, which will be insulated including wing insulation, so the water temp into the ex-changer should be in the low 60 degree range in the winter.
In fact the slab may warm the pex a bit more????

there still may be a better pump if tech support emails me back but this one is used in many geo and solar applications.

thank for your advice Martin

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