QA-spotlightheader image
0 Helpful?

Choosing a New HVAC System

A homeowner looks for suggestions in updating old heating and cooling equipment

Posted on Feb 5 2018 by Scott Gibson

Jill D has done her homework, and now it's time to choose a new heating and cooling system for her Climate Zone 5B home.

There are three distinct zones to consider: the main house, a sunroom addition, and an office addition. Neither the office nor the sunroom is ducted, although heating and cooling loads there are relatively low. In the main house, the heating loadRate at which heat must be added to a space to maintain a desired temperature. See cooling load. has been calculated at between 28,000 and 36,000 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. per hour, and the cooling load at between 24,000 and 36,000 Btu per hour.

Jill's existing furnace, which she estimates is 10 years old, is a 115,000 Btu/h variable-speed unit. Cooling is provided by a pair of 1-ton minisplits that have proved expensive to run. The existing water heater is near the end of its life.

What Jill has in mind is efficient central cooling with a blower that can run in circulation mode for filtration and to even out temperatures around the house. She'd like a system that's sized correctly so it doesn't cycle on and off frequently, and something that will provide cooling in her office.

"I've thought about minisplits or just an electric heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump.," she writes in a post in the Q&A Forum, "but everyone says the climate is just a little cold with no backup heat and that minis are the most expensive option. There is a lot of southwest glass for passive heat in the winter but it's problematic for summer."

Jill has been looking around for an HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. contractor but so far hasn't settled on one. "I'm not getting great proposals," she says. One refused to do Manual J and Manual D calculations, which are typically recommended as the necessary first step to specifying HVAC equipment. Another is willing to take Jill's suggestions, but doesn't seem to know which system would be the most energy-efficient.

In other words, she's stuck, and that's that's the story behind this Q&A Spotlight.

Let's do the math

In replying to a question from GBAGreenBuildingAdvisor.com editor Martin Holladay, Jill says that the cost of electricity in her area is about 10 cents per kilowatt-hour, while gas is between 47 cents and 50 cents a thermUnit of heat equal to 100,000 British thermal units (Btus); commonly used for natural gas.. Based on those numbers, Holladay says, the most economical choice will be natural gas. In a furnace running at 80% efficiency, 100,000 Btu of energy would cost 63 cents. That compares with 98 cents for a heat pump with a coefficient of performance of 3, and $1.47 for a heat pump with a COPEnergy-efficiency measurement of heating, cooling, and refrigeration appliances. COP is the ratio of useful energy output (heating or cooling) to the amount of energy put in, e.g., a heat pump with a COP of 10 puts out 10 times more energy than it uses. A higher COP indicates a more efficient device . COP is equal to the energy efficiency ratio (EER) divided by 3.415. of 2.

"When it comes to cost, gas wins," Holladay says.

Gas, however, isn't Jill's first choice even if it's less expensive than other options now.

"Gas won't be cheap forever and doesn't play well with solar (planned, none yet)," she says, "and [it] supports fracking (a problem in my area)."

Instead, she wonders whether a heat pump with an inverterDevice for converting direct-current (DC) electricity into the alternating-current (AC) form required for most home uses; necessary if home-generated electricity is to be fed into the electric grid through net-metering arrangements., capable of running at different speeds, or a Trane modulating furnace might be a better answer.

Yes, a ductless minisplit could handle the cold

One key question is whether a ductless minisplit would be capable of heating Jill's house in the dead of winter without some type of backup heat. On that issue, Holladay has no doubt she would be fine: "The contractors who are telling you that Climate Zone 5 is too cold for ductless minisplits are not telling the truth."

Dana Dorsett adds that it's possible to heat with minisplits even when the temperature drops to 25 below zero Fahrenheit. In Climate Zone 5B, he says, a correctly sized minisplit can equal the seasonal efficiency of a 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..

Several years ago, he writes, the Northwest Energy Efficiency Alliance monitored the performance of minisplits in Climate Zones 4C to 6B and found that even older designs functioned well, performing with an average COP of nearly 3. (There's more on that study here.)

"The typical name-plate HSPF (heating seasonal performance factor) of those units of old was about 10.0 Btu/watt-hour," Dorsett says. "Current model ductless minisplits designed for cold climates are now testing in the HSPF 13s and 14s. If the units are sized optimally, you should be able to beat the efficiency performance of the field survey units monitored by the NEEA in the 2010-2011 time frame."

He adds that living in a "B" zone (a dry region of the country in the West) means that a heat pump would use less energy in defrost cycles. And there's little or no latent cooling load.

Adds Calum Wilde: "Climate Zone 6A here. We had close to two weeks of approximately 0-5°F. My ductless minisplits worked great, they easily kept the house at 68°F."

Getting accurate heating load estimates

"Manual J" is a protocol developed by the Air Conditioning Contractors of America to produce accurate estimates of peak heating and cooling demands based on the characteristics of a particular house. The procedure is widely recommended, but Dorsett says that HVAC contractors rarely use it and, he adds, "only a fraction of those will perform [it] correctly."

It would be better if Jill hired a RESNET rater or an engineer. Or, he adds, Jill could use previous gas bills and weather history data to calculate whole-house heating loads herself, a process he describes as "not very time consuming." (For more on how to do that, read this article.)

"In a zone 5B climate with 'a lot of southwest glass for passive heat,' the fuel-use load calculation may hit somewhat to the low side," Dorsett says, "but not more than 20% unless the house had been meticulously designed for solar tempering, including optimizing the specifications for the low-e coatingVery thin metallic coating on glass or plastic window glazing that reduces heat loss through the window; the coating emits less radiant energy (heat radiation), which makes it, in effect, reflective to that heat; boosts a window’s R-value and reduces its U-factor. . Run the fuel-use load calculation, see how it stacks up against the Cool Calc freebie numbers. The fuel-use load numbers will almost certainly come in substantially lower, no matter what reasonable heating-degree day (HDDThe difference between the 24-hour average (daily) temperature and the base temperature for one year for each day that the average is below the base temperature. For heating degree days, the base is usually 65 degrees Fahrenheit. For example, if the average temperature for December 1, 2001 was 30 degrees Fahrenheit, then the number of heating degrees for that day was 35.) base temperature is used."

Using the furnace fan for filtration

One of Jill's goals is to have an efficient furnace fan that would be used to mix air in the house as well as provide some filtration. Her thermostat has a setting that runs the fan for about 10 minutes an hour at a less-than-maximum speed. It doesn't increase her power bill significantly, and it seems to help with Jill's seasonal allergies.

But, Dorsett warns, no matter how efficient the furnace fan might be, using it that way is "extremely inefficient." Instead, he suggests a heat-recovery ventilator set up to recirculate air would be more appropriate.

He adds that modulating ducted heat pumps are probably not the answer, either. Most don't have extended temperature capacity tables that go below -4°F, he says, and many don't go that low.

"They also don't have turn-down ratios more than about 2.5:1 (at minimum speed they're still delivering fully 40% of what it would deliver at maximum speed, no matter how many incremental steps it has), so very accurate load calculations and careful sizing is required for them to hit their high HSPF and SEER(SEER) The efficiency of central air conditioners is rated by the Seasonal Energy Efficiency Ratio. The higher the SEER rating of a unit, the more energy efficient it is. The SEER rating is Btu of cooling output during a typical hot season divided by the total electric energy in watt-hours to run the unit. For residential air conditioners, the federal minimum is 13 SEER. For an Energy Star unit, 14 SEER. Manufacturers sell 18-20 SEER units, but they are expensive. efficiency numbers," Dorsett says.

Plus, he says, the air handler for a ducted heat pump uses 10 times more electricity than 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. in recurculation mode. "So even running a 100% duty cycle on the HRV uses less power than cycling the heat pumps air handler for 10 minutes per day."

Our expert's view

GBA technical director Peter Yost added these thoughts:

Three of the conditions mentioned by Jill D strike me as important:

  • Lots of southwest-facing glass that creates problems in the summer.
  • A desire to filter indoor air with the HVAC system.
  • The fact there are three distinct heating and cooling zones in the house.

Jill D has a tall order. She wants to improve HVAC efficiency, deal with summertime solar gain, improve air quality, configure new systems so they can accommodate a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. system in the future, and integrate domestic hot water, if possible. The only conditioning not included in this mechanical system Rubik’s cube is dehumidification.

It would be hard enough to solve this entire puzzle in new construction; in a retrofit, it’s just too much. Here are my suggestions:

  • Keep looking for a qualified HVAC contractor or mechanical engineer. Your project seems well beyond the skills of the folks you've been dealing with so far.
  • Tackle the issue of domestic hot water separately.
  • Address summertime solar gain with shading. This is particularly true in a dry climate.
  • Address air quality with a whole-house mechanical ventilation system. It's hard to deal with allergies without whole-house mechanical ventilation. (For more information, see this article by Brian Just. It's possible to use CO2 levels as a proxy for indoor air quality.)
  • Stick with ductless minisplits as a solution to your three-zone problem.

I hate to end with something that might seem like a crackpot idea, but I'd like to think that somewhere out there you could find a high-efficiency, cold-climate, air-to-water heat pump system that will integrate hydronic space heating, domestic hot water, and (at least for some of Climate Zone 5B) even space cooling. It might be something along the lines of this new unit from Nordic Heat Pumps. I have heard whispers of this and am pursuing it; but so far, I have nothing solid to share. Stay tuned.


Tags: , , ,

Image Credits:

  1. Green Mountain Power

1.
Feb 5, 2018 8:05 AM ET

High efficiency, cold-climate, air-to-water heat pump?
by Yupster

And lo, there was a Canadian with a solution. Ecologix sells just such a unit. It has a COP of 3.0 at 17F and works at a reasonably high effieciency down to -13F. You can find it here. http://ecologix.ca/products/cold-climate-heat-pumps/air-to-water-heat-pump/


2.
Feb 5, 2018 9:10 AM ET

I was going to include an
by Stephen G

I was going to include an ecologix heat pump in my build but they never answered my emails. I even live in the same town they're assembled. Would have been a perfect fit, but no dice.

We everyday settled on a centrally ducted Mitsubishi unit.


3.
Feb 5, 2018 9:47 AM ET

Strange
by Yupster

That's odd, I've worked with them before on projects and found their customer service outstanding. I got direct answers on all my questions, often direct from product engineers. Used one of their products in an hvac design for our firm that is being installed this week actually.


4.
Feb 5, 2018 11:37 AM ET

Edited Feb 5, 2018 12:31 PM ET.

> possible to use CO2 levels
by Jon R

> possible to use CO2 levels as a proxy for indoor air quality.

I suggest that the link doesn't provide anything to show an accurate correlation between CO2 and the important indoor pollutants (eg, PM2.5, acrolein, and formaldehyde). Use CO2 level as nothing more than what it is.

I wouldn't label a system that is common in the UK a "crackpot idea".


Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!