Choosing HVAC Equipment for an Energy-Efficient Home
If you’re building a new green home, you’ll find that most available furnaces and air conditioners are oversized
Why are the smallest available American furnaces rated at about 40,000 Btuh? Back in the 1960s, a house in a cold climate may have needed such a powerful furnace — or even one rated at 60,000 or 80,000 Btuh. But these days, many new homes have design heating loads that are much smaller — as low as 10,000 to 20,000 Btuh. Over the past 30 years, building envelopes have become tighter and better insulated, but U.S. furnace manufacturers haven’t kept up with the times. For mysterious reasons, they don’t offer furnaces that are small enough for today’s energy-conscious builders.
I wrote about this frustrating problem in a 2009 article, “Heating a Tight, Well-Insulated House.” In the four years since the article was written, furnace manufacturers haven’t budged; their smallest models are still twice as big as most energy-efficient builders need.
John Straube outlines the problem
John Straube is a professor of building envelope science at the University of Waterloo in Canada, as well as a principal at the Building Science Corporation in Massachusetts. Straube has been wrestling for years with the question of how to heat small, energy-efficient homes. On February 7, 2013, Straube gave a presentation on the topic (“HVAC for Low-Load Buildings”) at the Better Buildings by Design conference in Burlington, Vermont.
Straube defines a low-load house as one with a design heat load of less than 25,000 Btuh or a design cooling load of less than 1.5 tons. Homes like this are increasingly common. “Peak demand for superinsulated houses of 2,000 square feet is often 20,000 btuh or less, and townhouses can be under 12,000 btuh,” Straube noted in Burlington.
Finding the right equipment is hard
If you install conventional (oversized) equipment in a low-load building, says Straube, “You get a choice of freezing or cooking.”
Builders in Ontario are now reporting comfort complaints from homeowners in new homes with oversized furnaces. “The furnace turns on for four minutes, and the living room thermostat is satisfied, so the furnace turns off,” said Straube. “But the master bedroom is at the end of a long duct run, and the bedroom is still cold. The furnace can’t even heat up the ductwork. Dozens of builders are telling us this is happening. The homeowners say, ‘I have one of those low-energy houses, and I could never keep my bedroom warm.””
Persistent shoppers may be able to locate low-load heating equipment — but the cost is high, and obtaining parts or competent repair service is often problematic. “Maybe you can import stuff from other parts of the world,” Straube suggested. “But that is fraught with risk, because there are no local suppliers. Or you can try using equipment designed for sailboats.”
Small furnaces, even when available, are expensive
Needless to say, if you buy a tiny boiler from Germany or a small stainless-steel stove designed for boats, the equipment will cost an arm and a leg.
“Standard units are cheap,” said Straube. “Small units are usually more expensive. You have to pay 50% to 200% more to buy a product that is half the size. The PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. crowd used to say, ‘We will downsize the heating system and save a lot of money and put it in the insulation.’ Well, that may work in Europe, but it doesn’t work here.”
Domestic hot water loads aren't dropping
Although heating and cooling loads in new homes are lower than those in older, leakier homes, most families use just as much domestic hot water as they always have. “You can’t ignore domestic hot water anymore,” said Straube. “For a townhouse, the domestic hot water load is often equal to the space heating load.”
If you want to use a tankless water heater, you need an appliance with a very large burner. “If you have two hot-water fixtures on at the same time, that’s in the range of 2 to 3 gallons per minutes combined,” said Straube. “If the temperature of the incoming cold water is 40 or 50 degrees F, now you need a large heating appliance — about 75,000 Btuh minimum. But you might only need 25,000 Btuh for space heat. If you install a tankless heater with a capacity of 90,000 to 125,000 Btuh, which is typical for Americans in a northern climate, you end up with an appliance that is four times the capacity of your space heating needs.”
Ask yourself a series of questions
There is no simple solution to the problem of heating a low-load house. Since all available solutions involve compromises, you’ll end up choosing the least-bad option. To find the right equipment, Straube suggests that you consider a series of questions:
- Is natural gas available? Homes without access to natural gas will probably choose an all-electric solution.
- If natural gas is available, is the minimum monthly charge worth paying? If the gas company imposes a minimum monthly charge of $12 or $15, even if you don’t use any gas, then it may not make any sense to hook up to a gas line.
- How high are your electric rates? An all-electric house is more palatable at 8¢ per kWh than at 16¢ per kWh.
- What fuel will you use for your water heater, clothes dryer, and kitchen stove? If you prefer to use gas for these appliances, it may make sense to use gas for space heating as well.
- Will the house need air conditioning? Most air-conditioned homes have ducts; if you need ducts anyway, you may want to heat your home with a furnace.
- What type of mechanical ventilation system will you have? What type of duct system will the ventilation system require?
The all-electric option
Homeowners without access to natural gas often prefer an all-electric house to one that depends on propane or fuel oil. “Some people in homes with really good envelopes are reverting back to electric resistance heat,” Straube noted. But this approach gives Straube pause. “Is the electricity low-carbon?” he asked. “In some areas, it’s hard to say. This is still a big issue. In many areas of North America, electricity is a high-carbon energy source. Heat pumps can help with this problem, since one unit of electricity can be turned into three units of heat.”
When I recently telephoned Straube, I mentioned Marc Rosenbaum’s preference for electric appliances over fuel-burning appliances. Rosenbaum argues that it makes sense to prepare for an all-electric future. After all, renewable energy sources — wind, 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., and tidal power — are delivered as electricity. Moreover, in areas of the country with high electric rates, it already makes economic sense to invest in a residential PV system.
“I think Marc is right but, as usual, he is ahead of his time,” Straube responded. “In the time that the typical furnace will take to wear out, we won’t even be halfway to decarbonizing the grid. I have no problem with an all-electric house with PV on the roof. Anyone who puts PV on the roof is accelerating the process of decarbonizing the grid, and I salute that. But I’m usually talking to people who depend on grid power. It’s typical for a client to ask, ‘I wonder whether we should choose natural gas or an electric 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.?’ They don’t ask, ‘I wonder whether we should choose natural gas or an air-source heat pump with $30,000 of PV on the roof?’”
Heating and cooling with ductless minisplits
One or two ductless minisplit heat pumps have enough capacity to meet the space heating and cooling needs of a well-insulated house — easily. “Fujitsu sells a ductless minisplit — the ASU12RLS2 — that is rated at 12,000 Btuh but actually has an output of 16,000 Btuh at 0 degrees,” said Straube. “The product overdelivers. I can buy it for $1,640, and it only costs me $500 to $700 to have it installed.”
The problem with ductless minisplits isn’t output; the problem is distribution of the conditioned air blowing out of the indoor units — what’s generally known as the “cold bedroom” problem.
“If you have a two-story home with three bedrooms, connecting heat to those rooms is a challenge,” said Straube. “Maybe you can install one unit upstairs in the upstairs hall, and one unit downstairs. Is it OK? How far can I push it? The master bedroom might get cold on the coldest night. With slightly tolerant customers, that might work. The best way to keep the rooms warm is to leave the doors open. If the doors are open, the temperatures will be relatively close. But if the doors are closed, the bedrooms will be 5 degrees colder than the hallway — even 10 degrees is possible. That’s OK for some people, but other people don’t like that. What temperature ranges are acceptable? The mass market is saying 3 degrees of variation from room to room. But in homes without any heat distribution to the bedrooms, when we measure the temperatures we get more than that. So, maybe you can install a little baseboard heater in each bedroom — but that can use a lot of electricity. These are the challenges.”
Some people have suggested that creative ventilation ducting — for example, a system that supplies fresh outdoor air to the living room and hallway, and exhausts stale air from the bedrooms — might equalize temperatures from room to room. Straube rejects that strategy, however. “Ventilation air doesn’t do much to move around heat,” he said. “Ten cfm of 72 degree air to a 65 degree bedroom won’t make any difference to the temperature in the bedroom at all. Open doors work better than HRV ducting.”
However, some homeowners don’t mind cool bedrooms. “There are hundreds of thousands of homes that have a single-point heat source — a wood stove,” said Straube. “It comes down to, ‘What are your comfort standards?’ Comfort expectations are varying. If you install this type of system, I wouldn’t say, ‘You will be comfortable.’”
If room-to-room temperature variations are unacceptable, it’s always possible to design a system with ducted minisplits. “There are units that can be attached to the ceiling — units with short ducts,” Straube said. “But then you have significantly lower efficiency with most current products.”
What about air-to-water heat pumps?
For heat pump aficionados, the ideal appliance would be an affordable, efficient air-to-water heat pump that could supply a home with both domestic hot water and space heating. According to Straube, it’s unlikely that such an appliance will ever be affordable.
“Designing an air-to-water heat pump is a challenge, because you need to heat the water to 120 degrees or more for domestic hot water,” said Straube. “Making hot water at a high 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. when it is -10 degrees outside is still tricky. For a ductless minisplit, you only need 100 degree air. If you have to heat water to 120 degrees, efficiency goes down.”
Straube noted that at least one manufacturer, Daikin, is selling an air-to-water heat pump (the Daikin Altherma) in the U.S. “The prices are in some cases insane,” said Straube. “Maybe $15,000 for most systems. Given the energy consumption of these low-load houses, it’s hard to justify this kind of investment. These units have the same problem as ground-source heat pumps: the investment costs are so high that they are hard to justify considering the low heating loads.”
Advice for homes that have access to natural gas
According to Straube, “If you have natural gas, a furnace is still a good choice, as long as you buy the smallest furnace you can find, and you design your duct runs carefully so the duct runs are not too long.”
At the conference in Burlington, Straube described one “solution” to the problem of oversized furnace: you can buy a two-stage or modulating furnace, and disable the high range so that the furnace can only operate in low range. “You can snip the wires so that the high stage doesn’t work — you lock out the top stage or the top two stages. You have to go in there with the wire cutters and make changes to the furnace. This can be done with a two-stage Goodman or Trane furnace.”
When I later telephoned Straube, he clarified his advice. “In public, I can’t recommend modifying manufacturers’ equipment at all,” Straube told me. “Whether or not you can even modify a furnace depends on the model you’re talking about. It’s possible to modify a Goodman two-speed furnace, but I do not recommend the modification."
Straube went on to explain why some contractors are tempted to disable the high range of a furnace. “The way these two-speed furnaces are usually set up is that they always begin at low speed. If it is still running after, say, 7 minutes, and the thermostat is not yet satisfied, it goes to high speed. In most homes, you only need the high speed setting once in a blue moon, so disabling the high speed allows you to design a low-demand duct system. If you can be sure the furnace won’t burn more than 24,000 Btuh, you can design a smaller duct system. That’s the reason to snip the wire or adjust the dip switches. Then you know the furnace won’t go into high speed.”
In theory, a combo system — that is, a system that uses a single water heater to provide both domestic hot water and space heating — makes a lot of sense. After all, you end up with just one appliance instead of two; that means that you have only one burner to service and one flue to install.
In practice, however, combo systems are tricky to design. Moreover, it’s hard to find the right combination of contractors to install them, and the systems are often quirky to troubleshoot and maintain.
If you want to install a combo system, Straube advises installing a hydro-air system (that is, a system that distributes warm air through ductwork) rather than a system with hydronic distribution system (for example, one that uses fin-tube baseboards). The reason is simple: most new homes include air conditioning, so you’ll probably need ducts anyway.
Fortunately, small air handlers with copper hydronic coils are readily available. Options include the Rinnai 37AHB series hydronic furnace or an air handler from Ecologix or First Company. Straube advises that low-temperature systems — for example, systems designed for 120 degree water rather than 160 degree water — are often best. “I’ve used an Ecologix air handler with 90 degree water,” Straube said.
Combo systems based on tankless water heaters are tricky, Straube notes, unless the designer includes a buffer tank of some kind. “Instantaneous water heaters have lag times and minimum firing outputs,” said Straube. “The initial slug of water is cold. So systems with tankless water heaters should have tanks. A tank provides a buffer capacity.”
Straube often recommends tank-style condensing water heaters, which represent “a very safe solution that is pretty affordable ($2,000) and pretty efficient (over 90%).” These systems work best if the flow rate to the space heating coil is under 2 gallons per minute.
When I spoke to Straube on the phone, he elaborated. “With a tankless heater, I have to worry about coincident draws of domestic hot water and space heat. If you include a 40-gallon tank, there is so much storage capacity that you can’t really screw it up, even if you have a completely out-of-sync heating demand. The tank just smooths it all out.”
However, even systems that use a storage water heater can be somewhat tricky to design. “Use a small pump so you don’t remove all the heat output of your water heater,” Straube advised. “Otherwise you overwhelm the heating capacity of the water heater.”
Straube is aware that the history of combo systems is littered with design errors. “You need to choose a low-watt blower and a low-watt pump,” said Straube. “This requires design. That means it’s harder to get these systems into the mass market.”
A variety of solutions, all with disadvantages
Straube summed up by noting that “there is no single answer” to this problem. He listed several possible solutions, along with their disadvantages:
- A condensing furnace plus a natural gas water heater: the furnace is too large, and the water heater is not very efficient.
- An air-source heat pump plus a natural gas water heater: a typical heat pump (even a 2-ton unit) is probably oversized, and the water heater isn’t very efficient.
- A tankless water heater with a small air handler: this system requires an oversized air conditioner — a 2-ton air conditioner is often too large — and combo systems are tricky to design properly.
- A ductless minisplit unit with a conventional water heater: this system may need backup heat in very cold climates, and you still need ductwork for ventilation.
What’s on the horizon?
With so many builders frustrated by the problem of oversized furnaces, are equipment manufacturers paying any attention? Not really, according to Straube. “We’ve talked to Trane, Carrier, Lennox, and we’ve told them that we want a smaller, cheaper alternative,” said Straube. “Their response is always the same: ‘No, we’re not interested. We are not seeing any demand.’”
Martin Holladay’s previous blog: “Green Building for Beginners.”
- First Company
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