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 load has been calculated at between 28,000 and 36,000 Btu 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 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 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 GBA 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 therm. 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 COP 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 inverter, 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 pump.
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 coating. 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 (HDD) 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 efficiency numbers,” Dorsett says.
Plus, he says, the air handler for a ducted heat pump uses 10 times more electricity than an HRV 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 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.