More Building Science
With all the incentives for heat pumps and the talk of electrification these days, you may be thinking of going with a heat pump. Heat pumps can work in both new and existing homes. But can you be comfortable with a heat pump? If you’ve been reading this blog for a while, you know my answer. Let me explain.
The temperature of heated air
First, let me say that air temperature is not the main factor that determines thermal comfort. Mean radiant temperature is more important. But for this article, I’m going to focus only on what forced-air furnaces and heat pumps do: blow warm air into the conditioned space.
A furnace burns a fuel (gas or propane) and transfers most of the heat of combustion to the air passing over the heat exchanger. That air then gets blown into the different rooms in your home to keep you warm in winter. The furnace-heated air coming out of the vents in your home is about 125°F. (There’s some variation, but this is typical.)
A heat pump doesn’t convert another form of energy to heat. Instead, it just moves heat. In winter, it moves heat from outdoor air to indoor air. And yeah, there really is enough heat to heat your whole house even on a cold day . . . assuming the system has been designed and installed properly. The air temperature coming out of a heat pump, though, is only about 95°F.
Why a lower temperature is better
We don’t need 125°F air to heat our homes to 70°F. Even if you set the thermostat higher, say 75°F, you can still heat your home with 95°F air. Think about it. You add 95°F air to 70°F air. The heat from the warmer air transfers to the cooler air and keeps you warm.
So why is lower temperature air better for comfort? Well, to get the same amount of heating from lower-temperature heat pump air, you need more airflow. That additional airflow is the key to comfort.
With high-temperature air, the furnace blasts you with hot air for a short time and then goes off. Then it comes back on a while later and blasts you again. So you’ve got what can be a noticeable variation in temperature over time. The big difference in temperature between the room air and the heated air also creates variation in temperature from one part of the room to another.
A heat pump, by contrast, adds a greater volume of lower temperature air. That extra airflow and lower temperature means a more uniform temperature. And that means you’re not as likely to notice temperature differences from place to place or from time to time.
The importance of good design
Now, the big caveat here is that you need a heat pump that’s sized properly for the heating load in the house. You also need a system that can provide enough heat even when the outdoor temperature is low. That means having the right kind of heat pump or having auxiliary heat available for the cold days.
Then there’s the distribution system (ducts plus supply registers and return grilles). The ducts and fittings need to be selected and sized to distribute the right amount of conditioned air to the rooms. And the duct system needs to distribute that air to the right places.
One of the most important rules for designing a duct system is not to blow air on people. You want to put the supply registers in places where they’ll blow the air into the room in a way that it gets well mixed with room air before entering the occupied zone.
That last point is the answer to what HVAC professionals call “cold blow.” Yes, heat pump air is cooler than furnace air. If you blow it right at a person in the room, they may feel a draft. The problem isn’t the heat pump, though. It’s the distribution system. (ACCA Manual T provides guidance for selecting, locating, and sizing registers and grilles. It’s rarely used.)
The reality is that properly designed heat pumps can provide more comfortable heating than furnaces.
________________________________________________________________________
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has written a book on building science. You can follow him on Twitter at @EnergyVanguard. Photos courtesy of author.
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
17 Comments
The 2 stage and modulating forced air gas furnaces with ECM motors are very quiet and subtle in the movement of air. I have had a 2 stage since 2007 and it has been wonderful. This furnace replaced an old GE 150k gas guzzler, I would compare my 2 stage furnace with ECM fan to that of hydronic baseboard for comfort.
Doug
Modulation AND right sizing is the key. The few furnaces with >1 stage are still laughably too big, they need more product offerings which actually fit American's needs.
Yes, there are a lot of two-stage furnaces that are oversized even with just the first stage running and have never fired up stage 2.
I'm not convinced that we should always equate uniform temperature with comfort. The ideal temperature depends on what you are doing (how active it is) and can also vary among family members. The old scenario of a wood stove in the middle of the house didn't always result in comfort, but it did allow pulling a chair up close to the stove to be very comfortably warm while reading a book, while you might do something more active in a more distant room and still be comfortable.
Charlie: Good points. Still, I'd rather have a heating system that provides more even heating than being blasted every few minutes by overheated air.
Hmm... with a heat pump providing cooler air at higher volume than a furnace, presumably it just cycles on and off less often. Surely a furnace can be set to provide air @ 95ºto reduce cycling?
user-723121 , I'd be curious to see specifics on your furnace, which sounds like it's a well-engineered and properly-installed unit.
I'm a solid fan of hydronic baseboard, because it provides silent, even heat without air blowing about. Perhaps by the time I'm ready to move in a few years, there will be affordable heat pumps that can provide 130º-135º heat for hydronic baseboard. and maybe even pumps that require much less electricity to run, although I think the compressor may be the main culprit here, and I'm not sure how that can be made more efficient.
"A heat pump blows lower temperature air into the house, and that’s better for comfort", says the text below what looks like a ceiling duct (?!?!) Not a good location for any forced-air system, I'd say.
Lennox 66k high fire, 45k low. There was a 45k high and a 30k low available (in 2007) but I chose the 66k because I had planned on a deep nightly setback. My design temperature heat loss is about 30k for Minneapolis for 3328 sf, a rambler with 1664 sf main and the same for the lower level. I set my thermostat for 3 cycles per hour, so run times would be less often but a bit longer each cycle. Only repair so far is a new inducer fan and have flame sensor cleaned. Would not change a thing if a replacement is needed.
Doug
Thanks, Doug. Very informative. (I'm somewhat familiar with Minneapolis temperatures as my go-to radio station is MPR Classical, playng as I type this.)
>"A heat pump blows lower temperature air into the house, and that’s better for comfort", says the text below what looks like a ceiling duct (?!?!) Not a good location for any forced-air system, I'd say.
Sure it is- it's a GREAT location (for a forced air COOLING system.) Most homes won't have separate duct systems for each. In Allison Bailes' region cooling loads are comparable to or often exceed heating loads, and a large fraction (most?) homes are built slab-on-grade, making it difficult to place ducts under the floor.
Agreed- for cooling. I've often wondered how well the cooling cycle of heat pumps works with floor registers! It seems having gentle circularion throughout the house is important for comfort. My current house is small (~1100 sf. main floor) and open plan with a cathedral ceiling and a 52" ceiling fan which runs on low 24/7 and provides barely-perceptible air movement throughout the main room.
Out here, heating is the main issue. Separate ductwork for heating and cooling would be a lot of work and expense. I lived in a couple of on-slab basement suites with ceiling ducts when I was a univ. student and a few years thereafter and it worked well enough, but if I were building a new house on slab now I'd find a way to bring heat in at floor level. Although with high-performance windows nowadays it's less critical to place the heat source underneath the windows.
Dana: Yes, providing heat lower in a room is generally better, but ceiling registers can work just fine in cold climates, too. If you have a good building enclosure and you follow the guidance of Manual T for placing, selecting, and sizing registers, it doesn't matter nearly as much. Of course, in a house with high ceilings, lower is better for heating.
Also, we do more heating than cooling here in Atlanta. My heating load is ~50 percent higher than my cooling load (see table below), and we heat for about five months and cool for about four.
And my ceiling registers keep the house nice and warm in winter. They worked even better during our December arctic blast that took the temperature down to 7 °F because our variable speed air handler ramped up to high, yielding more throw and better mixing.
Has anyone experienced heat pumps being less comfortable in mild winter climates where temps are often in the 30s or 40s? We're right on the border between climate zone 4 & 5, and our HVAC contractor says they've had clients complain about cold air coming out of the registers when the heat pumps are in their deicing cycles. He says the deicing cycles frequently happen because the coil gets just below freezing while the outside temp is just above freezing.
Yes. We have comfort challenges in Southern Oregon where morning temperatures are typically in the mid to low 30s. Defrost cycles lead to 7-15 minutes of cold blow. Our auxiliary booster electric strips assist with defrost but consume 10,000 watts when active. When not in defrost mode the system delivers 90F air and uses just 1500 watts.
@piperspace, "...Our auxiliary booster electric strips assist with defrost but consume 10,000 watts when active..." Aghhh! For me, this is absolutely unacceptable. Part of "going green" has also to be to use less energy and putting less demand on the electrical grid. Imagine thousands of houses eventually drawing 10,000 kW in the early morning...
Clarification - the booster pulls 10,000 watts (40 Amps) NOT 10,000 kw. The underlying issue is that we slapped a central heat pump into a leaky house without evaluating the ducts. Even so the overall energy cost did not go up. If I had it to do over I would have used ductless units. In newer construction the ducts can be larger to reduce noise and built inside conditioned space. As I understand it some of the ductless units manage the defrost cycle with less effect on comfort. I would love to see some data on the relative energy cost of central vs ductless defrost. All heat pumps have this challenge.
@piperspace: Oh dear.... where's my editor when I need one? I was deciding whether to write 10,000W or 10 kW and ended up writing both! Clearly anything that needs 10,000 kW will need its own hydroelectric plant. But even 10,000W (yes, I checked that this time) is way too much, although as you say, it is a characteristic of heat pump operation in cooler climates. (It was 34º here when I got up this morning, and only 43º now at 9:15 am.)
Aside from that, I don't much care for wall-mounted units and would certainly prefer the .larger, quieter ducts in a conditioned space. Fortunately I'm not yet in the market for a heat pump, and by the time I am, technology may have improved in a number of areas. Keeping an eye on GBA helps!
Initial caveat: I'm all for greening the grid, electrifying everything possible, and reducing the use of fossil fuels as fast as possible. And, let's weatherize as many existing homes as possible, so they're ventilated, air-sealed, and insulated optimally. (I know I'm preaching to a relatively small choir here.)
Question for Allison: What about cold-climate retrofit situations where the distribution system is NOT designed to avoid blowing cold air on people? "Ductwork is installed, not designed" was/is a maxim in the residential weatherization biz hereabouts. And, not all furnaces suffer from Allison's "With high-temperature air, the furnace blasts you with hot air for a short time and then goes off." Why not? Furnaces are available with continuously modulating firing rates and "variable speed" ECM air handlers to match the air flow to the firing (or cooling) rate.
For example, when we replaced our furnace and air conditioner several years ago, cold-climate air source heat pumps were bleeding edge technology here in Madison, WI. (Most local contractors still aren't selling cold-climate heat pumps, but many are willing to sell heat pumps when replacing an air conditioner - sometimes at an inflated price.)
We chose a Bryant Evolution furnace and air conditioner. (See disclosure below). The 60,000 BTUH furnace can fire at 40% of capacity. The 5-speed, 24,000 BTUH inverter-controlled air conditioner can cool at 20% of capacity (4800 BTUH - about the same as a small window air conditioner). I have to put a hand near a register to tell whether or not the furnace and AC are running. Or, I'll look out a window to see if the fan blade is spinning on the outdoor condensing unit to tell if the AC is running. (Once an energy nerd, always an energy nerd). We can briefly hear the inducer fan purging air before the furnace starts to fire, but once it's ramped down it's so quiet that I sometimes put my hand on the supply plenum to tell if the furnace is firing.
The 5-speed AC provides superior humidity control. The much longer cooling cycles at low cooling capacity remove more humidity on sticky days. The downside to purchasing this top-of-the line equipment, was that Bryant's proprietary ($400+) Connex thermostat is required to fully enable these variable rates for heating and cooling. (I preferred the user experience and energy reporting from our Ecobee thermostat that got replaced by the Connex. )
Attached is today's photo of our heating and cooling consumption via the thermostat. (It agrees with our actual consumption.) Our house was built in 1938. We're home during the day, and heat to ~70F when we're home and awake. We cool to ~76F whenever the summer humidity becomes uncomfortable.)
Disclosure: I'm not endorsing Bryant/Carrier/Payne (they're the same equipment - just 3 different labels) over competitors with similar variable capabilities. We received rebates from Wisconsin's statewide "Focus On Energy" program for the thermostat, furnace, and air conditioner, which reduced our cost.
Bio: I weatherized over 1000 homes from 1979-1996, and paid contractors to install over 1000 high-efficiency furnaces in the late '80s to early '90s, shortly after they hit the market. Home Energy Magazine published this 1994 article, "Condensing Furnaces: Lessons from a Utility": http://homeenergy.org/show/article/nav/heating/page/7/id/1086 (Yes, it would need updating 23 years later.)
Log in or create an account to post a comment.
Sign up Log in