These Ain’t Yer Grandpappy’s Heat Pumps
Earlier this month I went to NESEA’s Building Energy conference, and I think I heard three terms more than any others: heat pump, net zero, and passive house. (The second most popular trio was beer, wine, and whisky, but that may have something to do with the folks I was hanging out with.) So let’s get right to the important question here: Why do these people in the cold climate of New England love heat pumps so much?
Why heat pumps?
Marc Rosenbaum gave a presentation titled, Heat Pumps – Data and Lessons Learned – A Nerd’s Eye View. He started out by giving four reasons he likes heat pumps:
- No onsite combustion
- No chimney or venting
- Space heating & cooling, domestic hot water, and pool heat
- Cost-competitive
Most of his talk was about mini-split heat pumps, but he also spoke about ground-source heat pumps (GSHP) and heat pump water heaters (HPWH). He’s not a fan of GSHPs. He said that although air-source technology is less efficient than connecting with the ground in theory, he’s found the reality to be otherwise. So he sticks with air-source heat pumps, primarily of the mini-split variety, for space heating and cooling.
Rosenbaum has had good experience with heat pump water heaters. One thing he’s looked at is the effect of location on their performance. He wanted to know, for example, if a HPWH located in a basement might create more of a moisture problem, with an increased likelihood of growing mold. Yes, the heat pump will cool and dehumidify, but that doesn’t necessarily mean the dew point of the air will decrease. By datalogging the performance, he found that temperature, relative humidity, and dew point all went down for a HPWH running in a basement.
Also, as you might expect from someone who writes a blog titled Thriving on Low Carbon, Rosenbaum compared carbon emissions for water heaters using different fuels. He found that going from an oil-fired boiler to an HPWH dropped CO2 emissions by 89%.
Andy Shapiro spoke on high performance HVAC on the second day of the conference and echoed some of what Rosenbaum had said. Then he got into more details about zoning with mini-splits and when you might be able to get away with a single ductless indoor unit to heat the whole house—and when you can’t. (Coming from the Southeast, I found it a bit odd to be in a conference with so many people who mentioned air conditioning mainly as an afterthought.)
On that question of heating a whole house in a cold climate with a single ductless minisplit heat pump, I recall a couple of years ago hearing homebuilder Carter Scott speak at Building Science Summer Camp about the superinsulated, net zero energy homes he was building. They were 2 story houses in Massachusetts, and, as I recall, he started off heating the whole house with a single indoor unit. He later added another indoor unit upstairs, not because the unit couldn’t keep up in winter but, if I’m remembering correctly, because the upstairs didn’t cool so well in summer. (See Martin Holladay’s article on Carter Scott’s homes for much more detail.)
Can they provide enough heat?
We have more than enough evidence to state unequivocally that heat pumps work in cold climates. Air-source heat pumps naturally lose capacity as the outdoor air gets colder. That means that you may need a source of supplemental heat if the heting capacity drops below the heating load. No big deal. We know how to do that. Most standard air-source heat pumps are equipped with electric resistance heat to supplement what the heat pump provides.
One of the really cool things about mini-splits, though, is that when you pair them with a low-load—or, as Andy Shapiro said, a micro-load—house, you may never even need any supplemental heat. Many models actually have more heating capacity than cooling capacity and can continue pumping heat from cold outdoor air at temperatures well below zero (that’s Fahrenheit because so many cling to that outdated scale). And this isn’t just a hopeful statement from manufacturers. Rosenbaum reported that his projects handle below-zero weather with no problem.
And the clincher
Heat pumps work. One piece of equipment heats and cools. It works in cold climates. It’s efficient. But if you recall my list from the beginning of this article, one of the other terms I heard a lot holds the key to perhaps the biggest motivation for many in the NESEA crowd. (No, it wasn’t whisky, though a nice single malt can be very motivating.) If you want a net zero energy home, the clearest way to get there is by going all-electric.
Shapiro said it best: “We know how to make electricity onsite.” Build an all-electric, micro-load home, add enough photovoltaic modules, and voilà, you’re at net zero energy use, producing as much as you use. Richard Faesy repeated and amplified the drive to going all-electric as a panelist in the closing session. “We need to electrify all fossil fuel end uses,” he said, because that gives us the best chance of reducing the effects of climate change.
Don’t get me wrong. There are still plenty of people in New England and other cold climates who love furnaces and boilers. The folks at NESEA’s Building Energy conference, however, are in the vanguard of those who are rethinking how we design, construct, and operate buildings. And heat pumps are just fine with them.
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Photo by 51% Studios Architecture from flickr.com, used under a Creative Commons license.
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Glad to see more people are
Glad to see more people are catching on. In cold climates, ductless heat pumps make are a no-brainer for most super efficient homes. Here’s a study on the use of air source heat pumps in Alaska.
Mini-splits generally have more heating capacity than a conventional ASHP. This is because they have larger compressors than the nameplate capacity suggests (ditto with Carrier’s Greenspeed). In order to get the desired AHRI cooling rating (and thus their nameplate tonnage), the cooling capacity is limited electronically. Nothing more than a bit of marketing slight-of-hand. Not that there’s anything wrong with that, as long as we understand what’s going on.
But ductless is usually not a good solution for high performance homes in cooling dominated climates. Most people will forgive a few degrees of undershoot in a bedroom during winter, but in summer it’s important to deliver adequate supply air to bedrooms. Unfortunately, the smallest ductless heads have way too much capacity for the typical bedroom.
These new ducted mini-splits
These new ducted mini-splits are leaving conventional heat-pumps in the dark ages technologically. I build hurricane resistant homes along the NC coast using pre-cast insulated concrete panels that are extremely efficient. I call them my “small, tall and tight” homes because the lots are narrow and usually in the flood plain. This means we have to go multi-story with a small footprint. To help counter the stack-effect we absolutely have to used a zoned system. To keep our air quality up even during the swing months, we need to keep air moving. The low tonnage requirements per floor and the limited floor space for trunking makes a traditional system difficult to design.
Enter the ducted mini-split. They are a perfect solution. My personal home is only 1000 sf on a level. The lower level is all garage and storage. The next level in two bedroom suites and the top floor is a vaulted open plan for living room and kitchen. I used a 3 ton Samsung compressor unit. The bedroom level has a flat 1 ton handler set in the ceiling of the garage with ducts running to the outside walls above in a conventional manner. The top floor has a 1 1/2 ton flat unit set in a center closet ceiling with a lateral trunk running across the width of the room with vents directed toward the outside walls. When we decided to enclose the porch on the water side with glass, I added a 3/4 ton wall unit for occasional use.
The system has worked perfectly as the all-DC motors provide variable speeds both for the compressor outside and the air handlers inside. We do hear a little air noise at the return grill on the top floor unit due to the velocity needed to throw the air to the outside walls. But the bedroom unit is totally silent with just a whisper of air moving.
The only time I will use a conventional system is if I were building a single zone ranch house.
To move the direction of
To move the direction of electrifying all fossil fuel end uses – there is a huge educational effort ahead. I audited a home with its own Natural Gas Well. The meter was a valve. There was no incentive to upgrade. So the original 1940 furnace was still present. The heat rise on the Data Plate was 250°F – 275°F.
The incentive for the audit was the gas field was going dry. All the neighbors had been converted, at the expense of the Royalty Owner. GSHP, ASHP, Propane furnaces.
No one was happy. They liked coming in from the cold and getting that blast of hot air from the register.
This one is about the customer’s perception.
To David Butler’s comment,
To David Butler’s comment, when will we see more studies and anecdotes on DUCTED minisplits? Since the tech is essentially the same, aren’t these tiny air handlers designed for ducting a viable, even great, solution for better distribution?
I installed a 12kBTU Samsung ducted mini in an unvented crawl space 3 years ago. It was 25degF when we first fired it and somebody commented “why is the heat strip running when we didn’t wire it?”. When he put his hand over the supply register in the floor, the air was coming out warm. Not cool like a typical HP in winter, but WARM.
The kicker is that there was no aux heat strip in the Samsung. It was generating incredible heat from, yup, air.
Allison — Your picture above
Allison — Your picture above makes me nervous. It looks like a lot of air recirculation would happen driving down the HP efficiency. I am also skeptical about electricity being the solution to CO2. Burning gas at the location is perhaps 80 to 95% efficient. Burning it at the power plant generally is not and there are transmission and distribution losses as well.
Thomas wrote: “…noise
Thomas wrote: “…noise at the return grill on the top floor unit due to the velocity needed to throw the air to the outside walls.”
This is a misnomer. Return velocity has no impact on supply register throws. Ducted mini’s generally have very little available static (your Samsunng EH052CAV only has 0.16″) so it’s important to make the return side “disappear” in terms of static drop. The best strategy is to use a filter grille (as opposed to filter at unit) with a face velocity below 200 FPM, or at least 360 sq.in. for that unit.
In any case, supply velocity becomes a non-issue in super efficient homes. As long as the right amount of air is delivered to the room, enough mixing will occur naturally to keep temperatures relatively even.
@Harris, ducted mini’s are indeed a good solution for some homes, but units like the Samsung that have ridiculously low static require special attention to the duct system. This not only limits their application but (in my experience) reduces the number of contractors qualified to pull it off.
BTW, ducted mini’s typically top out at 15 to 17 SEER, whereas some of the smaller single-zone mini’s have SEER ratings in mid 20’s.
Great recap of NESEA&
Great recap of NESEA’s Building Energy 14 (BE14) conference, Allison. I enjoyed Jonathan Wright’s presentation about his experience building net zero homes. He moved into one of his units on the day a storm took out the power for several days. Even though it was winter outside, the temperature inside his home only dropped 3 degrees.
I also enjoyed the data-documented stories about clients who turned off their HPs to save energy only to have them run full out to play catch-up when turned on. The following year had lower electric usage due to leaving them on and letting them run at maximum efficiency rather than maximum output.
I’m looking forward to BE15.
David B.:
David B.: Thanks for the Alaska paper. I hadn’t seen that before.
Thomas D.: We like the ducted mini-split, too.
John N.: If no one was happy, the designers did a poor job. As you know, fixing the enclosure should be the first priority, but even so, you can get hot air from a heat pump. Marc Rosenbaum reports temperature rises of 40 degrees in heating mode, and Harris Woodward in the comment below yours reported nice, hot air coming out of the register.
Harris W.: The word is getting out about ducted mini-splits, but they aren’t without their drawbacks. Limited static pressure (sometimes as low as 0.125 i.w.c.) to work with is one. Lower efficiency than the ductless models is another.
John P.: Yeah, that photo’s not ideal. I grabbed it from flickr and thought I’d see who mentioned it first. I should probably change it, but sometimes I like to make people nervous. ;~) On the subject of electricity vs. natural gas, your point is certainly a valid one. I think what Richard was talking about was electrifying end uses and then using a lot more solar generated electricity.
Dale S.: Thanks! I didn’t catch Jonathan’s presentation. Nor did I catch Bruce Harley’s talk, but I’ll get a chance to hear his at ACI National next month in Detroit.
“…although air-source
“…although air-source technology is less efficient than connecting with the ground in theory, he’s found the reality to be otherwise…”
I am eagerly looking forward to a future article which gives more facts on this statement.
Thanks, Allison. But after
Thanks, Allison. But after watching you blow ping pong balls into the air and scouring the internet, this GC (jack of all trades, master of several) is striking out. High static is obviously bad news, but I don’t understand how having too little static pressure is detrimental.
My homes thermal enclosures are topflight (last one was HERS 54 with R-35 walls, R-32 basement, great windows, 1.25ACH50, a 15SEER Carrier HP, electric tankless DHW, NO renewables). So, I’m not sure I need a lot of velocity at the supply registers since perimeter rooms are pretty thermally homogenous.
Can you provide your Allisonian explanation?
@Harris, I don’t have Allison
@Harris, I don’t have Allison’s wit, charm or good looks, but I believe I can clarify the low static issue. I think there may be some confusion.
You’re absolutely correct that ‘high static’ is a bad thing, and you’re absolutely correct that you don’t need “a lot of velocity” at the supply registers in micro-load homes.
HOWEVER, the available static specification is a hard limitation of the blower. The ducts and filter create the static resistance, using up the available static, if you will. So if ducts and filter exceed the static budget of the blower, then the CFM will drop off, perhaps precipitously. That’s obviously a BAD thing. And I can tell you with certainty that 0.12″ is barely enough blower power to overcome the static drop for the typical media filter, let alone any elbows in duct run-outs or boots.
If you want to drill down into the specifics of your particular setup, let’s do that off-list.
Enthusiasm is a great thing
Enthusiasm is a great thing and keeps us all going. It also can blind us to the realities when they occur. We recently tested two ductless multi-splits with two heads each. The airflow on high was 52% of specified airflow on one head and 70% in another with clean filters (only two of the four heads were tested). To determine is the only problem was airflow, we forced the full specified airflow through the units and retested. One unit had a maximum COP of 2.9 at an ambient of 68 degrees F — well below the anticipated COP at that temperature. Unit is rated at 3.44 COP at 47 degrees F ambient.
On top of that the units are controlled by non-adjustable algrythms that appear to be aimed at comfort without apparent regard to efficiency.
I’ve been living with a
I’ve been living with a single 1-ton mini-split LG heat pump in my house for some 4+ years now. The house is a conventional wood-framed house from the early 70’s. I’ve dramatically improved the attic and crawlspace insulation, and extensively air-sealed the home (plus added a 40CFM ERV). I haven’t used any other sources or auxiliary heating. While it’s in the mild Pacific NW, it’s worked flawlessly in my 960 sq.ft. house.
I recently installed a similar unit in my brother’s newly built, high efficiency home. It also works flawlessly.
A single ductless mini-split
A single ductless mini-split for a high-performance two-story house seems like it’s aided by natural convection and the stack effect. How do you think you would make this work in a house with a similarly high-quality building envelope but a one-story ranch layout? Put the unit above the hallway to the bedrooms? Use a ducted unit in a dropped ceiling with very very short ducts to the bedrooms? Give up and use two units–one for the living area and one for the bedrooms at the end of the hall?
Hi Nate. To begin with, I
Hi Nate. To begin with, I would never specify single zone ductless for a two-story home.
A single head may work in a super-efficient ranch home if there’s little or no cooling load. A lot depends on the diversity of the design loads. If bedrooms have any significant cooling load, you need direct supply air. Ambient transfer won’t cut it. Of course, the main factor that must be considered is homeowner expectations.
Instead of “how to make this work”, we should always start with… what’s the best hvac solution for this home given all the particulars.
That makes sense, David.
That makes sense, David. Comfort-wise, the occupants fall pretty far to the side of “efficiency and $$$ savings” on the continuum of comfort vs efficiency. This house (mine) is about to begin the process of getting a deep energy retrofit. It’s in north-central New Mexico (Albuquerque area), so there are heating and cooling loads. There’s already an evaporative cooler which works very well and will probably remain the primary cooling appliance for as long as the weather cooperates. No A/C right now. Grossly oversized 125k BTU 80% gas furnace with bad uninsulated ductwork in an unconditioned attic.
I’m not fixated on the idea of using only a single mini-split, but the way I’m thinking about this is that if I can spend more money on the building envelope in order to downsize the HVAC requirements such that a single ductless mini-split will work–especially as a replacement for the furnace–I’d much rather do that. Of course if a ductless mini-split will never work due to uneven temperature distribution throughout the house, then I’ll start looking in the direction of ducted ones or multi-head units with a single compressor.
Hello
Hello
This ducted mini heat pump sure has my attention. I live in WV. Below zero right now. I burn coal for a backup. My heatpump is 3 1/2 ton American Standard. I’m getting to old to shovel coal into the stove. Looking for another solution. I’m all electric. Thanks