Seems to me as more people go the heat pump route that we are getting more than a few posts a week where the complaints are "lack of heat pump performance in heating mode".
Seems like they are not able to keep up. No surprise to me.
Maybe shoddy installs
maybe inadequate sizing
maybe not set up properly
I will give them the benefit of the doubt
We will see what happens
It has been and continues to be a cold winter.
If you poorly engineer and execute the installation of a forced air furnace and central ac it will still sort of work. If you do the same with mini split heat pumps it won't. It is the same contractors and same market conditions doing both. Add to that the poor quality of refrigeration products(i have bought 4 dehumidifiers over the past 15 years, all of them have leaked, i know an anecdote isn't a statistical study but seems like i should still have at least one that works if they aren't all junk). the technology isn't the problem, the problem is in the execution (granted the efficiency falls to a point where they are just resistance heaters around 0f). remember how the us toilet manufacturers spent a decade obstructing instead of figuring out how to make a toilet flush with 1.6 gallons? now many of them work just fine on 1.6 gallons.
It’s a “new” tech and a lot of installers are resistance to any sort of change. They’re the 2nd most common heating method, I’m not surprised we hear from them! Boilers are a tiny minority and have all sorts of problems.
I am sure they are fine in more moderate climates. @mattmia, I think they have toilets that will flush at 1.25 gpm.
Reminds me of a job I helped start up in the early 70s. It was a brand new church with electric heat.
We started the heat up and I can't remember if it was duct heaters or AHUs with resistance heat doesn't matter. It was probably 40 degrees in there and all the walls were cement block and brick so it felt even colder.
The air discharge felt freezing cold with the heat on full blast because the air was moving.
I guess it must have worked because we never went back but it sure felt like it would never warm up.
When I had my current house built in the mid 1980s, I had a heat pump with gas backup installed. At that time, the lower end of heat pump performance was around 40F. I decided early on to use the "Emergency Heat" function on the thermostat for almost all of my heating. That's the gas backup, but "Emergency" is a lousy name for it in the upstate NY area.
A year later I bought my rental and over the years, as I wandered through the Orange and Blue stores, the folks trying to sell heat pumps and I have had short discussions revolving around "how low in temperature does the heat pump work?" Sometimes the answer is -13F. I then ask "What do you do when it is colder than that?".
One answer was "Wait until it warms up." Can you imagine telling tenants to do that?
I think the 2020s version can go as mentioned, but I question the "efficiency" at that low of a temperature. Isn't that the coefficient of performance (COP), which I think is the ratio of energy supplied to the building divided by the total energy to get that energy delivered?
I'm using an LG minisplit I installed in our office at the shop about 5 years ago and it never has a problem keeping up. It was right around 0F here a few times this year and the office was fine. Funny thing is, I do hear it defrost when it's cold out, but I've never actually felt it. Doesn't seem to ever blow cold out inside the building, ever.
I'd say anytime a heat pump can't keep up it was obviously sized wrong or installed poorly. But there's also those odd posts where someone expects it to maintain 85F when it's 0F out and that just seems unreasonable to me.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
We go below -13F up here near Saratoga Springs.
There is an underlying problem here. Physics is a b___h. She just doesn't always want to do what people want her to do.
And the problem is this: it is astonishingly difficult to find refrigerants which have a reasonable phase diagram to allow for the hot deck to be much more than 100 F to 120 F warmer than the cold deck given practical restraints on pressure differences (water, by the way, does have a wider feasible range — but water is otherwise very strange stuff, and the minimum cold deck temperature is around 35 F anyway…). Different refrigerants have different usable ranges, of course.
This has never been a problem with mechanical air conditioning, as the sold deck is normally not much colder than 60 F, and the hot deck — even in some pretty horrid climates — is not much over 140 F or so — so no problem with the correct refrigerant. This is also not a problem with food storage and such applications in most cases, although freezer storage (large buildings) can be pushing the limit unless evaporative cooling can also be used on the hot deck side.
But.
Now you want a heat pump, and in order to make it saleable in general you want it to be reversible so you can get some air conditioning in warmer weather but heating in cooler weather.
Well, for the AC side, you want the cold deck at 60 F and the hot deck at say 140 F. Great. Select a nice refrigerant and compressor and off you go. Then it's October and January, and you want the hot deck side at 120 F but the cold deck side goes to -20 F. Ooooops… you have now asked a refrigerant which had acceptable characteristics at 60 F to work down to -20 F — effectively acceptable properties with a range of -20 F cold deck to 140 F hot deck.
An incredible amount of research is being devoted to finding refrigerants which have acceptable cold deck/vapour pressure hot deck/vapour pressure combinations which aren't toxic, explosive, bad for the environment, whatever. There is also the workable combination of two stage heat pumps (two separate circuits with an intermediate heat exchanger which is the hot deck for one circuit and the cold deck for the other) but they are complicated — and expensive.
In the meantime, so long as folks can be persuaded or compelled to buy single stage heat pumps with a working range between 0 and 100 F, we'll have what they have —but no amount of hope or stardust is going to make them work well beyond their design range, and what you gain on the cold deck side you are going to lose on the hot deck side.
It’s a solved problem yeah? For 90% of Americans? How many of us experience both 110F summer and -20F winter?
If equipment is undersized, or installed in a location where it is unable to function properly, that all goes back to the installer, no?
It's certainly not the equipment's fault. All of this info is provided by the manufacturers.
You wouldn't use a swamp cooler in Florida either, but that doesn't mean it's a bad product.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
When the OA drops below 30 the outdoor coil will begin to frost. There is no fix for that. The heat pump will have to defrost and run the electric back up just when you need it to produce the most heat.
This has to do with outdoor temp and has nothing to do with the refrigerant selected.
The colder the oA at the installation site the more it will need to defrost.
The other problem is an indoor comfort problem. Air moving at a decent velocity at 100-110 or below will feel cold.
Defrost is a solved problem. Modern heat pumps can defrost without any resistance AND not blow cold air.
Like I said previously,
I hear the minisplit in the office defrost but I've never noticed it other than that. It never blows cold air.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
Well… I'd agree. Very few of us. However, you have to use a different refrigerant or compressor or both to cover both ends of that range — so the heat pump you manufacture for many people won't be suitable for at least some other people. So — you build the best heat pump that you can afford to sell the majority of the people. I have no problem with that. Just don't try to sell it to the people for whom it simply won't do the job.
Not to mention the varios quite valid concerns that @EBEBRATT-Ed mentioned.
@Jamie Hall Manufacturers build different machines for different purposes. They've been making cold climate and not-cold climate heat pumps for a decade now. I'd argue this is solved.
Fujitsu’s run the indoor fan on low and the louvers straight out during defrost.
Worst conditions for a heat pump is between 35 - 40* and raining.
The LG seems to close the door and then I don't know what it's doing.
Doesn't seem like it runs the indoor fan at all, but I have no idea how that would work.
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
they drop to low. The system needs some heat for defrost.
Anything more than one speed was insanely expensive 20 years ago. 2 stage is probably they way to go once someone figures out how to do it cheaply. Maybe 2 rotors and pumps in the same hermetic housing separated by a partition and hx so the evaporator with one gas connects to one side and the condenser with another gas to the other side. Maybe it uses an antifreeze solution on a condenser and evaporator so it can reverse with a common fluid.
State of the art is variable speed compressors and fans.
that doesn't solve the working temp of the refrigerant problem
Precisely
When the temperature drops, heat pump output drops, and efficiency drops.
The output drop is very predictable, it's just incompetence to install something that doesn't have the needed output.
The biggest issue I'm seeing is people who were led to believe the heat pump would save them a lot of money and then are shocked when they get the bills the first winter. Fuel costs vary a lot regionally, but in a lot of the country, particularly in places with climates that are well-suited to heat pumps, the cost of electricity and other fuels are very close. The only time it would make any economic sense to switch is when a system needs replacement. And even then it's at best a wash.
I'm in the north edge of zone 5 and heating a couple of places with heat pump only. No backup. These are cold climate "hyper heat" units rated down to -22F. That rating doesn't mean they shut down though, they continue to run at reduced output.
Even using a mini split to heat a cottage further north in zone 6 with real winters where -25C to -30C (-10F to -20F) is common. Works no problem, blowing above 100F air.
With the current vapor injection compressors the working temperature issue is solved.
Most units are above COP of 2 at 5F, and still well above 1 bellow that. ie:
https://ashp.neep.org/#!/product/65014/7/25000/95/7500/0///0
I know this will make me sound like a zealot, but in any place that needs cooling, fuel burners for heat will be dead in a decade. Not because of some green hug the planet reason but simple economics. BOM cost is comparable, install is easier and operating cost is about the same.
California has its own state energy code, and it's instructive to see what they're doing. They require that heat pump installations be engineered, and they require a more rigorous process than Manual J. They require that local historical weather records be used to get a typical distribution of temperatures, then a calculation of COP at each temperature so that a weighted annual COP can be calculated.
They also require that the design temperature be between the 99th percentile and the average annual minimum. This results in larger sizing than what you'd get just using Manual J and 99th percentile. With modern variable speed heat pumps oversizing isn't the concern it used to be with fixed-speed compressors. By oversizing they reduce the dependence upon supplemental heat. They're worried about the effect on the grid if a cold snap happens and everyone turns on their supplemental at once.
There are a few places that are just too cold for the current crop of heat pumps, some of them require air conditioning in the summer.
I do think that within our lifetimes the idea of burning oil in your house for heating will seem as strange as burning whale oil for lighting.
or coal…
My challenge still stands…
I do find it curious, though, with all of the enthusiasm I read here, that friends and relatives of mine in the UK, particularly the north of Scotland, report that despite generous subsidies and rather draconian regulations, heat pumps are — to put it mildly — a flop, at least in older properties. The climate is no worse than southern New England… and I would think that they would have access to units at least as good as we do…
A few families I know personally, having been obliged to tear out their perfectly good natural gas fired heating systems and install heat pumps find that all that lovely hardware and glistening controls and so on… don't work. And they now have resorted to burning peat, the way folks did 200 years ago.
Edit to add: i'm not opposed to heat pumps in principle. In fact, there is one in an apartment in Cedric's home which works well enough to take the edge off down to about 5 F above. Cedric does the heavy lifting. What I am driving at, and politely decline to stop, is that they work well and are an excellent choice in some — indeed, many — situations. But they aren't suitable everywhere, and they must be chosen, engineered, and installed with great skill.
so vapor injection basically uses a low temp refrigerant and a 2 stage compressor?
Drill a vertical hole and then they work in any climate
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Vapor injection basically runs some of the refrigerant through the compressor twice on each cycle.
And can improve efficiency and hence COP. What it CAN'T do is change the temperature/pressure phase change relationships of the refrigerant, and thus the practical hot deck/cold deck temperatures and temperature difference of a particular refrigerant compound.
Vapor injection makes the low-temperature output higher while reducing the COP.
This is good info about how EVI works:
@Jamie Hall I'm not sure what temperature limit you are worried about. People use heat pumps even in zone 7, doesn't get much colder than that around here. The newer stuff with R32 and R290 does allow for a bit higher temperatures which does make life easier.
if i'm thinking about it the low end is going to be much more of a problem than the high end. the high end eventually at some pressure it will condense. at the low end you might end up needing a deep vacuum for it to evaporate if you choose the wrong refrigerant.
That seems like it would reduce the output of the compressor, but what else does it do?
Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.
Lowers the discharge superheat.
Exactly. It is almost entirely a matter of determining how low an absolute pressure you can create on the cold deck — and how high a pressure you can tolerate on the hot deck. In principle, the choice of refrigerant is more constrained by the cold deck temperature, since purely mechanical considerations start getting in the way as you approach an absolute pressure of zero. One can raise the hot deck temperature simply by raising the pressure (in most cases — a few refrigerants start to do some very odd things at very high pressures). Of course, as the pressure difference across the compressor increases, so does the power needed to drive the compressor — which reduces the COP of the system.
Interesting game.
To understand vapor injection, think of how the heat pump cycle works. When a gas condenses into a liquid it gives off heat. When liquid evaporates into a gas, it absorbs heat. The heat pump uses a compressor to increase the pressure of the gas to where it condenses at the temperature you want heat, and then allows it to expand to where it vaporizes at the temperature where heat is drawn from.
In a residential heat pump, the refrigerant might come off the compressor as a high-pressure gas at 110F. As the refrigerant passes through a coil room temperature air is blown over the coil, warming the air and condensing the refrigerant, which leaves the coil as a liquid at 70F. The refrigerant is then sent to an outdoor coil that may be at 0F, it is allowed to expand which drops its temperature to -20F. Air is blown over it to warm it to ambient, 0F, and it goes back to the compressor and the cycle repeats.
When the refrigerant comes off the indoor coil at 70F, it contains heat that is lost when it's sent outside to 0F. Vapor injection captures that heat and makes use of it. After going through the indoor coil, some of the refrigerant goes through an evaporator and cools, it then goes through a heat exchanger with the rest of the refrigerant. That refrigerant then goes outside already cooled, and the refrigerant that cooled it is warmed to room temperature. That room temperature refrigerant is mixed with refrigerant returning from the outdoor unit to raise its temperature.
So to the compressor it appears that it's warmer outside than it really is, which helps capacity and efficiency. The downside is that only part of the refrigerant is going through the outdoor coil and extracting the heat which warms the house, which would reduce both capacity and efficiency. Empirically what's been found is that a compressor running vapor injection at full output will have a higher output and lower efficiency than the same compressor running at full output without VI.
Good explanation. Thanks, @DCContrarian
