I keep seeing homes for sale with heat pumps. It's been a while, but I have seen -25F in winter, so I am puzzled why folks use heat pumps here. Had a h.p. in Florida, and when the temp got down to the forties, it wasn't warm enough indoors with the h.p. Is there new technology?
Are those heat pumps air-based or geothermal? Efficiency has increased over the years and air based ones can be efficient sources of heat even down to 20F or so. But if you see -25F in winter, unless it's geothermal, or dual fuel, forget it. Here in NJ, where temps only reach the single digits rarely, teens sometimes, I don;t know of anyone that has an air-based heat pump.
Ground source heat pumps work fine in colder climates.
I have a 3 yo Amana HP with propane backup here in western NC. It was the most efficient unit at the time. You get very little heat when the temp is in the mid 30s. It does use the propane during the de-ice cycles periodically. At about 30, it switches over to 100% propane. During the winters of 09 and 10, we used lots of propane, probably more in 10. In 11, we used very little, as it was warmer. I'm sorry we didn't put in geothermal because there were a lot of tax incentives at the time, which probably would have made it economical. BTW, I don't know about all the geothermal units, but my niece has a geothermal unit and there is nothing visible outside. The heat pump compressor and heat exchangers are all in the inside unit (Carrier).
I am in PA and friends put in a Mitsubishi split system a few years ago. My friend is a really sensible guy who is analytical and not a story teller. They decided to go with the version that also can be used as a heat pump mainly because it wasn't that much more and for a backup. They also have an oil fired boiler for hydronic heat.
They used it for the only source of heat the past two winters and the house was comfortable. We haven't been down to -25F though and I think the lowest we see here is -10 F in February.
He also ran the numbers and they were able to heat their house for less than using oil.
Where is here?
More to the point than having once seen it, what are the averages? Certainly a modern properly sized air-exchange HP will handle into the lower 30's w/ no problems. The one thing is that they will tend to have longer recovery time as the exit air temp's will not approach those of conventional furnace (or even a geothermal unit).
They certainly have improved even air-exchange but in very cold climates either they use geothermal (ground loop/well/deep reservoir all possibilities) or have a conventional backup.
We replaced an early air-exchange in E TN w/ geothermal and it was night/day...this is almost 20 yr ago now and the original HP wasn't but a cheap contractor-supplied unit in the late 70s when even the good weren't doing all that well, anyway. But, TVA power was cheap then so the resistance heaters weren't a big deal--probably 70% of heating in the area was electric at the time. By the early 90s the HP was on last legs and power rates were much higher and NG wasn't yet available so went w/ the higher installation cost of digging the trench and never regretted it while were still there. AFAIK the system is still functional w/ no problems--saw the new owner just a couple of years ago and he's also quite pleased.
It has a thermistor in-line w/ the aux heat that keeps them from being able to come on above about 18-20F so the 'Emer Heat' logic doesn't try to kick on if, for example, the house has been turned down and then come home and reset to normal so the dT is large and it thinks it can't catch up. Newer t-'stats may also have better logic to know the difference between chronic failure to keep up and change in demand.
A side benefit of the geothermal was essentially free hot water in summer using reject heat from the cooling cycle...replacing electric water heater inputs is _a_good_thing_ (tm) in most places given rates...
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Ground source heat pumps work fine in colder climates.
Usually when it gets very cold (say somewhat below 25 deg C) ( I am in the south so that is very cold to me) the heat pump will turn on the resitive heating elements if it can not keep up. It may be the heating elements turned on, but even at that, the electric heat may be cheaper than the price of oil in an old furnace.
He has a Mitsubishi inverter multi zone heat pump (split system). It doesn't have any resistive elements.
According to the graph it has 100% heating capacity down to 5F outside which tails off to 73% at -13F.
That sure seems like an impressive unit. I did see what appears to be some resistive heating elements in the individual units that seem to be an option.
Being a multizone unit, that should save a lot on the heat cost. I wish my heat pump was set so I could cut off part of it. I have 2 bed rooms upstairs that are not used any more and it would be nice to eliminate all the heat and cooling going to them. Also a room in the basement has some ducts going to it that would be nice to completely close off when I wanted to.
I think this is just another sales job and you're on to the truth. It only shows that it has 100% CAPACITY down to 10F, not how it gets that capacity. Unless the laws of thermodynamics have been repealed, the efficiency of ALL heat pumps declines as the outside temp drops. It's a matter of physics that no manufacturer can avoid. They can still deliver more heat than a resistive heating element would at 10F, but the amount of heat you get out of the heat pump drops on a steady curve as the temp goes down.
The only way they can get 100% capacity across that broad temp range is either by:
A - using resistive heat to supplement
B - Downrating the whole thing so that it's rated by what it produces at 10F.
Option B is nuts for obvious reasons.
I think you would want to contact Mitsubishi and explain to them that the systems they sell can't work as described (hint, Mitsubishi makes really well engineered stuff and they tend to be really anal about describing actual performance)
Then you contact my friend and let them know the performance he actually witnessed isn't possible in his system since it doesn't have resistive elements.
And maybe you just don't see that everyone who posts in this group isn't heybub just telling stories to screw with people...
I looked at the brochure in more detail. It appears that they do not use resistive heating elements. Now I suggest you look at the actual spec data in the brochure. Look at page 12, about 1/4 of the way down. It states that the rated capacity for the first model is 10,900BTU at 47F and
6,600BTU at 17F. For the largest model, it's rated at 18,000BTU at 47F and 11,300 at 17F. And then maybe you can explain how that jives with the performance curve in the graph that shows 100% capacity down to 10F. It is however consistent with physics and what we know about heat pumps, regardless of who builds them.
Isn't that a little different than it can't work and it must have resistive elements?
Heat pumps can work at low temperatures, but depending on the unit, the efficency is such that below a certain temperature, the resistive elements cost less to run than the heat pump compressor system. I don't know what it is now or with their system, but it used to be around 25 deg F. was the break even point.
Where I live, it seldom stays below 20 deg and most of that time it is night and warms up during the day. It has gotten to just below zero, maybe to minus 10 about every 15 years or so during the night. The heat pumps work fine in this area, with the resistive heat strips comming on only if the heat pump can not maintain the house at the setpoint.
The issue was how these Mitsubishi units could maintain
100% output all the way down to 10F. In my post, to which you objected, I said there were TWO ways they could do that:A - They use resistance heating to supplement the output at lower outside temps so the output remains 100% as the temps drop.
B - They downrate the output at the higher temps so that it's rated the same as the lower temps.
And I started off the post by saying it looks like another sales job.
So, after you objected and defended Mitsubishi, I go back and look at the actual performance data in the specs in the same brochure. And those specs show that the rated capacity at 47F is 10,900BTU while at 17F it's only 6,600BTU. It has 60% the heating capacity at 17F that in has at 47F. Exactly what we expect from a heat pump. So, it was not option A or B, just another sales job, ie marketing BS.
I never said it had resistive heating elements for sure. And apparently I was spot on when I said it looked like another sales job. You bought the BS hook line and sinker and when I pointed out that it doesn't add up, you defended it as an example of great engineering from Mistsubishi. The issue wasn't whether it has resistance heating elements. The issue is that graph is some kind of marketing BS, and it is not some miracle unit that has the same heat output at 10F that it does at 47F.
You still think these are some wonder units that defy the laws of physics?
Let's see. You get 3.413 BTU per watt hour of electricity for resistance heat. For the outside unit, that comes to about 1630 watts or 5570 BTUs if you put the power into a resistor instead of the compressor and fan. Plus you have the add the indoor fan. And, I know, I didn't add anything for power factor. But, it shows that you are almost getting the same amount of heat as you would if it were a resistive heater.
You didn't document the numbers you were using, but I don't believe the analysis is correct. Any heat pump system is going to give you a lot more heat out than a resistive heater, 2 to 4 times as much depending on the design, temps, etc. If they only produced heating similar to resistive heating, you wouldn't see them used and you wouldn't have folks bitching about the high electric bill that occurs when they go to resistive heat mode.
His analysis is correct, as I understand the data. What's missing here is that not all of the heat is useful. Some of it is created by the outside unit so doesn't contribute. As noted (by everyone here) the efficiency of a heat pump goes down with rising delta-T. At some point the "heat pumped" = "wasted heat", so resistive heat would be equally as efficient.
No, resistive heat is still 2-3x the cost of heat from a heat pump. People do bitch when their bills go up 4-5x. How many really know what it would cost to heat with resistive heat?
His "analysis" doesn't make any sense. He doesn't even make it clear which heat pump unit he's talking about so we can see what numbers he's using.
His bottom line is that he is saying that with a heat pump you get about the same amount of heat as you would with a resistive heating element that consumes equal electricity.
Why do you think people bother with heat pumps at all if you get about the same amount of heat from a resistance heater? Everyone would just use a cheaper resistance heater where ALL the heat would go into the house.
Obviously what is missing here is that you are wrong. OK so according to the two of you, the total heat generated by a heat pump is about the same as a resistance heater that uses an equivalent amount of electricity. And according to you, some of that heat doesn't make it into the house with a heat pump because it's lost outside. Therefore, a heat pump system would produce LESS heat into the house than a resistance heater? Not in my world of physics. Nor in the world where you see heat pumps used because they deliver a lot more heat for the same electricity usage as a resistance heater.
Now, why on earth would that be? You agree with the faulty analysis that says the heat pump only generates heat about equivalent to a resistive heater. And you say the other issue is that with the heat pump, some of that heat is also lost outside. So, how could resistive heat cost 2 -3X as much?
Now go ahead, instead of admitting you made a mistake, double down and as usual start with the name calling.
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