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Heat Pumps Take on Cold Climates

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Re: Heat Pumps Take on Cold Climates

#61

Post by throw0101b »

cwoolfe wrote: post #11

As someone who switched to a heat pump recently, you should know "your mileage may vary." Don't confuse physical efficiency for cost efficiency. In many places, natural gas is still more cost efficient, even if it isn't as efficient in terms of physics.

> In many places, natural gas is still more cost efficient, even if it isn't as efficient in terms of physics.

As page where you can enter some numbers:

* https://www.heatpumpswork.com/cop-target

Also, given a US zip code, how often the temperature may drop below 15F / -10C (which is a cut-off for many units):

* https://www.heatpumpswork.com/cold-days

A good HVAC contractor will create a design based on your building and location, and it will cover 99% of average days efficiently for heating and cooling:

* https://www.greenbuildingadvisor.com/article/design-temperat...

* https://www.airequipmentcompany.com/2021/what-does-design-da...

For the remaining 1% it will basically have to run all the time to keep up. (In any given year you may not even get below/above the average outlier 1%.)

Re: Heat Pumps Take on Cold Climates

#62

Post by throw0101b »

electric_mayhem wrote: post #32
cwoolfe wrote: post #11

As someone who switched to a heat pump recently, you should know "your mileage may vary." Don't confuse physical efficiency for cost efficiency. In many places, natural gas is still more cost efficient, even if it isn't as efficient in terms of physics.

As someone who switched four years ago to a heat pump plus conventional furnace, I can state from experience that my thermostat takes electricity cost per kWh and gas cost per therm (other units are available, these are just the ones that match my utility bills) and efficiency of both the heat pump and furnace and runs whichever is cheaper. Gas cost has varied a bunch over the years I’ve had it, electricity less so.…

> As someone who switched four years ago to a heat pump plus conventional furnace, I can state from experience that my thermostat takes electricity cost per kWh and gas cost per therm […]

What is the make and model of the thermostat you are using?

Is it from the heat pump OEM (Trane?) or a third party?

Re: Heat Pumps Take on Cold Climates

#63

Post by solardev »

sgerenser wrote: post #57
solardev wrote: post #34

If you're considering a heat pump, be sure to check the minimum operating temperature before you buy, and/or make sure you have a backup heat source. I live in a rented apartment with a mini-split, and it worked great for 363 of the last 365 days. Those two days, we had a sudden cold spell and temperatures went down to -6 F (which isn't that cold), but we found out the hard way that our unit was only rated to -4 F. O…

I don’t think it’s normal for a heat pump to just stop working completely like that vs. just getting more and more inefficient. Wonder if perhaps it iced up and the defrost wasn’t working.

It's entirely possible? Mitsubishi makes a bunch of different models, and ours is a cheaper one. Our area rarely gets under 0, so it's understandable why the landlords chose this model (and it's also an older model, for an older apartment). They have other models rated down to -15 or lower.

But even if it's because the defroster stopped working, or it's just an auto-shutoff feature to keep the equipment safe... the end effect is the same: There is no more heat when you most need it, and you'll freeze. We were wrapped up in multiple layers that day and tried to warm the house however we could, but ended up just needing to stay with a friend cuz it was so cold.

In some areas, "two sources of home heating" is mandated by law, but not where I live now. It still seems like good advice, specially if you can do that with two separate fuel sources (like electricity vs natural gas vs propane vs heating oil), in case one infrastructure goes down, as they often do during the worst storms.

Re: Heat Pumps Take on Cold Climates

#64

Post by _yb2s »

cwoolfe wrote: post #11

As someone who switched to a heat pump recently, you should know "your mileage may vary." Don't confuse physical efficiency for cost efficiency. In many places, natural gas is still more cost efficient, even if it isn't as efficient in terms of physics.

Gas powered heat pumps also exist, using the same principle as gas powered refrigerators… from what I’ve heard they are already used in some countries but not on in the USA.

Re: Heat Pumps Take on Cold Climates

#65

Post by DannyBee »

sokoloff wrote: post #55
DannyBee wrote: post #23

Earlier quoted context omitted.

this feels pedantic. They are clearly not talking about the theoretical bounds, but instead in practice. The claim can be literally true. It may be they are at x% efficiency in both cases, where x is optimal for bitter cold but not for mild winter.

> The claim can be literally true. It can be. Would you take an even-money wager that the CoP curve will be flat [or increasing] with decreasing outside temp vs my wager that it will be downward sloping with decreasing outside temp?

The claim is true. You've transformed their claim into something else.

What they wrote is " But heat pump manufacturers say they now have the technology to heat homes just as efficiently in bitter cold as they do in milder winter temperatures."

That is about whether they gained the technology to make the finished units as efficient in bitter cold as they are currently efficient in mild winter.

Given they were inoperative in bitter cold, and fairly inefficient in mild winter, this is almost certainly trivially true.

Here is a real curve: https://hvac-talk.com/vbb/attachment.php?s=f7eb5f83efc880440...

If we all mild winter 32F, and bitter cold -10F, they would only have to improve efficiency by 2x at bitter cold to make it as efficient as current heat pumps as mild winter.

That is easily possible.

As for whether they are strictly flat/decreasing, they are not.

Here is more data: https://connect.fujitsugeneral.com/3cfb13a7-a386-4697-aa3e-5...

You can see capacity, input power, and efficiency do not have a strictly linear (flat or decreasing) relationship with outdoor temperature. This is an artifact of it being impossible to good at all things at once.

It is also obvious to every reader you can not make a "maximally efficient in all" cases heat pump as efficient in bitter cold as in mild winter. So rather than assume that is what they are claiming by adding a lot of implicit things to it, i would just read it in a straightforward manner and when I do, it seems pretty easily true.

Re: Heat Pumps Take on Cold Climates

#66

Post by atoav »

solardev wrote: post #51
atoav wrote: post #19

Earlier quoted context omitted.

My father has a heat pump that works fine in alpine climate (half of the winter snow outside) for more than a decade now. Not sure about the precise english terminology here, but his hear pump works using the deep ground water next to our house. The temperature of that water slightly dips in winter, but it is no more than a degree. It is a very old house that before had a oil heating system and I think the replacemen…

This is common in some parts of the world (which is great!), but not in the US. They're quite a bit more expensive, especially compared to modern mini-split air-source heat pumps that can cost just a couple thousand dollars and don't require any digging or duct installations, just a small hole in the wall.

I guess my father was just lucky, because 1m through the wall from our oil cellar we had an old well that could be used, so there was no need for much more digging (except 1m sidways).

He added a ton of solar as well, which is a nice combination.

Re: Heat Pumps Take on Cold Climates

#68

Post by timbit42 »

sgerenser wrote: post #57
solardev wrote: post #34

If you're considering a heat pump, be sure to check the minimum operating temperature before you buy, and/or make sure you have a backup heat source. I live in a rented apartment with a mini-split, and it worked great for 363 of the last 365 days. Those two days, we had a sudden cold spell and temperatures went down to -6 F (which isn't that cold), but we found out the hard way that our unit was only rated to -4 F. O…

I don’t think it’s normal for a heat pump to just stop working completely like that vs. just getting more and more inefficient. Wonder if perhaps it iced up and the defrost wasn’t working.

My heat pump is 12 years old with a CoP of only 3 so when it gets down to -15C, it's less efficient than a resistive heater so it shuts off and the display shows P3. It was designed that way so you can have a secondary heat source set a few degrees below the temp you normally want so that kicks in when the heat pump shuts off. My backup is the electric baseboards I used before I got the heat pump.

Some heat pumps have a resistive heater built into them which kicks in when it's too cold for the heat pump to have a CoP of greater than 1.

Re: Heat Pumps Take on Cold Climates

#69

Post by simonblack »

_yb2s wrote: post #64
cwoolfe wrote: post #11

As someone who switched to a heat pump recently, you should know "your mileage may vary." Don't confuse physical efficiency for cost efficiency. In many places, natural gas is still more cost efficient, even if it isn't as efficient in terms of physics.

Gas powered heat pumps also exist, using the same principle as gas powered refrigerators… from what I’ve heard they are already used in some countries but not on in the USA.

They are very likely denigrated as 'obsolete technology', but like all technology there are use-cases where a particular technology works best.

Re: Heat Pumps Take on Cold Climates

#70

Post by simonblack »

NDizzle wrote: post #60
sgerenser wrote: post #57

Earlier quoted context omitted.

I don’t think it’s normal for a heat pump to just stop working completely like that vs. just getting more and more inefficient. Wonder if perhaps it iced up and the defrost wasn’t working.

It's my experience that they do stop working completely at some temp. The defrost cycle creeps up to 100% and there goes the heat.

That's because the heat-exchanger is not efficient enough to prevent too much frost from building up. The answer there is a better (probably bigger) heat-exchanger.
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