3 ms·

To be clear: "zero heat gain" isn't the right threshold to think about unless your house is a closed thermodynamic system. Real houses lose heat in proportion

by ajross 4y ago

To be clear: "zero heat gain" isn't the right threshold to think about unless your house is a closed thermodynamic system. Real houses lose heat in proportion to the temperature delta between inside and outside air. So what you really want to know is the temperature at which the heat pump is no longer capable of maintaining that delta. That's a whole lot higher than -20C. FWIW: we installed a heat pump recently here in Portland, and conveniently had a once-in-a-decade cold snap (17-20F for 3 days -- the PNW isn't that cold) in its second month. The system was just-barely-heat-flow-positive. A six hour power outage dropped the house temperature to ~64F, and it took a good 30 hours to get back to the 69F set point of the thermostat. So... on the whole we're pleased and the system did just fine in extremis. But no way would it handle -20C/-4F.

xenadu02 4y ago
This is extremely dependent on the efficiency of your system and how well it is designed. A 15 SEER heat pump can't compare to a 21 SEER (all else equal). There are heat pumps that do well below freezing, they're just top of the line and so more expensive.
philips 4y ago
What heat pump model did you install?
ericpauley 4y ago
This is just a matter of pump size. People in colder climates size their pumps larger in expectation of these cold temperatures.
- ajross 4y ago
  Of course, and on home insulation state. I'm just pointing out that the threshold to look at during isn't the one described. The specific numbers were just to illustrate the principle.
  - shkkmo 4y ago
    Neither is the one you are proposing since "big enough system to heat a house at cold temperatures" is just a question about what size of system to install. The 1:1 threshold tells you about the temp that everyone needs to start thinking about alternative heat sources. If you system is undersized and/or your house is poorly insulated, that threshold may be higher, but those numbers are per installation and don't answer the broad question about if heat pumps can "work" in cold climates.
    - ajross 4y ago
      This kind of nitpicky debate gets so tiresome. Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C". And I thought that was misleading, because that's not the threshold you use to size a heat pump as is doesn't reflect a steady state (at that temperature your home will be inexorably losing heat). And as I happen to have had personal experience in the recent past with measuring exactly this threshold on a newly installed heat pump, I thought people might be interested. I'm deeply sorry if this offended you for some reason, but I still don't see how you're disagreeing with anything I wrote.
      - shkkmo 4y ago
        > Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C" I did not read it that way at all. To me they are clearly talking about the point at which your heat pump no longer provides a efficiency gain over a purely resistive heater. I think where you went wrong is reading "gain" as "heat gain" rather than "efficiency gain". While the usage of "gain" is sort of ambiguous there, the rest of the comment thread makes it clear that the term 1:1 is not discussing the point at which your heat pump stops producing any heat but the point at which it becomes inefficient. > that's not the threshold you use to size a heat pump as is doesn't reflect a steady state I don't think anyone except you was talking about sizing the system, but about selecting the system type given the climate. If you had come into this just saying "here's my relevant experience with sizing heat pumps", that would have been productive. However you strongly asserted that the OP was using the wrong metric, which they weren't. > I'm deeply sorry if this offended you for some reason, I'm not sure why you think I am offended.
- throw0101a 4y ago
  > People in colder climates size their pumps larger in expectation of these cold temperatures. Both over- and undersizing them each have their own issues; see §2.1.1 and §2.1.2: * https://yukon.ca/sites/yukon.ca/files/emr/emr-air-source-heat-pump-pilot-project-technical-report.pdf https://yukon.ca/sites/yukon.ca/files/emr/emr-air-source-hea...
dashundchen 4y ago
That's not a flaw in heat pumps, that's just the sizing of the system. A good designer will not oversize HVAC for edge cases like a once-a-decade weather system. You will size it to meet typical conditions, saving money and increasing efficiency by having a 4 ton heat pump vs 6. For heating conditions, the typical workaround to meet demand in edge case extreme or long term cold is to add resistive heat pumps, which are much cheaper than upsizing the whole system. Though they are less efficient and therefore cost more to operate, they will only be used a few days a year when the heat pump loses ground. I have a 60kBtu/h ground-source heat pump backed up by 7kW heat strips that replaced a 110kBtu/h gas furnace. The heat pump meets our needs for probably 51 weeks a year. Typically we will have the strips kick on during a few days in February. But it saves us a ton of money not having to have a second heat pump or larger ground loop system.
- ajross 4y ago
  > That's not a flaw in heat pumps, that's just the sizing of the system. No, it's not. The temperature at which the heat pump produces zero net energy flow is a thermodynamic property of the system. You can make the system as big as you want and it will still produce zero energy, because zero times anything is still zero. That's why that temperature isn't interesting: yes, your heat pump "won't work" below that temperature, but it also "won't work" at a range of higher temperatures. And that range, as you point out, depends on installation details and insulation. But that's not what the upthread comment was saying. And as it happens, I got a chance to measure that for my system. And it turns out to be somewhere around 15-17F. I genuinely thought folks would like to hear about that as useful info, but instead it turned into this giant "actually" subthread...
  - saltcured 4y ago
    It sounds like you are claiming that a heat pump cannot be sized to meet the kW requirements of a given structure. It might not always be the wisest economic decision, but you can certainly install a larger capacity heat pump to meet whatever kW rating you would get from a given furnace. Consider the large systems used on office buildings or datacenters which greatly exceed the capacity needed for a normal home. It is true that a given insulated structure will have heat loss varying based on the differential between indoor and outdoor conditions. This is a power curve, i.e. a certain kW rate of energy loss for given conditions. The rate will increase as inside and outside conditions diverge. But for anticipated conditions, it is a finite value you can use to plan your system. This is what people mean is "just sizing": Install a system with a sufficient kW output rating to support your desired indoor and outdoor operating conditions. You would size it to exceed the real loss rate of the structure, so that you have head room to increase the differential within an acceptable recovery period.
sokoloff 4y ago
GP's "zero gain" was a statement about efficiency, not heat loss. ("Zero gain in efficiency as compared to resistive heating" rather than "zero gain in net BTUs inside the house".)
Sohcahtoa82 4y ago
I'm in the Portland area and got a heat pump installed in 2021. I overbought for sure, as I got a hybrid system that has a gas furnace backup. I've never needed it, but I've had a couple times where I wanted it and it was nice to have. If I switch it from "Heat" to "Emergency Heat", then it overrides the automatic determination of which heat source to use and just uses gas. I didn't lose power during that cold snap, but if I did, it would have probably brought the temp back up in under 4 hours.
- dev_tty01 4y ago
  We have quite a few power outages, so the best thing about gas furnace backup for me is that I only need a generator large enough to run the furnace fan to get heat in a power outage. Without it, I have to run a heat pump compressor and that is a lot more power than a fan.
sbbr 4y ago
Air-water heat pump provides heating for our house (170 m2, Estonia, well insulated) all year round without engaging support electric heaters. In fact, breaker for support heater is always off. Temperature went to -22C previous winter for couple of days, heat pump was able to handle it. Average winter temperature is about -4C.

by ajross 4y ago

xenadu02 4y ago

This is extremely dependent on the efficiency of your system and how well it is designed. A 15 SEER heat pump can't compare to a 21 SEER (all else equal). There are heat pumps that do well below freezing, they're just top of the line and so more expensive.

philips 4y ago

What heat pump model did you install?

ericpauley 4y ago

This is just a matter of pump size. People in colder climates size their pumps larger in expectation of these cold temperatures.

ajross 4y ago
Of course, and on home insulation state. I'm just pointing out that the threshold to look at during isn't the one described. The specific numbers were just to illustrate the principle.
- shkkmo 4y ago
  Neither is the one you are proposing since "big enough system to heat a house at cold temperatures" is just a question about what size of system to install. The 1:1 threshold tells you about the temp that everyone needs to start thinking about alternative heat sources. If you system is undersized and/or your house is poorly insulated, that threshold may be higher, but those numbers are per installation and don't answer the broad question about if heat pumps can "work" in cold climates.
  - ajross 4y ago
    This kind of nitpicky debate gets so tiresome. Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C". And I thought that was misleading, because that's not the threshold you use to size a heat pump as is doesn't reflect a steady state (at that temperature your home will be inexorably losing heat). And as I happen to have had personal experience in the recent past with measuring exactly this threshold on a newly installed heat pump, I thought people might be interested. I'm deeply sorry if this offended you for some reason, but I still don't see how you're disagreeing with anything I wrote.
    - shkkmo 4y ago
      > Look, upthread comment implied pretty clearly that a particular heat pump as installed would "work" until zero net energy flow at "-20C" I did not read it that way at all. To me they are clearly talking about the point at which your heat pump no longer provides a efficiency gain over a purely resistive heater. I think where you went wrong is reading "gain" as "heat gain" rather than "efficiency gain". While the usage of "gain" is sort of ambiguous there, the rest of the comment thread makes it clear that the term 1:1 is not discussing the point at which your heat pump stops producing any heat but the point at which it becomes inefficient. > that's not the threshold you use to size a heat pump as is doesn't reflect a steady state I don't think anyone except you was talking about sizing the system, but about selecting the system type given the climate. If you had come into this just saying "here's my relevant experience with sizing heat pumps", that would have been productive. However you strongly asserted that the OP was using the wrong metric, which they weren't. > I'm deeply sorry if this offended you for some reason, I'm not sure why you think I am offended.
throw0101a 4y ago
> People in colder climates size their pumps larger in expectation of these cold temperatures. Both over- and undersizing them each have their own issues; see §2.1.1 and §2.1.2: * https://yukon.ca/sites/yukon.ca/files/emr/emr-air-source-heat-pump-pilot-project-technical-report.pdf https://yukon.ca/sites/yukon.ca/files/emr/emr-air-source-hea...

dashundchen 4y ago

That's not a flaw in heat pumps, that's just the sizing of the system. A good designer will not oversize HVAC for edge cases like a once-a-decade weather system. You will size it to meet typical conditions, saving money and increasing efficiency by having a 4 ton heat pump vs 6. For heating conditions, the typical workaround to meet demand in edge case extreme or long term cold is to add resistive heat pumps, which are much cheaper than upsizing the whole system. Though they are less efficient and therefore cost more to operate, they will only be used a few days a year when the heat pump loses ground. I have a 60kBtu/h ground-source heat pump backed up by 7kW heat strips that replaced a 110kBtu/h gas furnace. The heat pump meets our needs for probably 51 weeks a year. Typically we will have the strips kick on during a few days in February. But it saves us a ton of money not having to have a second heat pump or larger ground loop system.

ajross 4y ago
> That's not a flaw in heat pumps, that's just the sizing of the system. No, it's not. The temperature at which the heat pump produces zero net energy flow is a thermodynamic property of the system. You can make the system as big as you want and it will still produce zero energy, because zero times anything is still zero. That's why that temperature isn't interesting: yes, your heat pump "won't work" below that temperature, but it also "won't work" at a range of higher temperatures. And that range, as you point out, depends on installation details and insulation. But that's not what the upthread comment was saying. And as it happens, I got a chance to measure that for my system. And it turns out to be somewhere around 15-17F. I genuinely thought folks would like to hear about that as useful info, but instead it turned into this giant "actually" subthread...
- saltcured 4y ago
  It sounds like you are claiming that a heat pump cannot be sized to meet the kW requirements of a given structure. It might not always be the wisest economic decision, but you can certainly install a larger capacity heat pump to meet whatever kW rating you would get from a given furnace. Consider the large systems used on office buildings or datacenters which greatly exceed the capacity needed for a normal home. It is true that a given insulated structure will have heat loss varying based on the differential between indoor and outdoor conditions. This is a power curve, i.e. a certain kW rate of energy loss for given conditions. The rate will increase as inside and outside conditions diverge. But for anticipated conditions, it is a finite value you can use to plan your system. This is what people mean is "just sizing": Install a system with a sufficient kW output rating to support your desired indoor and outdoor operating conditions. You would size it to exceed the real loss rate of the structure, so that you have head room to increase the differential within an acceptable recovery period.

sokoloff 4y ago

GP's "zero gain" was a statement about efficiency, not heat loss. ("Zero gain in efficiency as compared to resistive heating" rather than "zero gain in net BTUs inside the house".)

Sohcahtoa82 4y ago

I'm in the Portland area and got a heat pump installed in 2021. I overbought for sure, as I got a hybrid system that has a gas furnace backup. I've never needed it, but I've had a couple times where I wanted it and it was nice to have. If I switch it from "Heat" to "Emergency Heat", then it overrides the automatic determination of which heat source to use and just uses gas. I didn't lose power during that cold snap, but if I did, it would have probably brought the temp back up in under 4 hours.

dev_tty01 4y ago
We have quite a few power outages, so the best thing about gas furnace backup for me is that I only need a generator large enough to run the furnace fan to get heat in a power outage. Without it, I have to run a heat pump compressor and that is a lot more power than a fan.

sbbr 4y ago

Air-water heat pump provides heating for our house (170 m2, Estonia, well insulated) all year round without engaging support electric heaters. In fact, breaker for support heater is always off. Temperature went to -22C previous winter for couple of days, heat pump was able to handle it. Average winter temperature is about -4C.