3 ms·

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 syste

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.

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.