3 ms·
Heat pumps can heat in cold temperatures, but at decreasing efficiency or COP, which is how much heat you pump from the outside divided by how much heat it took
by thsksbd 3y ago
Heat pumps can heat in cold temperatures, but at decreasing efficiency or COP, which is how much heat you pump from the outside divided by how much heat it took to run the compressor. The theoretical COP is infinite if $\Delta T = 0$, and you can use Carnot to calculate the upper limit otherwise.
Now, the whole point of having a heat pump is to have a COP (much) greater than 1 (an excellent furnace has a COP just below 1). Once the difference in ambientT-outsideT becomes large, you approach COP = 1. The heat pumps manufacture will report the actual COP vs $\Delta T$; that is including losses, vs theoretical.
Now, a heat pump runs (almost certainly) on main power, and the alternative (usually) on natural gas.
It's a simple matter, therefore, to calculate the CO2 and financial cross over point for a heat pump vs. a furnace. I did that calculation when I had one installed in my house (for ductless AC; heating is an added bonus). It's somewhere between about -5 or -10 C, if I recall correctly.
Sure, a heat pump can work at -25C but it'll cost more and emit more CO2. This is because, dumb Joule for dumb Joule, natural gas piped to my home is cheaper and cleaner than the same natural gas piped and burned in a thermal power station [1]
In my small corner of the world this means that the heat pump is usually better to run than my furnace. But I keep my furnace on anyway, basically set on idle because there is no way that a heat pump can compete with a furnace on reliability. I don't want burst water pipes.
In Europe, heat pumps make 100% sense. Even though Europe is significantly north of the US, the winters are mild with few stretches below 0. In contrast, in some parts of the midwest long stretches of -20 C are not impossible. A week at -20 is not unheard of. In this situation Id be loath to have to rely exclusively on heat pumps.
[1] actually coal in my case so I fudged it in favor of the HP. Coal is also the case for many of you as well because coal is typically used as marginal power during the winter since it's easy to store and gas pipelines' pressure drop in the winter due to home demand.
- jotato 3y ago> It's a simple matter, therefore, to calculate the CO2 and financial cross over point for a heat pump vs. a furnace. I did that calculation when I had one installed in my house (for ductless AC; heating is an added bonus). It's somewhere between about -5 or -10 C, if I recall correctly. How can I do this? I know my heat pump can only maintain (not raise) my house temp at 36F/2C. But I have yet to figure out at what temp it is cheaper to run my natural gas furnace. I suspect ~40F/5C but that is just a guess (see my comment on parent)
- bobchadwick 3y agoIf you happen to have an ecobee thermostat, https://beestat.io/ https://beestat.io/ is an amazing tool for figuring out how your system performs.
- jotato 3y agoI don't :/ I do have a smart home setup but it is local-only. I think I am capturing most of the data I need. I know when my HVAC is running. I know how much power it is using. I know the temperature of every room, I have a stat outside so I have realtime temp/humidity of my location...I just don't have a way to capture and record how much heat is being generated. I'm wondering if putting a temperature sensor directly on an air vent would let me capture it over time
- thsksbd 3y agoSimple in the sense that high school chemistry/math is all you really need + definition of COP. Its a few steps. 1. You need to get your pump's COP vs T from the manufacturer. This is the hardest part. 2. Now find out what chemistry is used for your mains power (nat gas, coal, etc). 3. Assume (or get) the efficiency of your furnace as COP = 0.9 4. Calculate for each delta T how much input each heating Joule of energy takes both methods. 5. Calculate the cost of each Joule for each method at each T. If you are charged electricity based on consumption, use the marginal rate. For CO2 emission: 6. Apply the appropriate generation/transmission loss for electricity. 6. Calculate the CO2 from chemistry.
- jcalvinowens 3y agoThe manual for mine just gives two COP values: 3.63 at 47F, and 2.44 at 17F. That puts the cutoff for me at 30F, which is absolutely as cold as it ever gets in the bay area, so I only have the heat pump. This is a longer explanation I wrote up when I replaced it: https://www.reddit.com/r/bayarea/comments/10lxyso/replacing_my_gas_furnace_with_a_heat_pump_a/ https://www.reddit.com/r/bayarea/comments/10lxyso/replacing_...