[harg]

You get more heat out of using a given quantity of gas to generate electricity which is used to to power a heat pump than you do by burning the gas directly for heat, even when considering generation and transmission losses etc.

Given that even during a time of low output from renewables we still only rely on 53% gas for electricity generation it's still much more beneficial environmentally to use a heat pump.

The monetary cost is another story though, and I agree we do need to work on weaning ourselves off gas.

[StillBored]

In areas with moderate outdoor temperatures. Anywhere that the heatpump is going through defrost cycles, that statement is likely false because the COP is likely 1 or less when that starts happening.

Then on the generation side, its worse if the NG generation isn't a modern combined cycle plant. which also tends to nix places with a lot of renewable generation because the NG plants are just peaker gas turbines with much lower efficiency than plants designed for continuous use. So, its all situational, but at the same time if one has the choice for cheap NG using that as a second stage and setting the crossover heatpump temperature at the cost/BTU intersection between the heatpump and NG second stage is a rough approximation of the enviromental costs as well as the actual cost.

[triceratops]

> In areas with moderate outdoor temperatures. Anywhere that the heatpump is going through defrost cycles, that statement is likely false because the COP is likely 1 or less when that starts happening.

Modern heat pumps have a COP close to 2 at 5F/-15C while still delivering 50k+ BTUs. Here's one example: https://ashp.neep.org/#!/product/68628/7/25000/95/7500/0///0

[StillBored]

While heating, That isn't even a particularly efficient unit (EER 9), and not all all unusual for r410 (which is basically banned for new sales in the USA since jan 1st)

But it has a 150W pan heater, but I don't think that is its primary defrost mode, at least its not going to be at 50k btu at those temperatures unless its also really dry. The pan heater is probably to just assure that the coil melt water doesn't build up in the base pan.

For most of the forced air systemms in the USA, the deforst mode is as I mentioned triggered via outdoor coil temp/runtimes and somewhere below an ambient of ~30F, which will be a coil temp of ~20F. The exact algorithm changes from AC unit to unit, but you get say 20 mins of heating, and then it will flip to 5 mins of AC while not running the oudoor fan, where an electric (or maybe gas) furance will heat the indoor air after it flows over the indoor coil which is cooling the air. So its a double wammy, its taking 5+ mins of operation back at an even higher wattage.

AKA the COP goes negative for 5 minuites... Which will pull the SEER numbers down pretty hard, and that unit actually has pretty poor numbers for being variable speed compressors/etc.(edit: should be SCOP, since SEER(2) is just the ac side, that unit looks to be optimized for heating. Either way, depending on which efficienty spec you pay attention to, the 'problem' tends to be that the SCOP/HSPF/etc numbers are calculated using 'moderate' temp data, so below freezing temps are a minority of the calculation)

I poked around a bit looking for the install/service guide for that unit but didn't find one detailing the defrost algorithm.

[tharkun__]

Not your parent. We had a Fujitsu AOU15RLS3 installed ~10 years ago. 25.3 SEER apparently, EER 13.9. Also r410A. We mainly got it for AC in summer but we'll take the heating of course. Pretty cold winters here.

    Minimum Outdoor Temperature for Heating: -5°F

Yeah that's about as cold as it's right now actually. During the day!

We'll definitely use the propane fireplace in winter, especially when the power goes out, which has happened more frequently in recent years, as "proper winter" has given way to more freeze / thaw type stuff with ice storms.

[broken-kebab]

Can you elaborate on the statement in the first para? Is it a guess, calculation, or there are real-world data?

[Tostino]

A heat pump can be well over 100% efficient based on the energy input for equivalent resistive / chemical heating. E.g. your heat pump could use 100w of electricity to move 400w worth of heat (if generated resistively) from the outside to inside.

There have been multiple studies done that show that current generation heat pumps are quite a bit more efficient for a given volume of gas to burn it in an electrical generation plant and use a heat pump than it is to burn it in the house / building.

[broken-kebab]

I see, but it's still mostly a theory unless we count in all small nuances. Like for a place where winters are really frosty heat-pump usually can't help enough (my own experience), and it seems they come with resistive heating built-in nowadays. Which may change the picture. Also, I'm not a specialist, but my guess is that delivery (or how they call it in the industry) of electricity can be priced in potentially wide range.

[Tostino]

Yup, that was all accounted for in what I read (well over a year ago, can't find the link...sorry). It was also for a northern climate country (possibly the UK, maybe Norway, memory is failing me here), so it was not "best case" for heat pumps. They took into account losses from generation, transmission, number of days so cold that it is acting as close to a resistive heater (100-120% efficiency), etc. It was pretty comprehensive.

It was from a "what is the best way to lower our use of fossil fuels" perspective, and acknowledged that switching out all boiler systems for heat pumps would be a high cost, but it wasn't really a study on the economics of it. Just a resource usage perspective.