Central Heat Pump vs Variable Central AC Unit
While researching options for updating mechanical systems in my Massachusetts home I find marketing stating that a central heat pump plus furnace is the most efficient way to heat a home. Considering a furnace burning natural gas at 90+% AFUE, I can’t find math to back that up. Am I missing something?
I read Martin’s article from Jan 2019 (Which heating fuel is cheaper, electricity or natural gas?) and his numbers estimate electricity to be ~1.5x more expensive per BTU than nat gas. I did the math with my current fuel costs and potential efficiencies and have a similar result.
For a house with an existing condensing furnace and natural gas, would a central heat pump ever make sense over a variable central AC unit? Maybe the small extra cost is worthwhile for a redundant heat source that would allow me to run my old 92.5 furnace until it dies?
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Would a heat pump ever make sense? Yes:
1. If the heating cost is similar, and you care about the climate, you might be willing to pay a modest premium for lower CO2 emissions and for avoiding upstream methane emissions.
2. If you only care about your cash outlay and not the climate, you can play the dual-fuel game optimally by figuring out the COP where the costs per BTU become equal and only running the heat pump when you are at a high enough temperature that you'll be winning according to that game.
Also, gas prices are likely (though not certain) to increase more than electricity prices in the coming years.
Thanks for your input. I'm hoping to find a solution that can reduce CO2 emissions and operating expenses. Unfortunately, it looks like that would involve a heat pump COP of 4+. I'm not sure if such a product is available.
My challenge is finding information on the COP at particular indoor/outdoor temps for contending heat pumps. Does anyone have a suggested resource for this info? The Bosch IDS heat pump is interesting, and the spec sheet has some performance data, but I'm not sure if that can be converted to COP.
https://www.bosch-climate.us/files/Bosch_IDS_BOVA_product_specification_en_03.2018_US.pdf
Thanks
You'd need tables like that with power input as well as heat output to calculate COP. Or just tables of COP.
A great resource is the neep database:
https://ashp.neep.org/
Unfortunately the data there jumps from 47 outdoors to 17 outdoors. Many will be >4 at 47; but few (any?) at 17. Linearly interpolating won't help that much.
Thank you, that resource is great!
It depends what you mean by efficiency. The coefficient of performance for a heat pump is a lot greater than that for a gas furnace (hence claims that heat pumps have greater than 100% efficiency -- which isn't of course literally true, if you consider all their effects, but as good as true if you consider the effect on the temperature of your house). That doesn't necessarily mean it's the most cost-effective solution considering the cost to install, but it may well be cheap to run. And whether dual-fuel is even necessary depends on your climate and the kind of heat pump you get (several brands work well in cold climates). We've gotten by so far with no backup furnace, nor even heat strips, in Seattle. Averaging the last two years' electricity usage I get about 8,400 KWH/year total, a little over 4,000 of that being for the heat pump, in a four-bedroom house that has had some energy retrofitting, but is far from ideally insulated/air sealed/etc. (That's essentially all heating; we haven't yet needed very much in the way of a/c.)
I'm just looking at cost efficiency per generated BTU as I already have an efficient gas furnace and I'm looking to add either central AC or central heat pump.
With my Massachusetts electric and nat gas rates, I can't find a heat pump that would be more cost efficient at generating heat that the furnace.
Generally if you live in a heating dominated zone and have a city gas connection gas is the economic say gas.
My heat pump is an economic choice vs. propane.
Heat pump could make economic sense if paired with solar panels and net metering.
From a comfort point of view it is hard to beat gas.
From a carbon point of view a heat pump is a winner.
The dual fuel is a loser by most any way you look at it. Given twice the upfront and maintenance costs on the off chance that there will be big changes in the relative prices of the two fuels.
Most people can adjust to the fact that a heat pump will be supplying 85° air to heat the home while a gas unit will supply 130° if you have a dual fuel system you would need a will of iron to not flip the switch and disable the heat pump.
Walta
You need to find specific performance data for the particular unit you want to compare. Typical ratings are not given at a range of temperatures, and they aren't linear, either, so you can't just extrapolate out to where you need to be. Heat pump effiency in terms of input energy to useful heat output drops as outdoor ambient temperatures drop. This means if you're in a warmer climate, a heat pump may well blow away a gas fired furnace in terms of raw effiency numbers. In a more northern climate with lower average wintertime temperatures during the heating season, that same heat pump will be much lower in effiency -- but maybe still higher than a gas fired furnace. That's where it can get tricky. Without real data, you can't make a good comparison.
In terms of total cost, natural gas is often cheaper, but not always. You also have to consider the installation cost, which is especially a concern if this would be a retrofit replacing an existing system that is currently functional. Gas fired furnaces will typically have a much lower purchase cost. Minisplits might actually have a lower INSTALLATION cost though on a new project since they don't usually require any ductwork, and refrigerant lines are cheaper/easier to install than large duct systems.
In terms of operating costs, this will depend on your local electric rates. If you're in the pacific northwest, you're in good shape. New england, not so much -- they have some of the highest electric rates in the country. But to make an accurate comparison, you need data on the heat pump, specifically the CoP numbers at some various outdoor air temperatures.
I don't think we're likely to see gas prices increase more than electric prices in the near future. Most new electric generation has been from new natural gas generation, and that means rates will likely track somewhat. "Renewable" generation IS NOT going to make electric rates fall compared to other things. At best, rates will continue at their historical trend. More likely would be everything increases and that will all depend on the regulatory enviornment.
BTW, if you are interested in the overall system efficiencies and costs of the utility side of this, Dana Dorsett and I have had several exchanges on GBA about exactly that. Dana tends to be more of the "other side" from me, but he has well-reasoned arguments and info to back up his side too, so you can get a better idea of what is going on "behind the scenes" in the utility world. You can probably find some of these discussions with a search of the GBA Q+A section if you're interested.
Bill
I think the point of a dual fuel system is supposed to be that then you don't need either the most expensive heat pump (because it doesn't have to work perfectly for the few coldest weeks of the year) or the most efficient furnace (because you're not running it for enough of the winter for 90% versus 95% or 97% efficiency to make much difference). A lot of people just keep their old furnace as a backup, if it's in good enough shape, which could change the equation quite a bit. We had a terrible old gas furnace, and a relatively mild climate, plus wanted to get off fossil fuels entirely in the long run, so it wasn't an option we considered anyway.
If you take what you would've spent on the furnace, and turn it into air sealing and insulation (and maybe ductwork improvements). You may be further ahead economically with the heat pump and have a more comfortable house, even if the the heat pump is a lower than 4 COP since your heat loss is lower, you can have a lower COP for the same operating cost.
Also, if you can ditch the gas meter you save the meter charge every month (for me it's about $22).
Have you looked into solar? Net Metered solar and heat pumps are a really awesome setup. Massachusetts has some of the best solar incentives and policies in the nation. Last I checked the payback period in Mass was around 5 years.
I'm in CT with net metered solar in an all electric house with a Heat pump water heater and 2 cold climate mini splits. My electric bill is $9.62 for just the connection charge every month. My panels will have paid for themselve next spring after 5 years and my entire electrical consumption including heat, hot water and cooking will cost me $9.62 a month. I use zero fossil fuels in my house for heat.