Hybrid heat pump and gas furnace HVAC system being recommended
Nicko23
| Posted in General Questions on
I am looking for information about a ducted hybrid HVAC system that has both a gas furnace and a heat pump. I am in CZ6B and this system has been recommended by several contractors who think this is the way to go in our environment where we often have several days of -30 degrees each winter. This will be for a new build. This system would allow the heat pump to provide cooling and heating on mild days and then the gas furnace would kick in when the weather is 35 degrees or so. Does anyone have experience or knowledge about a system like this? Thanks!
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Replies
This is a great set up. One note though: installers usually don’t know your heat loss and are overly conservative with the crossover point. It might be 35 degrees, it might be -5 degrees, they’re just guessing. You can change it once they leave.
Another note: depending on your utility situation, it might make more sense to skip natural gas and go with propane/oil. They're usually pricier to burn, but if you don't burn much, then they often make sense if connecting to the gas grid is expensive or the monthly fixed fees are high.
My parents have this in Vermont. Bosch Bova and a propane system. I would like to fool around with the change over temp as they have extra solar that is given up each year for nothing.
Yeah it’s just laziness, which I understand because the contractor isn’t getting paid for determining the right balance point, even if the utility bills suffer as a result.
One advantage of not ditching the natural gas supply for such a hybrid system is that it can be fed to an automated emergency generator.
These feel like it should be a good idea but it is mostly there so HVAC folks would be comfortable installing a heat pump in cold climate.
The reality is that modern hyper heat units are no problem even in colder climate. There is no need for the extra cost of adding a gas service and associated build costs into a new build.
I would figure out what your heat losses are and then head to here to look at options:
https://ashp.neep.org/#!/product_list/
Probably something like this is in the ballpark:
https://ashp.neep.org/#!/product/34582/7/25000///0
In the top of the page there is a box labeled "Advanced Data - Sizing for Heating" . When you click on it opens up a window where you can plug in your heat loss numbers and see how the system will run.
If you are worried about those polar vortex cold snaps, you can include a strip heater.
In my area that the cost delta between running a heat pump or a natural gas burner is pretty much noise, even if you have to run the strip heat here and there, the operating cost delta is not worth the complication and cost of dealing with gas.
Hey Akos, thanks for this perspective. I am struggling with ditching the gas since in our area has is significantly cheaper than electricity unfortunately. So I am concerned about trying to heat only with the heat pump on our coldest days from a cost standpoint. Ideally though I would love to ditch gas altogether. Every other system or appliance in the house will be electric.
What are your local energy costs including fees and delivery?
For example, my gas meter fee is $25/month so $300/year. Even with more expensive electricity, that is probaly the cost delta between running a heat pump VS gas in colder weather.
Those cold days last, well, for a couple of days. In terms of total heating degree days, that is a small fraction of your yearly heat cost, not something I would worry too much about.
Plus with a heat pump you can always add a PV array to offset some of the operating costs, if you have good net metering arrangement, including an array as part of the new build is generally decent ROI.
I agree they are often training wheels, but I'd rather have them than no heat pump at all! I wonder the upfront cost of the gas hookup and ongoing cost of having gas onsite. Might make other options more appealing.
I agree, and I think the 35F cutover point is way too conservative.
But I'll throw in one caveat: if this is a retrofit the capacity of the ductwork may be a factor. BTU's delivered is determined by the air flow multiplied by the temperature difference between the delivered air and the room. A gas furnace is capable of producing hotter air than a heat pump, so for the same heat delivery it can have a lower air flow. If the ducts were sized for a gas furnace originally you can have issues with a heat pump being able to deliver enough heat
True but this is new build! Should be designed to work with lower temp air, whether furnace or heat pump.
It seems the hacks usually say 32 degrees to cross over, as if waters freezing temp matters at all here!
OK missed that detail.
I'm sure their thinking is 32f is where you could live without heat if you had to, any colder than that and you have to worry about pipes freezing.
Heat pumps spend most of the time in defrost near freezing temperature. On older heat pumps, the output starts falling off the cliff around that temperature. Combine the two and you can see COPs near 1, so it made no sense to run them much bellow freezing.
Hyper heat units are a different beast, on some units you get a COP above 2 even near 0F weather.