Can heat pumps continue to run while supplemental heat provides the remaining heat?
guidoism
| Posted in General Questions on
I had been under the impression that as it gets colder outside and the heat pump loses efficiency that you can incrementally start adding supplemental electric resistance heat to make up the difference.
Is this not the case?
I talked with a (Bryant) contractor who said that they can’t do this, that it’s it shuts the heat pump off completely at a certain point and just run a furnace.
But this doesn’t make sense to me since electric resistance is perfectly variable.
What am I missing here?
Note that I would prefer to move to all-electric over the coming decade.
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Replies
Depends on the unit. Some can be programmed to use to supplement or to switch over.
Another option is to go with a hydronic coil downstream of the unit. Some thermostats can run an aux heat strip directly, you would wire the hydronic coil to that and let the heat pump do the best it can. If you have an NG or gas hotwater tank near by, the aux coil can even run directly from that in an open system (provided your local code allows for this).
The reason a lot of the domestic non-hyperheat heat pumps switch to resistance only is the COP of the unit can go bellow 1 in cold weather. No point in running the unit when resistance heat is more efficient.
Systems with electric heat strips usually have two modes: auxilliary heat, where the heat strips adds additional heat if needed because they heat pump can't keep up, and emergency heat, where the heat pump shuts off because it's too cold for it to work right (too low COP, or bad for the components to run at that temperature) or maybe if it is broken. It used to be common to install a heat pump that couldn't run at all at the lowest expected temperatures, but there's not much reason to do that anymore, as you can find heat pumps that work down to very low temperatures.
Systems that combine a gas furnace and a heat pump can't do the combined mode, simply because the heat pump coil is located after the furnace in the air flow path. The hot air coming out of the furnace would be hotter than the heat pump coil, and the heat pump could not add heat to it.
As Akos noted a hydronic coil can be located downstream of the heat pump coil and can be run in either mode. Here's a blog post on that configuration:
https://www.energyvanguard.com/blog/35270/Heat-Pumps-and-Hydronics-A-Great-Team-for-High-Performance-Homes.
The refrigerant going through the compression or decompression is one of the limiting factors. Some systems shut off at -22f where others shutoff at -3f for freeze protection. The Enertech Advantage have a 3 stage heater coming off the heat pump for supplemental heat. The Artic heat pumps has an electric boiler that stages the heat based on the outdoor temperature. Both have outdoor temperature sensors that feed into their controls.
There is a good video on you tube in the coffee with caleffi series. Look for ones on air to water heat pumps that covers some of that material.