Heat Pump Water Heater and Tankless in combination?
I live in Down East Maine, and have a house with an existing propane-fueled boiler and tankless water heater combo. My intention is to eventually use the heat from that system as a backup to some mini-split heat pumps (to be installed early this fall). I have been approved for solar panels which will eventually provide a lot of my electricity needs.
The guy who evaluated my house for solar recommended installing a heat pump water heater, which seems like a good idea. For most of the year, my wife and I live here by ourselves, but in the summer time we will often have a lot of guests. I would like to get the efficiency and green advantages of the heat pump water heater, but how do I set up the system so that the heat pump water heater is the primary hot water source, and the tankless only kicks in as needed?
I get the impression that it’s really simple…some way to connect things in series, with maybe something called a mixing valve, but I haven’t been able to formulate a search engine query that yields a definitive answer.
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You're on the right path. If you look at Caleffi's Idronics journals, they'll probably have a diagram which shows you exactly what you need.
There are two ways you can do it. If your tankless unit is fine with hot intlet temperature (check manual) you can plumb them in series with the tankless unit after the tank. Set the tankless bellow the setpoint of the tank and this way the tankless will only run if the tank runs out.
The other way is to parallel them through a thermostatic mixing valve. You put the tank to the cold side and the tankless to the hot side. With this setup the mixing valve will automatically switch over to the tankless unit once the water out of the tank is bellow the temperature setpoint of the mixing valve.
Would there be a problem as the HPWH runs out but isn't cold? Say 85-90* water is coming out and the tankless needs to kick in. I thought some of the tankless heaters weren't great with smaller temp rises such as that.
High temps can be a problem with some units. Most rated for combi heat are fine with higher temps but you can still get cycling when it is not doing much of a temperature lift. Also a condensing unit fed with hot water will also loose efficiency but I doubt that would matter much in this case.
The mixing valve based connection would be the better option as the tankless is fed with cold water, the only issue there is that the mixing valve won't be cycling much so it can get stuck from scale buildup.
For the OP. You are searching for solar thermal water heater with gas tankless backup. The issue with most diagrams you'll find is they generally have pretty complicated setups with lots of valves and pumps.
Thank you all so much. These are very helpful answers. It sounds as though the mixing valve is probably the way to go, but I will check the manual for my tankless.
I recently attended a seminar where they recommended this HP tank + tankless, installed in series approach. They explained the issue people have is the slow recovery rate of the HP tank, and not having space for two tanks. They explained you can buy a device that switches which unit has power, so that once the temp of the water in the HP WH drops to x, you switch power to tankless. Once flow stops, you switch power back to the HP WH. Sounds like an ideal setup, and even easier in your case since the tankless is propane rather than electric.
It seems like keep the heat pump on constantly and turning the tankless on as needed would be cheaper?
That sounds reasonable...did they mention a name or model number for the switching device?8