I remodeled a house a few years ago with a propane-fired Vitodens 100-b1KA Combination boiler/tankless hot water, but I got tired of the inconsistent hot water temperature and the exhaust venting into the backyard during during the summer when children are playing outside. I then installed a A.O. Smith Signature 900 50-Gallon HPS10-50H45DV Heat Pump Water Heater. This achieved consistent temps and no stinky exhaust, but I ended up with energy use 5x higher then Energy Guide estimates and ran out hot water on heavy use days.

SO: I have two modern, "energy efficient" hot water heaters that don't quite meet my needs. Can I combine the two to work together? Ideally I would like the heat pump to do most of the work and have the tank-less water heater fire up when the heat pump's hot water is depleted. Has anyone tried a combination like this before? Will my tank-less water heater suffer for being fed preheated hot water?

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    I'm about 99.99% certain we've got several questions like this here already. Did you check the "related" questions?
    – FreeMan
    Dec 30, 2023 at 18:54
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    Generally yes, but specifically read your own appliance manuals, I don't want to read them for you to see if they are normal or abnormal. As for the energy guide, that's based on an assumed amount of hot water use, so if yours is greater than that, the operating cost will be as well. Likewise, most hybrid heat pumps have an electric element - if that's enabled, it's not any more efficient than a stock electric heater while using that. And on most it can be enabled or disabled.
    – Ecnerwal
    Dec 30, 2023 at 19:42
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    Many heat pump water heaters have two modes - a heat pump mode that is 2x-3x as efficient as a traditional electric water heater and another mode where it functions exactly the same as a traditional electric water heater - higher energy use but faster recovery time. If your water heater is undersized for your usage pattern (and at 50 gallons that is quite possible) then it could be running in resistance (not heat pump) mode a lot of the time, which can (depending on electricity vs. gas costs) cost you anywhere from 2x-5x as much per gallon of water heated compared to gas. Dec 31, 2023 at 0:08

1 Answer 1



I setup a 3 hpwt pre-heat system before a 180k btu gas hot water tank. Used the rheem proterra 80 gallon tanks. Set the hpwt to heat pump only mode.

The system used to be 2x 180k gas btu tanks. Given the hpwts are about 4000 btu equivalent each = 12,000 and we replaced a 180,000 tank with them you might wonder how it all still works.

The 2x 180k btu were oversized to start with. The other thing that really saves us is that we have a recirc pump so generally what happens is that the hpwts pre-heat as much as they can, supply the gas wt with the preheated water which is then put up to 140. The 140 water circs through the building and returns to the hpwts which store the 140 water and keep it at ~120 ( there are different time of day settings for different temps ). In the summer we can turn off the recirc pump and generally things are still good. In the winter we probably aren't maximizing the potential savings since the recirc will input them with gas heated water but they are installed in a boiler room and are able to capture a lot of the otherwise waste heat.

Still fiddling with it but if the recirc is off then the hpwts show empty a lot of the time and get some reports of not enough hot water. We do have a ST01 timer on the recirc pump so we might set that up such that the recirc is not running when people aren't typically using the hot water maybe 10pm-4am. I've got some energy monitoring devices installed but haven't done a deep dive into the data yet. The gas usage appears to have dropped by about 20% ( haven't checked into how much the ele has gone up though ).

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    Thanks all for the help and the comments. I have since plumbed my hot water to run through my heat pump hot water tank and then to the gas combi boiler. It seems that my particular boiler has flow meter as well as a thermostat for domestic hot water, and the flow meter causes the burner to start heating the water flowing through the boiler even when the water coming from the hpwt is already above the set point programed for the boiler, resulting in scalding hot water way above desired temps. I am going to reach out Viesmann to see if the boiler can be reprogrammed to behave differently. Jan 9 at 16:48
  • @CarsonMichaels interested to hear what you find out! Jan 12 at 8:13

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