propane tankless heater that can handle pre-warmed water

[SilverState]

We're putting an 80 gallon hybrid / heat pump tank in but I'd like to also have a propane instant heater that can kick on if needed. I've heard some instant heaters can and can't handle pre-warmed water. Does anyone have a recommendation on some manufacturers / models that would do a good job of this?

 

[ArkCabinMan]

We're putting an 80 gallon hybrid / heat pump tank in but I'd like to also have a propane instant heater that can kick on if needed. I've heard some instant heaters can and can't handle pre-warmed water. Does anyone have a recommendation on some manufacturers / models that would do a good job of this?

Marey would be my answer, by direct recent experience.
I built my off-grid cabin with two Marey propane-fired tankless water heaters (optionally) connected in series to each other via a home-made manifold so as to be able to take a truly steaming hot soak in the tub in the cold winter months. This is accomplished by minimally pre-heating in the first heater, the output of the first heater being routed via the manifold to the cold water input of the second heater.
In normal operation, the water heaters are separated from each other, one heater supplying the kitchen, the other supplying the bathroom.
Via the manifold, in the event of one heater failing to operate, the other heater can take over the duties of both.
Redundancy is key..!
The Marey units are sensitive to water flow, not water temperature. However, a thermocouple on the heat exchanger output pipe will shut off the gas flow, if there is an ignition failure of several seconds duration.

 

[newbostonconst]

About 20 years ago when instant water heaters started getting available. I tried doing heated floors with one.....once the floor got hot and started sending warm water to the heater...the unit would try throttling back because it didn't need to heat the water as much but eventually it couldn't throttle back enough and would heat the water too much and would error/lock out itself....

There are units the can do this now though....search/ask on www.greenbuildingtalk.com

 

[littleharbor2]

Not sure if the big brand, whole house type models have it but the smaller "Import" models have a Summer/Winter option. I would think the heater receiving the pre warmed water would be switched to summer. This setting seems to use less gas or fires only part of the burners.

 

[ArkCabinMan]

Not sure if the big brand, whole house type models have it but the smaller "Import" models have a Summer/Winter option. I would think the heater receiving the pre warmed water would be switched to summer. This setting seems to use less gas or fires only part of the burners.

Agreed, for sure. I forgot to mention that...!