Tankless to supplement existing conventional heat pump water heater?

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New Member
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North Carolina
I'm building a house for a vacation rental, with 4 bathrooms, which can sleep 8-10 people. I don't want to run out of hot water too soon, nor force people to have to wait too long for more hot water once the tank runs cold.

My ground water temp is about 55 degrees here.

I don't think people will expect to have endless hot water for 4 simultaneous showers, followed quickly by 4 more, but I can certainly see this situation happen with 4 couples trying to all get ready to go to dinner while on vacation.

I like the idea of the heat pump water heaters for efficiency, but I believe they take quite a long time to re-heat.

So, I'm thinking of adding an electric tankless heater under each pair of bathrooms (side-by-side, on each side of the house) to help, but I'm not sure how helpful this will really be.

I'm not sure how hot the incoming water can be in these tankless units, and once the main tank runs cold, they probably won't heat the large flow very well anyway.

I can't use just electric tankless as the size required for this flow simply requires too much electricity to be practical.

The other option i see is a second heat pump water heater, but that's a more expensive option, and still suffers from long recovery time.

There is probably no IDEAL option here, but I'd love to hear your thoughts on how to handle this (probably unique) situation.


Clinical Trail Failed. New therapy starts June 20
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Orlando, Florida
An electric tankless requires a lot of power and your electrical panel may not be able to handle the additional load and the cost can get quite high. Hybrid water heaters do use electric heating elements, the same as the standard tank. The heat pump is very efficient when little or no water is being used to bring up the water to the set temperature and maintaining the water temperature. Once the hot water demand exceeds the recovery rate for the heat pump, the electric elements kick in.

Most likely the best setup is to use the mixing valve. Set the WH temperature to 140 degrees and the mixing valve reduces the temperature to 120 degree for a safe level. This is how hotels provided enough hot water but the tanks may just be larger with larger heating elements. The mixing valve is usually called a tank booster. Using it it can increase the hot water supply about 40-50%. A 50 gallon tank with a booster would be like a 90 gallon tank.

Since you are in the build phase, also consider changing the heating elements to 5500 watt units for a faster recovery rate for a standard water heater. I'm not sure if a hybrid can support 5500 watt elements. Tankless WH take more maintenance and requires an annual flush or more per year depending on your water hardness. It's not just a flush but a cleaning process using vinegar or other cleaner to remove hard water deposits from the heating changers. For a tankless you'll need to look at the chart for recovery rates VS inlet temperature and demand.

My take on this is stay with one type of water heater to keep things as simple with little maintenance as possible.


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Peace valley missouri
Heres a mixed water calculator just change settings to gpm's and f °. Lp out of the question? Two heat pump heaters should work.

John Gayewski

In the Trades
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You could circulate water from a tank water heater to a hot water storage tank. This is how a hotel I worked on had endless hot water. They had two storage tanks that were insulated and designed to store hot water, with the three waters they had that circulated the hot water to and from the storage tanks.

Also keeping them stored at higher temps and using antiscald protection at the points of use.
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