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Question DetailsAsked on 5/29/2014

Can heat pumps be used with tankless water heaters

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Odd you should ask that - I was just reading a technical article on just that subject, regarding utilities for remote site facilities.


A tankless heater is designed to heat water from ambient to use temperature - so say to 120 degrees or so, in a short time. Obviously, the hotter the incoming water temp, the less time it takes to reach desired temp. Some tankless heaters sense the incoming temp and adjust flow rate or energy consumption rate to suit, others assume a 40-50 degree incoming water temp and cannot adjust to hotter incoming temp and will actually overheat if fed hot water.


Generally speaking, the money you would save would be from the temperature change effected by the more efficient heat pump (assuming it is operating in heat pump and not heating coil mode) versus the energy used by the tankless heater theat the incoming water the same amount.


Operationally, IF the heater can handle hot incoming water safely, then essentially using it as a topping-off heater could provide a much larger source of hot water, drawing off a water heater (or combined service hot water tank). This is commonly done on industrial jobs where most hot water demand is 120 degrees or so, but you need hotter water for an industrial process or steam for a process or steam shower or such.


Rigging a combined system takes some careful engineering to avoid unintended consequences (like blowing up your tankless heater), and I have heard of some problems doing this because the hot water in the hot water heater tends to accumulate sediment and minerals that contaminate and damage the tankless heater, whereas incoming cold water, due to the demand volume and more frequent use, tends to be cleaner.


If you meant geothermal or solar thermal heating heat pumps to preheat the water, then definitely yes, subject to the same tankless heater incoming water constraints, especially in very cold incoming water environments. In that case, the vastly higher efficiency of the heat pump would, at least on an operating cost basis (neglecting higher capital cost), be cheaper than a direct-fired tankless heater. Some geothermal systems, and even a few high end very high efficiency combined-cycle hydronic heating/combined use storage tank systems use this type of topping-up methodology to eek as much benefit out of the waste heat as possible, albeit with a loss in simplicity and capital cost.


If you meant a heat pump system for conventional hot water, but a tankless for a specific use like shower or kitchen faucet, then if they are seperately plumbed to different hot water piping, certainly and they would not influence each other at all.


So much for my shot in the dark - if I missed the target, use the Answer This Question button under your question to reply back with more specifics on what you are trying to accomplish.

Answered 5 years ago by LCD




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