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Tankless Water Heating Update

As we move closer to settling on actual equipment for the solar house, I have done some more detailed research into what models of tankless heaters would be appropriate to our designs. After considering a max instantaneous usage of around 2.0 - 2.5 GPM (clothes washing machine and an intermittent sink) and an inlet temperature of 50 degrees F, the solar house will probably need a 21 kW unit. This power rating would allow the solar house to get all of its hot water from one unit, reducing the overall cost (from using 2-3 point of use units).

A 21 kW unit used for 80 minutes a day continuously at 2.0 gallons outputting at 120 degrees F would consume 28 kWh. This would obviously be the worst case scenario. A sink will not use 2 GPM and hopefully, the inlet temperature will be above 50 degrees. The thermostat on board the tankless heater will only use as much power as is necessary to raise the outlet temperature to the desired level (about 125 degrees F in our case). If the flow rate is lower or the inlet temperature is higher, the unit would use less power.

One final consideration in choosing a tankless heater is the GPM draw at which the unit turns on. As heaters go up in size, the threshold at which they turn on also rises. This is to prevent the water from being heated too much by the larger heating elements under low flow conditions. This problem can be rectified if the proper controlling circuitry is in place, but a vendor with the technology would have to be located. Most high power units turn on above 0.5 GPM, which is about the usage of a sink. Therefore if the sink is turned on, the tankless heater may not supply any hot water because the threshold has not been reached yet. A heater with a lower turn on threshold will have to be located.


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