DIY Heat Pump Water Heater From a Dehumidifier – Part 2

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by Tim Fulton on October 21, 2013

In our last article, Hv23t took a scrap electric water heater and dehumidifier and put together a DIY heat pump water heater. However, there are some issues to work out. Mainly, the cold side heat exchangers were freezing up like bricks of ice which decreases efficiency quite a bit. There were also some other features to add to it.

However, even with the setup as is, when Hv23t tested things, he was seeing a COP of 3.1-2.1 depending on the water temperature. The higher the water temperature, the lower the COP as is the nature of heat pumps.

The first enhancement was to insulate the refrigerant lines going to the tank separately. Hv23t measured their temperatures and one was always 30-40F warmer than the other.

recycle timer

The second enhancement was to use a recycle timer to stop the compressor during the defrost cycle. This will stop the ice from forming on the cold side heat exchangers. What this basically does, is stop the compressor from running when ice forms on the heat exchanger. The natural heat in the system and around it melts the ice, and then the system can start running again. Previously, the system kept running until the block of ice had formed, by this time the efficiency is already being hurt. With the recycle timer, the unit will run for 45 minutes, and then turn off for 5 minutes. Previously it ran for 1 hour and 40 minutes before turning off to defrost.

time delay circuit

The next thing Hv23t did was install a time delay circuit for the resistance heating element in the heater. Once the heater calls for heat, this timer is engaged and if the heater is still calling for heat in 2 hours, the electric resistance heater kicks in to help warm the water.

temperature controller

The last electrical feature Hv23t added was a temperature controller. This little unit stops the heat pump from running if the ambient temperature is too low. Once the air temperature gets too low, its more efficient to just run the resistance heating element.

heat pump water heater

Last, but certainly not least, Hv23t installed the heater in his parents house. They previously had a propane hot water heater that was costing them approximately $775 per year to run. With a few measurements with a kill a watt, Hv23t estimates the new heat pump water heater will use around $175 worth of electricity to supply their hot water. A $600 per year savings isn’t too bad for using a bunch of scrap parts!

For more information, see Hv23t’s forum thread.

{ 1 comment }

1 Peter Roberts February 9, 2016 at 4:20 pm

Looking for sources for retrofit heat pump/dehumidifier to be hooked into the threaded drain hole in an existing off the shelf electric water heater.
I have seen one being installed on “This Old House”, so I know they’re available.
Would seem to make more sense than paying the big $ for an off the shelf combined unit such as Geo Spring, because probably the water tank will let go first, leaving the entire package useless.
Can anyone help?

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