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Community and Q&A

Tempering water for domestic hot water

Sfair74 | Posted in Mechanicals on

HI, I’m looking to build an Insulated Raft or Frost Protected Shallow Foundation single story in zone 6. I have a few questions regarding waste water heat recovery and tempering incoming cold water for domestic hot water.

I am interested in a grey water heat recovery but since i’m not planning to build a basement so I can’t install the standard vertical mount heat exchanger.

We don’t have natural gas in our area and I would like to avoid punching more holes in the building envelope. My wife and I are currently satisfied using shower heads that use 1.5 GPM and have a switch to change them to .5 GPM. I have grown accustomed to showering on the .5 GPM setting and take between 10 and 15 minute showers. Overall our household does not use much DHW. I was planning on just installing a well insulated but cheap electric hot water tank.

Is it worth my effort to install a horizontal grey water heat exchanger? I’m guessing that it is a poor idea the return on investment is not favorable in my situation.

While I was looking into radiant floor heating I stumbled on the open direct radiant infloor heating system. This system uses the in slab radiant floor piping to temper the cold water for the domestic hot water. Here is a link to an explanation of an open direct radiant floor system.

Since i’m planning on building a super insulated house radiant floor heating does not make sense. I think we would be better with a few ductless Mini Splits.

Now on to my question. Would it be a good idea to include some drinking water approved PEX piping in my slab to temper the water before it reaches the water heater?

My thoughts are that during shoulder months and the summer it would provide a slight advantage with free slab cooling. In the winter months it would steal some heat from the house but that heat would be replaced by the mini splits that have a much greater return on energy than the cheap electric water heater. I’m thinking it would be best to use two manifolds and run several equal length shorter loops through the slab to temper the water instead of one long run. Would the minimal amount of water used for domestic hot water would cause any discomfort issues with the slab? How much piping should be included in the slab to temper the water?

Does this sound like a good idea for my situation?


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  1. GBA Editor
    Martin Holladay | | #1

    The only disadvantage to your proposal is that the energy savings are likely to be too low to justify the cost of the Pex tubing and the labor to install it.

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