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Plate Exchanger for Radiant Heat

x_navyfc | Posted in Energy, Heating & Insulation on December 29, 2021 03:45pm

I recently installed 1/2” pex under a new cement slab in my 50+ year old home. 2 loops covering 500sqft total. 4” slab with 2”xps underneath. Climate zone 5.

 I’m trying to think of the best way to provide hot water for the slab while not taking up much space in the utility closet where my furnace and 40 gallon hot water heater is located. There isn’t much real estate available for a second HWH. Buying a tankless HWH is an option but it would be nice to utilize what I already have to keep costs down.

I’m not a fan of the idea of using potable hot water for the slab heat.

My thoughts are to use a plate exchanger to separate the hydronic system from the potable water such as:

Plate Exchanger 75 Kbtu at Menards
https://www.menards.com/main/p-1444433727819.htm

What I don’t know is if a domestic hot water heater set at under 120 degF is hot enough to transfer the heat needed to be used for in slab heat.

Is a plate exchanger 100% efficient in that an input of 115 degF will result in an output of 115 degF?

Any other ideas on what could work well?

 Thanks for your time.

Reply

Replies

  1. calvin | Dec 30, 2021 06:48am | #1

    Radiant heat technology has come a long way since we built 33yrs ago. My best recommendation is to consult with a knowledgeable radiant heat designer.

    Being an older dumb carpenter who enjoys the feel of a warm floor is about how far my knowledge goes.

    We submitted our floor plan and the rest of the design, heating load and location to a company that designed and sold hose. One thing I do remember was that length of hose and layout of zones was important. Water temp going out from our boiler needed to get back to the boiler before all the heat was transferred to the slab. Of coarse on an initial calling for heat, the transfer was quick and the return temp dropped quickly……so, hose length was fairly critical.

    As the slab was brought up to set temp, the water temp drop was less drastic.

    Layout was critical to produce an even temp around the rooms. The outer wall perimeter had a narrower hose pattern than the interior layout. They also were the beginning of the runs.

    Throw in a passive solar home layout and damned if it didn’t work as hoped!

    A pretty long while ago there was a contributor here on the board that would have been a big help for you, but as times goes here so do the members. NRT Rob I think was his forum name. Might search that and peruse, you could get lucky and find some info.

    Our boiler runs at about 125-130.

    Best of luck!

    1. x_navyfc | Dec 30, 2021 06:55am | #2

      Thanks for the reply and insight. I’ll keep researching and try reaching out to those supply and design companies you mentioned.

  2. User avater
    unclemike42 | Dec 30, 2021 07:39am | #3

    https://www.supplyhouse.com/Heat-Exchangers-821000?gclid=EAIaIQobChMIucetxcOL9QIVFFNyCh0G7AmvEAAYAiAAEgL7nfD_BwE

    These folks sell a variety of hydronic heat exchangers of different sizes. each has info on hydronic floor applications, with flow rates and input and output temps.

    Included is an application guide
    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/BP400-20LP-Selection-Guide.pdf

    If you operate with a lower input temperature, the heat transfer will also scale, so you would probably want to put in a larger rated exchanger than your heat load would otherwise call for.

    as you re-use your domestic hot water heater, you may want to increase the temperature setting on the water heater, and install a mixing valve to reduce the temperature for the domestic hot water.

    You will, of course, need a couple circulation pumps to make this all work, with suitable controls. You also will need an expansion tank. Probably want a pressure gauge for the hydronic loop. And faucet to let you drain the loop. And valves, lots of valves.

    Sounds like a fun project.

  3. Hydro_Tech | Jan 05, 2022 12:34pm | #4

    These guys can have lots of experience with these types of situations, I've found they are very helpful when it comes to radiant heating.

    https://www.janesradiant.com/

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