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thermostatic valve & pipe size with tankless

I have a new Rheem 64DVN tankless to heat DHW in a small Philadelphia rowhouse gut rehab.  The heater will be in the basement, and supply the kitchen directly above, and the bathroom on the 2nd floor, directly above that.

When trying to buy shower valves, I learned that individual hot/cold are now out of code, and have read conflicting info about thermostatic valves with tankless.

Has anyone had problems with thermostatic valves shutting off the tankless due to low water flow when lukewarm water is demanded?  (The valve I was looking at is Cifial 289.715.999 thermostatic with separate volume control.)

Are individual hot/cold controls recommended instead?

Also, has anyone had any problems meeting minimum flow from the tankless if 1/2" water supply piping (i.e. leaving the unit, up to the fixture) is used from a 3/4" manifold in the basement, up to the 2nd floor?  (Is this bad in particular if the mixing valve has 3/4" connections?)

Many thanks. I'll pass on the good karma.



  • Charlie from wmass
    Charlie from wmass Member Posts: 4,322

    I do not install tankless units much but maybe they guys who do can answer.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • EricAune
    EricAune Member Posts: 432
    Minmum flow rates

    Here's a link to the specifications for your tankless heater:


    You'll see the minimum .40 gpm to activate the burner is listed and .26 gpm to keep it running.

    You will then need to compare that to the output of the shower valve/head while in operation. 

    I have had issues with minimum flow rates and shower valves in the past, mostly in homes operating off private wells and lower than normal pressure.  Rheem advertises this particular model as having the "lowest in the industry" so you will likely be up against the constraints of your supply pressure/flow rate.  One thing to note, it would likely be best to use PEX tubing for the supply piping to the valve and eliminate any unneeded fittings as this will provide the least amount of pressure drop when compared to copper or other piping materials.

    There was an error rendering this rich post.

  • CMadatMe
    CMadatMe Member Posts: 3,086
    edited January 2011

    The use of that word key's me to ask some questions. Is this shower unit a car wash?

    When using thermostatics the only issue I have seen is that you may have to boost setpoint of the heater higher then normal. You may have to run 130 degree water versus 120 degree water but that depends on the customer.

    As far as flow rates shouldn't be a problem provided the unit is sized correctly. That unit on a 70 degree rise is only going to make 4gpm.

    There was an error rendering this rich post.

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