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Tankless WH as booster

SWEI Member Posts: 7,356
Years ago, there was one particular Aquastar model that ended up in a lot of solar installs, since it worked well with preheated water.

Looking through I&M manuals for current tankless offerings I don't see any mention of minimum rise or maximum inlet temp.  Anyone have specifics, or a pointer?



  • heatpro02920
    heatpro02920 Member Posts: 991
    Cross Rinnai out

    I install a lot of Rinnai tankless units and they do NOT like low delta t's. What do you use for heat, I have installed Water to water plate exchangers as booster for solar hot water, if you have an existing boiler that heats your house...
  • SWEI
    SWEI Member Posts: 7,356
    edited February 2013
    I did get one answer

    From A.O. Smith (Takagi) but it's not quite as simple as I thought.  I've attached an example for the GTS310 (TK-4) below.

    The application is for occasional booster heating of an existing SHW storage tank.  Since Takagi includes an "H" stamp on them, I can comfortably specify them (once I add proper controls and safeties.)
  • Jack
    Jack Member Posts: 1,047
    The thing about tankless and Solar

    Actually let me rephrase that. The thing about Rinnai and solar...I can't speak to other manuf as I don't know all of their specs, but it all pretty much starts with GPM X Delta T X 500= BTU. In order for the Rinnai's to initiate operation the unit must see a min of .4 gpm. Once it starts it will hold operation down to .26 gpm. On the RL94/75i the min btu is 10,300.

    In the worst case you have to have a pretty hi delta T (around 50*) at the. .4gpm rate to initiate fire. You can push those numbers around and get operation at higher initial flow rates with a lower delta T.

    A good solar system design in my opinion is to by-pass the tankless until you get a low enough solar tank temp to allow the delta T and flow rates to begin to let the tankless do its thing. I also don't like to simply run the water thru the tankless as all you are doing is driving the limits due to the high temps the solar tank can provide.

    If I am not mistaken, and perhaps Mark will chime in here, Caleffi makes a 3 way for this type application. By-pass the tankless when there is sufficient hot in the tank. When the tank drops to a low enough temp, and you have to spend some time on setting this up, the 3 way diverts back to the tankless and it peaks the hot water.

    I'm just re-modeling the house and want to install a system to do this. My problem is I have to do a sun-map to see if it is worth doing. My house sits under a very large Black Walnut, a 100' tall Ponderosa Pine and about 125' Sequoia, so it may not be worth doing;)
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