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tankless back up to solar

geo_29 Member Posts: 2
I am wondering how to use my existing tankless water

heater as a back up to a new solar water heating system. It is a Bosch 125HX

that modulates based on water flow not temperature. It picks up water temps by 90 degrees. We are planning on a three

way valve before the tankless to bypass it when the solar water is hot enough

for use.

But the question

we need a creative answer to is how to avoid super heated water when we have

90F water going into the demand water heater? 

Another question is how to get even

temperatures from the system, even at variable flow rate.


  • Robert O'Brien
    Robert O'Brien Member Posts: 3,422
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  • Jack
    Jack Member Posts: 1,045
    Check out

    Bonomi valves. You can lay them out in any flow configuration and they have the lowest turning torque of about any valve. That means ypu can use a small Bonomi actuator. I believe they are at You then have to co-ordinate the tank temp to bypas the tankless with the flow capability of the tankless.
  • hot_rod
    hot_rod Member Posts: 15,363
    tankless in Europe

    is a huge market. So Caleffi has developed a few valves, thermostatic and motorized to work with all types of tankless either modulating or not.

    Issues with flowrates, water quality. etc need to be addressed for the US market. Maybe....

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    the simplest answer

    for part load DHW in many cases is just use an electric on demand that can modulate. your service has to be beefy enough to handle it. but by the time you jump through the hoops required needed to make gas boosting worthwhile, you don't save that much.

    any cost differential between electricity/gas is a minimal concern when it is only doing part load on DHW like this. the solar will do the heavy lifting in most cases.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • lchmb
    lchmb Member Posts: 2,997
    edited July 2011

    If you read some of the tech literature with this unit it only allow's for "cold water" to the inlet. It clearly states you cannot use pre-heated water in this system and it will cause issues...

    "Verify the heater is being feed with cold water only. This water heater is not designed for preheated water or recirculation applications"
  • meplumber
    meplumber Member Posts: 678
    I completely concur with Ichmb.

    This series of Bosch units is particularly prone to problems if the entering water temp exceeds 60 deg F.  Bosch makes an tankless unit that is "solar ready".  But this is not one of them.  Time to regroup.
This discussion has been closed.