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Indirect DHW with slightly undersized boiler

deek Member Posts: 21
Hi Folks

I'm designing a system for my home that will have an indirect hot water tank.  I did my heat load calculations and bought my boiler and water tank. I failed to account for the fact that my altitude requires high altitude jetting on the boiler reducing the output considerably.  Here are the specs...

Viessmann ECD 140 non condensing boiler  116k btu DOE output, 104 500 btu with high altitude jets installed (NG)

Bradford white RTV-52-L Tank IBR heat source output of 113 000 btu. (priority DHW)

I understand that a smaller heat source will reduce the "continuous draw rating" but do I have to be concerned about flue gas condensation with this match up?  Does that rating mean that it can transfer 113000 btu of heat at normal operating temps?



Calgary, AB


  • Zman
    Zman Member Posts: 7,573
    Thermostatic bipass valve

    I think you are correct that the boiler will condense for a good part of the DHW cycle. I have a TT175 condensing boiler with a smart 60 tank. I have watched the return temps at startup and seen condensing temps for a considerable time under load (5 to 10 minutes). I my case I did it intentionally as the boiler is a mod/con. I think the simplest fix would be to install a thermostatic bipass valve on the boiler. This will assure hot return temps and have no effect on DHW production. I think it is great that you are thinking of this. Most folks would not have paid attention to a detail like this.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • deek
    deek Member Posts: 21
    Thanks Zman

    Thanks for the reply. I am wishing more and more that I had a mod con boiler! I purchased my boiler and tank used (never installed) from a guy that changed his mind about installing a hydronic system and went to a traditional forced air system instead.  I saved a lot of money but as a result, I have to build my system around the components I already have.

    I guess a three way mixing valve on a boiler bypass might provide boiler protection?

    Lots to learn....

  • deek
    deek Member Posts: 21
    another option?

    What about a boiler bypass with a variable speed pump like the Taco 00-vs with boiler protection? Seems like an effective though possibly expensive solution.
  • kcopp
    kcopp Member Posts: 4,443
    you will be....

    fine. You could also go w/ a tekmar control to help w/ the boiler protection. kpc
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