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Boiler return too hot from indirect DHW tank to allow condensing efficiency?

jch Member Posts: 29
Considering installing a Lochinvar WHN055 mod/con boiler with an indirect DHW tank (Squire SIT050) on a priority circuit.

Our DHW usage is quite low because we use a drainwater heat recovery unit that raises the tank input to 70F (from 40F).

My understanding is that:

- I need to run the DHW tank at 140F+ to prevent Legionaire's growth; and

- The return water to the boiler will be at least as warm as the water in the DHW tank, but

- The cooler the return water to the boiler, the more efficient the boiler will run (and it needs to be less than 120F to condense).

So does this mean that, when heating DHW, the boiler will always be running in its non-condensing mode (and therefore much less efficient)??

Thanks for helping me wrap my head around this.


  • SWEI
    SWEI Member Posts: 7,356

    Return water does not always have to be as hot as the tank temp -- if incoming water is 70, returning boiler water should be somewhere between that and the supply water temp.  I would experiment with lowering the supply temp for DHW priority and see what you can live with.

    The difference in efficiency between 120ºF return water and 130ºF is negligible.
  • jch
    jch Member Posts: 29
    What about in standby?

    Ah, I see what you're getting at: if you are actively drawing DHW from the tank, then the return temp of the boiler water would be lower than the steady-state DHW temp.

    But what happens when DHW is not being drawn from the tank (i.e. for 95% of the day)?  If the tank's temp probe says that the tank has dropped to, say, 130F and therefore the boiler fires up to bring it back up to 140F+ then wouldn't the return water to the boiler put it out of the condensing range?

    I haven't built the system yet -- that's why I'm asking.  Most indirect tanks I see are asking for boiler water at 180F (40F delta-T) which makes me wonder whether we'd be only getting 80% boiler efficiency rather than 95%.

    Or am I totally missing something here?
  • Zman
    Zman Member Posts: 6,807

    Mod/con boiler have a ton of adjustment that will help with efficiency. If you use very little hot water you can keep the margins very close.

     I am not sure about the lochinvar, but the triangle tube has an anti legionella feature where it  will raise the tank temperature at a set time to kill bacteria and then go back to a lower setpoint. You get the best of both worlds.

    Check out this presentation on condensing boilers‎

    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • SWEI
    SWEI Member Posts: 7,356
    Efficiency and temperatures

    Mod/con boilers have efficiencies in the high eighties when used in high temperature applications.  As the return temperature drops, efficiency goes up and flue temperature drops.  The exact condensation point depends on fuel composition and humidity.  There's no sudden jump in efficiency when condensing starts.

    Well insulated indirects typically have standby losses in the range of a degree or two per 24 hour period.  The cost difference between recovering this at 87% as opposed to 92% is really tiny.