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condensing boiler with indirect tank, getting best efficiency?

weil_fail Member Posts: 84
edited August 2019 in THE MAIN WALL
I am planning to get a condensing boiler with an indirect "water heater". I am a little bit concerned that my radiators (old fashioned standing models) are designed for high temp water (180°) and may end up returning water to the boiler above the temp where I can get efficiency gain from the condensing mode, at least part of the season. is there a way to have the return water go through the indirect tank on the way back, to scrub off a few more degrees to give it the best chance of being cool enough for condensing? obviously, I wouldn't want to run hot water through the radiators in the summer just to have hot water, so I would need some kind of clever way of routing the return water through the tank while heating season and then using a dedicated "zone" for the indirect the rest of the time.
I suppose it could be done with some clever check valves, but that's adding some failure modes.



  • GroundUp
    GroundUp Member Posts: 1,905
    Are you certain the rads will require 180 degree water at design temps? You could sure route the water the way you're thinking with a well placed 3 way zone valve, but the odds are your return water would still not be into condensing temps and it'd be pretty cobbled. I'm all for condensing boilers, but it's cases like this that I will always go back to an 80% unit when possible. I see no benefit to a mod/con unless you really need the space or have no means of venting conventionally. If you can get away with lower SWT through your rads, there may still be hope
  • weil_fail
    weil_fail Member Posts: 84
    SWT? ____ water temperature?

    I'm not sure that the return water would be warm, I'm just concerned because it was designed for hotter water, not with condensing/low-temp in mind. it may turn out that a lower speed pump could return cool enough water to stay condensing most of the time.
  • flat_twin
    flat_twin Member Posts: 350
    edited August 2019
    You need to figure the output of your radiators. See if you can find them on these charts. Add the total EDR for all radiator sections. Compare the total output for your rads to your heat loss. Then you'll know about what your SWT, (supply water temperature) will need to be.


    In my case, our 10 cast iron rads have a total output at 170 degree SWT of 102k btu. My house requires only 57-62k btu depending on how I do the heat loss calc. So my SWT only needs to be 90 degrees when it's 65 degrees outside, ranging to 140 degrees when it's -20 degrees outside. The water coming back to the boiler is always less than 140 except for when it's making domestic hot water with the indirect. That cycle only lasts 12-15 minutes three times a day so not condensing isn't a big deal.

    If your house has had envelope improvements over the years, doors, windows, insulation etc, odds are you can make good use of a modcon boiler. You have to do the math to find out for sure.
    GroundUpJean-David Beyer
  • weil_fail
    weil_fail Member Posts: 84
    thanks, that sounds like a good way to move forward. how do I get my heat loss number? it's a very old house with some pretty leaky windows, so I think it will be lossy.
  • flat_twin
    flat_twin Member Posts: 350

    I'm told the slant fin app has about a 20% fudge factor built in and when I compared to an actual fuel usage heat loss calculation this seemed to be true. Anyway it's somewhere to start.

  • Canucker
    Canucker Member Posts: 722
    The slantfin app has a fudge factor even if you're aggressive with the ACH. It's a good tool but consider it the upper limit when it spits out the results
    You can have it good, fast or cheap. Pick two
    HVACNUT Member Posts: 5,828
    Your free standing cast iron radiators are prime candidates for a mod con. No need for a bypass through the indirect. With a good turn down ratio and outdoor reset, the boilers flue gasses will condense if set up properly. And do the heat loss calculation, or make sure the contractor does one.
    delta T