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Combo Indirect + Direct Water Heating Solution

SJV Member Posts: 1
Long Story... Purchased my home as an abandoned short sale and 1 month in, the boiler cracked. Turns out the previous homeowner somehow disabled the low limit and operated it as a cold start - all to keep the house at 45 degrees with the 25 or so gallons of oil left in the tank... I guess it could have been worse - burst pipes, mold, etc. Anyway, I took the opportunity to do a heat loss calculation and it turned out that boiler was almost 2x (133k BTU) the required size. I replaced it with a good condition used Burnham 88K boiler that I picked up really cheap (friend just happened to be converting to gas). My original system has a 50 gallon indirect, and from everything I read, this was a better option than the tankless coil that this replacement boiler also happened to have. So I left the indirect in place and just capped the tankless coil. So now we're living here, but we always run out of hot water if we take consecutive showers, and the problem is exacerbated if someone decides to wash dishes as you can imagine. So I decided to study the problem. The boiler has no issue ramping up to the high limit (180) and maintaining while the indirect is calling, it's the heat transfer rate of the indirect that can't keep up, or I guess put another way, our flow/consumption rate must be too high. This makes sense, the shower that this happens to be a problem with most is a high flow design. So, me being the problem solver type, decided that I would use what I have to fix the situation - use both the direct and indirect. I plumbed the direct to feed the indirect tank, and then installed a thermostatic mixing valve on the output of the indirect just to be safe along with a thermostat so I can set/monitor the temp. So far this is working great, a truly endless supply of hot water. So, for the professionals, what are some of your concerns with this setup?


  • Eastman
    Eastman Member Posts: 927
    It sounds like the indirect coil is fouled or there is some other issue preventing the indirect tank from performing to it's full capacity.
  • BiggA
    BiggA Member Posts: 7
    Either something's not right with that indirect, or your shower is a firehose. I've lived mostly in houses with oil indirects for about 15 years, one with a 95k boiler, one with a 191k boiler, and over that time span, each ran out of hot water once, and both of those were on bitterly cold days with at least one heat zone running for a while and once-in-a-blue-moon hot water demand, neither went completely cold, and both recovered within 5 minutes. Part of the selling point of an indirect is that it never runs out of hot water, which is, 99.9% of the time, true.

    Those things are generally about 100k, so in that setup, it should have 50 gallons already in it at 120F, or whatever it's set at, then, if the boiler is always hot, it should run around 100k until the heat transfer hits equilibrium with the 88k boiler, and then run at the boiler's full output, at which point, assuming you have really cold input water at 50F, you should be able to generate ~2.5GPM of 120F water continuously.