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Steam boiler and indirect hot water with radiant

scott_blockscott_block Member Posts: 15
Hi All,

New to the forum and wanted to get some advice.

I have an old house with a gas fired steam Weil Mclain boiler thats about 25 years old. There is also an addition on the house thats about 650-700 sq ft that has a separate Weil Mclain hot water boiler with 2 radiant zones and 1 hot water baseboard zone. Lastly I have a stand alone hot water heater thats about 14 years old and have 1/2 water lines through the house to the fixtures.

What I am looking to do is condense this all into one boiler. In total the two boilers and 1 hot water heater are about 350-400 btu and my gas bills are pretty high in the winter. I also know the hot water heater needs replacing and the steam boiler is giving me some trouble and will most likely need replacing and or addressing in the next few years anyway and lastly need the space to make the utility room more comfortable for the laundry machines that are in there.

A plumber / heating company came to my house and made the suggestion to rip out the hot water boiler all together, put a new steam boiler in and have an indirect water tank as well as the two radiant zones and one hot water base board zone off of the steam boiler with priority zone valves for the hot water, expansion tank, mixing valves etc.... I really like the idea because then I would have 1 boiler that services everything and would have way less btu / gas bill as well as more hot water with better recovery and more space for the laundry machines that are in there.

Does it seem like too much to have the two radiant zones, one hot water baseboard zone and indirect heater of the steam boiler? BTU wise it seems like I will be about 1/3 or less of the total on a a new steam boiler for the hot water zones and indirect hot water.

In general are there any caveats to this setup? I have read a bunch about in general using the steam boiler for the indirect hot water is not so great and will most likely have to replace pumps every few years because of the dirty water (maybe if the supply line is high enough to the water line it would be clean water and not be so bad) and i'm concerned that the radiant and baseboard getting the right temps etc...any suggestions to make sure the plumber is installing the system properly with certain things like y strainer etc...

I have read some things on the forum but not really directly relating to this situation so any advise would be appreciated or reasons not to go this route.


  • STEAM DOCTORSTEAM DOCTOR Member Posts: 1,174
    You need to know the numbers. Technically it's doable. But hot water heating and/or indirect loads need to fall within the steam boiler's pickup factor. You would need a heat exchanger for the radiant zone. Baseboard may or may not need heat exchanger.
  • STEAM DOCTORSTEAM DOCTOR Member Posts: 1,174
    You might be able to use a tankless coil instead of the heat exchanger
  • STEAM DOCTORSTEAM DOCTOR Member Posts: 1,174
    If you want hot water with better recovery, you can put indirect off of water boiler.
  • motoguy128motoguy128 Member Posts: 138
    Might almost be better to just use a wall mount modulating boiler for the hot water and indirect. Might only need a small 80k for those loads. Would be more efficient and you don’t have summer standby losses on the steam boiler.

    Other option is baseboard off the steam
    Boiler, and radiant and domestic off a combi. Would reduce the all the controls and mixing valves needed.
  • scott_blockscott_block Member Posts: 15
    None of the combi tankless options really can work for me. I have 1/2 water lines and seems like with 1/2 water lines the combi or tankless is not recommended. I really want to eliminate having 2 boilers and a water tank with a total of 320,000 btu to a properly sized steam boiler that can handle the entire house and domestic hot water.

    I spoke to a master plumber / engineer that does a lot of boiler replacements in Brooklyn on steam systems and he said this is absolutely doable.

    I am not sure though what something like this should cost. I am removing 2 boilers and a water heater and doing a bunch of repiping and an indirect stainless tank as well as filling this with nyc.

    Does anyone have experience with this in NYC and what something like that may cost?
  • Jamie HallJamie Hall Member Posts: 13,118
    Talk to John Cataneo ( @JohnNY ) or Danny Scully ( @Danny Scully ).
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • HVACNUTHVACNUT Member Posts: 3,471
    Yes, that ☝️, and there's no price discussing on the forum for a majority of reasons.

    Obviously the boiler needs to be resized.
    The new steam boiler should have a diagram on installing an indirect. I would use that layout to pipe in a plate heat exchanger sized for the hydro zones and indirect.
  • motoguy128motoguy128 Member Posts: 138
    Here’s another idea. DO a indirect with a double coil. The upper coil does there floor radiant and lower coil heats that tank. Maybe still need a mixing valve depending on flow rates needed and target water temp.

    For the baseboard, run a 2nd zone off the hot water loop from the boiler. You may want priority on this so both are not calling at the same time.

    The advantage here, is you add mass/storage for the radiant micro zones.

    For controls, depending on boiler size vs. steam EDR, you may want to do DHW as a priority call. The reason is that if the DHW consumes for example a max of 120,000 BTU, then a 200k input boiler only have 40k in reserve for Steam output. That might cause imbalance issues during DHW calls. You could oversized the boiler, but during steam calls, you now have an oversized boiler that short cycle on pressure.

    All that being said, if you have a vapor system, I have noticed any issues being “short” on steam for a 1-5 minutes DHW call. Standby recovery calls (Due small use to recirc And or normal Tank heat loss) only consume 20-50k BTU’s and last 30 seconds. Longer draws form showers will consume 70-100k on my system and my reserve steam is only 60k, but that;s enough to keep the header and laterals full and the calls are so short that any imbalance is negligible. Most showers Or clothes washer fills are only 5-10 minutes long.
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