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Old Radiators, New Boiler

RobinInCali Member Posts: 38
edited August 2021 in THE MAIN WALL
I just purchased a four unit building built in 1928. Cast iron radiators in two large units, in-floor radiant slabs in the other two ground floor units. No leaks anywhere, great condition, so I have no desire to replace the radiators. (Besides, the pipes are wrapped in asbestos, which I have encased and want to leave the hell alone.)

(San Francisco. You run the heat most of the year because it's always chilly, but the design day is never for more than 40 degree winter weather for a couple weeks in Dec/Jan.)

However, each unit has a boiler that is almost laughable. The things date from 1928, and each boiler still works, but looks like it could explode any minute, has leaks, has had lots of repair issues. Way past time to replace. So, I am replacing the boilers. Each unit has its own huge gas water heater, too, which I also want to get rid of and gain space.

I'd prefer to put all the systems in the large basement area, with good venting options, because maintenance would be easier and I would have to enter the tenant units for repairs.


Is it better to go with individual wall mounted combi mod-con boilers? Though not a lot of efficiency would be there, because the water temp has to be at least 160 for two of the units. Or should I go with one super efficient HTP Pioneer type boiler with a large indirect tank and btu sufficient for the whole building?

Heat loss for all units is 118,550 btu. I also want an indirect sufficient to supply four bathrooms and kitchens.

Again, Two of the units have in floor radiant slabs. Two of the units have the old cast iron radiators.

I'm leaning to the larger HTP Pioneer with one large indirect tank. Having four boilers seems like a recipe for more repairs down the line, but they would be plumbed separately so each tenant could pay for their own heat. My goal, however, is to be as green as I can.


  • mattmia2
    mattmia2 Member Posts: 9,467
    edited August 2021
    Another option would be 2 cascaded boilers, you could pick the boilers to have a 10:1 turndown and sizing that better matches the load most of the time with one boiler firing near its maximum turndown but both could fire full to pick up some of the DHW load so you wouldn't have to do it all with quite as enormous an indirect. It would also give you some redundancy if one failed you would still have some heat.

    Oh, and it is very likely you could run an outdoor reset curve that kept the boilers in the condensing range most of the time.
  • Steamhead
    Steamhead Member Posts: 16,730
    Are all the systems hot-water, or do the radiators work with steam?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Hot_water_fan
    Hot_water_fan Member Posts: 1,799
    Where does that heat loss come from? Seems high for somewhere so mild.
  • tim smith
    tim smith Member Posts: 2,743
    A lochinvar WHB 155 easily does it all. Or maybe an 055 for the 2 radiant and an 085 for upper floors radiators and indirect. That way one boiler for low temp one for higher. Plus could possibly set up some redundancy. Pretty simple.
  • RobinInCali
    RobinInCali Member Posts: 38
    Great suggestions so far. To answer questions: The radiators are hot water, 180 degrees. The heat loss is big because the square footage is pretty big and also, the original windows leak, windows cannot be replaced because they’re architectural.

    So nobody wants me to put in the single Pioneer boiler? I like the idea of two wall hung units.
  • Jamie Hall
    Jamie Hall Member Posts: 22,941
    The windows can't be replaced -- been there, done that many times. They can and should, however, be repaired -- and they can have interior storm windows added (several manufacturers) which are easy to put in and out and which bring them up to nearly the best of the modern triple glazed units. They're also cheaper.

    Do that and you will be very happy
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • woobagooba
    woobagooba Member Posts: 186
    edited August 2021
    Allied might be able to help with your storm windows. They are often used to satisfy historic district requirements.


    They are pricey, but will provide better protection for your windows than interior storm panels. Restoration / maintenance of old windows is expensive (unless you have unlimited free time).
  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 3,942
    edited August 2021
    If they are one bedroom units, you could go with combi boilers that do both heating and DHW. That would save a lot of space.

    [email protected]
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab