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Connecting a new boiler to the old cast iron pipes and radiator

RobinInCali Member Posts: 36
I have a three story building, and each floor has a 1928 cast iron boiler that is way past dead. Inefficient and broken and unsafe. When they work, the radiators put out plenty of heat. The old hot water radiators are in fact, gorgeous, in great shape, and all work well. I want to install a single boiler to work both all three floors—one zone for each floor—and add an indirect water heater. Ideally, I’d like to keep the old radiators. At least for now. If it ain’t broke…

The other option is to remove the old radiators and install modern radiators. I want to see if I can reuse the old piping and radiators. They work great and there are no leaks anywhere.

The boilers are rickety junk.

My question: how do you go about connecting pipes to each floor? Do you simply run a new zone where the old boiler pipes connected— to the inflow and to the outflow — on each floor? Seems basic. I’m just not sure how the old system worked other than to heat the water and have gravity and nature do the rest. Hot water rises, cool water sinks. There are no pumps. Just a boiler that heats the water, the water makes its way through the radiators, then cools, and the water sinks to the basement then gets drawn up through the floor again. I think.

There are air chambers are on the roof, the drains from all three floors are in the basement, and I’m not sure how the old system really works. I’ve installed lots of mod con boilers with zone pumps, and they worked fine. My question is, should I worry a lot about the current piping that’s worked well since 1928? Shouldn’t just repiping and adding a pump for each zone work?

Thinking logically, I should just hook up the zones to the existing pipes near the old boiler, call it a zone and call it a day.

Where could I read on old systems like this? Any ideas?


  • Jamie Hall
    Jamie Hall Member Posts: 20,488
    This has come up from time to time, and I think some of the Caleffi articles address it.

    Basically, you are on the right track. The existing boilers are in the basement? Then you would them up as zones coming off the single boiler. I would drop the old air tanks on the roof and use a conventional -- and single -- expansion tank on the system. You will have to pump the system, and it should be set up primary/secondary. There are arguments in favour of a single secondary pump and zone valve, and there are arguments in favour of zone pumps instead. Chevrolet vs. Ford. Since each zone is its own floor, you won't have anywhere near the balancing hassles which usually happen when going from your type of system to a pumped system -- which is one good thing! Further, since your air tanks are on the roof already, the pressures in the system will be similar to what has been there, so leakage shouldn't be too much of a problem, if any.

    More details?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • RobinInCali
    RobinInCali Member Posts: 36
    Thanks. It doesn’t seem to be complicated, but I want to make sure I’m not missing something before I start tearing into the walls in each floor near each boiler. It’s a gravity fed boiler system. No pumps. A boiler on each floor. And the tanks on the roof are air, but also water. I forget what that’s called in a gravity feed system without pumps…

    I want to convert it to a single modcon boiler, while rescuing the old piping, and having one zone per floor, with each zone being pumped. All the piping and radiators appear to be fine, no leaks, (and the former owner had just installed lovely hardwood which is why I don’t want to touch the radiators piping.)
  • Jamie Hall
    Jamie Hall Member Posts: 20,488
    Boilers on each floor? Odd setup, but then -- something new every day. Still, since the air tanks were on the roof that won't change the pressures, which is a good thing.

    You should be able to pipe that with a feed and return for each floor from a manifold in the basement, fed by the mod/con. As I note, either zone valves or zone pumps. Oddly enough, once you get the system filled and purged, the extra height to the third floor, for instance, won't make any difference.

    I would use fairly generously sized risers and returns, and I certainly would go primary/secondary.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 809
    3 boilers on 3 floors. This sounds like multiple "tenants". Do you pay the heat for all floors or the "tenants"? If the tenants pay, are you sure you want to combine them and pay the heat, with no control over temperatures on each floor? What about heat in common areas?

    How is hot water currently configured?

    I realize 3 new boilers may be an overriding concern, but had to inquire anyhow.
    JUGHNE Member Posts: 10,320
    Is there a boiler physically on each floor or a boiler in the basement for each floor?

    To be a gravity system it seems they would have to be lower than the radiation.
  • RobinInCali
    RobinInCali Member Posts: 36
    It used to be three units. It is now one house. Yes. 

    Each floor is separately plumbed. 

    The new system will have two high temp zones and two low temp zones (in floor).

    The original radiators and piping are in excellent condition. No leaks. Works great. Everything on the two floors with radiators was wrapped in 2” thick plaster cast asbestos which has preserved it. Was high tech back in the 20s. It insulates great. 😬 This is another reason I don’t want to touch it. 

    I’m will be replacing the other two zones, 1with staple up and 1 with slab. 4 zones plus the DHW indirect. This is most efficient and I no longer have tenants to worry about. Just extended family. 

    Hope that answers the questions.