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Second Circulator on Old Cast Iron Rad System

We have an original cast iron radiator system currently on a Keystoker K6 and are switching to propane. There are three sets of main lines, supply and return each, with the radiators tapped into them. 1 line is running 2 rads. 1 is running 4. The last is easily three times the length of the others, “L”s around half the cellar and feeds seven rads including three on the second floor (12 foot ceilings). The first rad in this line is a massive, 17 fin three column 52 in tall behemoth in the front hall. It gets blazing hot but nothing diwnline from it gets any heat. If I shut it down at the valve to only about a quarter turn open, the next two rads gets hot and the rest get lukewarm. So I don’t think it’s airlocked, I just think it’s too far for our single taco on the main supply to provide proper flow.

I’m considering adding a circulator to the return on just this line with an independent thermostat. We don’t have the time or money until next year to do a real overhaul. We’re already seeing freezing temps.

Is this a possible fix or am I way off?


  • jrhughes
    jrhughes Member Posts: 3
    Sorry, two part question. If the second circulator works, how do I control it from a propane boiler with its own internal pump? I’ve seen there are taco circulator zone control panels, but how does that tell the boiler when to fire up? I’ve watched some install videos by taco but they only address the pump to panel, not the panel to boiler. Currently we have a Honeywell aqua stat that does this but the boiler tech support will only say that doesn’t work, hire a pro. Well we are in the middle of nowhere with pros talking six weeks to install.
  • Wouldn't it be easier to replace the existing circulator with one that's sized properly if that's the problem? It's tempting to add a circulator on the return; I've seen it unsuccessfully done many times. It doubles the head, not the gpm.

    Some questions for you to properly figure this out:

    1) What kind of piping system do you have? Monoflow series loop? Two-pipe system?
    2) Pipe diagram showing pipe size at the boiler and on each loop.
    3) BTU load of each radiator. If you give us the height, number of sections and number of columns, we can figure this out for you. Pictures would help.

    It sounds as though you have one pump and three zone valves. Is that correct?

    What is the model number of your existing Taco circulator?
    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