Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Steam -> Hydronic Conversion Question

Hey all,

I have a pretty old boiler (installed ~1981) that appears to have been converted from steam to hydronic at some point.

I say this because the main "trunk" of the system is 3" pipe, with various 3/4" pipes off of it for each radiator. It's a two pipe system.

The boiler's input/outputs are 1.25" copper, and whoever did the conversion just used some kind of reducer to make the transition between the two sizes.

My question is this: would a standard Taco 3/4 HP circulator be enough to move the water throughout the system optimally?

I ask because I'm worried it isn't, resulting in me paying more for fuel in the winter than I need to and wasting energy and money.

LMK if you need any more details to give me a gut sense, and thanks in advance!


  • Jamie Hall
    Jamie Hall Member Posts: 23,554
    Are you sure it was ever steam? That sounds a lot more like a gravity hot water system. Some photos of the main and return showing radiator takeoffs, and a couple of radiators themselves, would help a lot to know.

    A 3/4 hp pump should be ample to provide circulation, depending on the exact model of the pump (flow rate and head differ for different pump models of the same horsepower). In fact, it may well be more pump than is needed. There are many ways to determine that, but one of the best is also simple: what is the temperature difference between the supply going out of the boiler and the return coming back to the boiler?

    One thing to note: the amount of fuel you burn to heat your house is only very loosely related to things like flow rate. It is much more closely related to how much heat it actually does take to heat the house. Just changing pumps or even boilers rarely makes more than a small percentage difference, unless the boiler is really old or poorly maintained.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Steamhead
    Steamhead Member Posts: 16,948
    This might have originally been a gravity hot-water system. Are the original flow and return mains the same size?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
    JUGHNE Member Posts: 11,093
    Pictures of the typical radiators showing both ends may help.