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Control for Steam Boiler with Hot Water Loop

Apologies for the duplicate post; I put it in the wrong forum.

Gas fired steam boiler with a hot water loop to heat the basement.

I've been all over the Honeywell and Taco spec sheets and don't think I've turned up the right control yet.

What I want is that the boiler should fire whenever the steam zone is calling for heat, or when the hw zone is calling for heat and the boiler water temp is below the high limit, say 180F. The circulators should run whenever the hw zone is calling for heat and the boiler water temp is above the low setting, say 100F.

The tricky part is that for the burner, I want the high limit to be ignored when the steam zone wants heat but paid attention to when the HW zone is calling for heat.

It's obvious to me how to wire this if I were building it from scratch with relays: circulator is run by the HW zone thermostat and the low temp aquastat in series, both close on rise. Burner is run by the steam zone thermostat in parallel with (the HW zone thermostat (close on rise) and the hi temp aquastat (open on rise) in series).

Rather than wire this up myself and leave a confused mess for the next guy, I'd prefer to put in a control that's designed to do this. I think this is the same control you'd use if you had, say, a temp-stor run off a steam boiler.

Can anyone point me at the right solution?



  • paul_91
    paul_91 Member Posts: 62

    use a honeywell 845a relay. terminals t & t connect to your t-stat . you bring 120 v to terminals 1 & 2 and have a jumper going from 1 to terminal 3... terminal 4 goes to the hot leg on your pump..[terminals 3,4 & 5,6 are all dry contacts]. use terminals 5 & 6 in series with your high limit [180 f] , lwco & p-trol. with this control you can turn the boiler on when there is no call for steam heat...
  • chris_69
    chris_69 Member Posts: 29
    Good start - What about steam thermostat?

    Clean and simpler than I thought I needed. Thanks.

    I guess I don't need a low limit aquastat to delay running the circulators until the boiler comes up to temp (small Burnham Independence) -- OK to turn on the circulators and fire the boiler at the same time?

  • paul_91
    paul_91 Member Posts: 62

    its ok to start burner and circulator @ same time ...use any t-stat
  • paul_91
    paul_91 Member Posts: 62

    the steam t-stat is wired as is... in series with your safeties[ lwco & p-trol]...if your boiler has a fan center[or just has a regular transformer] just bring terminal ''r'' to terminal 5 on the 845a relay when the hot water zone calls for heat it will close terminals 5 & 6 sending [24v] power out terminal 6 to your high limit then through your safeties[lwco& p-trol] to your gas valve ...with no call for steam heat the boiler will cycle off of the aquastat...but when theres a call for steam heat the boiler will run until it satifies the steam heat t-stat.
This discussion has been closed.