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Gas steam boiler controls?

Rock Member Posts: 44
I have an old Dunkirk gas steam boiler that was designed for steam radiators that was modified by adding two hot water zones. The system has a Taco SR502 2 zone switching relay and a strap on Honeywell L6006 aqua stat, in addition to the normal steam controls. My steam radiators are still hooked up, but im only using the hot water zones, 1 room with baseboards and the rest of the house , hydro heat ( coil inside air handler). The water zones were working good unlit late last winter, when steam started coming out of the radiators, and some times in the middle of the night when the thermostats weren't calling for heat. I had a hvac company come over and he couldn't figure it out. Some observations: When I turn on the switch of the cold boiler the flame comes on, if I immediately turn on the thermostat the circulator pump comes on(with cold boiler). Should the aqua stat maintain the proper water temperature and should the circulator pump only come on when the boiler reaches temperature? If I remember when the system was steam only, the flame came on as soon as the thermostat was closed.


  • Big Ed_4
    Big Ed_4 Member Posts: 1,698
    I will assume the Aquastat may be set too high and breaking steam ... Not sure which pipe the aquatstat is strapped to to give you an number, but turn it down as to not let the system break steam ...
    I have enough experience to know , that I dont know it all
    HVACNUT Member Posts: 3,917
    The L6006 is a DT aquastat. Are all 3 terminals wired?
    I dont see how a clamp on aquastat could be directly wired to the burner circuit. Theres no other aquastat in an immersion well in the boiler?

    The L6006 isn't wired for "make on temp rise" to control the fan on the hydro coil zone?

    Some pics and knowing electrically how it's all wired up will help.
  • Rock
    Rock Member Posts: 44

  • Rock
    Rock Member Posts: 44
    Only two wires coming off aquastat, one looks like it goes to low voltage transformer, the other goes to the honeywell box with red plug, the igniter?
  • JohnNY
    JohnNY Member Posts: 2,657
    It used to work and so I'm inclined to believe your aquastat wiring is fine. However, I've never had the results I've hoped for to control hydronic loop temperatures when strapping the aquastat to a pipe. It works far better, more accurately that is, when it's set in a well in the boiler's block (heat exchanger). It sounds like your aquastat is losing its calibration. That happens. It's not reading the temperature right and it's allowing the burner to fire long enough to make steam when you're only try to make hot water. Turn it down 20° and live with it a week or so of run time.
    Yours is a simple aquastat and so there's nothing in it to control the pump. The pump will run whenever there is a call for hydronic heat and the aquastat will cycle the gas valve.
    Contact John "JohnNY" Cataneo, Master Plumber
    in New York
    in New Jersey
    for Consulting Work
    or take his class.
  • Rock
    Rock Member Posts: 44
    edited November 2018
    Thank you for the replies. Is there any way to create a well inside the boiler for the aquastat probe, or to insulate the strap on to make it more accurate? Assuming this system was never modified( when it was a pure steam system), without the strap on aquastat, what regulated the boiler flame? If I remember correctly the flame came on with the thermostat.
  • Jamie Hall
    Jamie Hall Member Posts: 15,828
    If it was a pure steam system, there were at least three controls involved. The thermostat, which told the system that heat was wanted. The pressuretrol, which kept the pressure from going too high. The low water control, which kept the system from operating if there wasn't enough water in the boiler. These can be wired up in various ways... but the overall operation was simple: the thermostat called for heat. If the boiler was cold or at least not steaming, and there was therefore no pressure, and there was water in the boiler, the burner fired up (often with a short time delay). As steam was created, if there was too much of it, eventually the pressure rose to the cut off setting of the pressuretrol, and the burner quit. If the thermostat still wanted heat, once the pressure had dropped enough the burner came back on. If at some point in the proceedings the water level dropped too low, the low water cutoff shut the system off.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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