Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

Gas hot water boiler short cycles

Hey all,
Our gas hot water boiler has been short cycling and I can’t figure out what’s wrong. It started more or less after I replaced the original Honeywell thermostat with a programmable Honeywell thermostat, so I spent time there first. Nothing seems to make a difference on that end.
Boiler is a Weil McLain, 1985 vintage, single-zone, gas boiler feeding 7 rads in a 1,500 sq. ft. house.
It kicks on, fires up and the temp. Gauge will rise from 140 to 160 and then it shuts down. This takes about three minutes. Pressure is at 15 throughout.
I pulled the two rubber vacuum hoses off and cleaned some soot out, re-siliconed the box they plug into and connected everything back up. No change.
When I connected the thermostat, I wired it wrong at first, and I’m wondering if that might have ruined the vacuum switch or something else?
Running out of ideas.
— Saintpaulguy


  • JUGHNEJUGHNE Member Posts: 6,796
    How many wires at the T-stat?
    If only 2 and you tie them together will the boiler stay on and go up to the high temp cutout?

    Are you sure your pump is running?
    The new Tstat instructions may have directions for system set up. If so you need to set that function for hot water heat not forced air as it usually comes set for.
  • SaintpaulguySaintpaulguy Member Posts: 2
    It was the pump. Your question made me take a second look at it. I opened the junction box and the pump kicked in. Short version is that the neutral wires were taped together and covered with a plastic cap, but weren’t fully connected, and just opening the box allowed the wires to change position and connect. I must have done a bad job reconnecting things when I first pulled it all apart.
    If I had just replaced the thermostat, none of this would have happened, but I considered replacing the old thermostat wire as well, and started to disconnect things at the boiler.
    Operator error! But at least it was a simple fix.
    Thanks for the comments.
Sign In or Register to comment.


It looks like you're new here. If you want to get involved, click one of these buttons!