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.

boiler is firing when there is no call for heat or hot water

michael_34 Member Posts: 303
I have a potential SHW customer that has a Vaillant boiler (about 10-12 years old).

When I was looking over the boiler system, the boiler would fire every 10-15 minutes. All zones were closed and t-stats satisfied (it was about 70* outside). The temp in the tank was at the set point temp.

The boiler will fire for a minute or so. and stop.

What are the potential causes of this?




  • kcopp
    kcopp Member Posts: 4,306

    Is the boiler control a "tankless" one...

    Are there Taco zone valves? Sometimes I have run into a case where a 571 head is about to go and they will fire the burner for no apparent reason...

  • John Reynolds
    John Reynolds Member Posts: 22
    ghost firing

    check the gas valve. I have run into this a couple of times in the past. Someone does some work and a drop a solder crosses the TT terminals causing a call for the boiler.
  • Gordan
    Gordan Member Posts: 891
    Along the same lines...

    Any other short in the thermostat wiring, including at the terminals in the back of the thermostat. Can you check the call for heat terminals with a multimeter to determine whether something's actually switching it on?
  • MacArthur
    MacArthur Member Posts: 2

    I had a similar situation where the boiler was firing all the time and bumping the high limit.  noticed one of the zones was always calling.  What I found was a sheetrock screw straight through the t-stat wire inside the wall, shorting it out.  You said nothing seem to be calling, so based on  that, I would suspect the Aqua-stat is going bad.  Try disconnecting all t-stat wires that can make a call for heat.  Get and accurate water temperature, note the High and Low limits settings and watch the boiler for awhile and see if it comes on when it shouldn't have.  I it does, it's probably a bad aquastat.
  • Techman
    Techman Member Posts: 2,144
    The heat,

    that the boiler makes,where is it going? Feel the lines and trace it out.
  • World Plumber
    World Plumber Member Posts: 389

    It's time to start testing for voltage and continuity where it shouldn't be. You find what bring the boiler on you find your culprit. You didn't have someone install spill switches in parallel with the controls did you? I found that one in a school no less. I was there in the middle of July working on the air conditioning and heard the boiler running. As mentioned zone valve heads going bad something across terminals. OH!!!! I found bad wire once the system worked for 4 years then started acting up. How I don't know. When I cut the thermostat wire open 4 of the wires were bare fore over ten feet. Any control could be doing it. If you have an older mechanical thermostat. Sometimes dust will build up on the bulb and push it down. 
  • HydroAirJoe
    HydroAirJoe Member Posts: 27
    short cycle

    I have this issue but it only happens 4 times a day.  Found out it was  a built in feature of the airhandler(Firstco).  It wants to circulate the water 4 times a day at 30 second intervals.  This is fine  if the airhandler directly controls a circ pump, but in my application the airhandler controls zones valves and the zone valves end switch connect to the boliers aquasat via TT terminas which will fire up the boiler for 30 seconds.  I was told by Firstco this is a law and the reason for this is because the air hanlder can be in a potable water loop and the idea is to not have stagnat the water in the heating coil.
This discussion has been closed.