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.

Electrical Schematic

Alan (California Radiant) Forbes
Alan (California Radiant) Forbes Member Posts: 4,102
edited September 2023 in THE MAIN WALL
I've never been good at electrical schematics and I need some help understanding this Viessmann boiler.

I replaced the sequencer already 12 years ago and suspect that this one is being troublesome as well, but it's an intermittent lockout and I have yet to be there when it's happening. The owner has been cycling the power to get it running again.

The attached diagram shows the sequencer below middle with a relay up a little to the right. Is it safe to say that a sequencer is a relay, in this case a double pole, single throw relay? And that when there's 24 volts to the coil, the relays close, in this case, sending 110 volts to the inducer fan and 24 volts to the ignition control module?

Now, there's that 24 volt coil to the right that also closes when there's a call for heat (independent of the two flame rollout switches) that will turn on the inducer fan, but not send 24 volts to the ignition control module.

I'm just wondering why they add the relay. Perhaps an aid to troubleshooting, i.e. if the fan is running and there is no power to the ignition control module, then the problem is the pressure switch? Is this a common design?

8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    your reading is correct -- and why they used two relays I'm not sure, except that two pole relays are more common...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,887
    edited September 2023
    It's Viessmann, so I'm going to assume there's good reason for it, I just don't know what it is. It seems redundant. During post purge, 4 and 5 stay closed on the sequencer while 2 and 4 open on the relay. 

    Is the sequencer Delay On Make as well as Break?
    If DOM the 24 volt circuit will be delayed but the inducer will still run through the other relay. When the timer hits 0 and 1-3 close in the sequencer, the pressure switch is already (hopefully) closed and 24v continues to the ignition module.
    JUGHNE Member Posts: 11,102
    Sequencers have a time delay between the sets of contacts closing and then reopening.

    The blower contacts 4 & 5 would close first for pre-purge.

    Then some seconds later 1 & 3 would close for ignition.

    When tstat satisfied the 1&3 open and 4&5 opens after post purge.

    The second 24 volt relay might hold the blower on after the timing out of 4&5 opens.....if tstat still calling and one of the FRS opens.

    This would make the inducer fan run, possibly cooling down the FRS to reset (if auto) and then refire.

    Doesn't explain the problem.....not sure if my theory is correct either. Another WAG.

    What is used for for tstat, probably not the Chronotherm. (which dates the boiler BTY).
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    What happens before and after the power is cycled? 
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
  • Teemok
    Teemok Member, Email Confirmation Posts: 561
    edited September 2023
    Looks like the sequencer is both a delay on break and a delay on make. Maybe thermally activated. When TT circuit is first closed there's a delay on make while the heater warms to close the sequencer contacts. At the same time the second relay that is handles 120v for the fan in parallel has already closed spinning up the fan giving pre-purge time. The heater in the sequencer gets warm and closes its contacts starting the ignition attempt. If it fires great, if not, the fan just runs.

    Then when opening TT it breaks the yellow wire circuit, the flame goes out and the sequencer heater is de-energized but it is still holding the contacts closed because it's still warm and the 120v keeps the fan on for post purge even though the second relay's contacts opened when TT was broken. The fan runs till the sequencer cools and opens it's contacts. Yes, I'm guessing
  • Alan (California Radiant) Forbes
    @Jamie Hall @HVACNUT @JUGHNE @Solid_Fuel_Man @Teemok

    I just learned a lot. Thanks to EVERYONE!
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab