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Munchkin 80M problem

zircote Member Posts: 2
The boiler is set up with a primary and secondary heating loop with a 25 gallon buffer tank and a DHW 40 gallon tank.
The system has been in place 13 years.
Combustion analysis has been done. Regular cleaning of the combustion chamber including cleaning the probes is done.
The problem is new and seems to arise when there is a simultaneous call for heat and DHW.
During a call for DHW the boiler sounded like it was over heating the circulating water and cavitating, it also rattled the over pressure valve causing it to leak. Thinking the pump, B&G three speed set to high as required, was nearing its end of life I replaced it with a Taco three speed and also replaced the over pressure valve. The non return valves are in place.
That seemed to solve the problem.
But, of course I would not be asking for your help if it did.
I have now replaced the heating circulator as well.
Usually being the second person in the shower before bed, the call for hot water happens at a very inconvenient time.
This does not happen every day and has been rather rare. It seems like both DHW and heating circulators are operating at the same time.
Other notes:
It does have a concentric intake/exhaust
The two pumps are connected directly to the Munchkin and controlled by its program. Zone valves and circulators are controlled separately.
Post purge on the pumps is set to 0.

Any ideas?
Thank you


  • "DHW and heating circulators are operating at the same time."
    If both the DHW pump and the primary pump are connected to the Munchkin, the programming will normally turn to DHW priority, turning off the primary pump.

    Turn up that post-purge number to 4 minutes or so. All that heat in the heat exchanger can cause the water to boil.
    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
  • HomerJSmith
    HomerJSmith Member Posts: 2,309
    Does the indirect HW tank have a mixing valve on the domestic hot water output? If so, I would suspect that the tank is heated above 125 deg. What does the display console say the temperature is when you are experiencing the problem? In other words, when you are heating the DHW. What is the tank temperature set at? Do you use a aquastat or a sensor to monitor the tank temperature? Was there any work done on it lately, before the problem ocurred?

    Replacing parts willy-nilly to fix a problem and hope for the best is an expensive undertaking. I'm not one for the shotgun approach.

    A picture is always helpful, then I don't have to ask so many questions.
  • zircote
    zircote Member Posts: 2
    The DHW does have a tempering valve at the output.
    The tank is heated to 160 degrees.
    It uses a temp probe.
    I have noticed that the hot water at the faucets seems hotter than usual.
    No work has been done beyond what is described.