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LAARS Mighty Therm 2 Help...

YammiR1 Member Posts: 3
edited October 2019 in Domestic Hot Water

My company took over a contract for a medium sized condo, they have 3 LAARS Mighty Therms for Heat, and 1 Mighty Therm 2 / Storage Tank for DHW.

I walked in first day today for PM on all 4 boilers and found the floor of the mechanical room covered in water, the Pressure Relief Valve on the DHW boiler is lightly but consistently going off as boiler temps rise.

Expansion tank is a 4.5 gallon Watts, I put a pressure gauge on the air side of tank and got 80psi.

The pressure gauge on the heat exchanger manifold pins out just before the relief goes off, the pressures are getting way to high during expansion. Even when the DHW 120* is satisfied the pressures are still abnormally high, just not high enough to pop the relief valve.

I have narrowed things down to either the Expansion tank is under sized, or the Pressure Reducing Valve is giving out and letting Boston Ma utility pressure in un-regulated.

I haven’t been able to track down the PRV yet due to recent renovations done on the building, also worth noting is the Pressure Relief Valve is rated for 125psi.

Any opinions on what might be causing this high pressure/pressure relief valve leak would be greatly appreciated.


  • Zman
    Zman Member Posts: 7,542
    Bad (or no) reducing valve, bad expansion tank, bad relief valve?
    You need to start by finding the reducing valve and verify that it is working correctly. The expansion tank needs to be isolated from the system and drained in order to check precharge pressure.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    HVACNUT Member Posts: 5,740
    At 125 psi, the leaking relief valve is in the DHW storage tank, not any of the boilers, correct? If so, then either the pressure is too high (125 psi) or the tank tank temperature is too high (210°?). Is there a domestic extrol? It should be charged to the same pressure as the incoming water pressure. Is there a domestic PRV? Is DHW temp exceeding 210°? Being a condo, I'd assume its code to have a thermostatic mixing valve on the domestic hot so hopefully the residents are safe from scalding. Bad aquastat on the storage tank or constant power to the tanks HX circulator?
  • YammiR1
    YammiR1 Member Posts: 3
    I was there for the first time Friday and haven’t been able to find the DHW Pressure Reducing Valve, it is possible there isn’t one as the 4 Heat Boilers are in the same room and there PRV is easily visible.

    I will work on tracking down the PRV Monday morning, also the Pressure Relief Valve is on the Boilers Manifold (Copper Fin Heat Exchanger).

    When I check the Expansion Tank further, If I find it Bad or Undersized I plan on sizing with Static Cold Fill Pressure, BTUH, Max Temp and Storage Tank Size.

    I appreciate the responses and will keep posts on the outcome.

  • Mike_Sheppard
    Mike_Sheppard Member Posts: 696
    When you checked the air pressure on the bladder tank, did you isolate it and relieve the water pressure? You must relieve the water pressure from the bladder before you measure the air pressure. If not, you’re not reading the actual air pressure.

    They generally lose a pound or two of air pressure per year.

    How many stories is the building? 4.5 gallons seems a bit small for a condo building. What is the input of the water heater and the storage capacity (gallons) of the storage tank?
    Never stop learning.
    delta T
  • YammiR1
    YammiR1 Member Posts: 3
    edited October 2019
    It is a 4 story building, and I also find the Expansion Tank to be undersized.

    I put in a Service Request for the DHW System and am returning Monday, I will 100% check the Tank correctly and track down the PRV.

    I plan on sizing a new tank need be with Cold Fill Static Pressure, BTUH, Max Temp and Storage Tank Size, also worth noting this is a very long standing issue as determined by Standing Waterlines and Corrosion on Mech Room floor in this specific area.