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How high temperatures should get on oil furnace


I recently put in a slightly used wtgo4 weil mclain gold oil burner.
The aquastat works well and shuts the burner down appropriately.   Right now I have it set at 190 for the winter. 
Once it shuts down the burner at 190, the temperature continues to rise to about 225.

Is this normal?



  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,597
    It can be. There are controls that can circulate that over run of temperature. The problem is that it does not save a ton of $$$ and that control adds to the cost of the equipment. What brand and model number of Aquastat does your WTGO have?

    Second Q. Are you using the tankless coil for Domestic Hot water? If yes, did the installer provide a mixing valve on the hot water?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • thekrazykat18
    thekrazykat18 Member Posts: 4
    Thanks for the reply.
    Aquastat is Honeywell l4006a.

    No I am not using the coil.  I have an indirect water tank fed through the main outlet of the boiler using a taco pump.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,597
    edited November 2022
    Then the OEM Aquastat on your WTGO is incorrect for your purpose. Can you list the part numbers of the controls on your Weil McLain?
    Primary control part number
    Aquastat Relay (that operates the circ pump and burner)
    Honeywell L4006 location?
    Low Water Cut Off control (If any)
    Are you using a zone controller for the different pumps

    Can you take a picture of the controls with the covers off so I can see the wires inside. Far enough back so I can see connecting wires.

    ALSO, That WTGO has a tankless coil (or at least a blank plate) that has a gasket that will start to leak in about 2 years. After 500 hours of operation you should take a wrench to the 6 bolts and snug them up. Then again in another 500 hours. (That may take about 2 to 5 months depending on how long the off cycles are)
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • thekrazykat18
    thekrazykat18 Member Posts: 4
    Thanks again.

    This boiler was given to me by my dad 5 years ago. He received this from a friend that had done a gas conversion.  It was used only 2 years prior. 

    The original boiler was a steam boiler wm sgo5 which was converted to a hot water when the house was renovated 6 yrs ago.  It was leaking since the cast iron block was cracked.  We poured a sealant into it but it finally gave way last week.

    My dad was going to change this out for me but he developed cancer and passed away 2 yrs ago.

    The boiler guy used the old go5 control since the go4 pump was leaking.  

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,597
    edited November 2022
    This is the reason you have poor temperature control.
    The WTGO boiler was designed to have the temperature control located in the center hole in this tankless water heater.

    For your safety I would set the L4006 to 160° until you can get someone to put the proper controls back in the correct place

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • thekrazykat18
    thekrazykat18 Member Posts: 4
    Thanks.  I will call the boiler guy to fix this.
  • Jersey2
    Jersey2 Member Posts: 166
    Mine does the exact same thing, but a gas boiler and indirect. I have it set to 120, the boiler clicks off when the water in the tank reaches 120 or HL. Usually the boiler has turned off at 180 high limit, and then the residual heat from the run makes the boiler temperature creep up to 225 or so.
    I'm not a plumber or hvac man and my thoughts in comments are purely for conversation.
    HVACNUT Member Posts: 5,756
    The GO and even the 66 and 68 Series Weil McLain's tend to override a bit. However, as Ed mentioned, the aquastat is in the wrong location. Once in the correct location, I would suggest turning it down to 160°, and the differential (the white dial inside) to 15°. Or replace the L4006A with one that will help save fuel, rather than a simple on/off. Ask "the boiler guy". Also ask him about the benefits of a modern primary control.
  • Dave Carpentier
    Dave Carpentier Member Posts: 579
    Mine will continue to raise temps about 20deg after it shuts down.
    I always thought it was the residual heat in the exchanger. When the burner shuts down so does the burner's blower, so there's stranded heat there too.
    30+ yrs in telecom outside plant.
    Currently in building maintenance.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,597
    There is one thing that everyone is forgetting. When the thermostat's call for heat is over, on most boilers, everything stops. The burner stops, the circulator pump stops, the moving water stops. Without all this movement, the heat does not go away. It just moves by conduction and convection much slower. (Gravity). As you all remember from this illustration from an earlier post of mine, The cast iron holds on to the heat and it takes longer to give it up. "see the hands"
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • hot_rod
    hot_rod Member Posts: 21,874
    The wall temperature of the cast iron on the flame side runs much hotter than the water inside sensed by the aquastat control. So that is how the temperature inside overshoots, absorbing the temperature from the hot cast iron.
    Natural draft through the sections will cool it, as will any convection or gravity induced movement out of the boiler into the system.

    If the piping is tightly checked on supply and return, that overshoot is more noticeable and stays longer.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream