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Adjustment or new Aquastat & settings

wolfwagon Member Posts: 1
Hello Folks I need some help.

I have a Weil- Mclain Gold Oil Boiler( model P-WTGO-4) that provides Heat and Domestic HW (tankless).
In December of 2022 due to a severe cold snap the house was cold the tech came out and made an adjustment he said" I raised your DHW so you should be fine." Heat was good and DHW was fantastic.
Fast forward to December 2023 I get couple minutes of HW then it turns lukewarm and gradually warms up over 10-15 minutes but not hot like before. I had yearly service done in Dec and told tech (different) about the problem. he cleaned and serviced but still have the issue. I keep my house thermostat at 70. If i raise it to 72 before i jump in the shower hot water is excellent.
My Aquastat (Honeywell L8142A) settings currently are (have not been touched since the tech did it in 2022):
High 190
Low 160
Diff 15
So the question is should i adjust my current aquastat and if so to what or should I replace the Aquastat and what should the settings be? I cannot keep the house thermostat to 70 due to the price of heating oil.

Other thoughts?

Any help is much appreciated.


  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,818
    edited March 2

    Do you know what the temperatures were set at after the Dec 2022 adjustments were made?

    The settings you have now are where I would normally set them. If you are not getting enough HW all the time, then you need to increase the temps gradually to get it where you want them. Try High 190 and Low 165 with DIF 15 and see if that helps. After 3 days, if it is not enough, then go to High 190 and Low 170 with DIF 15.

    If you need to go higher after 6 days, then you must also increase the High so it is at least 20° higher than the Low. High 195 and Low 175 with DIF 15.

    With the WTGO hot water control you MUST have a mixing valve that has 3 openings. One from the hot out of the coil, one from the cold going into the coil, and a MIX out to the spigots and faucets. Without that valve you have the danger of scalding.

    Based on the age of the WTGO Boiler you may have some calcium build up inside the tankless coil that is inside the boiler. That will act as insulation between the hot boiler water and the cold water flowing thru the tankless coil pipes. That will make the water heater less efficient and it will end up with the symptoms you described. Adjusting the temperature settings higher will increase your hot water, but it will also increase your fuel usage.

    If your tankless coil is more than 10 years old or if you have very hard water, a cleaning service on the tankless coil will get that water heater back to operating like new. The first one may be more expensive if the coil was not installed with the necessary valves for flushing cleaning chemicals thru the coil. But once those valves are in place, then you may be able to do the service yourself in the future.

    Message me if you want more information on tankless water heater maintenance.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Panheadsforever
    Panheadsforever Member Posts: 21
    Clogged coil sport
  • Big Ed_4
    Big Ed_4 Member Posts: 2,780
    Back flush or bounce the coil to remove the mineral build up . Increase heat transfer .

    Also make sure the mixing valve is not bypassing ..

    Running off a coil for hot water . Low volume shower heads work ....

    I have enough experience to know , that I dont know it all
    Larry Weingarten
    HVACNUT Member Posts: 5,828
    Shouldn't be that big of difference in one year. To verify, the aquastat is an L8124A, not a 42, correct? 
    My bet is on a whacked Lo limit differential. Only the 1K1 contacts close on a heat demand, but the R-B contacts do not close to maintain the Lo. Not exactly uncommon with that control. 
    Ask the tech to measure outlet water temperature at the coil with a clamp thermometer without any heat zones running. Compare it to the Lo setting, less the differential. With the Lo at 160° and a diff. of 15°, the burner should kick in at 145°. Not too hard to verify. 
    A better replacement if that's the issue is the L7224A.

  • PRR
    PRR Member Posts: 121
    edited March 3
    > I cannot keep the house thermostat to 70 due to the price of heating oil.

    The way it works, the house temp-set isn't that important.
    The Aquastat keeps the boiler water hot, winter and summer. When you want to pay for indoor heat, another thermostat turns-on a water pump to circulate to your radiators/baseboards. But even if you never call for room heat, that boiler is full-hot and the in-tank coil should be delivering water very nearly that hot.
    Keeping the boiler hot (as Ed tells you) is not going to be a big hit on fuel costs, because it is only an insulated pot of water, not a whole cold house.
    I remember different numbers but that was a steam system. Instead of a water circulator it brought the water over 212F and steam rose to the radiators. This was before tempering valves were common and yes it could scald bad. But it was steady so once you found an H/C balance it could be comfy.