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My dual boiler is heating the radiator pipes when hot water is requested

Erion1983 Member Posts: 2
So, I have an oil boiler that generates both heat and hot water. In the summer the thermostat is not calling for heat, but when I open the hot water and the boiler starts working to generate hot water, the heating pipes leading to radiators start heating too. How do I resolve that so that hot water only is generated and no heat in the radiators?


  • Kickstand55
    Kickstand55 Member Posts: 103
    The system should have flow check/control valves or electric zone valves to prevent this from happening.
    Flow checks/controls can harbor dirt, rust, solder etc. on the seat and prevent them from holding back hot boiler water.
    Check to see if there is a small screw fitting on top with a flat or round for opening/closing. Maybe it was left open during service or installation. If found with the screw post in the exposed position, turn it to the right until it stops. Don't use a wrench!
    Send pictures for us.
    Go here for more information. https://www.tacocomfort.com/product/flo-chek. This is only one brand of check, for example only. There are others. I'm not interested in favoring one product over another.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,501
    edited August 2022
    @Kickstand55 may be correct and that will solve your problem. There is another cause that may also be happening to you. Starting on page 9 of this booklet http://media.blueridgecompany.com/documents/ZoningMadeEasy.pdf you will find a detailed explanation of what may cause your issue. The Flow Valve may be stuck open OR you may be experiencing Ghost Flow. (also explained in the booklet)

    My question would be "When did this problem start?" Has it always been a problem or is this something new? Was any repairs completed on the boiler that required the boiler to be drained? Perhaps the valve was opened for the repair and never set back to automatic.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • Erion1983
    Erion1983 Member Posts: 2
    I should have added more details. It is a steam system with 1 zone heating and a tankless coil hot water heater combo. I don't know when it started as I recently bought the house. The problem is similar to this post but the suggestions offered there is not working.

  • mattmia2
    mattmia2 Member Posts: 9,398
    Is the dhw a tankless coil in the boiler or an indirect water heater connected to the boiler or an indirect using a tankless coil as a heat exchanger. Pictures will help a lot here. On a dhw call the boiler should run on an aquastat that keeps the boiler below boiling but in your case either the aquastat is bad or isnt seeing the boiler temp so it is running off the pressuretrol/vaporstat on a dhw call.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,501
    If you have the tankless coil then the minimum water temperature aquastat that is in the tankless coil portion of the boiler is set too high or is defective. To make hot water only all the boiler needs to do is to get to about 180°. If the water is allowed to get any hotter, there is a chance that the burner will start to make steam. If the burner stays on long enough to make steam, then you will get heat in the radiators. Check the control in the tankless water heater (where the hot and cold water pipes are connected) to see what temperature it is set to.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    EBEBRATT-Ed Member Posts: 15,297
    Also, there could be mud or sludge around the aqua stat well