Disconnecting Tankless Hot Water Heater From Oil Boiler

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    • #273567
      Avatar photoMasterPlumbers

        Help…My hot water originally came from a tankless hot water heater in an oil boiler, but the water was no longer hot so I switched to an external hot water heater. What I need to know is how to disconnect the temperature sensor which keeps the boiler water hot for the tankless hot water heater. I tried disconnecting the wire to the “low” side temperature select knob(in the control box), but now I have no power to the circulating pump. What is the PROPER way to remove the tankless form service?


      • #288052
        Avatar photoHarold Kestenholz

          You probably removed the black wire to the Honeywell block in the 8124. Put it back and remove the blue wire; the blue one controls the burner low limit. Also turn the low limit knob all the way down – the one on the right – so the circulator will come on sooner. This will prevent sending very hot water out to cold piping and radiation expanding them suddenly.

          One way is to change the control from an 8124G or H control to an 8148A which has no low limit and circulator temperature controls in it. But allowing the boiler to use the low limit feature of the 8124 reduces the temperature swing of the boiler and may help prevent leaks due to the ‘working’ of the seals from temperature changes in mild weather. So you could leave all wires connected and just turn the low limit side all the way down to 120F or so.

        • #288053
          Avatar photoHarold Kestenholz

            The above answered your question. IMO the proper way to make hot water while putting a tankless coil out of service is to use an external water heating storage tank, either one with a heat exchanger built as part of the tank or through use of a heat exchanger.

            The boiler already has a burner and flue. Using the boiler to make hot water makes sense because only one burner has to be serviced and no changes to the flue are necessary.

            A 3-gallon-per-minute tankless-coil boiler with a 120,000 btuh burner can make 180 gallons of hot water in one hour. Nothing else comes close to that, including a residential external boiler-fired water heater. But a 60 gallon boiler-fired water heater can make 100 gallons of hot water in the first hour, and that is a good amount of hot water.

            A separate oil-fired water heater has a smaller input. it makes a good amount of hot water, but it requires another burner to service and flue modification.

          • #288054
            Avatar photoGuest


              thanks so much for the info…the hot water system you are speaking of acts as its own zone off the boiler, dosen’t it?? who manufactures such units???


            • #288055
              Avatar photoHarold Kestenholz

                Many manufacturers make these units today because it is such a good idea. I prefer the stainless steel tanks for longevity. Your local plumbing/heating supply has the popular brand they feel is the best.

                Yes, they act as a separate zone off the boiler. The R845 or R8445 circulator relay sends power back to the 8124 ZR terminal to fire the burner from the hot water thermostat in the tank.

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