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Any Harm If System Supply and Return Are Reversed?

flat_twin Member Posts: 290
Last November we had a WM Eco modcon installed by the heating contractor who has serviced our fuel oil boiler for the last few years. The boiler has run fine all winter. Nice even heat throughout the house, no trapped air issues, very quiet and best of all the cost to heat our home is down 40% over last year.
Today I found a photo of our basement from last summer when the city water expansion tank broke and flooded the basement. Our old fuel oil boiler was in the photo. When I zoomed in I could see the supply and return piping were backwards of what they are now.
Our system is one zone, 10 cast iron radiators with large piping dating back to the 1930's. It's a two pipe direct return layout. The main trunks, both supply and return, are 2.5 inch dropping down to 1.25 inch near each radiator. The piping near the boiler is direct with one circulator, not primary secondary. It's been this way going back to the 1960's.

My main question ... Does the system care which way the water runs? I checked the radiators thinking the position of the valve would be on what should be the supply side but they're not uniform in that respect. Some are, some aren't. There are no arrows on any of the old piping to indicate direction of flow.
As it is now, the circulator is on the return side of the modcon boiler, pumping away from the expansion tank. The old fuel oil boiler had the circulator on the return side pumping into the boiler then toward the expansion tank directly overhead. From what I understand, the correct piping scheme is to pump away from the expansion tank as it is now. The contractor that installed the modcon is not the one who installed the fuel oil boiler 12 years ago. This has me wondering if the fuel oil boiler was piped backwards? The heating system worked fine with either boiler. Never a failure, no odd noises or lack of heat. If I hadn't gotten interested in hydronics while dialing in the ODR and taking time to study the pipes in the basement I might never have noticed when I saw the photo today. Thoughts?


  • Jamie Hall
    Jamie Hall Member Posts: 16,559
    On a system such as yours, no -- it really won't make any difference. The only way it might -- and there is a way -- is if you have control valves on the radiators, and if the valves are now on the outlet side, and if the washer or disc in the valve is loose, you could hear a rattle. Three ifs. So I wouldn't worry about it.

    Before the general audience gets carried away -- there are systems where it could make a big difference: monoflow, where the direction of the flow through the diverter Ts is critical. But that's not the case here, from your description.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • lchmb
    lchmb Member Posts: 2,997
    nice even heat...40% savings...I'd say your all set...
    rick in Alaska
  • flat_twin
    flat_twin Member Posts: 290
    edited March 2017
    Some of the radiator valves are on the outlet side... now. Strange they're not all one way or the other but it's been like that for years. No rattling sounds. I had a hard time believing the old boiler photo when I saw it. Had to go check the basement again to make sure. I'll subtly ask the heating contractor about it next time they're here for service. Thanks for your input.

    Yes 40% less! Mission accomplished. We've been waiting for natural gas for a long time. Finally happened so we're free from fuel oil. I was thinking this winter has been milder than last but the heating degree days over the same period last year are almost the same. We spent $1720 in fuel oil last winter and fuel oil was relatively cheap compared to other years. We've used 510 ccf NG so far this winter. I think we can finish the heating season with 800 ccf. At the current rate, that's $1000. And we keep the thermostat up a little higher now and no night time setback.