Have just bought a house in which we've inherited a hydronic system - so just beginning diagnosis and diagramming, etc.

It seems that radiators use only one line for both supply and return; presumably valves in each radiator regulate flow to that radiator? Apart for a need for balancing the whole system - some rooms are too hot, others cold - it basically works.

Longer-term vision for system is to break it up into 'n' zones; individual valves, and I imagine, dedicated return line. At this point, house has only two zones, so much work to be done in warm weather!

Does this sound like a crazy setup? An ad-hoc kind of installation?

  • It sounds odd, since most water type systems need two lines to make a loop, or one line in a loop(radiator to radiator then back). Having two lines in one pipe seems inefficient since the cooler return will cool the supply. Can you provide pictures.
    – crip659
    Commented Mar 28, 2023 at 20:21
  • A single line doesn't make any sense. How could you possibly replace the cool water in the radiator with warm water through a single line?
    – brhans
    Commented Mar 28, 2023 at 20:25
  • That doesn't work and makes no sense, because the hard part is finding the route/pathway for the pipes - once you have that, 2 pipes is no harder than 1. The only way 1 pipe can work is if the radiator has huge volume (large enough to absorb the cool water in the supply pipe and still receive enough hot water to usefully heat), and then the furnace pushes this slug of water back and forth. Commented Mar 28, 2023 at 20:36
  • 2
    Another possibility (apart from the monoflow system Ruskes mentions) is that the system is steam, not hydronic, where a more literal "one- pipe" (not even a loop) is quite common - steam rises to the radiators, condensate flows back from them. Look Ma, no pumps! But steam systems are becoming rarer with time for logical reasons....
    – Ecnerwal
    Commented Mar 29, 2023 at 3:05

1 Answer 1


You have a One Pipe System and it looks and works like this

one pipe heating

one pipe

Suggest to Read the article, it nicely explains the details.

The main drawback is there is a temperature drop at each radiator feeding back in the single line thus reducing overall heat supply. Thankfully each radiator has a thermostatic valve for individual control.

  • 2
    Usually called Monoflow and it's important to know the tees are not just tees, they are special and the flow and return ones are installed differently. There's nothing wrong with this system but it's not easily cut up into zones.
    – jay613
    Commented Mar 28, 2023 at 21:20
  • 1
    It does have "per radiator" zoning ability with the thermostatic control valves, though. Won't suit people who buy into "$mart thermo$tat$," works fine if you like your thermostats dumb and the valves are in good repair.
    – Ecnerwal
    Commented Mar 29, 2023 at 15:19
  • 1
    Thanks so much for all responses - you guys are great! Yes, indeed - that 'one pipe' diagram above is schematically what we have... $Ruskes comment I think summarizes my findings; that heating is wildly different at different points in the circuit. $Ecnerval - Yes, a sophisticated 'n' zone system, under IoT control, is precisely the goal of this exercise - though not this week! I'm thinking - eventually - individually-controlled pumps(?)/valves(?), perhaps even on a per-room basis. I think a lot of efficiency could be achieved in this house...
    – DrLou
    Commented Mar 29, 2023 at 21:15
  • That's a really good article thanks.
    – jay613
    Commented Mar 30, 2023 at 1:24
  • @DrLou Use the @ to ping someone, no need to ping the answer author - they're automatically notified. Also, be sure to say "thanks" properly but up voting answers, and giving a check mark to the one that helps the most (you may want to wait 24 to 48 hours to see if additional answers show up).
    – FreeMan
    Commented Mar 30, 2023 at 14:29

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