My house was built in the 1920s, the heating source was a hot water/radiator system. Originally the basement was probably used only for a coal furnace (we have the drop-off door that the coal guy used to deliver through). We are looking at putting the basement to use and one of the major issues is the presence of 2" pipes for supply and return to/from the now-gas furnace. The supply and the return are not the same pipe. These pipes drop down from the ceiling and slope towards the furnace. Because these pipes aren't running through the joist bays and because of their downward slope, they can be as low as 6' from the basement floor surface.

The pipes are probably steel or cast iron (they're really thick, so unlikely to be copper).

I'm wondering if anyone has replaced their basement hydronic piping with something like PEX, and if so what was the required slope back to the boiler?

Edit: Based on comments from @SteveSh the slope was probably due to this system being steam originally. It must have been retrofitted a while back to use hot water instead. The slope is now not as important, but I'm still interested in hearing if people have replaced their hot water pipes with PEX. Any challenges that come up with that type of work?

Location where old steam pipes meet new copper hot water pipes Furnace close-up Picture of the spaghetti in my basement

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    Sounds like this is a steam system, not hot water (hydronic). The return pipes slope towards the boiler so that the water from the condensed steam can run back to the boiler. – SteveSh Feb 11 '20 at 19:49
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    No, just plain old hot water, steam would require those little valves on my radiators. It's possible that the system was originally steam and may explain why the pipes are sloped, but the presence of both supply and return makes me think that the system was never steam. – rajan Feb 11 '20 at 20:43
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    I don't think you can go by the one-pipe vs two-pipes configuration. Steam systems come in both varieties. One of my former houses was a two-pipe steam configuration. And it was built around the same time as yours - mid 1920's. – SteveSh Feb 11 '20 at 21:37
  • In your house what was the second pipe used for? So you had two pipes going to each radiator (which had the bell valve like the image here) heatinghelp.com/assets/Uploads/_resampled/… – rajan Feb 11 '20 at 22:05
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    Yes, one pipe at each end of the steam radiator. One pipe supplied the steam. The steam would condense to water in the radiator, fall to the bottom, and run out the second pipe back to the boiler. The one pipe configuration used one pipe for both steam supply & condensate return. – SteveSh Feb 11 '20 at 23:07

I don't have any first hand experience, but I don't think there's anything wrong with using PEX for a hot waater heating system. I base this on two recent observaations.

1) A new $million+ house I was in uses PEX to distribute hot water from a central boiler & manifold to places of use.

2) My son's old house used PEX to send hot water from the boiler up to the air handler that was in the attic. The heat exchanger in the air handler heated the air moving through the air handler. Sort of like having backup or auxiliary heat for a heat pump system.

You should really check with your local building code authority to make sure they're OK with it.


Here in Italy pex is widely used for new heating system (both radiators and underfloor), just check the maximum allowed temperature is high enough (I'd say over 120°C) and you could use it for both flow and return. The story is different if your heating is steam.

The slope could be there because originally it was naturally-circulated hot water (hot water) system with no pump, only the (relatively low) specific gravity of hot water 'moviing' the system.

In my opinion, if your new boiler have a circulating pump, you could hide your pipes anywhere and replace them with PEX

  • That makes sense that old hot water systems didn't have pumps, I was thinking something like this originally. So it sounds like the sloped return pipes are either because the system was steam or because it was hot water without a pump, and now that it is hot water with a pump I should be able to replace with PEX and hide the pipes into the joist bays. – rajan Feb 16 '20 at 18:32
  • I like that you mention the temperature limits on the PEX. Radiator vs Baseboard vs Radiant have different operating temperatures. Anything past radiant floor heating I would think could be very close to the recommend limits of PEX. It's too bad you didn't get the accepted answer. IMO "I've seen this used in two houses" is no reason to assume it's safe to use in this system. – QueueHammer Oct 31 '20 at 0:54

Yes, you can replace parts of a hydronic system with pex. Just make sure you use the right kind of pex pipe. There is special pex for hydronic heating systems. In addition to that the wall sizes for pex pipes may or may not line up with the wall size of the various kinds of pipes in your system, so when converting you would need to make sure that you kept the inside diameter appropriately sized.


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