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Rad pipe minimum slope - Settled floor

My question is:
- could the lack of consistent slope in the return piping be the cause of a lack of flow across a radiator
- is 1/2" slope over 14' enough to account for the inconsistencies that might arise in the pex pipe expanding or drooping over time?

100+ yr old CI radiators. old gravity system, now with circulator pump. It was extended onto a porch which had settled quite a bit. Hot water supply was getting to the radiator, but didn't seem to be flowing through as only the coupling and first section would get warm. Piped bottom-to-bottom. after plenty of pondering, I realized that the return has the farther run and due to settling, was now pitched up to where the pipe connected through a wall before dropping down back to the main system. The pipes are running through shallow sleepers on top of concrete slab. I had to replace the wood flooring so once done, the pipes will be inaccessible again.

I replaced the iron pipe with pex and with a lot of work I was able to get a fairly consistent, but very minor slope into the return pipe. It is only about 1/2" over 14' I tried to secure shims and channel the pex in foam insulation to allow for expansion, but not drooping.

the only other option I can think of is daylighting the pipe earlier above the floor and piping exposed around the back of the rad to the return spud.

Thanks for any comments!!


  • Jamie Hall
    Jamie Hall Member Posts: 23,446
    The possible problem which I can envision is that no matter how you support that pex it will develop high points and low points. These may accumulate air and be a bear to purge. In general, on a forced circulation system, if you have bleeders at the high points you should be able to purge the system and it should circulate, regardless of pipe pitch. But trapped air can kill you...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Steamhead
    Steamhead Member Posts: 16,902
    What size are the pipes?
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • PorterPlumber
    PorterPlumber Member Posts: 2
    edited August 2019
    @Steamhead, the pipes are 1/2" Pex. The pipes they replaced were 1" .
    @Jaime Hall, thanks for that. I was thinking that the pressure would be enough to clear. Rad is on first floor of two story system. It's not too late to go back with hard pipe. I just want to make a foolproof choice before putting flooring down and it becomes inaccessible
  • hot_rod
    hot_rod Member Posts: 22,262
    Some small ups and downs should not be a problem. A high point manual or auto vent is a good idea to assure the radiator is filled.

    1/2"pex in a slab has a lot of ups and downs.

    Be sure you have enough circulator to present 2-4 feet per second velocity, no partiality closed valves when power purging.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream