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Discuss UFH pressure when running. in the Underfloor Heating Forum area at

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  1. Reven

    Reven New Member


    I've laid wet UFH in the new extension (suspended floor) as follows:

    1) 3 loops (about 80m each) in the zone.
    2) 2 port valves for 3 zones on the flows (UFH, upstairs rads, downstairs rads).

    The pressure test indicates no leaks in any of the loops.

    When the valve to the UFH is closed, the pressure indicated on the manifold gauge drops to zero over a period of time. The boiler pressure remains at a steady 1.5 Bar. As soon as the valve is opened, the UFH manifold gauge pressure goes back up to 1.5. The zones are operating and heating effectively aside from this.

    Can anyone suggest other avenues of investigation or perhaps a theory on what may be happening?

  2. oz-plumber

    oz-plumber Plumber

    First option - replace pressure gauge on ufh manifold and see what happens.

    If it's doing the same thing, test each circuit individually.
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  3. Daveplumb88

    Daveplumb88 New Member

    As long as you don't have to top up through th filling loop, I wouldn't worry. It's still connected hydraulicly (?) by the return pipework. If leaking you'd lose all your pressure from whole system. It's a bit of an anomaly that you'd have to see the system to explain (possibly). So, as long as there's no topping up required, best thing is STOP LOOKING AT THE GUAGE, and worry about something else, like what the wife gets up to while you're at
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  4. Dan Masey

    Dan Masey GSR

    Sorry to kind of hijack this post. Kind of on point, but when adding UFH to an existing system, does this out a big amount of pressure on the expansion vessel as suddenly the system has a lot more water within it?
  5. Daveplumb88

    Daveplumb88 New Member

    As long as the TPR doesn't discharge and you lose pressure/system water, then the existing expansion vessel will and is coping. Pressure vessels can take the pressure, it's in the name. If, however, the system loses pressure through the TPR (guage at 3 bar or thereabouts), then you can just cut in another small vessel rather than paying mote for a bigger one.
  6. Daveplumb88

    Daveplumb88 New Member

    I've just worked it out (I'm such a freakin nerd sometimes) and for every 100m of 16mm ØD UFH pipe laid, you add 15.4 ltr..So, allowing for 5% expansion, you need to allow 0.77 ltr of expansion space for every 100m UFH pipe laid.
  7. chris watkins

    chris watkins Plumber

    Unfortunately Dave it doesn't work like that when sizing vessels, there are a few more things to take account of.
    Hope this helps
    Heating Vessel Sizing Guide.png
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  8. chris watkins

    chris watkins Plumber

    The above is care of RWC
    Static Head in metres = cold fill pressure normally 10M (1bar) so p1 would be 2.
    Safety valves are 3 bar on domestic systems so p2 = 4.
    e = 90deg C therefor 0.0359
    just need C total system volume

    Take off what is already there if you like (boiler) and then buy next biggest vessel for remaining.

    Nerd's of the world unite.
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    Last edited: Mar 7, 2017
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