Replacing TRV without draining by isolating radiator possible?

Discussion in 'Plumbers' Talk' started by muffinbubble, Nov 13, 2018.

  1. muffinbubble

    muffinbubble New Member

    Hi, I'm very new to plumbing but try anything and have replaced all the TRV's in my house without too much problem.

    I have a question regarding draining the system and TRV's though. So far I've drained the system to replace the TRV's and spent some time reading about whether it's possible without draining the system.

    My understanding (which I'm guessing is wrong) is that if there is no pressure in the boiler (combi) that if the radiator is isolated, at that point the water is locked into the radiator.

    I have pipes coming through the floor into the TRV. The TRV (and lockshield) is turned off to prevent the water going into the radiator, so this means if I remove the TRV then water from the pipe going into it from the floor will be exposed. However, if the boiler has no pressure then wouldn't the water level remain at the top of the pipe and not spill out?

    I hope that makes sense. Then it would be a case of just draining the radiator itself, replacing the TRV and reconnecting everything back.

    I'm guessing that the pipe from the floor will squirt water everywhere which is why everyone says to drain the system, but with no pressure I don't get how (as no water is being pumped in when the boiler is switched off, right?!). Could someone please clarify for me? The problem is fixed, it's more I'm trying to understand why it wouldn't be possible this way.
  2. terrymac

    terrymac Screwfix Select

    Just because boiler pressure gauge reads zero that does not mean there is no water in radiators / pipework. Of course its possible to change a valve whilst the system is full of water ,but the water has to be held under vacuum. I would advise a diy'er to drain down as it negates the risk of water damage if things go wrong. To answer your question , water will come out the pipe with pump not running ,if vacuum is broken ,gravity takes over.
  3. muffinbubble

    muffinbubble New Member

    Hi, I should have mentioned the radiator is upstairs too, if that matters. I know how to drain/add inhibitor etc, as have done this already, I just didn't understand how water would come out of the pipe with no pressure but that's my lack of knowledge hence the question :).

    I am also aware the pipes have water, but as mentioned in original post expected that once the boiler is off, that water would remain where it is with no pressure :oops:.
  4. Heat

    Heat Screwfix Select

    As above^^^ If you drop pressure to zero then water will not come out of the pipe going into the valve if no other part of system is open, including auto air vents that would need the little manual cap on them tightened temporary.
    You will need to turn other valve on radiator off and also expect water to come out of valve tail unless you bung a bit of rag in it while you fit the new valve on pipe.
    Use plenty old towels and a baking tray or similar
  5. Heat

    Heat Screwfix Select

    Upstairs radiator valves are easier if boiler is on lower floor.
    Just check auto vents are off on that and higher floors

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