We recently had our boiler replaced with a heat pump. To help the circulation of the water in the radiators, the engineer installed an external pump.

Most radiators now make a constant running water noise, probably due to the circulating pump being set too high?

One of the radiators has now started to make a new noise: it sounds like filling up a bottle. There is no other way I can explain it. It lasts a couple of seconds (until it sounds "full"). After a while it will start again. It goes on for a few cycles. Also, possibly related, the radiator is now only heating on a vertical band near the IN valve and at the bottom. A bit like this:


Before the installation of the HP, the engineer cleaned the whole network with sludge remover. The affected radiator was working fine at the end of the last season (apart from the common running water noise).

I tried to bleed all the radiators in the house and only one had air in.

What can be the cause of the weird (filling up) noise?

UPDATE:this morning I noticed another radiator making the weird noise. So maybe is something in the system?

  • 1
    When did you bleed the radiators? Immediately after the work, or more recently? When I Curran my system out they need bleeding at least twice, but a couple of them are likely to need another bleed.
    – Chris H
    Nov 14, 2023 at 9:00
  • @ChrisH the initial bleeding was done by the engineer after the installation. I have done another bleeding now that I have restarted using the heating
    – algiogia
    Nov 14, 2023 at 9:14
  • @FreeMan it's two different noises. Take an empty bottle and fill it up from the tap: this is the new noise the radiator is making now, in addition to the constant 'running water' noise
    – algiogia
    Nov 14, 2023 at 14:12
  • Ah, sorry, my bad.
    – FreeMan
    Nov 14, 2023 at 14:16
  • Air can move around the system. Retry bleeding all again after it's been run
    – Mark W
    Nov 15, 2023 at 15:08

2 Answers 2


Radiators should be completely full of water if it is not heating up evenly then there is a bunch of air in it.

At the other side of the in valve there should be a bleed port. While the heat is running crack it open to let out the air until you get water out of it. Do this for all radiators. Only using the bleed port at the boiler will not work given the geometry of the radiators being almost ideal for trapping air inside them.

You may need to top up the loop as volume taken up by that air now needs to be filled by water. And you may need to repeat this bleed on some radiators should they tend to accumulate the air bubble over other radiators.

  • As said in the question, I have already done that.
    – algiogia
    Nov 14, 2023 at 11:11
  • Did you bleed at the radiator until water came out? And you probably need to repeat that as the air migrates through the loop. Nov 14, 2023 at 11:20
  • You may need to bleed them several times, @algiogia, until you get all the air out of the whole system.
    – FreeMan
    Nov 14, 2023 at 13:52

It really sounds like it needs another bleed. I expect to bleed until water bubbles out, but maybe you need to let out more than this. That's with the system running, and a rag to catch the water even if the system has just ben cleaned.

Only as you're having trouble, you should make sure the valves are quite open at least while you bleed. Turn the flow valve up to maximum, noting wither the number it's on to start with, or how many turns you open it. The balancing valve on the return often needs pliers or a spanner to turn it, after removing a cap (though not always, as on my towel rail radiators). Count the turns, and open it all the way. After bleeding, restore these valves to where they were before - unless either valve was very nearly shut, which would give the further symptom of very slow heating on the bits that do get hot.

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