One radiator seems to need bleeding weekly ?

[Chris Hogg]

I my previous property, a bungalow with the c/h piping around the edge of the loft and downpipes to the radiators. There were two automatic air bleeds in the system at the highest points (can't remember why two; possibly in the feed and return pipes). They had a little floating ball inside, which rose as the system was filled, and sealed a small vent at the top of the valve. When air accumulated in the system it would gather in the valve, the little ball would no longer float and the air would escape through the small vent. It was something like this

but black plastic, not brass. Never had a problem with them.

[Jethro_uk]

My Vailllant combi boiler has something in the innards with a screw that lets air out until the water seals it up again. To bleed the pump I think.

[Tim+]

Be warned, they can weep. Ours did and killed our boiler. I believe the screw cap should be tightened after the “post-refill bleeding” is done.

Tim

[nothanks]

I can believe they can leak (or, more likely, stop working because they get gunged-up) but it's a struggle to see how they could "kill" a boiler. What did the death certificate give as the fundamental cause of death?

[Tim+]

In our case the vent wasn’t built into the boiler but mounted above it. Water leaked into the circuit boards.

Tim

[nothanks]

Hmm, I hadn't thought of that ... pause to check mine, one of which is also above (ish) the boiler ... the cap is tight. The others are in one of the roof spaces so I'll wait until the next time I check the mouse traps!

[mm0fmf]

A hot leak. A joint is water tight when cold but develops a minute leak when hot. A tiny amount of water emerges and the joint is hot enough it evaporates quickly so the joint appears dry. When it cools it seals tight again.

I had problems with a rad filling with air, always the same one and it needing bleeding every 2 weeks. This was an old style open system. When the boiler was replaced with a sealed system and flushed and new inhibitor added there was no need to bleed any more. It used to leak pressure fast enough such that it needed topping up 4 times in the first year. Then twice the second year. After that it will keep sufficient pressure it needs no topping up between services. It drops about 0.5Bar in a year. Most of the rads/pipes are 35 years old.

[The Natural Philosopher]

Or its a pinhole that is slowly scaling up as new mains water is added to the system

[RJH]

I'd check (if you can) that the pressure gauge/readout is reasonably accurate. On mine (sealed combi) the gauge under reads a fair bit until given a couple of firm taps.

In my old place a couple of leaks remained concealed because (I think) the open system would automatically top up through the CH header. One was a pinhole leak that must have been active for years - the nearby joist had almost rotted through.

OOI, where does the air get in?

And if water isn't escaping, doesn't the pressure in a sealed system keep rising?

[alan_m]

In a sealed system the pressure fluctuates depending on the temperature of the CH water. When cold mine is set to 1.25 bar and when hot it gets to around 1.8 bar. If the there is a leak the pressure will drop.

The system is pressurised from the mains via a filling loop. The loop is just a (flexi) pipe with an isolation valve on both ends. Open up both valves and mains water is fed into the system. When the pressure in the CH reaches a nominal 1 bar* both valves are shut off.

• Boiler manufactures may recommend anything in the range 0.8 to 1.5bar. A safety valve set to a 3 is included in the system to prevent over pressurisation.

[RJH]

Yes, and in the OP's case the pressure doesn't drop. But if he didn't bleed the radiator, and air continues to enter the system, wouldn't the pressure keep rising until the system pops?!

I ask because, to me, air entering the sytem would indicate a leak. What I don't understand in this case is how air is getting in.

Yes, but it'd be safe to assume the loop is and has remained closed.