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Hey all,

Really hoping for help. My boiler won't stay on for more than a couple of minutes (I think self-defense, overheat prevention) and my grundfoss pump makes noises as if it has air trapped in it. However, opening the bleed valve on the face of the pump doesn't seem to fix anything. I can't think of what else to try.

A bit of background:

I drained down the central heating to replace a rad and install TRVs on a number of rads. The system is has an F&E tank in the loft, feeding an ideal classic boiler. The boiler heats the water tank and the central heating via Grundfoss Pump. No new pipework, just new valves and a one in one out on the radiator (slightly bigger rad).

I refilled from the bottom up with each rad bleed screw closed until I filled that rad. Rads all appear to fill from both feed and return sides. But as above, I can't get the boiler to stay on for more than a couple of minutes and water proceeds around the system at snails pace. 3 or 4 upstairs rads get hot, but I'm left with the overriding impression that the pump isn't really pumping and has air trapped in it. I've tried twiddling the bleed screw relentlessly with the heating both on and off, but still it sounds like air is in there

I've re-drained and refilled the system a couple of times and am now at a complete loss.

Any help would be amazing, from a desperate and soon to be cold showered DIYer.

Its either an bad air lock or pump has given up.
Lets start with the assumption its an airlock. How can I force water through the system? hose pipe from the mains would work, but not something I've ever done and I don't know how to regulate the pressure. Any suggestions?


Staff member
S. Mod
Gas Engineer
Is there a drain off above the boiler ?
Is there a drain off above the boiler ?
There is and I can't budge it, it's been on there unopened since the house was built as far as I can tell (about 25 years). I'll see if I can dig out some plusgas or GT85 and give it a squizz. Reasonable odds that if there is an airlock its in that part as I can't see any way to bleed that loop of the system. That said, it is above the boiler. That makes no sense to me. How can I get water out of the boiler with the drain off above it?

RIC213, I re-read my post and can see how I may have caused confusion. Refilling method was.

1. Close drain cock
2. Close every bleed valve on all rads
3. Open the feed from the F&E tank and let it start to fill the system
4. Open the bleed valve on the lowest rad on the house, wait for rad to fill, close bleed valve and move to next lowest rad. Repeat until all rads are fil
5. Bleed pump, and the bleed valve in the airing cupboard.

Some thoughts that occurred overnight.
1. Hot water does reach the pump quickly
2. The boiler shuts off irrespective of whether its the central heating or HW calling for heat.

Couple that with the fact that some rads do get hot and my interpretation is that the likely cause if the return pipe to the boiler has air in it. The seized drain cock is on the flow side of the boiler, but I suppose that doesn't make a difference.


Simple test of pump.

Undo the bleed screw and stick a screwdriver on the impeller shaft.
If it slows down or stops with minimal pressure, most probably the pump capacitor is stuffed - or the pump itself is stuffed.

It rally doesn't sound like an airlock to me - rads upstairs working
( Thermosyphon ).
Boiler heats up quickly - not enough flow through heat exchanger.
Update. Pump appears to be fine, rotates freely and as far as I can tell continues to rotate properly as you described oz-plumber. I part drained the system, refilled, and noticed that the pump runs near silent right up until the hot water reaches it, at which point it starts to make crunchy cavitation noises. I'm jumping back in the "air in the boiler" camp. All efforts to undo the drain cock above the boiler have so far failed. Next option is to part undo the lock nut on the return pipe to the boiler and let water out that way: that'll be fun...
GT85, a hammer and lots of force and closed eyes praying to Zeus and the drain cock came free :). Needed a new washer as it wouldn't reseal, but I'd prepared for that.

Upshot of it all is that I now have most of the rads in the house working, but the downstairs ones are insanely slow to heat up and one of them remains icey. All upstairs ones get hot, but a couple are slow to do so. It seems are in the return is the culprit: The question is, how to expunge it?

There are three four exit points that I can see.
1. Drain cock from lowest rad
2. Drain cock from boiler
3. F&E tank
4. Any radiator with an open valve.

Question for the experts: is the logic/plan below sound.

I'd like to apply pressured water to the system. I can attach a hose to any of the points above as long as one of the others is open to prevent over pressure. The end result should be that I force water between those two points and any air between them gets taken along for the ride. I'd obviously need to get lucky enough that the trapped air is between two of these points.

Any obvious flaws in this?


Staff member
S. Mod
Gas Engineer
Turn all the down stairs rads off and turn the boiler on

They should all heat up if they don't turn all but one off

Wait till that one gets hot hot and then air that rad

Once that one is done just repeat with the rest
Huge thanks, that worked, expunged air as I went with several rads gassing like mad, and all rads in the house now get toasty as hell.

Next job is balancing, but I think a beer is in order for this evening.
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