I am a complete amateur trying to get my central heating going again.
I reattached an old radiator, bled it and all seemed fine then I noticed
that the downstairs radiators were all cold. Upstairs were lukwarm all of a
I assumed an airlock but could not find air in any of the rads. Then I
looked in the attic and found that header tank was empty (and dry). Fixed
valve on stop cock and that seems to be ok now.
I ssumed system airlock. We don't seem to have a bleed valve on the system
in this house so I drained it down and refilled it bleeding all downstairs
and then upstairs.
Now when switched central heating on again, the water does not seem to flow
and boiler does not fully ignite (pilot only).
Can anyone detect anything screamingly obvious that I have neglected to do?
Pump is definitely working by the way.
It's a grundfos selectric.
Can't see any screw except the big one in the middle which I have removed
and replaced to no avail.
It spurted out a little water and the pump rotor is visible. Seemed to be
With my pump there is a big black plastic screw in the middle of the cover.
If I unscrew this a few turns then jiggle it water comes out.
This effectively bleeds the pump.
Had to do this last time I drained and refilled because of symptoms like
yours - pump running but no heat to radiators.
After some jiggling and spurting (plumbing can be fun) the note of the pump
changed, there was a 'shoosh' noise, and everything started working again.
If your setup is like mine (pump in airing cupboard by hot water tank,
higher than highest radiator) then you probably got into the situation where
the upstairs radiators had water but the pump was running on air.
This can cause an airlock in the feed down from the header tank to the
system, and/or in the pump.
(1) Try jiggling that central screw a bit more - from what you said you
didn't get a lot of water out so you may still have mainly air in the pump.
If the pump was full of water and pumping at presssure I would expect water
to come out quite forcefully. Also, if you have a speed switch, try
increasing the speed of the pump.
(2) Try to clear out the feed from your header tank to the system. You can
do this from the top or the bottom.
(i) from the top:
tie up the stop cock and then drain the header tank using whatever drain you
have at the bottom of the system.
If your header tank will not drain because of an airlock or blockage, then
siphon it out into the bath using a hosepipe.
This also cleans any crud out of the header tank.
Disconnect the feed pipe from the bottom of the tank, and connect a hose
pipe to it with a jubilee clip.
Other end of the hosepipe on a mains cold tap. This will pressurise the
system and then you can bleed all the radiators and hopefully clear all the
This is especially effective when gunge has been sucked out of the header
tank into the pipework because you let it empty.
(ii) from the bottom:
attach a hose pipe between a cold mains tap and your drain tap at the bottom
of the central heating.
Open the drain tap and turn on the cold tap.
This will pressurise your system and should drive water up the filler pipe
to the header tank, then out the overflow (check carefully first that your
overflow is working).
This should shift any air out of the top end of the system above the level
of the upstairs radiators.
Do this slowly and carefully!
Too much oomph with the cold mains, or a poorly tightened jubilee clip, can
rain on your parade.
I would be mildly surprised if you boiler had overheated because the
thermostat should shut it off as soon as it is up to temperature, and you
haven't been running it dry as far as I can tell because your upstairs
radiators were still warm, therfore at least partly full of water.
My (virtual) money is still on an airlock in the pump.
Er.. what about the expansion pipe? Unless I'm missing something(!), rather
than pressurise the system, mains water will flow up the expansion pipe into
the header tank - and will promptly pee out through the (now open) bottom
Good point - I am now racking my brains to remeber how I did this when I had
to clear my system out.
My first guess would be that I put a bung in the hole out of the tank, or a
stop end where the copper was disconnected from the tank coupler. However to
pressurise the system you would have to close off the expansion pipe as
Second thought is that I had my trusty helper working the tap, and once
water came out of the expansion pipe then the top end of the system was
clear of air and crud. Probably best to put a bucket in the (empty) tank
under the expansion pipe to save having to siphon the water out again.
I describd this correctly (I think) a few months back - will have to Google
back and find out.
Thinking further, if you attached the hose to the expansion pipe with the
down pipe still connected, then this would force water round the system and
water should come up the down pipe.
This again would clear air and crud out of the top end of the system.
However ISTR my expansion pipe is 22mm so would need some adapting to take a
hose pipe (which just fits 15mm).
Apologies to the OP for my senior moment :-(
With little or no flow, the boiler may have overheated and tripped out on
its over-heat stat if it has one. This has to be manually reset by pressing
a button before the boiler will fire again. Your boiler User Handbook should
Have now sorteed this problem though. I ran the system with the drain open
and my wife in the attic keeping an eye on the header tank level. Did this
for about 20 mins and then ran it as normal.
The return pipe got warm fairly quickly which was a good sign, the boler cut
in and cut out when the water got to hot (another good sign) about an hour
later the rads started to warm up slowly.
This morning it is back to normal with the extra radiator.
Much obliged to all.
or phone their
Could be a proper airlock. Hosepipe on the drain and a watchful eye on
the header tank can fix that...
...or, more likely, the boiler's locked itself since it overheated
when you ran without water. Take cover off and look for an obvious
reset button. Failing that, rifle drawers and then google for manuals.
Failing that, ask in here giving make and model.
Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
Actually I let it cool down and it started up again but stopped after a
while when the water coming out of the boiler was very hot.
Now only the Pilot stays on but the pump keeps pumping.
When you say hosepipe on the drain I suppose you mean run the system with
the drain open and somebody in the attic making sure that the header level
doesn't get too low?
Nono, the other way around. The method is the same as backflushing a
hot water tap.
You connect a hosepipe to the drain connector of the CH, open the
drain and turn on the hosepipe's tap. Forces water back through the
system (although by the shortest route, so won't get an airlock in a
badly designed system).
The reason I say to have someone watching the header tank is indeed to
stop it overflowing. It should have an overflow pipe, but quite often
these are undersized and/or blocked. Don't trust just this alone!
Simon Avery, Dartmoor, UK
uk.d-i-y FAQ: http://www.diyfaq.org.uk /
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