For the past 4 or 5 days we have had a problem with our central heating
system; it is pumped with a valve for the heating and a separate valve for
the hot water. There is one pump.
All the controls work properly but they fail to either bring the boiler on
heat or to keep it going.
It appears to me that air is getting into the system and maybe into the pump
so stopping it working even although it is still running.
I have bled the radiators on each of the past 3 days and get some air out
each time. There is no apparent leak in the system but of course there may
be under the floor. I checked the header tank in the loft and it is up to
level.
Can anyone think of a reason for air to be getting into the system or how it
can be doing so?
Thank you

Stewart explained :

It appears to me that air is getting into the system and maybe into the pump
so stopping it working even although it is still running.
I have bled the radiators on each of the past 3 days and get some air out
each time. There is no apparent leak in the system but of course there may
be under the floor. I checked the header tank in the loft and it is up to
level.

Try bleeding the entire system with the pump turned off.

Can anyone think of a reason for air to be getting into the system or how it
can be doing so?
Thank you

Pump perhaps set to too high a speed and drawing air in via the vent
going up to the tank. A leaking pipe joint, or valve etc. which is
under vacuum with the pump running.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

In an earlier contribution to this discussion,
Stewart wrote:

For the past 4 or 5 days we have had a problem with our central
heating system; it is pumped with a valve for the heating and a
separate valve for the hot water. There is one pump.
All the controls work properly but they fail to either bring the
boiler on heat or to keep it going.
It appears to me that air is getting into the system and maybe into
the pump so stopping it working even although it is still running.
I have bled the radiators on each of the past 3 days and get some air
out each time. There is no apparent leak in the system but of course
there may be under the floor. I checked the header tank in the loft
and it is up to level.
Can anyone think of a reason for air to be getting into the system or
how it can be doing so?
Thank you

The air could be a red herring. Have you considered the possibility that the
pump could be running but not actually pumping because (say) its impellor
has disintegrated or come loose on the shaft?

Does the boiler fire at all? If not (assuming an S-Plan system from your
description) then the pump shouldn't run either - in which case you could
have a problem with the zone valves, thermostats, programmer or wiring. If
the boiler runs for a minute or so and then cuts out on its own stat, that's
a sure sign of lack of circulation - in which case I would suspect the pump
rather than air locks unless the system has recently been drained and
refilled.
--
Cheers,
Roger
______
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Stewart wrote:

For the past 4 or 5 days we have had a problem with our central heating
system; it is pumped with a valve for the heating and a separate valve for
the hot water. There is one pump.
All the controls work properly but they fail to either bring the boiler on
heat or to keep it going.
It appears to me that air is getting into the system and maybe into the pump
so stopping it working even although it is still running.
I have bled the radiators on each of the past 3 days and get some air out
each time. There is no apparent leak in the system but of course there may
be under the floor. I checked the header tank in the loft and it is up to
level.
Can anyone think of a reason for air to be getting into the system or how it
can be doing so?
Thank you

It sounds unlikely that air in the system would be stopping the pump
from working. I've had several systems where radiators have had as much
as 50% of their volume occupied by air, and they still worked fine as
far as the boiler firing and the pump working were concerned. The only
symptom of air circulating in the system as far as the pump was
concerned as the occasional "gushing" noise from the pump with a change
in the pump noise as it thrashed the impeller in air for a time.

I'd look elsewhere for the problem. We had symptoms like those you
describe with the previous boiler. The problem turned out to be a faulty
optical sensor used to monitor the boiler flame. It decided the flame
was too yellow (smoky) and cut the burner out at random intervals. But
that's not an attempt to diagnose your problem, just an indication that
even though all thermostats, timers and valves are behaving properly
that there can still be a problem elsewhere.

Since it was old and there were other problems we replaced it with a new
condensing boiler.

As to where "air" comes from, there are several sources. Yes leaks can
admit air but often it's dissolved gasses from the water that cause the
problem. Principally CO2 from hard water. Another source is hydrogen
caused by galvanic action between steel radiators and copper tube. A
plumber I once knew amused himself by setting fire to the "air" when he
bled radiators.

Thank you both; I shall check further to-morrow but let me add that this
morning I bled air from most of the radiators and the pump etc worked fine
with no "knocking". When we came home at 1930 the heating was off even
although the pump was running and the radiators were just luke warm.
I bled all the radiators again and a significant amount of air escaped.
Once this was done the heating came hack on and the house is warm again.
The pump is now around 15 years old and I wonder if it is possible to get
cavitation in such a small domestic pump?
There is no evidence of a leak anywhere in the house but of course it could
be under the ground floorboards.
Unfortunately my brother, who is a Corgi registered plumber, had just left
to go on a 3 week holiday to South Africa so I either struggle on until he
returns or employ another plumber to change the pump.




"Stewart" wrote in message
...
For the past 4 or 5 days we have had a problem with our central heating
system; it is pumped with a valve for the heating and a separate valve for
the hot water. There is one pump.
All the controls work properly but they fail to either bring the boiler on
heat or to keep it going.
It appears to me that air is getting into the system and maybe into the
pump so stopping it working even although it is still running.
I have bled the radiators on each of the past 3 days and get some air out
each time. There is no apparent leak in the system but of course there
may be under the floor. I checked the header tank in the loft and it is
up to level.
Can anyone think of a reason for air to be getting into the system or how
it can be doing so?
Thank you

Harry, if anything I am more puzzled than ever. As you suggested I turned
off the pump and then tried venting the radiators again.
Those on the ground floor emmitted a little air but when I tried the
upstairs radiators I got nothing at all, not even water. I have taken some
of the plugs right out and yet no evidence of water or air.
I shall try to-morrow when I can monitor the feed cistern.


"Harry Bloomfield" wrote in message
k...
Stewart explained :

It appears to me that air is getting into the system and maybe into the
pump so stopping it working even although it is still running.
I have bled the radiators on each of the past 3 days and get some air out
each time. There is no apparent leak in the system but of course there
may be under the floor. I checked the header tank in the loft and it is
up to level.

Try bleeding the entire system with the pump turned off.

Can anyone think of a reason for air to be getting into the system or how
it can be doing so?
Thank you

Pump perhaps set to too high a speed and drawing air in via the vent going
up to the tank. A leaking pipe joint, or valve etc. which is under vacuum
with the pump running.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

In an earlier contribution to this discussion,
Stewart wrote:

Harry, if anything I am more puzzled than ever. As you suggested I
turned off the pump and then tried venting the radiators again.
Those on the ground floor emmitted a little air but when I tried the
upstairs radiators I got nothing at all, not even water. I have
taken some of the plugs right out and yet no evidence of water or air.
I shall try to-morrow when I can monitor the feed cistern.


You said earlier that you had checked the 'header tank' in the loft. Which
one? You should have a large cold water header tank for the domestic hot
water system and a smaller fill and expansion tank for the primary
circuit.[1] It's the small F&E tank which you need to check. Because they
need topping up so infrequently, it's not uncommon for the ball valve to
stick closed. Then, when the level drops due to a slight leak or
evaporation, the water is not replaced and air can enter the system. If
nothing - not even air - comes out when you bleed a radiator, it could well
be that there's no water in the F&E tank. Also check that any taps or gate
valves in the feed pipe from the F&E tank into the primary circuit are
actually open, and not turned off.

[1] Unless, of course, it's a sealed (non-vented) system - in which case
there will be a filling loop and pressure gauge, and a probable need to
re-pressurise the system.

--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

On Sun, 17 Feb 2008 23:30:34 +0000, Roger Mills wrote:

In an earlier contribution to this discussion, Stewart
wrote:

Harry, if anything I am more puzzled than ever. As you suggested I
turned off the pump and then tried venting the radiators again. Those
on the ground floor emmitted a little air but when I tried the upstairs
radiators I got nothing at all, not even water. I have taken some of
the plugs right out and yet no evidence of water or air. I shall try
to-morrow when I can monitor the feed cistern.


You said earlier that you had checked the 'header tank' in the loft.
Which one? You should have a large cold water header tank for the
domestic hot water system and a smaller fill and expansion tank for the
primary circuit.[1] It's the small F&E tank which you need to check.
Because they need topping up so infrequently, it's not uncommon for the
ball valve to stick closed. Then, when the level drops due to a slight
leak or evaporation, the water is not replaced and air can enter the
system. If nothing - not even air - comes out when you bleed a radiator,
it could well be that there's no water in the F&E tank. Also check that
any taps or gate valves in the feed pipe from the F&E tank into the
primary circuit are actually open, and not turned off.

[1] Unless, of course, it's a sealed (non-vented) system - in which case
there will be a filling loop and pressure gauge, and a probable need to
re-pressurise the system.

Another thing which is a fairly common fault is that joint between the
fed pipe and the main circuit plugs with lime/gunk and won't let much
water join the circuit on refilling.


--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at http://www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html
Choosing a Boiler FAQ http://www.makewrite.demon.co.uk/BoilerChoice.html