I have Baxi Bermuda gas fire, with back boiler, which supplies central
heating and hot water to my home.
A few days ago the water heating failed. I checked the header tank
which was O.K. and, on the highest thermostat setting, the central
heating appeared to operate satisfactorily although, when at less than
maximum setting, it failed to operate the pump.
When the water heating was tested on the same maximum thermostat
setting, the boiler lit for just over a minute, rested for 3 mins then
reilluminated. I allowed it to operate in this way for over an hour
but, although the flow and return pipes behaved as expected, the water
temperature at the tap was unchanged i.e. cold.
It seems that no heat exchange is taking place between the primary and
secondary water supplies. I'm reasonably sure that the boiler
thermostat is faulty, but that does not explain this odd behaviour.
Can anyone offer any suggestions as to what I should explore next,
please, or am I wandering down a blind alley?
Thanks.

Tony.

wrote:
the boiler lit for just over a minute, rested for 3 mins then
reilluminated.

It seems that no heat exchange is taking place between the primary and
secondary water supplies.

Pump failed?

In an earlier contribution to this discussion,
wrote:

I have Baxi Bermuda gas fire, with back boiler, which supplies
central heating and hot water to my home.
A few days ago the water heating failed. I checked the header tank
which was O.K. and, on the highest thermostat setting, the central
heating appeared to operate satisfactorily although, when at less than
maximum setting, it failed to operate the pump.
When the water heating was tested on the same maximum thermostat
setting, the boiler lit for just over a minute, rested for 3 mins then
reilluminated. I allowed it to operate in this way for over an hour
but, although the flow and return pipes behaved as expected, the water
temperature at the tap was unchanged i.e. cold.
It seems that no heat exchange is taking place between the primary and
secondary water supplies. I'm reasonably sure that the boiler
thermostat is faulty, but that does not explain this odd behaviour.
Can anyone offer any suggestions as to what I should explore next,
please, or am I wandering down a blind alley?
Thanks.

Tony.

Can you tell us a bit more about your system? Does the primary hot water
circuit (from the boiler to the indirect coil in the hot water cylinder)
circulate by gravity (convection) or is it pumped?

If the latter, is there a cylinder thermostat and zone valve for the hot
water (or maybe a 3-port valve controlling both HW and CH)?

When you mentioned the thermosatat above, were you talking about the knob on
the front of the boiler, or a room stat somewhere?

--
Cheers,
Set Square
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Can you tell us a bit more about your system? Does the primary hot water
circuit (from the boiler to the indirect coil in the hot water cylinder)
circulate by gravity (convection) or is it pumped?

Gravity feed.

When you mentioned the thermosatat above, were you talking about the knob on
the front of the boiler, or a room stat somewhere?

Before testing the central heating I turned the room and boiler stats
to maximum because it was a warm day. The heating worked.

The boiler stat was also set to max for the hot water test, with the
results described in my previous post.

Tony.

In an earlier contribution to this discussion,
wrote:

Can you tell us a bit more about your system? Does the primary hot
water circuit (from the boiler to the indirect coil in the hot
water cylinder) circulate by gravity (convection) or is it pumped?

Gravity feed.

When you mentioned the thermosatat above, were you talking about
the knob on the front of the boiler, or a room stat somewhere?

Before testing the central heating I turned the room and boiler stats
to maximum because it was a warm day. The heating worked.

The boiler stat was also set to max for the hot water test, with the
results described in my previous post.

Tony.

OK, so it's the gravity circulation which isn't working - suggesting an
air-lock or blockage in that part of the circuit. You've probably got two
header tanks in your attic - a large cold storage tank which feeds the
domestic hot water system, and a small fill & expansion tank which feeds the
radiators and primary hot water circuit. How much water is in the *small*
tank? There should be a pipe from the bottom of this tank, comnnected into
the primary circuit somewhere - probably near the hot cylinder in the airing
cupboard. Is there a tap or gate valve in this pipe, and is it fully open?

When did the hot water system last work properly - and did it suddenly stop
working?
--
Cheers,
Set Square
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It stopped working, suddenly, about a week ago. At that point the
header tank (5 gall) seemed to be empty or, at least, too low for me
to reach its base (it's rather inaccessible). I replenished it to about
2/3 capacity.

Have just checked the tap feeding from it. It wasn't fully open but is
now.

The airlock sounds a likely possibility. Can it be cleared without
disconnecting anything?

I'll run the system again now to see if it clears itself but, if not,
I'll wait for your further advice.

Many thanks for your help so far.

Tony.

In an earlier contribution to this discussion,
wrote:

It stopped working, suddenly, about a week ago. At that point the
header tank (5 gall) seemed to be empty or, at least, too low for me
to reach its base (it's rather inaccessible). I replenished it to
about 2/3 capacity.

Have just checked the tap feeding from it. It wasn't fully open but is
now.

The airlock sounds a likely possibility. Can it be cleared without
disconnecting anything?

I'll run the system again now to see if it clears itself but, if not,
I'll wait for your further advice.

Many thanks for your help so far.

Tony.

Hopefully it will purge itself and then be ok.

It's quite a common problem for the ballvalve on the F&E tank to seize up
through lack of use - then when some of the water evaporates or leaks, it's
not replenished - and the level drops enough for the gravity circuit to get
air in it. The pumped radiator circuit often continues to work.

If it doesn't purge itself, you may be able to refill it from the bottom. Is
there a drain cock near (or on) the boiler? If so, you may be able to attach
a hose to that - with the other end connected to a mains cold water supply.
Attach the hose, open the drain cock and then *gently* open the cold tap -
with someone up in the attic watching the F&E tank to ensure it doesn't
overflow.
--
Cheers,
Set Square
______
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You obviously know your stuff. Thanks a million. It has indeed purged
itself and we now have hot water once more. Thanks for the last tip
too; I'll keep it in mind for future mishaps.

May I pick your brain a little more? Because of the awkward situation
of F&E tank I cannot access it in such a way as to be able to reach the
base to check depth. Is there a way to monitor this from outside? I'm
thinking of something like 2 'U' tubes fastened together in such a way
that one is inverted over the rim of the tank, reaching almost to the
bottom, connected to the other which extends below the tank base and
rises again to the rim. If I then syphon some water from the tank it
should find its own level and provide a water gauge. Does that sound
feasible?

If I'm pushing my luck with that question, please ignore it. I'll just
have to experiment.

Tony.

You obviously know your stuff. Thanks a million. It has indeed purged
itself and we now have hot water once more. Thanks for the last tip
too; I'll keep it in mind for future mishaps.

May I pick your brain a little more? Because of the awkward situation
of F&E tank I cannot access it in such a way as to be able to reach the
base to check depth. Is there a way to monitor this from outside? I'm
thinking of something like 2 'U' tubes fastened together in such a way
that one is inverted over the rim of the tank, reaching almost to the
bottom, connected to the other which extends below the tank base and
rises again to the rim. If I then syphon some water from the tank it
should find its own level and provide a water gauge. Does that sound
feasible?

If I'm pushing my luck with that question, please ignore it. I'll just
have to experiment.

Tony.

In an earlier contribution to this discussion,
wrote:

You obviously know your stuff. Thanks a million. It has indeed purged
itself and we now have hot water once more. Thanks for the last tip
too; I'll keep it in mind for future mishaps.

May I pick your brain a little more? Because of the awkward situation
of F&E tank I cannot access it in such a way as to be able to reach
the base to check depth. Is there a way to monitor this from outside?
I'm thinking of something like 2 'U' tubes fastened together in such
a way that one is inverted over the rim of the tank, reaching almost
to the bottom, connected to the other which extends below the tank
base and rises again to the rim. If I then syphon some water from
the tank it should find its own level and provide a water gauge. Does
that sound feasible?

If I'm pushing my luck with that question, please ignore it. I'll just
have to experiment.

Tony.

It doesn't actually need much water in it - as long as there's enough to
cover the outlet which feeds the heating system. [The level rises, of
course, when the system is hot - so there needs to be plenty of room for
expansion before it gets anywhere near the overflow].

I can't see into my tank very easily. I usually check that it's healthy by
reaching over the top and pushing the ball down. If water flows in when I
push the ball, but immediately stops when I let go of it, that shows that
everything is ok. If it carries on running, it suggests that the level was
too low - and that the ball valve was stuck.

If you really want a visual check, could you mount a mirror somewhere where
it would enable you to see into the tank? If you want a sight tube, all you
really need is a hole in the side of the tank near the bottom, and a
right-angled tank connecter in it - with a length of rigid transparent tube
extending up from the connector. Or you might be able to rig up an
electrical alarm of some sort - using a float connected by a piece of string
to a micro-switch - in such as way that if the level dropped too much, the
string would operate the switch.
--
Cheers,
Set Square
______
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Thanks again, Set Square. Here endeth the topic.

For information, I tried my idea of a giant, clear, tubular butchers'
hook, with the hooks as long as the body. It works; I now just have to
find a way of fixing it.
It looks something like this, with the line representing the tubes::-
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Regards.

Tony.