Gas central heating query

[tonyboten]

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.

[Aidan]

Pump failed?

[Set Square]

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?

[tonyboten]

Gravity feed.

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.

[Set Square]

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?

[tonyboten]

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.

[Set Square]

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 c*ck 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 c*ck and then *gently* open the cold tap - with someone up in the attic watching the F&E tank to ensure it doesn't overflow.

[tonyboten]

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.

[tonyboten]

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.

[Set Square]

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.

[tonyboten]

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::- ___ | | | | | | | | | | | | | |___| Regards.

Tony.