Gas boiler - last legs?

[Grumps]

Hello

We've got a 28 yr old Potterton conventional gas boiler serving a hot water cylinder and 11 rads. Over the past couple of weeks our old boiler has started boiling the water. It has kettled for a few years, but this boiling is a new development. The British Gas man (we have maintenance on this old thing) said it was most likely sludged up either in the HE or pipework around the boiler, and thus the water is not circulating fast enough so it boils. I thought we had a Y-plan system but can't seem to figure out the pipes in the airing cupboard. I have posted a picture here:

the boiler is in the direction of the arrow shown. "This looks like it comes from the boiler" is a pipe that goes to the coil of the HW cylinder. "This doesn't go to the boiler" is a pipe that doesn't appear to be in the direction of said boiler and connects to the base of the pump. I was expecting the hot from the boiler to go to the base of the pump! I know it's difficult looking at just this one picture, but can anyone explain if it is correct? Ta.

[TheScullster]

"Grumps" wrote

Hi Grumps

Like you I would expect the boiler feed to be at the base of the pump. The pipe heading towards the cylinder from the valve would be the central heating feed. The pipe heading away from the cylinder from the valve IYSWIM then tees down (via some strange elbow arrangement) to the top cylinder connection; and tees up to a reduced 15mm pipe - is this the cold water make up for the heating circuit? At this age it's likely to be vented with a loft header tank? It would depend on the piping layout, but with our system (prior to re-work) there was a serious build up of crud around a motorised valve and at the vent/liquid interface. It could be that you have a similar issue and the restricted vent is causing the boiler to pressurise a little. Have you replaced any piping/valves to know the general condition/cruddiness of the internals? Have you kept a reasonable concentration of inhibitor in the system? Have you ever flushed the system out (either chemical or power flush)?

Having had an old boiler replaced with a modern condensing jobbie, I would hang on to the old girl as long as possible. New models are far too complex for their own good IMHO.

Phil

[Grumps]

Hi Phil

Ta for reply.

The strange elbow arrangement was my plumber's best attempt at fitting a replacement cyclinder many years ago. The new couplings didn't line up with the exiting pipework. The tee up to a 15mm pipe is for the bathroom rad which is in the hot water circuit.

Yes, vented with a loft header tank.

The vent was coupled with the cold feed about 10 years ago due to another blockage. So the cold feed is through the vent pipe. This, apparently, is quite normal as a 'bodge' to give it some more years.

The system was drained but not power flushed about 4 years ago. The last bits of water out were dark coloured. It was refilled and inhibitor added then.

I'm sure people will have their opinions on the new(?) condensing units, but have you reason to suspect that they'll not last as long? According to Mr B.Gas, we'd save about £450 a year if we put in a new A-rated boiler. According to my calculations, if my boiler can be brought back to its peak performance of 65%, then a 91% efficient boiler would save us £186/year. I think 91% is the max listed in sedbuk. It'll take a good few years to recover the costs involved in a boiler replacement, TRVs, flue re-positioning (it's too close to a window), etc.

Is it possible to just power flush the boiler and close-to pipes?

[Grumps]

Oh, and the other thing was that the BG man put a "do not use sticker" on it due to the fact that hot/warm water is getting into the header tank and making it, er, warm - which appraently it's not designed to do. The risk being that the tank deforms and splits and the water comes out which will be replaced by fresh water from the ball-c*ck. I could of course do big bodge #2 and direct the vent to the overflow thus disposing of the hot/warm water outside.

[funkyoldcortina]

If that happens the never-ending supply of cold water coming from the ball c*ck will only be a minor comfort to anyone who happened to be under a split tankful of boiling water pouring through the ceiling onto them...

People have been killed in this circumstance.

[Grumps]

I'm not sure. I thought the only deaths have been due to the main cold water tank boiling, not the boiler header tank. I could be wrong though. Anyway, there's only a gallon in the header tank, and if it split then the contents would end up in the airing cupboard. Almost as if it were designed that way to be safe!

[Mike Tomlinson]

In article , Grumps writes

BG man did the right thing.

The tank might fill with very hot/boiling water which will soften it and if/when it collapses, that water will then come crashing through the ceiling onto some luckless person below. A toddler died some years ago when this happened.

[Roger Mills]

The pipe connected to the base of the pump *must* go to the boiler. It's almost certainly the main flow pipe - going on to the 3-port valve which splits the flow into two circuits - HW circuit going into the top of the cylinder coil, and CH circuit going down under the floor to the radiators. The returns from the HW and CH circuits will combine somewhere under the floorboards, and go back to the boiler.

Even if everything is the wrong way round - with the pump and valve on the return rather than flow side, the pipe beneath the pump will *still* go to the boiler - but will be the return rather than the flow. Which way is the arrow on the pump?

Are you sure that the pump is working? That would be my first port of call if the boiler is boiling due to insufficient flow.

[Grumps]

Indeed. That is his job which he executed perfectly. No complaints.

Not the same situation here though. That poor child died as a resuilt of the water tank containing the boiling water. In my case, it'll only be the boiler header tank. This holds about a gallon and sits above the airing cupboard.

[Grumps]

Ta, yes. I've had a much bigger look with the torch and feeling under the boards with my fingers. Indeed the pipe going to the bottom of the pump comes from the boiler. There's a short loop under the floor which doubles back and makes it look like the pump pipe goes the wrong way.

The pump's arrow points up. So everything is as I originally expected.

The pump sounds like it is whirring when required, and the speed switch seems to change the whirring sound. When central heating is switched on (just for testing), all rads get nice and toasty. Maybe if the three-port valve is stuck somehow, that might slow the water flow. But would that also make the pump sound like its struggling?

If I disconnect the pipe at the base of the pump, and have access to the cold feed and vent pipes, doesn't this mean I can isolate the boiler and do a separate power flush on just the boiler?

[TheScullster]

"Grumps" wrote

Provided that the vent is 22mm not just the 15mm (usual make up water feed size).

I had a Worcester Bosch system boiler fitted in 2006 - still using a vented system. The only problems so far have been due to the plumbers not setting up the auto bypass valve properly and the boiler constantly over-running the pump as a result. That and the varying casing noise caused by the oscillating fan levels. Basically they are exactly what you would expect of modern technology - built with loads of lights, gizmos and sensors (plenty to go wrong) and minimum material thickness etc.

If the boiler is in a room that benefits from the heat loss from your current boiler then the figures are further exaggerated. Our boiler is in the kitchen, so (for most of the year) heat lost from the boiler is useful and not lost "efficiency".

It may be worth chemical flushing by running the system with cleaner yourself for a couple of weeks (Fernox tech support v good).

On the subject of warm header tanks, my understanding is that they are likely to get warm via the action of expansion. As long as you don't have pump-over via the vent then some warmth in the header tank is usual.

Phil

[cynic]

You could try some "boiler noise silencer" gel to deal with the kettling, and you should also check the boiler thermostat is working. When these get old and tired they often do not switch the heat off at the original set temperature but their calibration drifts, Welded contacts in the 'stat are another possibility. A replacement stat is significantly cheaper than a new boiler by a couple of orders of magnitude

[Roger Mills]

It's not uncommon for the impeller to part company from the shaft - so the motor whirrs but it doesn't pump any water. Having said that, if the rads get hot, it suggests that the pump *is* working. Does this boiling problem only occur when you're just heating the HW, with the CH switched off? If so, there could be a blockage in the HW circuit. there were one or things I couldn't quite make out in your photo. Is there a manual gate valve in one of the pipes to the cylinder coil, to balance the HW & CH circuits? If so, could that be closed up too much?

It depends where the fill and vent pipes are connected into the system. Mine are both teed into the flow pipe just under the pump, and so only give access to *one* side of the boiler.

[Grumps]

No, no manual gate valve. Just a poor photo.

Hmm, I'll have to investigate my pipes some more. Thanks.

[Grumps]

You could try some "boiler noise silencer" gel to deal with the kettling, and you should also check the boiler thermostat is working. When these get old and tired they often do not switch the heat off at the original set temperature but their calibration drifts, Welded contacts in the 'stat are another possibility. A replacement stat is significantly cheaper than a new boiler by a couple of orders of magnitude

OK. Any idea what this stat looks like? I guess it's mechanical and connected to my front panel dial in some way.

[Grumps]

Because they charge £700 for the flushing service! They don't do a freebie anymore.

I'm sure that is certainly true.Several years ago I changed a rad and the system wouldn't re-fill. That's when BG joined the cold feed and vent together.

How do I test the boiler stat? If the boiler is hot and I turn the stat to min, it still runs. Is that a good indicator of a faulty stat?

The only control valve I have is the 3-port valve. Is this what you mean? Ta.

[cynic]

the thermostat "works" is behind the adjustment dial and there will be a thin capillary tube with a vial on the end which tucks into the boiler shell, usually in front at the top but sometimes further back. Some boilers have an overheat stat as well although this is often a manual reset device.

[Roger Mills]

Yes - either that or a wiring fault which is effectively short circuiting the stat.

To test it properly you need to remove it from the boiler (pull the bulb at the end of the copper capilliary tube out of its pocket in the heat exchanger, and disconnect the wires (making a careful note of the connections!).

You then need to dunk the bulb in an old saucepan of water on a gas or electric hob so that you can vary the water temperature - and, preferably, also measure it with a mercury in glass thermometer - and check whether the electrical contacts open and close when you would expect them to for various knob settings.