expansion tank overflowing - a big puzzle why

[Jennie]

Hi all,
My customer's f&e tank overflow has had a steady trickle. When the hot water is turned on, it gushes.
I initially changed the ball valve (shutting off perfectly) and stiffened the side of the tank (it was flexing out, but otherwise been in for years with no issues), then realised the excess water was coming up the feed pipe.
When calling for hot water only, the heating pipe flow from the 3-port valve was burning hot. So that's been changed. But in retrospect, that wasn't the cause of the problem.
More investigation - the circulating pump was definitely knackered. So that's been replaced. I drained the system, so I could also replace the 'original and corroded' gate valves on the pump, and flush out the dirty central heating water and re-fill with inhibitor. No trickle from my open pipe ends once drained, so it can't be the coil in the cylinder. Cylinder immersion is turned off (hasn't been used in 30 years).
But still a steady trickle from the overflow pipe.
The boiler is a wall-hung old Thorn, with a flow, return and also a vent pipe connected to the boiler. The feed pipe is teed on to the return pipe.
When the boiler is fired up, on hot water only, the pipes and boiler casing get very, very hot. I suppose it would eventually reach similar temperatures on the central heating circuit, given long enough.
I suspect the issue lies with the boiler, and possibly a boiler stat has gone.
The main temperature stat knob on the front of the boiler seems to work - when turned down to 1, the burner goes out. But I'm supposing (I'm not familiar with this particular boiler), that there is a high and lo stat in there as well, to monitor flow and return temperatures.
The boiler has a permanent pilot light. Even when there's no demand for hot water (they're not using it until it's been fixed, and so the pump hasn't been running or pumping over), there is a steady trickle from the overflow. The flow pipe is tepid. Surely expansion from just a pilot couldn't produce so much water overflowing?
British Gas are going out tomorrow to check out the boiler (it's under one of their service plans).
Any ideas anyone?
Jennie.

 

[SavageW]

It does sound like it could still be a split coil Jennie, the only way I can seem to suggest it is that when the DHW demand is on the coil slightly expands? Maybe try isolating the coil to the cylinder for day or two an monitor wether the overflow still trickles? To be fair I'm only suggesting to try it I can't say it's definitely that just can't see another way it could happen, your probably get some more replies from more experienced guys

 

[Jennie]

That's what I keep coming back to. But if it was the coil, shouldn't an equal amount of rater be fed into the CWST to keep rate with the trickle? The CWST is generally full and ball valve up shut (except when a WC is flushed, etc). Unless, of course, half a pint of water (or whatever) can trickle through before the CWST ball valve drops down sufficiently to re-fill. I could isolate the cylinder to find out. Possibly both the boiler and the cylinder have issues. How much stuff can be knackered in one house at one time?

 

[Jennie]

I think I've cracked it.
The boiler is getting too hot because…
when the split on the coil opens up, water is pushing back into the heating circuit - both into the flow and return. Its going up the return, and through the overflow pipe. And in addition, it’s going up the flow and against the pump. The hotter the water in the cylinder, the more resistance to the pump - which is what knackered the last pump out. And by then, the boiler is struggling to get rid of the hot water, so is overheating and making the vent pipe and boiler casing also hot.
Water likes to find its own level. So an easy test would be to put a bung in the outlet from the CWST (hence bringing its level below the expansion tank), which should stop the dribble from the overflow.
Wow, what a puzzle.
Jennie

 

[king of pipes]

Puzzling jennie i would expect the domestic hotwater to looking dirty ? If its mixing with the primary heating water hope you get it sorted bud

 

[dan_the_plumber]

Either you need to bend the ball valve down to set water level in FE tank lower. Or you could try Food colouring in a radiator. If colour comes out of DHW taps you know your coil has gone.
Also if boiler control stat has gone, leading to boiler staying on too long, I would expect the Hi Limit stat to trip. You can test the control stat with a resistance test and the Hi Limit stat with a continuity test.

 

[Ric2013]

But I'm supposing (I'm not familiar with this particular boiler), that there is a high and lo stat in there as well, to monitor flow and return temperatures.
The boiler has a permanent pilot light.

I'm not an RGI, but would suspect that a boiler old enough to have a permanent pilot probably does not monitor flow and return temperatures. It probably has one stat either on the H EX, or on the outlet.

Surely expansion from just a pilot couldn't produce so much water overflowing?

I would agree: once the water has expanded, the water will no longer be expanding, so the water level would not continue to rise once the water is warm. In any case, I'd expect the temperature of the water to be lower on the pilot than when the boiler is actually firing.

The hotter the water in the cylinder, the more resistance to the pump - which is what knackered the last pump out.

I don't follow this line of thought. I can't see a leaking coil changing conditions to the point that the pump is running without flow. Was it seized? Could have just seized at some point with disuse (or debris) and then burned out when switched on.

So an easy test would be to put a bung in the outlet from the CWST (hence bringing its level below the expansion tank), which should stop the dribble from the overflow.

Assuming it is a Combined Feed and Cold Water Storage Cistern, that makes sense.

So you have a fully-pumped system with an indirect double feed (not Primatic) DHW cylinder? The water level in the F&E is higher than that in the CFCWSC?