C-plan hot water problems

For ages the hot water didn't work, then we had a plumber come and clean an epic amount of sludge out the system.
After that CH and hot water worked, but now its getting warmer we need to turn off the CH.
The plumber said he couldn't understand our timer, whenever its on water only then the boiler switches off, but the pump still runs. Surely this is the opposite of what you want. The timer is ancient and may well have gone crazy.
So last night, figuring I wanted to stop the pump but keep the boiler on, I physically unwired the pump, then left the time thinking it was running hot water and CH. Sure enough the boiler got hot, so did the pipes to everything, radiators and tank even though the pump wasn't running. But within a minute or two the pipes were banging and clunking unhappily. Like the boiler was too hot or something. I turned down the boiler stat a little but it still isn't happy. It is now OFF. I could reconnect the pump and switch off all the radiators but this isn't really a good enough fix. Especially as all their valves are old and will probably leak if I adjust them.

There is no tank stat, there are no motorised valves that I'm aware of.
Just a boiler, a pump with a flow and return on narrower pipes, and a gravity feed to the tank on wider pipes.
The timer only had 6 wires going to it, 2 supply, 2 to the boiler, and two to the pump.

Any help would be much appreciated. I don't know much about heating systems but am able at most DIY stuff and really want to sort this.

Who told you it is a Cplan, you'd need a motorised valve on the cylinder flow or return (NOT the vent) and a cylinder 'stat.
If it's banging as you describe it I would say there is a lack of water in the system.
As for the pump running when on HW only it's either the wiring wrong or a faulty timer, I would imagine.

thepinkwaffle said:

There is no tank stat, there are no motorised valves that I'm aware of. Just a boiler, a pump with a flow and return on narrower pipes, and a gravity feed to the tank on wider pipes. The timer only had 6 wires going to it, 2 supply, 2 to the boiler, and two to the pump.

Click to expand...

For a start, what you have is not a C Plan as you do not have a tank stat and motorized valve. It's called a pumped heating, gravity hot water system.

With this system the hot water temperature is effectively controlled by the boiler temperature. The controls allow hot water only (summer) or heating and hot water (winter), but not heating by itself. The difference between winter and summer settings is that in the winter the pump in the radiator circuit runs, controlled by the wall thermostat.

It sounds as if your boiler thermostat may have failed, so the water is overheating.

Converting to C plan would allow you to control hot water temperature independently of the boiler. Depending on which timer you have, you may also be able to have the CH on separately from the HW. Here are some pics to explain:

View media item 827 View media item 73

ok great, that's useful stuff. Previous plumbers had suggested c-plan but didn't really seem to know what they were on about.

I don't really want to convert anything, I just want to get it working as it should.
Having hot water and CH in winter is no problem. I just need to find a way of stopping the CH in summer and keeping the water hot.

I agree the timer is probably crazy, but as I said its only 6 wires so presumably I've faked what the system should be seeing by unwiring the pump motor.

I turned the boiler stat down from 5max to 3, but it still clunks and bangs.
Just to be clear, this is a dial under the bottom cover on the boiler, when I turn it down the boiler cuts to pilot, then once its cooled for a min is switches back on full blast, cooks away for a bit, clunks and bangs and then cuts to pilot only again.
Is this what the thermostat is supposed to do or should it be able to reduce the intensity of the burn instead of just cutting it off?

twgas: I doubt there is a lack of water as it is a vented system and we've never had a problem with that. I'll nip in the loft tonight and make sure the header tank is full. Is this why things usually bang?
It never makes a noise when the pump is running.

thepinkwaffle said:

Having hot water and CH in winter is no problem. I just need to find a way of stopping the CH in summer and keeping the water hot.

Click to expand...

That's down to how the system is wired. The boiler should be connected to the Timer HW ON terminal and the Pump connected to the room thermostat, which is connected to the timer CH ON terminal.

I turned the boiler stat down from 5max to 3, but it still clunks and bangs.
Just to be clear, this is a dial under the bottom cover on the boiler, when I turn it down the boiler cuts to pilot, then once its cooled for a min is switches back on full blast, cooks away for a bit, clunks and bangs and then cuts to pilot only again.

Click to expand...

Did you plumber flush through the tank pipes as well as the radiator pipes?

Is this what the thermostat is supposed to do or should it be able to reduce the intensity of the burn instead of just cutting it off?

Click to expand...

Yes; the thermostat is nothing more that a temperature controlled switch. When the switch is closed, the boiler runs at full output until the water reaches the set temperature. The switch then turns the boiler off until the water temperature has dropped enough and the cycle repeats. Modern modulating boilers do adjust the boiler output so there is not so much turning on and off.

I'm pretty sure the plumber flushed the cylinder pipes as well as the CH as that was why he was called in the first place. The hot water tank wasn't getting hot but the CH worked. He may have just had to drain down and refill to clear the blockage, but I remember something about jacking in a full mains water pressure pipe to help clear the blockage.
He came back a week later and swapped out the water again as he'd left cleaning chemicals in it for the week.
Either way, with the pump running, the gravity feed works just fine.
Since unwiring the pump I think the gravity is still working but we get this clunking and banging.

Big thanks for explaining the boiler thermostat. That puts my mind at ease.

There is no room thermostat and no thank thermostat. The only thermostat is on/in the boiler.

I'd heard that there should be some sort of passive valve that the motor overcomes when switched on that prevents the radiators getting hot when the motor is off. The radiators (especially the first one or two) are still getting warm with the pump off. Could this valve have failed? Am I even likely to have such a valve on this heating pumped, gravity hot water system? Is this just another tale from a plumber who didn't even know what C-plan was or wasn't?

What do you think I should do? Turn down the boiler thermostat further?
Is the clunking/banging damaging? Is it going to break if it keeps doing it?

What boiler, make and model, do you have?
The noise may be the water in the boiler boiling because of a faulty 'stat or lack of circulation - which obviously isn't the case when the pump is running.
The passive valve (usually known as anti-gravity) is, as you said, to stop summer operation of the rads and you are correct in the fact that it should only open uhnder pump pressure. Whether you have one,and if it's broken, or not, I can't say.

Its some old baxi, I'll have to look up the model when I get home.

I think I'll try the stat on a lower temp, see if I can stop it boiling.
There wasn't any valve looking things obvious on top of the boiler where the four pipes are and the pump are but maybe its an internal thing.

The Gas Valve could be 'letting by' slighty thus not fully shutting off and boiling the water in the boiler.

It wouldn't be an internal thing - assuming you mean internal to the boiler.
They were usually put on the flow to the upstairs rads, above the tee supplying the dowstairs.

thepinkwaffle said:

Either way, with the pump running, the gravity feed works just fine.
Since unwiring the pump I think the gravity is still working but we get this clunking and banging.

Click to expand...

Where, exactly, is the noise coming from?

There is no room thermostat and no thank thermostat. The only thermostat is on/in the boiler.

Click to expand...

You need a room thermostat, relying on the boiler thermostat to control the house temperature will waste gas. You should also consider converting to a C Plan as this would save more gas.

I'd heard that there should be some sort of passive valve that the motor overcomes when switched on that prevents the radiators getting hot when the motor is off. The radiators (especially the first one or two) are still getting warm with the pump off.

Click to expand...

That's an anti-gravity valve. It prevents the rads heating up when only HW is required.

What do you think I should do? Turn down the boiler thermostat further?
Is the clunking/banging damaging? Is it going to break if it keeps doing it?

Click to expand...

The cause of the noise needs to be found first. You should not need to turn the boiler thermostat down. In any case, if you turn it too low you will not get hot enough water in the tank.

If your current plumber is unable to help, get someone who can. But please explain the problem to him first, before he comes. Ask friends, relatives and neighbours who they use and would recommend.

The noise is coming from the boiler and the pipes directly above.
I'll have more of a listen/take the body off the boiler tonight.

I may well replace the timer with a wireless remote thermostat in due course but don't think I can stretch to C-plan.

As for the anti-gravity valve, I haven't seen it anywhere. The boiler sits about a foot below the ceiling, so there's only room for the pump and pipes which then go in to the loft. I've been up there before (not recently) and don't remember anything other than the pipes. I'll have to trace them and see exactly where they go to find if there's a T between up and down, though I doubt there is. I suspect they're all just on one giant circuit.

An anti-gravity, aka non return, valve is not very big. It looks like this: