Clattering noises from the hot water system

[UKnewbie]

Hi all, first time poster here, but enjoyed reading the forum up till now.

Anyway, we have a lot of noise coming from our CH system when it's in the hot water heating cycle. We have a combi boiler (glow-worm) and a gravity fed system. The noise has been with us for a couple of months now and we had a plumber out who suggested we replace the 3 way valve and actuator. I did that and have also replaced the cylinder thermostat (thinking that it had failed and wasn't stopping the cycle).

Recently I have put a cleaner through and this weekend I flushed out the system and replaced with inhibitor. It made no difference.

The CH system is quiet and seems relatively trouble free. It's when the hot water cycle is operating and the water is being heated up to the set temperature. At the end of the cycle it starts to gurgle and you can hear rushing in the pipes and then there's a whole crescendo of noises until it ultimately surges/purges into the F & E tank in the loft.

Could it be a build up of limescale in the cylinder? I'm not sure about airlocks, as I bled and bled all the rads this weekend. I also have an AAV. Could it be an electrical component in the boiler (thermostat, over-heat thermostat or even the PCB)?

Any ideas or advice would be gratefully received.

 

[JohnD]

... We have a combi boiler .... the cylinder thermostat...

Is it really a combi, or has it really got a cylinder? Very unlikely to be both.

My guess is that it's got a cylinder, and isn't a combi.

It's open-vented and it gurgles into the F&E. How much mud is in the bottom of the F&E, and what colour is the water?

What chemical did you use when you cleaned it? What came out?

 

[mcmoby69]

Hope you got your money back off the plumber who advised you.

 

[UKnewbie]

Thanks both, my F & E tank is spotless, well it was on the weekend. I ran 2 litres of PC400 Flushchem for 2 weeks then drained until all outlets were clear. I ran the pump and the incoming water kept up as I flushed out the cleaner. Then added inhibitor. Ok, yes, wrong with combi. It's a standard boiler.

 

[JohnD]

for overheating, feel the flow and the return pipes when it is just heating the cylinder. The flow should be "too hot to hold" and the return should be "too hot to hold for long."

if you turn down the boiler thermostat, the temperature of these pipes should be noticeably lower.

What temp is the cylinder stat set to? Use a thermometer and see what the actual tapwater temp is.

if the boiler is overheating, the overheat stat would stop it firing, eventually.

What colour is the cylinder?

 

[UKnewbie]

Once again, many thanks for the reply and consideration.

OK, checking the pipes as I tiptoed out the house this morning, the flow and return pipes were both almost too hot to touch. Hardly any difference in heat. BUT, it was getting near the end of the water heating cycle? Tonight I will drain off some hot water then isolate the boiler function to only heat the water and check with a cooler hot water cylinder. Then, I'd expect the return pipe to be somewhat cooler, yes?

The hot water stat is set about 57-ish. I will check the actual water temp tonight. It does get pretty hot, certainly too hot to have your hand under a tap for too long.

It's a 140 litre IMI herculag unit. The lagging colour is green. I'm guessing over 10 years old. If it is the cylinder, I have already checked new ones and the fitting locations match my current layout. Should be a simple enough swap (if that's the case).

Re the overheat stat, just to mention, it has tripped a couple of times (button popped up) when we first noticed the issue, but not in the last 7-8 weeks.

Oh, and I'll turn down the boiler stat as an experiment. When I do, will that greatly reduce the outgoing water temp of the boiler? So on the flow line to the cylinder, I should be able to hold it for a while?

 

[Madrab]

Re the overheat stat, just to mention, it has tripped a couple of times (button popped up) when we first noticed the issue, but not in the last 7-8 weeks.

Is the electrical immersion heater on the cylinder switched on too, as well as the boiler heating the cylinder? If so (the overheat stat has tripped a couple of times) There's your problem then. Switch off the immerser, it's only there if the central heating fails. If it's not then something is allowing your cylinder to overheat.

 

[UKnewbie]

Hi, no the immersion heater is only used in times of need.

 

[Madrab]

The overheat stat on the boiler, which john mentioned, would just shut the boiler down, it doesn't 'trip' as such, so what other stat's been tripping (button popped up)?

 

[UKnewbie]

Hi, this is a over heat thermostat, correct? The central 'button' (looks like a cross) on this popped up a few times. I reset (and got the boiler working again) by pushing it back down.

Anyway, thanks for your suggestions. Tonight I turned off the CH and water heating and ran down some hot water. I went out to the boiler (in the garage) and turned down the main thermostat to about half.

I came back into the house and just to make sure, bled the pump.

Then I started just the water heating and waited. The 3 way valve went over to 'W' and the water started pumping into the cylinder. Before long, both flow and return lines into the cylinder were too hot to hold.

I went out to the garage and listened to the boiler, as I have heard the 'rushing' noise coming from there on one occasion. The boiler cut out, and all seemed fine. As I was leaving the garage the boiler started up again. I thought this was strange as only the hot water circuit was operating. I got back to the water cylinder and felt the pipes, and again, too hot to hold. It continued pumping for another 10 mins or so.

But, no gurgling/clattering/crashing this time.

 

[JohnD]

do you happen to have a multimeter with a socket for a thermocouple?

The connector will probably fit a small plug with two flat prongs like this

and may well be marked "K type" or similar

 

[UKnewbie]

Hi John, I have a multimeter, but not a thermocouple. What item would I measure?

 

[UKnewbie]

I can probably get access to that item. I'm an Engineer in the Automotive sector.

 

[JohnD]

thermocouples are available for a few pounds. If you already have a multimeter with the socket, they are a cheap and easy way to measure pipe temperatures and observe rate of change. My BIL is an Australian HVAC engineer and tells me that when fault-tracing, the thermocouples are so cheap that they just tape one to any suitable part or pipe, and leave it there, with the plug dangling, ready to connect next time.

They may have moved over to IR heat guns now.

p.s.
I am a householder not a heating engineer, but I'm thinking that your boiler main thermostat is not regulating the boiler temperature correctly.

 

[UKnewbie]

I could probably get hold of an IR gun quicker than a thermocouple. Which pipes are you suggesting I check?