Hot water cylinder; something rattling

[donkeykong2]

firstly great forum here and lots of great advice.

I have a problem that has started a week ago. When the HW is heating only, i can here a rattle somewhere around the cylinder. it sounds like its coming from inside but not sure. However when the CH and HW are on its all quiet. What could this be? The only thing that has changed is having the pump recently changed from a 15-50 to a 15-60.

I have tried the pump on all settings and all the air seems to have been bled from the system. Recently the system was drained by problem still exists.

 

[delmel]

sure you have not also got the immersion heater switched on ?. If not perhaps its the water boiling to hot inside and stat set incorrectly.

 

[donkeykong2]

immersion is off and the water in the cylinder is near enough cold. though good point it might have initially started when the immersion was on, water was boiling and the boiler has heating the water.

Could something have broken and why doesnt it do it when the HW and CH are both on

 

[delmel]

check the stat temp setting on the tank. When they are both on, they are sharing the power from the boiler, if just hotwater its all going to that and if stat is high on the tank it will boil away.

Boiler will function using the tank stat, immersion heater will have its own stat, so if thats off you can rule that out for now and look at the tank stat

tends to be set between 50 - 60, if you have anything much higher that could be the water getting to hot

 

[delmel]

check the stat temp setting on the tank. When they are both on, they are sharing the power from the boiler, if just hotwater its all going to that and if stat is high on the tank it will boil away.

Boiler will function using the tank stat, immersion heater will have its own stat, so if thats off you can rule that out for now and look at the tank stat

tends to be set between 50 - 60, if you have anything much higher that could be the water getting to hot

I am not a heating engineer just my thoughts

 

[donkeykong2]

thanks for reply. the cylinder stat is on 50. the water is only heated twice a day, so at both times is when HW starts is usually only warm and rattle starts straightway.

Its almost like something is moving around, piece of debris or something and can be heard from 2 of the radiators at the same time

 

[donkeykong2]

I can't really tell where its coming from, but it can be heard through the pipework. It also continues to happen once the boiler has switched off and the pump is still running.

I tested yesterday by doing the following
1. Switch on CH - no noises
2. Switch on HW - no noises
3. Switch off CH - noises start
4. Turndown the cynlinder thermostat - boiler stops, pump runs, noise
5. Bled pump and rads - no difference
6. Switched on CH - heard water being filled into the boiler and pipework - noise stops

I haven't checked if water is being vented back into the loft tanks when the banging occurs but both tanks are full to their usual level.

Any further ideas?

 

[delmel]

Air lock maybe (you should bleed rads with system off)

 

[donkeykong2]

rads have been bled over a number of weeks with system off. still get the noise when HW only.