Banging noise coming from the thermostats

BredburyBlues · Oct 14, 2016

I've turned my central heating system on this week, yes i know its too early in the year and man points must be deducted. All the rads work fine and all is quiet….for a while, then a knocking starts to come from one of the rads. It generally happens once i've used the hot taps in either the kitchen or bathroom for the first time after the timers come on.

The noise appears to be located at the thermostats. If I adjust the thermostat slightly it can stop the noise but this will generally result in the noise moving to another rad meaning 1) I end up chasing the noise around the house and 2) the original rad might stop working until i put it back to the setting it was originally at. The noise does stop it I run the hot tap which makes me think theres air in the system but shouldn't it escape out of the taps if that was the case?

I turned off the system, all the rads and left them to cool down. I turned the system back on then the rads one by one starting with the nearest to the boiler. Everything was ok for about 30 mins until it started again.

This was a problem towards the end of the last central heating season so I've thought about replacing all the older thermostats as out of the 5 rads in the house the only one that didn't appear to be effected was the one that was installed a couple of years back (newer thermostat) but last night it turned out that one was playing up as well so i'm at a bit of a loss now.

Where should i go from here, all help is much appreciated.

N

finchy01 · Oct 14, 2016

What system do you have? Is it a combination or conventional boiler?

BredburyBlues · Oct 15, 2016

It's a Worcester Greenstar 24i Junior

Last Plumber · Oct 15, 2016

Check the thermostatic valves are on the flow.

From cold, switch heating on and walk around the house feeling which valve gets hot first on each rad. That's the flow. Mark it with a pencil so you remember it.
Some TRV's are bi flow / bi directional valves. Some are not. This is normally indicated by an arrow on the valve body.

It is true to say that even though most modern valves are bi flow, they can still be happier on the flow side, depending on your system and it's flow rates and pressures.

If they're on the return end it may be better to swap them to the other end if they're fairly new but for the cost of the valves, I would suggest replacing them. I would still make sure they're on the flow.

You may need a heating engineer to do this for you.

As regards man points, yes you have lost a few but I did glance at the stove the other night and considered putting a match to it.

I resisted.

Man points intact !

Azzer1704 · Oct 15, 2016

It's the rad valves, as the thermostats start to kick in they will bang if the water is flowing wrong direction. Hot water takes priority on a combi boiler which is why it stops when you run the hot tap. Is it a newish boiler? Either flow and return pipework were reversed when boiler was fitted or the valves need changing. As a temp fix set the valves to max or take the heads off