Glowworm38cxi - fault

Does this happen during CH demand or under HW demand or both?

Stephen.

It works fine on the central heating, then when I run the hot water it get too hot and blows the overheat stat

Don't know that boiler, but overheating on hot water is often caused by a scaled up secondary heat exchanger. If the hot water either isn't as hot as it used to be at a reasonable flow rate, or the flow rate is less than it was, that would also point to the same cause.

To descale it, you'll need a lot of descaler (something like Furnox DS-3), or you could replace it.

Its a pig of a job to remove the DHW secondary heat exchanger. You have to remove a short piece of gas pipe within the boiler to get the secodary heater exchanger out.

You need to descale both sides of the heat exchanger.

ALso there are trwo gauze filters within the boiler ot clean as well

The primary side shouldn't scale up - that's only cooling water down. If the water circulating in the system is dirty, it may block with rust particles and other debris, but that doesn't dissolve in descaler very well.

If those are easier to get at, might be worth trying to cleam just those to start with, and see if that increases the flow enough.

Thanks for your help Andrew & Stephen. Descaled the heat exchanger and it's working fine now, your right it was a pig of a job, I took the boiler off the wall as I couldn't get it out. I might still replace it while I remember how. Thanks again

that obviously means you had to disconnect the gas supply to the boiler in order to remove said boiler off the wall.... and then rehang boiler and reconnect gas....

I presume you're gas safe registered?

The DHW heat exchanger is at the back of the boiler and as mentioned earlier, you have to remove a short length of gas pipe within the boiler to get the DHW HE out.

The fact that you could not get the DHW HE out without taking the boiler off the wall suggests that the hot, cold, feed and return water pipes at the back were fouling the space needed to manipulate the DHW HE out.

Trouble is that the channels within the DHW HE are tiny. it only takes a few pieces of magnetitite to block up the channels.

the rads are on 10mm or 15mm or 22mm pipe that is several orders of magnitude greater than the DHW HE's channels.

These two filters are:

Cold water inlet filter

and

Return water filter.

They are supposed to protect the DHW HE from clogging up but do not do a very good job....