combi boiler intermittant domestic hot water

[jocojohn]

Hi can anyone help with the problem I have with a Glow worm 24 ci combi boiler. The heating radiators work ok although the pressure sometimes drops and needs filling suggesting a leak somewhere. The main problem is the domestic hot water. When using the shower or hot tap the burner will fire and run for about 7 seconds then slowly switch off for about 7 seconds, then back on, and so on. A plumber has had a quick look and checked that the flow valve turned freely. His guess is that the domestic heat exchanger may be scaled up. Does this sound a likely culprit or is there anything else that may cause this problem. Any advise would be great, thanks.

 

[Rob]

yes boiler cycling like this is usually down to the HE sludged up

 

[jocojohn]

Thanks for that reply Rob884, next question... Is it worth trying some kind of de-scaler in the system or is it best to replace the heat exchanger?

 

[Rob]

if you have time then cleaning it out would be worth a try. we usually just replace them for this reason

 

[jocojohn]

Thanks again, I have to drain the system this weekend to move 2 radiators. when re filling, what de scaling product would be best to try and for how long. I think it`s worth trying this before replacing the HE.

 

[GH blackpool]

hi there folks - similar problem - jocojohn do you mind if I hijack a little?

I've a ravenheat rsf85et, it's about 6years old.

I've emergency cover with DirectLine and have them out about once a year over the past 3yrs, have had a replacement DV and PCB in this time, and thermisters replaced Friday 17th after issues with hot water temperature (but the CH was fine).

Long story short, my hot water is now generally fine but occassionally I suffer the same prob as jocojohn - ie sometimes the hotwater turns cold, then heats for a few secs, and repeat, rpt, rpt.

this actually happened on Friday when the engineer tested the water after replacing the thermister and he said its likely the HE and that I could try a desludge for about £250, and if this failed then my insurance would cover replacing a knackered HE.

However, the boiler is inside a cupboard in a fitted kitchen, and he said to get to the HE, the cupboard (and hence attached cupboards too) would need to be ripped out.

2 questions if I may?

Is this the only way to get to the HE and replace it;

And can anyone suggest an approximate price for a desludge (he said that they inject a chemical via a filling loop, leave it two weeks, and then empty it out)? or is £250 quite accurate and reasonable?

Thanks in advance, we've a baby due in 2 weeks - this is all we need

 

[GH blackpool]

anyone....