Glow worm 30cxi - hot then cold water

[Timmyspicyman]

Hi everyone,

I've got a problem with my glow worm boiler (similar to other people on here it seems )

The hot water will run for a couple of minutes, then go cold for a minute, then hot again, so on, so forth.

Strangely the radiators seem to come on at the same time, even when the thermostat is down to zero, the rad temp on the boiler is down to zero, and the heating is switched to off.

I've checked the taps in all the rooms and bizzarely only the upstairs taps/showers are effected. The kitchen, downstairs loo and outside hot tap (yes I have an oustide hot tap, don't ask.....) all run fine.

My initial thought was the diverter valve. Had a guy out to look at it and he changed the cartridge, but no avail. He's quoted £400 to change the valve in its entirety (which seems a bit steep to me).

Any suggestions?

Thanks in advance.

 

[ChrisR]

Your seocondary heat exchanger is probably scaled up (boiler water, not tapwater limescale) and yes your diverter valve is "letting by".

To replace both of those would be expensive. It would be a lot less to do descaling in situ. There's a fair chance that would fix both, though the DV may still let water to CH when it shouldn't.
Unfortunately it's not something that many boiler blokes would do, and you'd be rare among diyers to work out how to do it, and carry it out right.

How old is the boiler?
Your system probably all has sludge it. If you do replace/clean the boiler parts, chances are that it'll recur in about the same time.
So the proper answer is to chemically power-flush the whole system, with special emphasis on the boiler. And hope the DV recovers in the process.

Changing the DV alone would be pretty much a waste of time and money. I suspect your man so far may have only changed the easy bit - the motor, which would have been easy to test and probably had nothing wrong with it. Or did he "get wet"?

For flushing you'll get quotes between a couple of hundred and many many hundreds.
Some of those will make a complete horlicks of it and flush the scale into the boiler where it'll nicely block it.

Good luck!

 

[Timmyspicyman]

Thanks Chris.

The boiler was installed almost exactly 5 years ago. We've been quoted £50 per rad for a flush, but given we've got 15 rads in the house, that could get quite pricey!! And if we need a new DV on top of that, that will be over a grand in total. May be worth just ditching it and installing a new boiler rather than throw good money after bad....