combi boiler problems please help

[higgsy]

I have a worcester bosch 24 cdi fitted by BG a few years ago, now I am having a problem, when I run my shower it goes from hot to cold then hot again so on & so forth. does anyone know why this is happening.it does not happen when I run the taps although the flow from the bath hot tap is quite slow. any help would be greatly appreciated, before I get someone in to look at it I would like some idea of what is causing it. so i dont get ripped off as usual .

[Christian McArdle]

If the bath tap is fine, then I'd eliminate the possibility of a faulty shower mixer before suspecting the boiler.

The bath tap is likely to be slow, as it appears that the installer correctly backed off the hot tap isolator to prevent you drawing more water than your combi can supply. A 24kW combi won't provide a high flow rate and will not fill a bath quickly.

Christian.

[Nicknoxx]

Depends what you mean by slow. If you live in a hard water area then it's likely that the heat exchanger in the boiler is scaled up which reduces the flow rate. If it gets too slow the boiler will cut out periodically and you will find the shower goes cold.

I've heard that it's possible to descale the heat exchanger but don't know if this is easy or effective. Certainly it won't be cheap to replace.

N

[higgsy]

when it was installed I enquired about fitting a device to stop it scalling up but was told by BG that it did not require one.

[Ed Sirett]

I tend to agree, some people beleive that electromagnetically conditioning the water prevents the build up of scale. I don't beleive that so I don't fit any. The only sure way to feed softened water to the CW inlet of the boiler.

Some models of boiler are fairly simple to replace the [secondary] heat exchanger. (The vokera Linea serie is one, the Vaillant turbomax series is not too bad either. Some are very difficult the floor standing Worcesters for instance.)

There is a small risk that descaling the unit will spring a leak, but if you are really looking to save then removal and acid descaling should work fine.