Combi Boiler - normal behaviour ?

[Rik Smithies]

I have an old (10 years+ ?) inherited boiler - a Vaillant COMBICompact VCW-GB 242 EH.

It generally functions well but when I run a bath in cold weather, at a constant medium tap flow rate, the water temperature fluctuates greatly. Scalding hot for a minute or so, freezing cold for another minute and so on. This makes it very hard to fill a warm bath as the cold outweighs the hot over a few minutes.

The bathroom is upstairs and the boiler downstairs and mounted on the outside wall of the main house, in a kitchen extension/garage type thing we have.

I imagine that the boiler is struggling, after all it is old and it is a cold time of year. But I would have thought at a certain flow rate the temperature would be constant, even if lukewarm.

A British Gas chap told me "all combis are like that"/"yours is too old"/"in the wrong place" etc etc. Is he right and it is time to fork out for a new one ? Or is this something that can be fixed ?

thanks Rik

[Colin Wilson]

This gets mentioned regularly, so no doubt one of the regular central=20 heating wizards will be along shortly to work their magic :-)

From what I remember of previous answers, British Gas are simply trying=20 to apply their "perhaps we can sell a muppet a combo" logic !

No nastiness intended there - it was a general comment about people who=20 buy from BG in the first place, like my ma-in-law who just spent over =A34k= =20 on a standard *combi* CH system from them... The same could be had for=20 =A32.5k maximum almost anywhere else

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[Ed Sirett]

Almost certinly the secondary heat exchanger has scaled up. Replacement or descaling is called for.

A possible work around which is tedious is to set flow rate quite low and turn down the thermostat to the point where the flames do not go off and on. Since there is only one thermostat this may mean turning it up for heating, that's inconvenient.

This happens on most combis sooner or later (VCWs are fairly robust and have a substantially secondary heat exchanger so they tend to take a long time (10 years) to get furred up. I would think properly maintained your VCW has some more years in it.

[Rik Smithies]

thanks for replies, that secondary heat exchanger tip might end up saving me a few quid. Rik

[Rik Smithies]

Another thought, I live in a soft water area (Birmingham), so is this scaling still a likely explanation ?

[BillR]

I'd be surprised. The soft water is more likely to eat away the heat exchanger than scale it up.

[Ed Sirett]

OK so we are not looking at limescale but the same effect could be had by debris build up on either the primary or secondary side of the heat exchanger. Or you might have a normally working boiler but the temperature sensor has drifted out of spec. Or the thermostat board inside the die-cast box - middle horizontal board has a problem.

To progress further you will need to see if the gas is cutting because the primary flow temperature has gotten to 80-90C (bad 2ndary HE) or because the sensor/PCB is over enthusiatic.

Another possibility is that the modulator on the gas valve has failed, you can check to see if that might be the case by looking closely at the flames to see if they reduce before the gas gets cut.

HTH

[Neil]

Apologies for hijacking this thread but my Ravenheat RSF100 is exhibiting similar problems except that the DHW output never gets above lukewarm. When the water temp. control on the boiler control panel is turned up from minimum the flames increase momentarily then drop down to previous level & then gas valve cuts the supply - is this indicative of a failed modulator ? Boiler then fires up again then cuts out after 15 seconds and on and on. DHW temp. sensor has been changed but this has not solved the problem. CH works fine.

Neil

[Ed Sirett]

Sorry, the way I worded it could seem to say that the flames going up and down means a failed modulator (or PCB control). No, the job of the modulator is to match the gas flame to the demand so when the flames go down before going out the modulator is doing its job and has not failed.

[geoff]

A few years ago, Vaillant changed the design of the secondary heat exchanger from one which looked like three black plastic tubes to one which looks like a sardine can.

So, if you do buy one and it doesn't look anything like your old one, that's why.

">thanks for replies, that secondary heat exchanger tip might end up >saving me a few quid."

yes, but prepare yourself for a bit of a shock though (£150?)

[geoff]

I doubt it, the old style ones are made of plastic