I know that there are some plumbers on here, I was just wondering if you could help?

It is a problem i am having with the Boiler , Baxi 105(combi)

When i am running hot water for a bath after about 30 seconds the water starts to run cold for about 20 seconds then warms up again , its as though the boiler keeps cutting off and letting the water go cold .

It seems ok when you are having a shower and the central heating works (i did notice theat the little LED above the flame symbal took a while to come on when i first started the central heating)

thanks for any help

 

It sounds like either the domestic hot water heat exchanger is either blocked with black carbon or limescaled or one of the NTC sensors (thermisitors) is playing up.

 

Thanks mate

do you just flush the exchanger through (if so how is this done?)

 

Just had this on mine was the divertor valve sticking causing the thermistor on the c/h pipe to cut the boiler out.
To test it leave your heating off until it cools down, identify the c/h pipe out from the boiler, then put your hot water on , if the c/h pipe gets hot then the divertor is sticking. Easy to get from most plumber supplies places PTS, Wickes etc and really easy to change, just did mine and what a difference
Must admit I did clean out the heat plate as well but it was definetly the divertor sticking for my problem.

 

To be honest, its not really a DIY job to flush the heat exchanger out if its limescaled, you can do it with black carbon scale and use caustic soda, but you will have to drain the boiler down and turn off the cold water supply to the boiler, it would normally take around 60 mins to clean properly.

You would be better off just replacing the thermistor(s) first, they are cheaper and easier to DIY and then you can see if its cured it or not with out a lot of outlay.