Gas Boiler not modulating Hot water

[sjones]

The instant hot water from my gas boiler cycles from hot to cold. If I turn on the tap the gas comes on, temperature of water goes up, as shown by the bar graph in the front of the boiler and hand on the hot water pipe, then there is a click and boiler goes off for about 20S. I think and the gas jet is not modulating and the over heat thermostate is cutting of the gas, however if I turn the water temperature down to half way it seems to work OK, could the water overheat thermastate be faulty? The bar graph on the front of the boiler is only getting up to

40 degress before gas cut off, I remember it going to 50/60 degress when running correctly. Any advice?

[sjones]

I Mean the DHW temperature setting on boiler seems to stop it cycling (Heatline S30)

[stevelup]

I had exactly the same symptoms when our secondary heat exchanger was clogged with limescale.

On our boiler, the LED display on the front shows the DHW temperature when you are drawing hot water and the primary circuit temperature at other times.

Because the DHW heat exchanger was clogged, the primary circuit would overheat after approx 15-20 seconds and cause the boiler to cut the gas supply.

The fact that your unit works OK if you turn the temperature down suggests that your unit *is* modulating correctly.

Regards,

Steve

[sjones]

Thanks, that could well be it, boiler only fitted 3 1/2 years ago mind you. If that is the problem I will leave it to the professionals to change. How would you know, short of dismantling it ?

[sjones]

Might have a go myself, how difficult is it to remove a secondary heat exchanger, does anything need draining etc. Thanks again for the advice

[stevelup]

Hi

It depends on the design of the boiler. I changed one on my Vokera Linea 24 a while ago and the process could not have been simpler.

I bought a new heat exchanger off eBay. It cost =A330...

1) Turn off boiler 2) Turn off mains water 3) Drain a little bit out of the heating system (in our installation, the heat exchanger is almost the highest part of the system) 4) Undo two screws. The heat exchanger then came out in one piece. No pipes or anything to disconnect - it used four o-rings recessed into the mounting place.

Reassembly in reverse.

I think the whole job took about 10 minutes. If you tell the group what make and model of boiler you have, someone will be able to explain the procedure.

Steve