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?

On 5 Jun, 10:07, wrote:
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?

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

On Jun 5, 10:23 am, wrote:
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?

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

On 5 Jun, 11:35, stevelup wrote:
On Jun 5, 10:23 am, wrote:

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?

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
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 ?

On 5 Jun, 13:46, wrote:
On 5 Jun, 11:35, stevelup wrote:

On Jun 5, 10:23 am, wrote:

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?

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

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 ?

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

On Jun 6, 10:00 am, wrote:

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

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 £30...

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