Hi,

I'm having a problem with my central heating boiler, the safety cutoff
keeps 'popping' out when ever the room thermostat is *not* calling for
heat. Looking at my system I can see the flow feed from the boiler
passing through the pump (high speed setting), through two 'T'
piece's, and then the motorised valve (and onto the rads). The two
'T' pieces are for (I think) a) a loop circuit and b) to feed the hot
water storage tank. There are also two gate valves on these two
pipes.

Can anyone give me ideas on why my system keeps cutting out like this?

How open should the two valves be? Should the loop valve be more
closed than the others?

Thanks in advance

Danny

PS: I've just flushed the system completely, used cleanser for a
week, flushed twice, re-filled and treated with inhibiter.
Boiler does not cut out when the room thermostat is calling for heat

"Danny" wrote in message
om...
Hi,

I'm having a problem with my central heating boiler, the safety cutoff
keeps 'popping' out when ever the room thermostat is *not* calling for
heat. Looking at my system I can see the flow feed from the boiler
passing through the pump (high speed setting), through two 'T'
piece's, and then the motorised valve (and onto the rads). The two
'T' pieces are for (I think) a) a loop circuit and b) to feed the hot
water storage tank. There are also two gate valves on these two
pipes.

Can anyone give me ideas on why my system keeps cutting out like this?

How open should the two valves be? Should the loop valve be more
closed than the others?

Thanks in advance

Danny

PS: I've just flushed the system completely, used cleanser for a
week, flushed twice, re-filled and treated with inhibiter.
Boiler does not cut out when the room thermostat is calling for heat

When everything *stops* calling for heat, and the boiler burner shuts down,
the water needs to continue circulating for a little while. If this does not
happen, the residual heat in the metal parts of the water get transferred to
the static water inside - causing the temperature to rise to a point where
the safety cutout operates.

In order to prevent this, two conditions must be satisfied:
1. The pump must be controlled by the boiler - so that it goes on pumping
until the boiler has cooled down sufficiently
2. The water must have somewhere to go! If you have individual motorised
valves for central heating and hot water, there *must* be a by-pass circuit
to allow the water to circulate when both of these are closed. [If you have
a single 3-port valve, this is not usually a problem - since it can never
close both of its outlets at the same time].

The first thing to check is that the pump is wired correctly, so as to be
controlled by boiler, as described above.

Then - if you have separate motorised valves for CH and HW - identify the
by-pass circuit, which should short-circuit the boiler flow and return pipes
without going through any motorised valves or through the CH or HW circuits.
The by-pass may well have a gate valve to stop *all* the flow from going
straight back to the boiler without going through the CH or HW circuits. If
this gate valve is fully closed, you ain't got a working by-pass!

Start by opening it fully, and check that it cures the problem. Then close
it progressively in order to find the position where it is open as little as
possible without causing the boiler to trip.

HTH,
Roger

Hi Roger,

Thanks for the info. The pump runs for approx 10 minutes after the
boiler shuts down, I guess this means its wired correctly.

The two valves (one for HW tank, and the other for by-pass) were only
slightly cranked open. My concern for opening the by-pass valve fully
was that the water would simply travel the easiest path, thus reducing
the efficiency of the CH when the motorised valve is actually open.
However I take your point about opening it fully and workng backwards
to find the 'best' position.

With regards to the valve that regulates the water supply to the HW
tank, do you recommend that this be fully open also? Can adjusting
this vavle help to stop my boiler from short-cycling? (I think thats
the correct terminology!)

Regards,

Danny

"Roger Mills" wrote in message ...
"Danny" wrote in message
om...
Hi,

I'm having a problem with my central heating boiler, the safety cutoff
keeps 'popping' out when ever the room thermostat is *not* calling for
heat. Looking at my system I can see the flow feed from the boiler
passing through the pump (high speed setting), through two 'T'
piece's, and then the motorised valve (and onto the rads). The two
'T' pieces are for (I think) a) a loop circuit and b) to feed the hot
water storage tank. There are also two gate valves on these two
pipes.

Can anyone give me ideas on why my system keeps cutting out like this?

How open should the two valves be? Should the loop valve be more
closed than the others?

Thanks in advance

Danny

PS: I've just flushed the system completely, used cleanser for a
week, flushed twice, re-filled and treated with inhibiter.
Boiler does not cut out when the room thermostat is calling for heat

When everything *stops* calling for heat, and the boiler burner shuts down,
the water needs to continue circulating for a little while. If this does not
happen, the residual heat in the metal parts of the water get transferred to
the static water inside - causing the temperature to rise to a point where
the safety cutout operates.

In order to prevent this, two conditions must be satisfied:
1. The pump must be controlled by the boiler - so that it goes on pumping
until the boiler has cooled down sufficiently
2. The water must have somewhere to go! If you have individual motorised
valves for central heating and hot water, there *must* be a by-pass circuit
to allow the water to circulate when both of these are closed. [If you have
a single 3-port valve, this is not usually a problem - since it can never
close both of its outlets at the same time].

The first thing to check is that the pump is wired correctly, so as to be
controlled by boiler, as described above.

Then - if you have separate motorised valves for CH and HW - identify the
by-pass circuit, which should short-circuit the boiler flow and return pipes
without going through any motorised valves or through the CH or HW circuits.
The by-pass may well have a gate valve to stop *all* the flow from going
straight back to the boiler without going through the CH or HW circuits. If
this gate valve is fully closed, you ain't got a working by-pass!

Start by opening it fully, and check that it cures the problem. Then close
it progressively in order to find the position where it is open as little as
possible without causing the boiler to trip.

HTH,
Roger

"Danny" wrote in message
om...
Hi Roger,

Thanks for the info. The pump runs for approx 10 minutes after the
boiler shuts down, I guess this means its wired correctly.

The two valves (one for HW tank, and the other for by-pass) were only
slightly cranked open. My concern for opening the by-pass valve fully
was that the water would simply travel the easiest path, thus reducing
the efficiency of the CH when the motorised valve is actually open.
However I take your point about opening it fully and workng backwards
to find the 'best' position.

With regards to the valve that regulates the water supply to the HW
tank, do you recommend that this be fully open also? Can adjusting
this vavle help to stop my boiler from short-cycling? (I think thats
the correct terminology!)

Regards,

Danny

The valve on the HW circuit is there to balance the flow when the motorised
valves for CH and HW are both on - i.e. to make sure that most of the water
goes to the radiators. You need to find the right position for this, so that
the HW gets hot in a reasonable time without stopping the radiators heating
up quickly. I would experiment with positions around half open - or maybe a
bit less.

Assuming that the boiler only trips when both motorised valves are closed
(i.e. when CH and HW demands are both satisfied) the position of the manual
valve on the HW circuit won't affect this. It only comes into its own when
both motorised valves are open. [If the boiler trips when the HW is still
being heated, this valve definitely needs to be opened more].

Incidentally, "short-cycling" is something different - and not what I
believe you've got. Short cycling is when the boiler runs for a few seconds
at a time and then go off. And then comes on a bit later, and does it all
over again. AIUI, your problem is that the overheat stat trips - and has to
be re-set manually before the boiler will light again. Is this right?

Roger

Again Roger, thanks for all of your help.

Thats right, it only trips when the CH motorised valve is closed. I
fully opened the by-pass valve, with the CH circuit off, I noticed
that the water wasn't passing through the by-pass valve, massive
temperature difference on either side of the valve. I'm thinking that
this valve may be faulty and is stuck closed (it's not motorised),
should I replace this or is there another method of fix?

Regards

"Roger Mills" wrote in message ...
"Danny" wrote in message
om...
Hi Roger,

Thanks for the info. The pump runs for approx 10 minutes after the
boiler shuts down, I guess this means its wired correctly.

The two valves (one for HW tank, and the other for by-pass) were only
slightly cranked open. My concern for opening the by-pass valve fully
was that the water would simply travel the easiest path, thus reducing
the efficiency of the CH when the motorised valve is actually open.
However I take your point about opening it fully and workng backwards
to find the 'best' position.

With regards to the valve that regulates the water supply to the HW
tank, do you recommend that this be fully open also? Can adjusting
this vavle help to stop my boiler from short-cycling? (I think thats
the correct terminology!)

Regards,

Danny

The valve on the HW circuit is there to balance the flow when the motorised
valves for CH and HW are both on - i.e. to make sure that most of the water
goes to the radiators. You need to find the right position for this, so that
the HW gets hot in a reasonable time without stopping the radiators heating
up quickly. I would experiment with positions around half open - or maybe a
bit less.

Assuming that the boiler only trips when both motorised valves are closed
(i.e. when CH and HW demands are both satisfied) the position of the manual
valve on the HW circuit won't affect this. It only comes into its own when
both motorised valves are open. [If the boiler trips when the HW is still
being heated, this valve definitely needs to be opened more].

Incidentally, "short-cycling" is something different - and not what I
believe you've got. Short cycling is when the boiler runs for a few seconds
at a time and then go off. And then comes on a bit later, and does it all
over again. AIUI, your problem is that the overheat stat trips - and has to
be re-set manually before the boiler will light again. Is this right?

Roger

"Danny" wrote in message
om...
Again Roger, thanks for all of your help.

Thats right, it only trips when the CH motorised valve is closed. I
fully opened the by-pass valve, with the CH circuit off, I noticed
that the water wasn't passing through the by-pass valve, massive
temperature difference on either side of the valve. I'm thinking that
this valve may be faulty and is stuck closed (it's not motorised),
should I replace this or is there another method of fix?

Regards

Sounds like you need to replace the valve. You might consider using an
automatic by-pass valve - which only opens when a certain (adjustable)
pressure is reached, so that nothing goes through it when either or both CH
and HW circuits are operating. Screwfix do a suitable valve for about £25.

Roger

Thats right, it only trips when the CH motorised valve is closed. I
fully opened the by-pass valve, with the CH circuit off, I noticed
that the water wasn't passing through the by-pass valve, massive
temperature difference on either side of the valve.

Sounds knackered, just like most gate valves ten minutes after installation.
When you replace it, use an "Automatic Bypass Valve" instead of a gate
valve. This stays closed when the zone valves are open, but when they close,
it detects the pressure differential from the pump and opens fully. This
way, you get an effective bypass and no short circuit when the system is
actually working.

Christian.