Boiler fires, goes off after a while, then doesn't re-fire

I hope you can help. I have a boiler that isn't re-firing correctly.

I have a Baxi Solo 3 "PFL80" boiler, a Drayton Lifestyle LP722 controller and a Drayton room thermostat.

It's currently winter (obviously) so without the heating on, the room temp is much lower than that demanded by the thermostat.

When the controller time first turns everything on in the morning at 6:30, the room temp (14 degrees) is way below that demanded by the room thermostat (22 degrees), and the boiler starts and fires for about 20-30 mins. The boiler then goes off. The room temperature drops and drops but the boiler does not re-fire.

During this "dormant" state, on the boiler, the "boiler on" light is on. No ther lights are lit on the boiler apart from that. On the controller, both the heating and water lights are on. The room thermostat is "clicking" at approx the correct temperature according to my thermometer.

This morning, the heating came on and warmed up the house a bit. It then shut off after about 20-30 mins and room temp started to fall and was down to 14 degrees, and the boiler was still not firing. The "boiler on" light was showing on the boiler. The thermostat was sitting at 22 degrees and it was getting pretty chilly in the house.

Then, after some random time, about 45 mins this morning, the boiler eventually re-fired and the radiators warmed up again.

This problem is repeated in the evenings.

No amount of twiddling the thermostat, turning the water and heating buttons on & off at the controller, or turning the boiler dial off, on, up, and down, seems to have any effect whilst the boiler is in this "dormant" state. It then turns itself on without anyone doing anything.

I've had a plumber over to look at it. Needless to say when he comes over, the problem doesn't happen so diagnosis has been difficult. However it's safe to say that this has become less random and is now happening every morning and evening (plumber to return...).

The room thermostat has been replaced. The boiler PCB has been replaced.

What is the best way to diagnose what is causing this, and what pointers can I give ot our plumber when he calls again?

Thanks!

edit: I should add that when the boiler is firing, there are no unusual noises from it - all seems normal and the exhaust is blowing out through the vent/flue in the wall as normal.

Having done further searches through this forum, this post looks like a similar problem, albeit with a different boiler:

//www.diynot.com/forums/viewtopic.php?t=213443

Hi, just going off the information in your post, I would guess this is a controls prob rather than a boiler prob. I'm gonna assume that were the pump is fitted there will be a motorized valve either with 2 pipes connected to it or 3 pipes connected to it (usually in the airing cupboard). Speaking from similar experiences I've attended, I'm gonna assume this is a 3 port motorized valve(mid position valve) and on the side of the domestic hot water cylinder there will be a thermostat strapped to it. Here is what I think the problem might be -
Your hot water temperature, in a morning, in your storage cylinder is under the required temp, but not massively, because some is still there from the previous day. Your demand for heating and hot water comes on in a morning and puts the mid position valve in the position so it heats both rads and hot water. Once your hot water reaches the set temp on the cylinder thermostat, the mid position valve shuts off the side of this valve feeding the cylinder and the valve moves over to allow hot water from the boiler to flow into the heating circuit only. At the same time the valve moves into heating position it operates a microswitch inside the valve which continues to fire boiler. If the motor inside the valve does'nt operate this properly and the miroswitch does'nt make, heating will not work independently. A quick test to see if this is the problem is turn your cylinder thermostat as high as it will go, or move it away from the side of the cylinder, so the hot water temp never satifies.If this is the case replace mid pos valve - Good Luck

Ah - yes that sounds very possible. Thanks for your reply.

So the hot water takes a little while to heat up, and when it does the fault in the 3 way valve thing means that when the hot water is hot enough, the whole boiler shuts off rather than just the hot water circuit closing.

So, I'll try your recommendation next time it happens to see if I can isolate the problem. Also I guess just running a load of hot water out of the taps & shower will have a similar effect i.e. cooling the hot water cylinder, and thereby waking the boiler up again.

Is it a difficult/expensive job for a plumber to replace said faulty thingy?

Further to TOYBOY's advice, test your stats.

With the boiler control stat up to max, switch room stat to min, cyclinder stat to max; does boiler fire?

Switch cylinder stat to min, boiler will shut down, switch room stat to max; does boiler fire? If so, see if boiler stays on long enought to heat room.

Super advice - will do that this evening.

I'll post back the results...

OK - some inconclusive results so I hope someone can help a bit further.

In the airing cupboard I have 2 Honeywell F8-9804 thingys attached to separate vertical pipes which are joined a bit higher up into one pipe, on which is what I assume is a pump.

One of the F8-9804's is on a pipe which goes into the side of the tall, thin water cylinder. The other is on a pipe which disappears out of site underneath the water cylinder.

With the boiler in it's "dormant" state (boiler-on light on, room thermostat demanding heat, boiler not firing), I turned the stat on the hot water cylinder clockwise to its max and I heard a click from it, and I could hear a faint whir from one of the F8-9804's.

The boiler didn't fire though.

I then ran a load of hot water through the taps to cool the hot water cylinder.

About 10-15 mins after turning the hot water taps off, the boiler did fire and the radiators started warming up.

Is there usually a delay between the hot water cylinder stat being triggered and the boiler waking up? If so then I think I'm on the right lines and it's one of the F8-9804's. If not, it could be back to the drawing board.

I'm now waiting for the boiler to go "dormant" again before trying word for word the tests above.

tobers said:

With the boiler in it's "dormant" state (boiler-on light on, room thermostat demanding heat, boiler not firing), I turned the stat on the hot water cylinder clockwise to its max and I heard a click from it, and I could hear a faint whir from one of the F8-9804's.

The boiler didn't fire though.

Click to expand...

if the valve opened then the states calling for heat so boiler should get a signal to fire up.

I then ran a load of hot water through the taps to cool the hot water cylinder.

Is there usually a delay between the hot water cylinder stat being triggered and the boiler waking up?

Click to expand...

no

why don't you just test each circuit on it own with a multi meter
test the heating circuit see if its getting power on the orange etc then test the hw, don't have to keep waiting then.

Click to expand...

seco services said:

why don't you just test each circuit on it own with a multi meter test the heating circuit see if its getting power on the orange etc then test the hw, don't have to keep waiting then.

Click to expand...

I am unfortunately a skill-free zone as far as electricity goes. I'm just keen to find out as much as possible so as not to waste the time of the plumber who I'll be calling in imminently.

you won't be wasting his time.
tell him its not firing when heating called for and he'll want to run his own electrical tests.

Latest on this:

For the last couple of hours, the boiler has been on and the central heating working fine, with the house nice and toasty. I had turned the boiler thermostat to max.

When the room temperature reached the required amount, the room thermostat clicked off and the boiler stopped firing.

Turning the room thermostat up higher so it clicked again resulted in nothing - the boiler sitting in the "dormant" state again with the "boiler on" light on but not firing. I turned the room thermostat right down to minimum, and the "boiler on" light went off.

Popping upstairs, I turned up the hot water tank thermostat until it clicked. Immediately I could hear water running in the pipes and the pump felt like it was running. On the boiler, the "boiler on" light was now on, but the boiler did not fire.

Turning the room thermostat back up above the room termperature didn't make any difference either - the boiler-on light was still on but the boiler remained dormant and would not fire.

Any ideas?

It seems you do not have one 3 port valve, but two 2 port valves and the one related to CH appears to be faulty.
The room stat provides the power to the motor and it opens the valve.
There is also a live feed to a micro switch and when this switch is closed it fires the boiler. The closing of the switch takes place at the end of the movement.
Problem could be (a) faulty micro switch or (b) micro switch not being triggered or (c) no power going to micro switch

I am always suspicious of somebody called a "plumber" trying to fix a heating system.

They are pretty simple and it should not be very difficult to see where the fault lies assuming he can use a test meter and perform simple logical diagnostic tests.

I hope you are not paying him until he can find the fault !

Tony

its not firing up on either heating/hw.

seco services said:

its not firing up on either heating/hw.

Click to expand...

Yep that is correct.

Same again this morning. Heating and water ran until the hot water tank triggered the cylinder thermostat, at which point both the heating and hot water went off even though the room stat was still calling for heat.

Raising the cylinder stat past the point where it clicked didn't fire the boiler again.