Gloworm 30cxi Boiler Short Cycling

richardc1983

Hoping someone can help. This boiler is really starting to annoy me now thankfully I have insurance on it but the engineers who come out have sometimes been clueless.

Over the last few years it's had main Pcb replaced, new heat exchanger, fan unit and various other things. The last repair was a month ago and was the main pcb that had blown. Now we have started to use the heating because it's got colder I've noticed the radiators take ages to get hot and then when they do they go cool for a while and the burner doesn't come back in for a while.

When it does kick back in the display shows the flow temperature going up very quickly (it's set at about 50c sometimes lower) and then the flow temp can hover at like 51c then you'll see it drop to 48c then back upto 52c and will jump up and down like that and then the burner cycles off.

It never used to do this the boiler burner could be on for about half an hour sometimes longer just ticking away modulated down low. All the radiators are fully on in the house so there should be plenty of flow. I've tried turning the flow temp up but it does the same pretty much.

When the burner is off the flow temp quickly starts dropping and then the fluttering of temp on the display occurs it will flicker between 42-38c then backup 40 then may go back down to 38 before going back upto 41c. When it used to work ok sometimes it did cycle quickly because we run the flow temp quite low anyway as low as 45c sometimes but the house was always toasty can't get the rads more than Luke warm at present. The temp on the display I am assuming is the boiler flow temp? The return pipe is luke warm whereas the flow pipe is hot.

I'm pretty sure there is no blockage it was all drained down the other year to replace the trv's in the house and inhibitor fitted. The heat exchanger was replaced after this though as it was blocked about a year ago.

Even when I reset the boiler at the power switch it is doing this thing where the fan surges - like a car revving when I turn the power off with a f12 fault code. It really doesn't seem well at all. Turning on the hot water seems to clear it but it does this for about 5 minutes before sorting itself. I have checked all PCB connections and they are all fine. During this time the pressure digits are going up and down like a yo yo.

What could possibly wrong with it - other than it being a rubbish boiler!

Thank you.

Rubidium

Check the pump - the "pressure digits are going up and down like a yo yo" should not be happening.

richardc1983

Rubidium wrote: »

Check the pump - the "pressure digits are going up and down like a yo yo" should not be happening.

The pump is working fine and sounds fine as well. You can hear there is plenty of flow.

DavidFx

Is the temperature of the boiler exit pipe also rising quickly? - if so, this would point to a low flow rate through the boiler. If the exit pipe remains cool then this would point towards the boiler temperature sensor misreading.

bondy01

I can't really help from a technical point but as I have a nearly 11 year old 24cxi I thought I would give you my 2p worth. I called glowworm out last year and they fixed various problems and thought the guys that visited (4 times) were excellent. (there was a lot of issues that needed parts) I run mine at a boiler temp of 74C and it only varies from 73-74 when it reaches this temp. The flow and return pipes are stinking hot , I'm not sure why you run yours much lower.
The burner light stays on on mine but you seem to be saying yours goes off?
F12 as you probably know is user interface connection fault - Check wiring between mainboard and user interface.
The pressure surely shouldn't be varying when you use hot water , mine doesn't.
Lastly if you are on a glowworm plan have they not sent you a letter to get a new Vailliant for less than 2k like they did me?

richardc1983

DavidFx wrote: »

Is the temperature of the boiler exit pipe also rising quickly? - if so, this would point to a low flow rate through the boiler. If the exit pipe remains cool then this would point towards the boiler temperature sensor misreading.

The exit "flow" is getting hot quick but thats no different to normal. It had a new heat exchanger last year so no idea why its doing this now.

richardc1983

bondy01 wrote: »

I can't really help from a technical point but as I have a nearly 11 year old 24cxi I thought I would give you my 2p worth. I called glowworm out last year and they fixed various problems and thought the guys that visited (4 times) were excellent. (there was a lot of issues that needed parts) I run mine at a boiler temp of 74C and it only varies from 73-74 when it reaches this temp. The flow and return pipes are stinking hot , I'm not sure why you run yours much lower.
The burner light stays on on mine but you seem to be saying yours goes off?
F12 as you probably know is user interface connection fault - Check wiring between mainboard and user interface.
The pressure surely shouldn't be varying when you use hot water , mine doesn't.
Lastly if you are on a glowworm plan have they not sent you a letter to get a new Vailliant for less than 2k like they did me?

Thanks for the reply, answer to your points:

1. The lower you have the flow temp the more efficient your boiler is, as long as it keeps the house warm. The house warms up and keeps the thermostat from overshooting at about 45-50C its only a small 2bed house and I have oversized radiators in each room so they kick out plenty of heat. Any hotter and rooms get stuffy, TRV's shut off leading to overshooting of room temps and the thermostat in the hall shuts off which then means temps fluctuate as they go up and down. I also hate walking into a room where the rads are pumping out red hot heat. The fact you say your return pipe is also red hot means your boiler is not running efficiently, your return temp should be 55C or below for most efficient running of the boiler.
2. When it was working properly and it was just ticking away the flow temp display would read say 45/46 and never lower or higher. The burner also would remain on and only go off if it reached the set point of the boiler, the lower you run the boiler if there isnt much heating demand at certain times of the year it is more likely to cycle off as it cannot modulate down quick enough. Currently though that is not the case and it should not be cycling off when radiators are still cool.
3. The pressure isnt going up and down when we are using water. The display that shows either the flow temp when the heating is on or pressure when the hot water is on is showing the pressure jumping up and down from 1 bar to 2bar when it does the fan revving thing. This happened last night when we got the F12 fault code and we wasn't even using the boiler the heating was turned off and no hot water was being used.
4. The service plan is with homeserve not glowworm directly so we didnt get a letter from them.

I think we are now going to have to get rid of this nasty boiler!

DavidFx

richardc1983 wrote: »

The exit "flow" is getting hot quick but thats no different to normal. It had a new heat exchanger last year so no idea why its doing this now.

Is the boiler return also heating up quickly? If so then you have a 'short circuit' in the system - if not then it would point to a low flow condition or that the diverter valve is stuck in the HW position.

richardc1983

DavidFx wrote: »

Is the boiler return also heating up quickly? If so then you have a 'short circuit' in the system - if not then it would point to a low flow condition or that the diverter valve is stuck in the HW position.

No the boiler flow return is just luke warm and corresponds to the temperature at the return side of the radiators which are barely hot.

Hot water is working fine but its this short cycling thats an issue. If I turn the boiler up to max output on heating then the radiators do eventually get hot as it stays on for longer but not sure why the pressure is fluctuating like it is when the fan is revving and also the the f12 fault code etc.

DavidFx

It sounds as if the diverter valve is stuck in the HW position. You are calling for heat in the CH circuit but the water is going to the HW heat exchanger. As the HW is not flowing - no heat is removed so the boiler quickly shuts down when the setpoint temperature is reached. There must be some leakage into the CH circuit as it does eventually warm up.
The F12 error needs to be sorted first.

richardc1983

The rads are getting hot, the boiler is just short cycling and when it cycles off the rads go cool quite quickly and its a while before they get hot again even though the boiler is firing up quite often but not for long enough. The F12 code occurs when you power it off and back on again but last night also noted that it happens when it cycles off sometimes and the fan pulses up and down... then the fault clears by itself.

I am pretty sure the diverter valve was replaced as well a couple of years ago as well.

Seems this post is exactly the same: http://forums.thedigitalfix.com/forums/showthread.php?t=618282