Hi there
My Vokera 24e Maxin combi developed this problem a couple of weeks ago: when HW is continuously demanded, the burner switches itself on/ off, every 30 seconds or so. It means having to step in and out the shower, waiting to catch the next stream of warm water, etc.
It's an old boiler that came with the house - we've been here 3 years. I haven't tested to see if the same problem occurs when running the CH.
Any ideas?
thanks
Daljit
Can be caused by a furred up plate exchanger (in which case it won't happen when running the CH). The scale in the plate exchanger reduces the heat it can transfer to the HW, so the primary circuit gets hotter than expected and the boiler cuts out.
Hi
I've just tested if the burner cuts out when demand is from the CH, and it does. Ignites for about a minute and then off for about 30 seconds.
thanks
Daljit
Different problem then. Possibly lack of flow through boiler. Maybe a blockage or the pump is on it's way out. Guessing now though. Is the flow pipe very hot just as it cuts out? (Be careful not to burn yourself if it is.)
Hi
Apologies for taking a week to respond, but daytime job's been murder.
I've checked the pump by removing the vent(?) cap, and it does seem to spin freely. However, i note that the flow pipe does heat up almost too hot to touch, just before the burner cuts out - but i thought this normal? It does then rapidly reduce in temperature, which again suggests pump working. Question i guess is, is it working correctly?
I found this page on pumps:
Other info:
- the water in the system looks clean
- the pump's a Grundfos UPS 15-50
- boiler must be about 5 - 8 years old
Taking a different tack, would it be worthwhile looking at the ignition box? Not quite sure what that is exactly, but a friend mentioned it as a possibility.
thanks
Daljit
The next step would involve measuring the gas pressure to the burner to see if it is being modulated. If the boiler is still cutting out on a reasonable (5-10 litre/min) flow rate or the gas pressure is always at maximum then other actions are needed.
I'd still go for the secondary heat exchanger being scaled up, as this is a stock fault on all combis - but not in a soft water area.
Although I would have expected the burner to stay on longer for the start of CH mode. This is where experience of just feeling around and measuring a few things to get an accurate diagnosis is difficult to put into text.
In a healthy system I would expect the initial burn in CH mode to be around 5-10 minutes depending on the size of the system. At the end of that period I would expect all the radiators to be hot. If the system has more than 5 or more than 10 rads the longer times are expected needed.
This indeed is the case - the burner's lit longer when demand is for CH - about 30 - 45 secs longer - altho doesn't stay on anywhere near long enough for eight radiators to get warm. And yes, it's soft (Scottish) water.
I need to do a bit more background reading before being able to report this (reckon it'll be next Sunday).
thanks for advice
Daljit
Ok - checked burner pressures and they matched those given in service manual - approx 10mbar and 1.5mbar for max/min DHW demand. I did need to remove the modulator coil to establish output at the minimum setting, but this isn't unusual (according to the manual). I noticed that the boiler didn''t cut-out at the min setting (which of course makes sense).
I also checked the soft light ignition pressure, and this too was fine, cutting out after 10 secs like the manual says its should.
Any other suggestions?
thanks
Daljit
What flow rate are you getting for the DHW? Pump problems (OK I accept they are more likely now we have eliminated the common stuff) temperature sensors. PCB problems.
Flow rate is approx 7 l/min when DHW control turned to max. I haven't noticed the flow in the
Is there anyway i can check pump is working optimally?
Temperature sensor - i've located one - the thermistor (part no 8484)
- but how do i check it's working properly?
PCB - I'm not that good with electrics, so would be lost if the problem's here.
thanks for your perseverance!
Daljit
OK with that flow rate the boiler should modulate and give a steady stream of HW at a steady temperature.
We have the surprising behaviour of the CH cutting out as well as the DHW.
Here are some of the possibilities in no particular order: 1) There is inadequate flow in the primary circuit; so the temperature rise in the primary heat exchanger is too great and the boiler cuts out.
To diagnose you need to work out what is getting hot and what is not. In CH mode make sure at least one or two TRVs are full on. Correct operation: The flow pipe is a little warm and continues to warm up steadily over 5 -10 minutes to untouchable. If the pipe is cold and then very hot there is poor primary flow. Either pump is bad or the heat exchanger is blocked or some other fault with the same effect.
2) The Sensor is over enthusiastic,T so the PCB thinks the boiler is over heating and shuts the burner down. raden/geoff can say better than me - but 10k Ohm cold andAlas, the game nowadays requires electrical, mechanical and IT skills.
IT'S FIXED! Magic! You'll never believe what the problem was - am still amazed myself.
This morning i phoned the Vokera tech support line - the one for CORGIs - and confidently described my problem, the checks i had undertaken, and the results. Told him i was completely stumped. He asked if the fan was working, and i said it was. Then he asked if i had checked the flue something-or-other. He said the name a couple of times but i still didn't catch-it, so i asked him what he meant. He said if i push the flue (at the top of the boiler) to one side and peer in, i should be able to see a 10mm pipe sticking up. He told me to scrape away anything that might be blocking this (think he said it sucks in air). Anyway, that's exactly what i done and found there was a tiny bit of white material on the lip (less than 4mm diameter). Scraped it away, assembled the flue, and tried it out - and bingo - worked first time!
Great stuff - have now just run a bath to celebrate. Wife thinks i'm a god too.
Anyway, cheers for all your advice. I reckon if i got a man in, he would have done all the same checks you suggested and then phoned Vokera for the solution - it's happened to me before (with a Vaillant). Anyway, i've saved some cash so name your charity - will send em a chq for =A325 - email your address to rev_funk@hotmail with a co. and uk. and i'll send you the receipt.
cheers
Daljit
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.