Vokera Boiler, Air in system? Central Heating Help?
FASTdan - 27/4/12 at 08:18 AM
Hi all,
Just wondering if we have any experienced central heating engineers on here? This might be a long one so apologies in advance, I will try to keep to
the main points:
Bought a new house 6 months back, 3 storey terrace with C/H installed approx 2008 by a British gas sub contractor. We've only just moved in
though as spent the last 6 months rennovating (old lady house). She had the boiler running just the C/H as she had an aga running the hot water.
So what I've done - under the guidance of a very helpful and experienced plumber - is to retain the original boiler (as only a few yr old) and
install a Y-plan system for running C/H and hot water. Hot water wise I have installed a plumb center 'centerstore' cylinder in the cellar
and made the system unvented (this was to free up space in the bathroom by getting rid of the old cylinder). Likewise the C/H is now unvented so no
header tanks and I have split the system into upstairs/ground circuits. Both systems have the appropriate expansions in place.
Unfortunately I am having issues getting the boiler to run. It will cut out on fault codes 28 and 74 numerous times following start-up, each time
having to be manually reset by me. Codes:
28 - something along the lines of return has become hotter than flow
74 - No flow
(they are my interpretations as I dont have the manual to hand). Basically both are indicative of flow issues - note I have had 3 pumps on it and
current one is brand new so unlikely a pump issue.
I've spent 3 weeks now f*cking about with it including getting an engineer out. It appears to be air in the system. The first week I spent hours
bleeding rads, then once the engineer had been in we came up with a self bleed using the nipple on the top of the boiler - basically cracking that and
letting water out to look at you'd think it was just water. But when a hose was connected and dropped into a bucket it bubbled away merrily -
proving there was a large presence of air. So for the second week each evening I would run the boiler and leave this bleeding (topping up the filler
loop as and when). For a full week it nearly constantly bubbled, but now in week 3 the bubbles are becoming infrequent. The boiler appears a
'bit' more consistent and once the initial 5 or so faults are reset it will tend to run ok-ish. It seems like when cold it cuts out lots,
once run for a while becomes more stable. It seems happier with more circuits open for some reason.
I still think its probably air - but its driving me mad. how long do you go on bleeding it?! Is this common? I cant believe it can be so hard to get a
system up and running again following a complete drain. Its not like huge airlocks though, its like the entire system is full of tiny bubbles. You can
stand at a rad and bleed it for 15 minutes and it'll still be spitting at you.
Just to note, the system does not loose pressure - therefore I think reasonable to assume no air is being introduced to the system.
Edit to say, boiler is a vokera mynute 20VHE. The reason I dont just get the guy who supervised my install is because he has a downer on vokera and
just blames the boiler. plus he's given me more than enough help for free so i dont want to burden him further. The other engineer I had out was
helpful, but didnt really have any revolutionary suggestions other than keep persevering and if it doesnt work we'll replace some sensors - he
removed both flow and return temp sensors from the pipework at one point and the boiler still cut. Perhaps this points to a PCB issue, as effectively
those sensors must then have been reading identical air temp?
[Edited on 27/4/12 by FASTdan]
[Edited on 27/4/12 by FASTdan]
MkIndy7 - 27/4/12 at 08:51 AM
I'm guessing something isint quite right on the whole 3 valve setup on the system and the boiler can run when the valves are closed.
What gives the boiler the signal to run?
Can't rememeber all the S plan Y plan setups off the top of my head as I do commercial stuff but basically I'd say:
If there 2 port valves, a timer should give out a feed to the thermostat,
if the thermostat (room or cylinder)calls for heat this then opens the 2port valve
When the 2 port valve is full open the end switch then powers up the pump and the boiler.
There's no way a heating system should need that much venting!
FASTdan - 27/4/12 at 09:05 AM
quote:
Originally posted by MkIndy7
I'm guessing something isint quite right on the whole 3 valve setup on the system and the boiler can run when the valves are closed.
What gives the boiler the signal to run?
Can't rememeber all the S plan Y plan setups off the top of my head as I do commercial stuff but basically I'd say:
If there 2 port valves, a timer should give out a feed to the thermostat,
if the thermostat (room or cylinder)calls for heat this then opens the 2port valve
When the 2 port valve is full open the end switch then powers up the pump and the boiler.
There's no way a heating system should need that much venting!
Hi thanks for your reply. They are 2 port valves, one for each circuit. Room stats and cylinder stat are the driving devices - however I am currently
running the circuits/controllers by switching from their off position to 'constant/overide' or whatever its called ie not running any
programs. So basically permanently calling for heat (with all stats set nice and high).
I cant believe a system can need so much bleeding either?
I also have a bypass circuit from the flow (pre valves) to return with the relevant bypass valve in place.
[Edited on 27/4/12 by FASTdan]
MkIndy7 - 27/4/12 at 09:25 AM
With everything on an set full however their wired shouldn't really matter then..
Has the boiler got an internal pump and there's external ones aswell?
Their definately all flowing in the correct direction and not fighting each other, usually when thinga get slightly over a very basic setup they tend
to run a "low loss header"
FASTdan - 27/4/12 at 09:40 AM
No internal pump on this boiler - basically I havent disrupted the pump set up at all. There was originally an external pump on the flow at the side
of the boiler, all my changes have been after the pump (apart from swapping the original pump out for 2 others, inc the latest brand new one). the
flow basically goes:
boiler > pump > bypass tee > christmas tree (3 branches each with motorised valve)
pump definitely pumping in the right direction (arrow on side) and I do get heat after each motor valve. the system essentially works fine - I can get
a full tank of hot water, and all rads hot. But thats only after I've reset the boiler numerous times due to fault codes. Once warmed up it seems
much more consistent. In the first 5 mins it'll cut numerous times, then will likely run fine for a couple of hours, but will still occasionally
cut.
Would you expect a system to run fine even with some air in it? I have yet to introduce inhibitor but I cant see this making much difference.
Im wondering on the PCB more now, surely if the engineer disconnected both temp senders from the pipework then they must have sensed the same air temp
yet it still cut. To me this says that the PCB is faulting regardless of inputs (there are no other sensors, flow etc on this model boiler).
Its one of those annoying situations. I can pay vokera circa £250 for a one-off repair covering parts to £400. But I do not want them coming out only
to say 'there's air in the system'. But likewise if I keep calling independents and then I have to replace the PCB (£138 plus fitting)
Im already nearly up to the vokera price.
adam1985 - 27/4/12 at 10:23 AM
Whats the temperature like on the flow pipe coming out of the boiler if its getting very hot straight away its a flow problem
If its just warming as expected it could be the themister they work on resistance call vokera they will tell you how to test it
It should be certain ohms at a corresponding temp.
MkIndy7 - 27/4/12 at 10:26 AM
Certainly sounds a strange one..
Pulling the wires off the boiler thermistors, the boiler might recognise it as a fault and revert to using the other sencor, or even if the whole
thermistor is detatched it'll still read the wrong air temperature if it's faulty.
Replacing them could be a good cheep place to start.
Does it actually feel like the return is too hot when it faults?
As it could be flying round the bypass rather than around the system and coming back red hot as that's easier than flowing all the way around the
system when it's cold but would tend to equal out when the whole system is warm and circulating properly
FASTdan - 27/4/12 at 10:47 AM
quote:
Originally posted by adam1985
Whats the temperature like on the flow pipe coming out of the boiler if its getting very hot straight away its a flow problem
If its just warming as expected it could be the themister they work on resistance call vokera they will tell you how to test it
It should be certain ohms at a corresponding temp.
Initially I had that - where the pipe would get ridiculously hot then the boiler cut, but that was weeks ago before lots of bleeding etc. Now it warms
as you'd expect. The engineer I had out tested the ohms but only compared the two sensors (which he said were very close to one another), he
didnt have data for what they should be giving. I think he also swapped them round. I will see if I can get the vokera values.
quote:
Originally posted by MkIndy7
Certainly sounds a strange one..
Pulling the wires off the boiler thermistors, the boiler might recognise it as a fault and revert to using the other sencor, or even if the whole
thermistor is detatched it'll still read the wrong air temperature if it's faulty.
Replacing them could be a good cheep place to start.
Does it actually feel like the return is too hot when it faults?
As it could be flying round the bypass rather than around the system and coming back red hot as that's easier than flowing all the way around the
system when it's cold but would tend to equal out when the whole system is warm and circulating properly
I see what you are saying re. thermistors, thats obvious thinking about it, if its a dodgy sensor its always going to read dodgy!
Good point regarding the bypass - I have been feeling the return pipe but only ABOVE/BEFORE the bypass line joins it - which feels about right
relative to the flow. I will try feeling after the bypass tonight. I had noticed that my bypass pipe gets warm quite quickly and wondered why - I
mentioned this to the engineer but he put it down to heat soak which surprised me as I'd expect that to take a while. I've not touched the
bypass valve though (so its at its factory setting - i dont know if they are adjustable?) for fear of cocking something up. Your theory of equalling
out when hot makes sense and rings true with the way it seems to be cutting out.
bob tatt - 27/4/12 at 11:19 AM
I'd be looking at the bypass my Self. If when it initially fires the bypass is fully open the boiler will think there is a problem as your not
getting the correct temp differential between flow And return so it thinks there's a flow problem and goes to fault.
The bypass should be just open it will be adjustable, Air shouldnt be a big problem on a sealed system did you vent from the lowest to the highest
rad?
Hope this helps
Rob
FASTdan - 27/4/12 at 12:27 PM
quote:
Originally posted by bob tatt
I'd be looking at the bypass my Self. If when it initially fires the bypass is fully open the boiler will think there is a problem as your not
getting the correct temp differential between flow And return so it thinks there's a flow problem and goes to fault.
The bypass should be just open it will be adjustable, Air shouldnt be a big problem on a sealed system did you vent from the lowest to the highest
rad?
Hope this helps
Rob
Ok, so when you say 'just open' which way is that? clockwise to shut? I have had a quick look at the thing but couldnt see much in the way
of markings on it.
Regarding venting, I didnt do any particular order unfortunately - is this important?
bob tatt - 27/4/12 at 02:19 PM
Clockwise to close then back it off half a turn this should be enough
As for venting always start at the lowest point working up by floor , air rises so it allways goes up meaning you get mOre out first time.
FASTdan - 29/4/12 at 08:10 AM
Ok so you wont believe what the problem was.........I certainly cant believe it!
THE F*CKING BOILER WAS ACTUALLY PLUMBED IN BACKWARDS!!!!!!
LOL no bloody wonder it wouldnt run and thought the return was hotter than the outlet, it would be!! Thats as installed by the original british gas
subcontractor, god knows how or if it ever ran right before. I can only assume that me adding lots to the system including the bypass has just
compounded the issue. Although I am told that the old lady lived in one room with the gas fire and of course her aga ran the hot water so perhaps she
never bothered with the C/H anyway.
As soon as we spotted that and swapped them it ran absolutely perfectly, quieter and consistently. 
To say I'm happy is an
understatement!
Its always the blatantly obvious that you miss - its only as my plumber mate was looking through the engineers manual I downloaded that he happened to
spot it.
Thanks to everyone who took time to reply though 
bob tatt - 29/4/12 at 08:34 AM
Good result indeed, and your right if all else fails start at the basics as work from there.
You can now get onto what ever the next niggle in the house is, hope it's not electrical ad I know sod all about electrics unless it's
boiler or heating related
Rob
David Jenkins - 29/4/12 at 08:38 AM
I don't know if there any British Gas fitters or sub-contractors on this forum...
...but, after previous experiences, I wouldn't trust any of them with any gas-powered kit in my house.
I've gone to a well-respected local company to get work done in the past few years.
Volvorsport - 29/4/12 at 09:07 AM
as above , british gas sub contractors get paid nowt ......
my old man has replaced lots of PCBs on vokera boilers .
FASTdan - 29/4/12 at 10:38 AM
quote:
Originally posted by Volvorsport
as above , british gas sub contractors get paid nowt ......
my old man has replaced lots of PCBs on vokera boilers .
Yes thats what I've been told - BG give them a shoestring budget to install such systems, hence the bodges.
Aside from being plumbed in backwards this one is also installed illegally - the flu was pointing downhill (which has now been corrected) and it is
too close to a return wall outside. Also, the condensate was plumbed to a pump (because its in a cellar) but the pump was not wired so did not run!
Consequently the pump would fill up and then spill out onto the cellar floor.
BenB - 29/4/12 at 11:05 AM
quote:
Originally posted by FASTdan
quote:
Originally posted by Volvorsport
as above , british gas sub contractors get paid nowt ......
my old man has replaced lots of PCBs on vokera boilers .
Yes thats what I've been told - BG give them a shoestring budget to install such systems, hence the bodges.
Aside from being plumbed in backwards this one is also installed illegally - the flu was pointing downhill (which has now been corrected) and it is
too close to a return wall outside. Also, the condensate was plumbed to a pump (because its in a cellar) but the pump was not wired so did not run!
Consequently the pump would fill up and then spill out onto the cellar floor.
Well at least if its a concrete floor you'll know it's been nicely acid etched for painting
adam1985 - 29/4/12 at 11:26 AM
Ive been called to a leak from a system installed by bg turned out there was no solder in the fitting at all and had held with just the flux for a
number of years
Also makes me laugh when i see a advert for a one man band with
"british gas trained" or " 20 years british gas experiance"
So what.
Sorry mini rant over