Prima F40, Alpha 2, Boiler Overheat (Intermittent Fault)

29 May 2013
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United Kingdom
After some quick advice... Sorry for the lengthy post.

We have had issues with our Prima F40 boiler overheating over the last couple of months... Just as we are about to move house (exchange is imminent - handy)!

Last year we had a similar issue and
- Replaced the shut of valves to the F&E tank (it was overfilling)
- Fitted a new pump (it was old and struggling, noisy)
- Drained and refilled with fernox cleaner, re drained and refilled with inhibitor

This was done through British Gas Homecare which we no longer have as it is a load of rubbish - they can't get parts which are easily available through plumbers merchants). There was a lot of air in the system which required a lot of bleeding etc. over the following week or two.

All seemed fine over the winter. Once in a blue moon the boiler has been tripping on the overheat for the last few months (maybe since the beginning of March once every two weeks or so). Last night this re-occurred but the pump was running after the boiler had tripped (sounded faster than normal) which we had not noticed before.

The only work we had done before this was the fitting of a new flue as the terminal guard had corroded to make sure it was compliant (yes all done by a gas safe guy). The airing cupboard, pump, valves etc were not touched as part of this process.

Current Settings:
Boiler: Set to "2"
Thermostat: set to about 18 degrees at the moment
Programmer: Heating and water set to come on at 6:00 and off at 8:30, on at 5:00 and off at 9:30
Pump: Set to "1" of the the 3 fixed speed settings (there are options for variable speed or constant flow rate and 1, 2, 3 fixed speed)

Observations so far:
- 3 port valve seems to be working happily (switching between HW, CH and both as called for).
- After pushing in the overheat switch on the boiler we cannot recreate the fault, it runs fine!
- Pump is running fine (although I wonder if setting 1 is too slow, it is more noisy on setting 2).
- I switched between speeds last night before deciding to leave it as it is so as not to cause more issues today while not in the house, it worked on settings 1 and 2 but 2 was a bit "wooshy"
- feed and expansion tank was fine, filled enough but not overfilled.
- I bled the valve in the airing cupboard last night

General observations:
- Sometimes the boiler will take a few clicks and fan whirrs before it fires on, most of the time it fires first time.
- My wife informs me that the boiler sounded like it was struggling to fire this morning (I was fast asleep)

I intend to bleed the rads this evening and see if that helps

If this does not resolve things what is my next step? It's very hard to pinpoint what the issue is!
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Just a thought we also recently noticed a leak from the garden stop cock (under kitchen sink) last week which i fixed at the weekend by repacking the gland nut.

Funny how these things happen at the most inappropriate times!


Not sure how long this had been going on (it was a very slow drip and we noticed when water began to drip at the front of the cupboard about a week ago). We assumed it was a spillage until it mysteriously re-appeared a couple of times and on further investigation the stop cock was dripping... Could have possibly taken a couple of weeks for the water to find it's way to the front of the cupboard.

Could this have introduced some air / sludge into the system?
Sounds, to me, your cold feed/open vent connections are at best restricted or even wrongly configured. Reasoning for this is the noise and continual need for bleeding. Run a magnet around the pipes in the airing cupboard, if it sticks you've got magnetite.

Will bleed first then see what the magnet says.. It is at the moment only a minor annoyance.

When we refilled the system last year the header tank was murky (at best - full of crud would be more accurate) so if we have a build up of magnetite then I guess the restricted flow would result in not enough flow to the boiler and therefore overheat tripping.

I had a quick look last night and there was a little brown film on top of the F&E tank water but otherwise looked relatively clear compared to last year.

Should the cleaning fluid and inhibitor introduced then not have prevented this happening?

If we do have magnetite is a power flush the best way to go given that it seems to have recurred in just a year?

Any idea why this is so intermittent?

I don't particularly want to drain the system again or power flush at this point as we may be moving out in the next 2-3 weeks. I just want to make sure it's all functioning ok (I guess I may not be able to without though)!
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Sounds like typical restricted cold feed to me. Turning the pump up to speed 2 or 3 will mask the problem of overheating but actually make the blockage worse in the long run as it introduces more air into the system by pumping over into the header tank. Cutting it out and cleaning is the only solution. Powerflush would include that but the rest may not be necessary if the radiators all work fine and no cold spots etc.
So a powerflush would likely help, draining, cleaning and refilling with inhibitor again might help, but installing new pipes would be an option too? Guess I'd better get someone to take a look...

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