Boiler Fan Problem (PING Geoff?)

[Roger Mills]

The fan on my ~20-year-old Baxi Solo WM70/4PF seems to be getting a bit hot!

We noticed a smell like food burnt onto a hotplate (the boiler's in the kitchen) and eventually tracked it down to the boiler. The fan in question was reconditioned by Geoff a couple (maybe 3 ) years ago. The fan spins freely, and works ok - and the boiler seems to work ok. The smell seems to be coming from the fan windings.

Also, the fan casing seems a bit discoloured (a darkish bluey green colour instead of bright galvanised) as though *that* has got hotter than usual.

I've got a spare fan (used, but in working order) which I could fit - but if the root of the problem is elsewhere, that could suffer the same fate.

Anyone got any ideas as to what may make the fan body overheat? Am I right in thinking that it's designed to blow hot (combustion) air *out* rather than cold air *in*?

[Andy Champ]

While we wait for Geoff - my glow-worm sucks cold in, and pressurises the casing. The only significant outlet from the casing is through the burners, then the heat exchange, then the exhaust. This design has the sensible benefit that any casing leak lets cold fresh air into the room, and that the burners won't run unless the casing is on and it is up to pressure.

It's not much pressure BTW.

Andy

[Heliotrope Smith]

my glow-worm sucks cold in, and pressurises

SENSIBLE BENEFIT??

Positive case pressure boilers are the most potentionly dangerous fan flue boilers there are.

The slightest leak in the casing or the casing seals will allow the products of combustion into room and and due to the poor combustion that is likely to occur, high levels of carbon monoxide (CO) could be produced Fresh cold air will NOT be blown into the room.

These types of boiler can be death traps if not properly maintained.

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[geoff]

In message , Roger Mills writes

Is the plastic cooling rotor on the end of the shaft still intact?

I presume it's plated, not painted and the bluish hue has appeared since it was installed - the boiler is running too hot

Yes - get someone in to sort it out, the boiler is overheating

or ring us tomorrow 01923 229224

[A.Lee]

While you are around. My mate ruined a PCB on a new boiler on Thursday - Potterton Gold H, found a leak on the pipes, had to drain down, found the drain point is above the PCB, then the hosepipe came off, and went over the PCB, only a small bit, but enough to fry it when turned on (he didnt think any water had gone on it). There were balck tracking marks on the back of the PCB.

Anyway, if such a thing happened in the future, could you repair it? And how fast?

Thanks Alan.

[Roger Mills]

Yes, it's either zinc plated or galvanised - somesuch. Yes, the hue has appeared fairly recently.

Yes, I guess it's overheating in one sense or another. How many modes of overheating are there? I checked the gas consumption rate in case something had happened to the pressure setting on the gas valve, but that's about right - so the heat input isn't excessive unless the calorific value has dramatically increased, which is pretty unlikely.

The boiler stat is cutting in and out ok to regulate the water output temperature correctly.

BUT, the exhaust coming out of the flue seems much hotter than usual. From memory, it has always been 'warm' but now, I can't hold my hand any closer than 500mm from the outlet.

From the installation manual, it is apparent that there are various baffles inside - presumably to direct the flame to the right places. If one of those had an unscheduled hole in it, could that explain the symptoms?

[cynic]

Have you checked the heat exchanger is clear and the spread of hot gas flow is distributed over the whole area? I havent picked up tools for some time now but it woulkd be a good start to ensure the airways/ burner/heat exchanger/flue exit is clear then if so check the gas rate.

[Roger Mills]

To add a bit more to my previous post . . .

Here's a picture of the discoloured fan and the spare one side by side. No guesses as to which is which!

here's a picture looking down through the hole with the fan removed.

's nothing untoward when looking down through the fan inlet hole, but maybe I'd need to dismantle a lot more to see any problems with the baffles?

[Roger Mills]

No, that's probably the next thing to check - I shall have to bite the bullet and take the front off the combustion chamber. If I do that, I can remove any obvious crud, but how can I check that the flame is evenly distributed? [I obviously can't fire it up when it's in bits!]

As mentioned earlier, I've already checked the gas consumption rate, and it's in the right ballpark - about midway between the max and min settings for the boiler.

[Andrew Gabriel]

Have you measured the flue gas temperature? Checking back over my service readings for Potterton Suprima and Profile conventional boilers, flue gas temperature readings are 145-160C, although probably neither were on their highest setting, but that's only going to add at most another 20C. The highest for anything I've measured is a multipoint water heater at 208C.

(Obviously, condensing boilers are very much lower.)

[Roger Mills]

No. I don't have any suitable measuring equipment, and I don't know what it *should* be.

As noted earlier, it *seems*[1] to be a lot hotter than I previously remember it being - and the fan body, through which the exhaust gases pass, has recently become discoloured, and there's a hot 'smell' when it's running.

What could cause the flue gas temperature to rise? [There's quite a powerful flow]

[1] Calibrated hand held in exhaust flow!

[Andrew Gabriel]

Off the top of my head...

Burner pressure too high, so flame (combustion) happening further through (or even past) the heat exchanger. However, that would also give a high gas rate, which you already checked. Heat exchanger partially blocked to flue gasses might also have the same effect of delaying full combustion, without using a higher gas rate. *

Heat exchanger has become thermally insulating, either on the inside or outside, e.g. a layer of soot (or other debris), or a layer of hard water scale (usually makes it noisy, like a powerful kettle).

Heat exchanger isn't full of water, e.g. an air pocket. (Don't know if that's possible with your boiler model).

*Is there a sight glass so you can see the flame colour? Even if there is, it usually only gives direct sight of the bottoms of the flames, but they get yellow/sooty at the tops first. However, you can sometimes tell that light is being generated further up the heat exchanger by seeing it reflected off the burners or something else visible at the bottom which has line-of-sight up the heat exchanger.

[Roger Mills]

Indeed.

That's a possibility. I shall have to dismantle it to check that.

Well, there *could* be soot on it. However, it's only a couple of years since it was serviced - having been previously untouched for at least 10 years - and the bloke who did it then said that it was as clean as a whistle.

It's a vented system, but it's had very few water changes during the boiler's 22 year life, and it was all washed out and re-inhibited a couple of years ago, so there *shouldn't* be too much scale.

Unlikely, I would have thought. The boiler is on the ground floor, with pumped circulation up to the airing cupboard - and beyond. There's plenty of water in the F&E tank.

Yes, there is a sight glass, and the part of the flame which I can see looks blue.

[geoff]

I'm always around

With a few exceptions, we only repair pcbs that we have a test rig for

Do you mean the Potterton Gold Promax 28HE ? Yes, we have a test set for that, but the problem with "water" damage is that the water is a highly corrosive ionic liquid which eats away at any component it comes in contact with. Sometimes these are not economically repairable

If its a board we can do, then we aim at a same day / in one day, out the next turnaround

[Andy Champ]

Hmm I'm trying to work out how this could be. And failing.

The air next to the case is fresh air. It entirely surrounds the burner, except for the flue. Any leak would allow that fresh air into the room.

How could that let combustion products into the room?

Andy

[Roger Mills]

I think he's saying that if you pump fresh air into the boiler, the combustion chamber will be at above room pressure. If there is a leak, the products of combustion could leak into the room.

Whereas, if the fan is pumping the exhaust *out*, the combustion chamber is *below* room pressure . . .

[Heliotrope Smith]

Thanks. Yes in a nut shell that's about it.

Do a Google on positive pressure boilers. Plenty of information on how dangerousthese can be.

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[gremlin_95]

[Andy Champ]

OK, I'm completely confused by this. There's widespread industry concern over the safety of these things - and yet I can't see how they (or mine at least) is a problem.

There is a large case which contains air pressurised by a fan (you know whose!) the fan sucking air on from outside through the balanced flue.

Entirely enclosed within this case, and touching the case nowhere except where the flue goes through, is a burner and heat exchanger.

The exhaust side of the flue is open to the outside.

Normal airflow is in, through the fan, into the case, up past the burner, through the heat exchanger, and out of the flue.

A leak in the case would result in fresh air from the main body of the casing going into the room.

A hole in the heat exchanger would allow air from the case to bypass the burner. Not good for efficiency, but it would have to be a damn big hole to make much difference.

The exhaust part of the flu is in the middle of the inlet part until it's outside the house. Any leak here would result in exhaust gasses mixing into the inlet air. That would undoubtedly be a Bad Thing, as the boiler would probably generate CO and blow it all over next door's garden. But there we have the only way I see a risk - case leaks AND flue leaks, flue leak to the extent that the boiler is starved of oxygen and generating CO. So TWO faults.

What am I missing?

Andy

[Roger Mills]

If the leak is above burner level, it won't be fresh air that comes out

- but the products of combustion.

The obvious!