Smell of gas after boiler has been off for an hour

Jungle95

Hi,

Apologies if this shouldn’t be posted here. New to the forum. 

We have a Worcester Bosch Greenstar 24i boiler in the house that we moved into at the end of Oct (the boiler would have been new with the house so Dec 2007). 

We get a slight smell of gas in our small utility room where the boiler is housed about half an hour to an hour after we turn the room thermostat down from an all day temp of around 20/21 down to about 16/17 degrees for night time (we have a little one who constantly kicks off the covers and it’s very cold in Inverness at the moment) and the boiler effectively shuts off as it’s not needed until the temp drops which must be well into the night. 

We don’t ever smell anything when the boiler is running and it’s always the same scenario. Boiler been running constantly and only when you reduce the room thermostat by quite a bit (say 4 or 5 degrees), it’s usually about half an hour to an hour where you smell something. I can smell it from the very top right of the casing / front cover. It’s very very faint but I can smell it. I’ve checked there when the machine is running and it’s not there.

We went out for an hour and a half yesterday for a walk so turned the heating down from 21 to 15 and when we came back, the smell was in the utility room again. 

You would think we would also smell it in the morning when we get up but we don’t. It’s really strange. 

We’ve had the boiler checked for leaks by a gas safe plumber who was using a Worcester detector and he couldn’t find anything at all after an hour and a half of checking with the machine off / on / running at full whack / cooling down etc. He had previously done some work to the boiler after we moved in and some parts had needed replaced.

We have a boiler & heating protection package with SSE (it was cheaper to include it as part of the energy package) so we can call for an engineer to come out again for a £$ excess but having a googled it for a long time, I have only ever found one post online with someone having the exact same problems as us word for word. Same boiler too. He had the Gas Safe people out and a further three gas safe engineers, all who couldn’t detect any leak. 

Hopefully someone might have a suggestion. 

Many thanks,

Dougie

EdTheHeaterMan

When that boiler is operating, the inside of the enclosure acts as a plenum for the combustion air. The air for combustion comes thru a duct from outside in most cases. The duct (usually a round plastic pipe) enters the top of the casing. When the burner operates the air for combustion is drawn in thru the duct into the casing and from inside the casing to the burner fan inlet. This causes the internal area of the casing into a slight negative pressure zone. Any leaking of air or gas will be from the room into the cabinet. When the burner is not operating, the pressure inside the cabinet is no longer under negative pressure. Any gas leaking from a loose fitting or gasket or pinhole in a pipe will be able to fill the cabinet interior. If the burner is off long enough the gas will start to leak out of any opening in the cabinet into your utility room.

That is why it happens as you describe. What you need to do about it is remove the cover, find the leak, and repair it. Or have a professional repairman remove the cover, find the leak, and repair it.

Hope this was helpful
Mr. Ed

Jungle95

Hi Mr Ed,

Many thanks for such a quick reply. 

Your explanation / diagnosis was great and easy to understand. It makes total sense. We’ll get someone on the case ASAP. 

Will let you know what the outcome is. 

Thanks again.

Dougie
Inverness

nicholas bonham-carter

Maybe a small obstruction in the exhaust flue is interfering with the combustion, causing only partial burning of the gas. A combustion test might confirm or deny that.
Good luck. What I would like to know is- what news of Nessie, or is she hibernating in the winter time?—NBC

Jungle95

Had the gas safe engineer round today. Sure enough a very small leak was found when the machine was idle but not until about 15 mins after it was off. No leaks at all when the machine was running. Fully tested. Leak was from unused gas in a small chamber in behind the fan. His meter only peaked at 100 parts per million.l which was a tiny reading He said the amount of gas left in there after the machine was off was so small that it wasn’t an immediate danger but would need repaired. This would have also been why we couldn’t smell anything in the morning as what little gas there was left would have likely went up and out the extractor fan grill just above the boiler outside. When the machine runs as Mr Ed said, any of that gas is sucked in and used when running. Looking at getting someone out to repair it ASAP and hopefully give it a service too. 

Nessie likely too cold to show herself. Absolutely freezing. 

HVACNUT

Which part was he referring to?

Jungle95

Hi. 

Just had the SSE subcontracted engineer round today (five and a half hours later). He knew what the fault was immediately and was very good.

The diaphragm was knackered which was his initial diagnosis over the phone (have a photo of it) but he also replaced the gas valve at the entrance to the unit as he detected / discovered a leak there too after doing final checks to the machine.

Turns out it has been leaking with the machine on and off. When the machine was on anything inside the unit was sucked away so not detected by the gas safe guys. He was amazed it wasn’t shut off by the gas safe guy.

To the best of my knowledge he replaced all the seals (7?) in the top left unit, replaced the diaphragm, replaced the main gas valve and some other pipe work just in case and he did all sorts of other tests and checks to ensure the machine was 100% as he didn’t want to have to come back to do any other work when he had the whole thing apart anyway. That’s why it took him so long. 

So ... fingers crossed all is well. It seems to be running fine now (quieter too) so will keep a close eye on it.

EdTheHeaterMan

Thanks for updating us. "We like it when a plan comes together"

bluto

I stumbled across this thread while searching for precisely this issue. I have a Worcester Bosch Greenstar 4000 which emits a smell of gas from the top of the casing about 30-60 minutes after the boiler has finished heating cycle. The smell disperses soon after. It never happens while the boiler is running.

Three engineers have been round to look (1 Cadent and 2 WB), and none could find any problem. They changed the gas valve just in case, but it hasn't improved matters. The smell is intermittent and often very faint. It may be a coincidence, but the times it has been most noticeable have usually been during cold weather (2 degrees or less outside).

The OP mentioned a diaphragm as being the main culprit. I sent him a PM a couple of days ago, but suspect he is no longer monitoring this thread. Where would such a diaphragm be located? (I am only aware of one in the expansion vessel, but this doesn't seem relevant for a gas problem.)