Smell around boiler only when off

We’ve got a Logic Combi boiler and have had issues with the water not properly heating - radiators fine, taps not fully heating up - and we’ve got a good engineering working on it. Used him a lot in the past and he’s always done us right with a good reputation locally.

But, we’ve noticed in the last few days a very faint slightly gassy smell in the area in front of the boiler. Its doesn’t seem to spread into the rest of the room but hangs in the air where it is. Its also not constant but does always sit in the same place. Its a narrow room (2m across) and the smell seems to sit in the corner directly opposite the boiler cupboard and along the windowsill between them.

Spoke to the engineer we’ve been using for advice and if it could’ve been caused by fault we experienced as this is new and we only noticed it after his last visit. He said that the only work that’d been done related to water diverter and heat plate and didn’t touch the flue so didn’t think so. Advised monitoring the problem and making sure our CO detectors were operational and in the right place but otherwise as far as he could think we could continue to use the boiler. He suspected that the smell could be from the vent blowing back in. He is due back later this week as had to wait for a part from supplier.

This hasn’t been an issue in the past and it feels unusual that the smell only seems to gather when the boiler is off. When the heating has fired up or the window its beside is open it dissipates quickly.

Any idea what it could be?

There is a CO detector in the room and its not sounded but I’m going to get a new one in the morning since that one will be six or seven years old now.

What do you reckon? I’m leaning toward getting a second opinion in if I can get someone short notice.

Have looked and sniffed all around the boiler itself and there aren’t any obvious noises or even smells inside the cupboard itself.

Would be prudent for the engineer to check for gas leak.
He may not have worked on gas side, does not mean there cannot be a gas leak or flue issue.
Any smell of gas has to be investigated.

Smell may linger when the door is closed and air not disturbed like overnight. Slight gas leak ( can only be confirmed by use of correct instrument), dissipates when fresh air enters the room as result of movement or and opening/ closing the door and window

Engineer was out today.

Checked the gas pipes feeding the boiler for leaks and it looked ok. Opened up the boiler checked the internal seals and tightened bits up where he could. Again no sign of leaks. Checked the connections around the exhaust flue and couldn’t identify any issues but again tightened up and whatever else just to be safe.

He’s satisfied there’s no natural gas leak but there could still be the possibility that the smell has been fumes from the flue escaping or being blown back.

Got new CO detectors and will continue to monitor.

Reassured that the house isn’t likely to explode but obviously far from ideal if exhaust gas has been filling up in the back of the room I use as a home office.

Still a fume-y odour in the room near the boiler this morning. Since the seals on the boiler were checked yesterday could this be a blocked/partially blocked flue?

Still no CO alarms triggered but its quite a high ceilinged openish plan old mid terrace the corner that smells is probably the only space confined enough for it to concentrate.

Again, when the boiler is fired up there’s no odour, only when its off, so it doesn’t appear to be actively pumping huge volumes of fumes into the house. But with a wife and child in the house I’m anxious of any unusual boiler smells even if the CO alarms haven’t gone off.

In the house there are two alarms that are six or seven years old and two that were bought this week.

Is it possible that the condensate pipe (which may be teed into a drain) is allowing odours into the room, which may smell like gas?

Eg, there may be an air gap installed on the condi pipework.

As mentioned above, condensate pipe might be a good place to inspect.

Where does the condensate pipe lead to? @Postmap

What CO alarm do you have? It should pick up something, even if diluted, if it is indeed fumes/products of combustion. Might be wise to post a photo of the boiler with the condensate trap and pipe arrangement