What smells like gas but probably isn't gas?

Hi everyone, I hope you don't mind my first post being a request for help, but I've been trawling the internet for days and read about every website/forum post there is about gas smells in buildings.
I live in a 140 year-old(ish) granite building that at some point has been converted into four flats. Because it was all one big house before, we do get noise and cooking smells etc travelling between floors.
Anyway, about 4 weeks ago now we noticed a gas-like smell in the communal ground floor hallway, and we also seemed to get 'clouds' of it moving through our (first floor) kitchen and hallway, above where the ground floor smell was. It dispersed quickly when we opened the communal hallway window, though.
A week later the smell was coming more frequently (3 out of 7 days) and our neighbour called the Emergency Gas line. We had a couple of SGN guys come out and test everything. Unfortunately the smell had mostly dispersed by the time they arrived and they detected nothing after a thorough examination.
A week later we called them out again, and more or less the same thing happened - smell had mostly gone by the time they came, and again they tested all along the gas pipes, everyone's boilers, cookers, etc - everything they could think of. They found nothing, and said it couldn't be gas, it's just something else with a similar smell.
Another two weeks on and we're still getting the smell, although not every day. It's almost always there when we come home in the evening, and seems to disperse as we open a window (or when everyone else in the building comes home and fresh air gets in). No-one is usually in the building during the day, and our boilers are not usually on either (yet it appears anyway). Once the smell disperses, it doesn't reappear during the night or early morning (also a time when no fresh air gets into the building).
I imagine that it's a small amount of some sort of gas (not necessarily natural gas since it tested negative) getting in and building up during the day, as it does go away quickly when fresh air circulates. I am concerned about the week over Christmas because no-one in the building will be at home. If whatever it is is left to build up over a week...
Of course we could leave a communal hallway window open over that week when no-one is around, but we are worried about pipes freezing. Also, I just hate not knowing what it is! The emergency gas guys told us not to worry, it will probably go away by itself in time, and it couldn't be a gas pipe leak as that would be constant. But I've read that sewer gas and so on can also be combustible, so I wonder if anyone has any ideas what this could be, so we can get it sorted before everyone goes away?
Thanks very much, and sorry about the long post. I just wanted to make sure I gave as much detail as possible :)
ETA: We have a carbon monoxide detector in the area of our flat where the smell is, and it's happy enough and staying quiet.
--
QuackDuck

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On 07/12/2012 17:03, QuackDuck wrote:

I smelt gas in a public park recently. Stopped and asked a dog walker and he said the gas co had been investigating the smell for a "few years". Every now and then they dig something up but so far haven't found the source.
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we used to get that close to home, The smell came from the old - disused - main pipe which was still in the ground. after heavvy rain a tiny bit of residual gas would come to the surface.
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From KT24

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On 07/12/2012 22:13, charles wrote:

One particular road very near home used to reek of gas on a fairly regular basis. As all I was doing was driving through, I rather assumed that the people who live and work there had reported it. (Anyway, I somehow always forgot by the time I got home!) But it continued for many months - maybe years.
After a few particularly smelly days, the gas people turned up and did some fairly significant digging. Thereafter they seemed to have one part or another of the road dug up for months - and not always the same patch! I'd guess there were at least six separate areas where the mains were replaced.
I would have expected that scale of gas loss to show up on some sort of metering. Maybe that is expecting too much of their meter accuracy and reading skills.
--
Rod

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Interesting. We get a very localised gas-type smell here after rain, and we've never tracked it down. I've entertained the theory that it's being washed out of the soil, and it's good to hear about something similar. But although there is an currently above-round gas pipe in the area concerned, which has been checked twice, it's unlikely that there ever was an underground gas pipe there.
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Mike Barnes

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Maybe a scary thought, but what was there before the houses were built? Any chance that there are some nasty chemicals that are getting lifted to the surface with the rain?
Any one who knows Widnes will know about contaminated soil :-) I always thought that one area smelled of cress, the sort that you have with salad, but it was just the mix of chemicals that had been dumped over the years.
--
Bill

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Open fields (in 1801).

Funnily enough the outside bogs were in about the right place.
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Mike Barnes

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That likely won't fix the problem. If the flue is faulty, it needs fixing. If the flue gas is leaking into your house, this is a very dangerous situation. You could be gassed.
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The smell from the Granox rendering plant was quite entertaining.
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When I was young in and around SW London. you got things like Strong smell of beer, coffee in Putney, many places smelled of gas and one particular place smelled like candles. Smells can be very evocative.
Brian
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On Friday, December 7, 2012 10:13:20 PM UTC, charles wrote: <snip>

I think you'll find the mercaptans they use for the smell can hang around in old pipes and in the soil around where there was a leak, even though the actual gas has long gone. Any disturbance or ground water movement can then bring them to the surface.
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On 07/12/2012 20:23, stuart noble wrote:

Not relevant as (i) CO doesn't smell (that's why you need a detector) and (ii) what you are apparently smelling is actual gas, not "burnt gas" which is where CO comes in.

It's been happening in our street for years. Every few months they come along and dig another hole outside our house and cause chaos for a couple of days, then bugger off again. Very odd.
David
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Many gas appliances release unburned gas into the flue when lighting up. If the appliance is burning the gas well, you won't get enough CO in the flue to be dangerous, so there's no guarantee a CO alarm will go off when exposed to the flue.

It can be carried a long way underground in other service ducts. There was an occasional smell of gas at a road junction near me. Gas board had been along several times making small holes in the road and pavement, but obviously couldn't find the source. One day I walked past, and all the BT inspection covers were off. They were digging up about 150 yds further down the road where the leak was, but it was being carried in BTs ducting and escaping from the manholes around the junction.
Telephone exchanges have special provision to prevent any gas which has got into the ducting from leaching into the building. Sometimes you'll see a tall vent pipe outside which is venting the ducts before entry to the exchange. A couple of ones I know it's just a streetlamp column with nothing on the top of it.
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Andrew Gabriel
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On 10 Dec, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

I remember visiting (Circa 1973) the one that led to that provision, together with a PO employee (who also posts here) a few minutes after the bang (we were queueing in the nearby chinese takeaway on the way back from the pub when the bang occured, he was the first BT employee at the scene). Gas Board person was swearing it wasn't gas (for insurance purposes) whilst his gas detector was screaming. then there was a roar from the ground which was eveloping to a crescendo of a bang. This was repeated with a louder bang, and flame from a manhole where people were digging, resulting on one being hospitalised with severe burns. For some unexplained reason, the assembled crowds retreated by several fathoms after that.
A section ofthe roof of the exchange re-appreared in the attic of a house at least a hundred yards away.
Telephone service took several months to restore with about 7 temporary exchanges located on the same site and at a neighbouring exchange.
Stench pipes started to appear at all exchanges shortly afterwards to vent the cable chamber, which then had updated seals into the building.
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B Thumbs
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QuackDuck wrote:

Natural gas has the smell added to it artificially. It doesn't smell like coal gas at all. Which type of gas does yours smell like? The smell added to natural gas is like rotten eggs. Could there be something rotting away under the floor or behind the fridge or in a void? I had a problem with a smell like that. Eventually it stopped. That must have been when all the flesh had gone. I found the cat's skeleton a few years later. I dimly remember the tapwater in a college hall of residence smelling slightly of hydrogen sulphide. Maybe that was sulphur. If you have sewage pipes behind boxing-in they might be leaking a bit. I had this problem in the basement. The smell was only there now and then. I only found out what was happening when the sewer blocked outside and that caused it to block inside, so the plywood had to come off, and the outside of the stack was damp.
Bill
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On 07/12/2012 20:36, Bill Wright wrote:

You are right about the smell being added, but it isn't like hydrogen sulphide at all. I think it might be methyl mercaptan. Something similar was added to coal gas too (although I agree, coal gas had a sort of "gas-works" smell, for those old enough to remember gas works!)
The normal sewer or drains smell is hydrogen sulphide: it's formed by bacteria in the black slimy sludge which you find in ditches and other places with stagnant water.
My money would be on it either being from a sewer or soil pipe with some sort of fault or a sink / washing machine drain without a functioning trap, or something dead and decaying. Rat under the floorboards? Pigeon in the loft?
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On 07/12/2012 21:04, newshound wrote: <>

Anyone in the house taken to eating asparagus?
Br J Clin Pharmacol. 1989 May; 27(5): 640641. PMCID: PMC1379934
Odorous urine in man after asparagus.
C Richer, N Decker, J Belin, J L Imbs, J L Montastruc, and J F Giudicelli
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1379934 /
--
Rod

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I've certainly had experience of a washing machine occasionally smelling rank during it's cycle. Never did really get to the bottom of it. Is one of the occupant running one during the day?
Andy C
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Too many "eco" washes at 30C. Run a boil wash through it once a month..
Gordon
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Thank you very much everyone for the replies! There are so many so I'm not going to quote them all, but that has given me a lot to investigate.
And yes, it smells like the cooker gas, so it's definitely that chemical that's added to gas (or something that smells similar - I have also noticed that garlic residue under my fingernails after cooking dinner smells like that!).
Dead animal: we did have mice in summer, so it could be that. It did occur to me but I didn't think it could last for four weeks and come and go, but if someone has experienced this with a dead rat then it's a possibility.
Washing machine: I've done a couple of "service washes" with mine recently using soda crystals and 95C water, hasn't made a difference but could be one of the neighbours.
A leak outside: another possibility - BT came and dug up our street a few weeks ago (1.5 weeks after smell had started) and seemed to be using water pumps. Our phone line had gone crackly so I think the heavy rain had flooded something underground. Surely if they had noticed gas they would have smelled it though, and we can't smell it outside (but then, it's not an enclosed space).
One of the neighbours' boilers is under the stairs, actually. The flue outside stinks of gas when it's on, but the gas guys tested the boiler along with everything else and said it was fine. I suppose it could be the flue smell, but I don't see why a) it would come back inside, and b) it would suddenly start now, after being fine for years.
--
QuackDuck


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