You should smell nothing. I would check both the exhaust duct and that the space where the heater is located is receiving sufficient make-up air. Both are important.

Thanks.

I's in a "closet" high and low wall grills to the living space, so I think it's fine in that regard.

I also learned that a chinmey liner was installed about 10 yrs ago (this is in an 80 yr old townhouse.)

I think I will tape all joints and elbows (aluminum tape), along with making sure there's a horizontal rise as it heads toward the flu. Sound about right? Anything else? (Oh, maybe also the tightness where it connects to the flu - might need something to snug it up.)

Thanks again for the reply.

Sorry about the spelling!

Two things 1) Vacuum the wall grills and make SURE they're not dust-clogged or obstructed in any way whatsoever 2) The chimney liner from 10 years ago doesn't answer the question. You must VERIFY that the duct is IN FACT clear all the way up and out. If you've got an old bird's nest or something up there partially obstructing it, that's an issue. Spider webs and dust can do it, too.

By all means make sure the joints are sealed. But don't be casual about examining the make-up venting and exhaust ducting.

I'm concerned about what you've said. You described a particular problem and now you seem to be assuming these first-look items aren't contributing. Both are the first things to consider. If they're not right, it won't matter what else you do.

Thanks for the concern. Message received, thanks! I will be doing all of the above.

Not sure how to verify a clear duct all the way up. Inspect from below or above? Should a flue be snaked from the roof like a plumbing vent stack? I'll try to take a look with a flashlight, as well. Or is a small hand-mirror inserted on a stick to see daylight above the way to go?

If you've got a straight shot and can see full-diameter daylight up there, that's great. Notice the "full-diameter" comment. Snakes are good. Probably the same diameter as your dryer vent. They make flexi-brushes that can be fed from the bottom, too, and they even follow the elbows.

That's true -- and very good advice. However, carbon monoxide has no odor. The original issue remains. Suspect their exam for one would also cover the other. And it would probably be free as well as informed and professional. I suspect if you tell them about the odor, they'd come right away.

Call the gas supplier.

They usually have someone on call 24/7 for reports of gas odor.

While it is true CO is orderless, it is very rare to have incomplete combustion only produce CO and no products without a smell.

When I worked for the fire department every CO call we went on was apparent as soon as you entered the house.

Going form clean outside air into a place with trapped products of combustion was obvious.

The hazard comes form the fact that when we are exposed to low levels of small after a while we can no longer sense it.
The anesthetizing affect of the CO only further feeds the problem.

did the people who lived down there ever complain of headaches?

Best way to check draw on the b-vent is to place a candle on top of the hw heater, next to the flue hat, and see if the flame draws towards the flue hat when the heater's heating. IF so, it appears to be drawing ok, but unburned gas is your problem.

I had exactly the same issue as you - a very slight gas smell with my old hw heater. I tried the soap test (bubbly soapy water applied to the gas pipe joints, if you see a bubble growing then you know there's a leak there. If you see one, I suppose you could try to tighten the joint, carefully (does pipe dope allow later tightening?)

When I replaced my hw heater, I remade all the gas pipe joints - cleaned and applied new pipe dope, and the smell disappeared, so there was a bad joint somewhere, very small leak, since you couldn't see it with the soap test. I suppose it could have been the valve thing on the heater, too.

Thing with the gas scenting agent mercaptan (it's added because natural gas normally has no scent) is it's easily detected by the nose in tiny, tiny quantities. Needless to say though if things are right there should be no smell there. I'd have it looked at as soon as possible and install a CO and gas detector nearby, they're well under $30 these days.

Thanks for all the replies. (I just got back in town and will be addressing this tomorrow.)

In no particular order:
a) No complaints of headaches by anyone over the years, b) I do smell the gas faintly at the edge of the "vent hat" of the boiler (next to the water heater), c) there's also a bit of "warm oil" smell from it's circulator pump, which I just realized was still running constantly with the heat off....and probably hasn't been oiled in a few years, so Ill have to address that issue, as well.

Thanks for all the recommendations, including calling the gas company (I may wait on that for a moment so the gas doesn't get shut off for house tenants just yet). I'll also take a CO detector and see what happens, try the candle test, soap the joints, etc., etc.

I will also see if the utility has a parts & labor plan - my dad's plumber in philly says many utilities have them, and they typically open up in the fall for enrollment (for around $100). That sounds like a good investment.

BTW, I am sorry for neglecting to mention the adjacent boiler (same utility closet) in my original post.

I realize this is a long shot to say the least but I had a strange odor in my utility room where the furnace/AC and water heater are. I also have a floor drain. I couldn't isolate the odor. Turned out it was a partially used, old garden spray bottle of malathion pesticide. It emits a rotten egg smell like gas does. So make sure there is nothing else stored nearby.