Intermittent gas smell in AC vents tied to hot water usage

[HomeownerAJ]

Seeking advice on a dangerous problem with an elusive cause.

After running hot water in the master shower for a few minutes, a strong rotten egg odor is emitted from the AC vent. The smell is present in all vents serviced by this AC unit. The smell dissipates and does not recur even with continued hot water usage. This will happen again on another day, but is not always repeatable.

The gas line and systems have been checked for leaks recently and come out clean. But the problem wasn't occurring at the time of the gas line test.

The Furnace/AC system is a dual system. Both units are old but have been under home warranty and have had many parts replaced. The water heaters are new and have been checked for the source of the smell.

The water heaters are installed in parallel rather than series. Based on speculation by the latest plumber to look into this problem could the cause be something related to small off-venting of gas pressure caused by the spike/dip in gas pressure based on both water heaters firing? This would fit the symptoms as the source of the gas leak would be the furnace/AC unit but caused by the gas usage of the dual water heaters.

What would cause a small discharge of gas in the AC unit tied to hot water usage?

[tipsrfine]

I never heard of gas that smells like rotten eggs. Maybe it's flu gas being pulling into the air handler's return. You should have someone test to see if your water heaters are venting properly, and/or whether your air handler is creating a negative pressure that causes your water heaters to spill flu gas. Could be something died in your ductwork.

[garychance]

what about the floor drain next to furnace? shine a light in the top and see if you see water, if the trap is cracked or dry its possible its sucking sewer gas into a loose return at furnace and through the supply vents. (also if you have a sewage ejector pit for basement plumbing, make sure the lid is sealed as it will do the same thing.

[BaldLoonie]

Water smelling like rotten eggs isn't that uncommon, particularly with well water and certain anode rods in the water heater.

[garychance]

Water smelling like rotten eggs isn't that uncommon, particularly with well water and certain anode rods in the water heater.

Being a plumber as well, i agree, and its not just limited to well water, if your public system is on a water tower, i have had same problems, you can turn the temp up on the water heaters, or better yet change out the magnesium anode rod for an aluminum. (Its bacteria attacking and feeding off of the magnesium that creates the smell. Turning the temp up some will kill them, or replacing the rod will get rid of there food. (that rod is in there to protect the tank from hungry water, so it still needs one, or it voids the warranty if they find out its out.)

[tipsrfine]

It is a definite possibility that your water heater is spilling flue gas at start up, and then vents properly after it has run for a while. The cause of this could be that your openings to your return ducts are depressurizing the basement. When the flue gas spills from the water heat flu pipe it is sucked into your air handler via the return side and then distributed to your supply vents.
Get someone out their to do some pressure testing of your basement and testing of the water heater venting.

The reason why this does not happen in winter can be due to the fact that it is colder outside, so your water heater flue gas vents quicker/easier. Also, your furnace runs in winter and that assist the venting process for the water heaters.

[HomeownerAJ]

It is a definite possibility that your water heater is spilling flue gas at start up, and then vents properly after it has run for a while. The cause of this could be that your openings to your return ducts are depressurizing the basement. When the flue gas spills from the water heat flu pipe it is sucked into your air handler via the return side and then distributed to your supply vents.
Get someone out their to do some pressure testing of your basement and testing of the water heater venting.

The reason why this does not happen in winter can be due to the fact that it is colder outside, so your water heater flue gas vents quicker/easier. Also, your furnace runs in winter and that assist the venting process for the water heaters.

Flue gas at startup sounds like a plausible cause - especially with two water heaters firing at the same time. Do you think the venting relationship to temperature would explain why this happens in the summer afternoons when it is over 85 degrees outside vs. in the morning when it may only be 60 or 70?

[tipsrfine]

Flue gas at startup sounds like a plausible cause - especially with two water heaters firing at the same time. Do you think the venting relationship to temperature would explain why this happens in the summer afternoons when it is over 85 degrees outside vs. in the morning when it may only be 60 or 70?

Have you tried shutting off the gas valve to the water heaters and then running the shower (or putting them on pilot)? The smell could be from the additive they put in your natural gas so you can smell a leak. When it is over 85 degrees outside and your water heaters spill flu gas at start up, the flu gas could contain some of the methanethiol (the additive) that wasn't burnt off in the combustion process. methanethiol has been described as smelling like rotten eggs.

[Joehvac25]

Water smelling like rotten eggs isn't that uncommon, particularly with well water and certain anode rods in the water heater.

Yes I have smelled some really bad well water, they may also not be drafting initially, then once the flu warms they start to draft.

[HomeownerAJ]

I don't believe the problem is the water. The water smells fine. The smell comes from the AC duct. Only in the summer, no problem with any smell in the winter.

I thought it was natural gas, but having just visited with the gas company, the smell is more sulfur like.

I don't think it is anything dead in the vents because the problem has persisted for over 6 years. The water heaters are new. Plumber has checked the anodes. Both plumber and gas company verified venting of the water heaters.

The smell only occurs in the afternoon on warm days when the hot water in the master bath is run.

[garychance]

you use hot water in winter, and on cold days, (Both the water heaters are used then as well as the furnace blowing air) Try not running hot water at all in this bathroom, and make sure you do not smell this smell. Or better yet if you smell this every day, now that its warm out, shut the water heaters of and see if the smell go's away, as a test.

[garychance]

if the sewer gas not from flue, but from plumbing is getting sucked into the air handler/furnace and the nearest supply is close upstairs to this, plus if this is the bathroom you use mostly for showers you spend enough time in there to notice it. (just a thought)

[HomeownerAJ]

if the sewer gas not from flue, but from plumbing is getting sucked into the air handler/furnace and the nearest supply is close upstairs to this, plus if this is the bathroom you use mostly for showers you spend enough time in there to notice it. (just a thought)

Yes, I think this is about the only thing left. There is a sewage smell that is getting sucked into the air handlers. Both AC units share the same return duct work so the smell is distributed through the vents on both floors.

The smell is triggered by the operation of the master hot shower. It also seems to only happen once. We reproduced it yesterday for the ductwork camera contractor but then about an hour later ran the shower hot water and no smell.

Today i will try running hot water in the tub, but according to the previous owners, this will not cause the smell.

So something causes a sewer smell when hot water is run. Bacteria in the pipes? Something on the pipes reacting to the heat then burning off until it rebuilds or recollects?

If it is anything related to the supply side of the hot water that will be replaced when we replace the polybutylene (another problem not disclosed until we did the inspection).

Anything else to do diagnostically? I'm told you can't really scope an interior drain line. Might be something outside the drain line anyway reacting then the drain pipes heat up.

[martyinlincoln]

How did they verify the venting? A combustion analyzer taking multiple readings and a draft gauge is the correct answer. Blowing out a match and sticking it under the draft diverter is the wrong answer.

[HomeownerAJ]

How did they verify the venting? A combustion analyzer taking multiple readings and a draft gauge is the correct answer. Blowing out a match and sticking it under the draft diverter is the wrong answer.

Match method.

[HomeownerAJ]

How did they verify the venting? A combustion analyzer taking multiple readings and a draft gauge is the correct answer. Blowing out a match and sticking it under the draft diverter is the wrong answer.

Would improperly combusting gas possibly smell more like sulfur than the smell of natural gas straight from the line? When the gas company was there they did a pressure test on the gas line and both outside and inside pressure was fine. In the process I got a good experience of the smell of the natural gas and think the problem smell coming from the vents is stronger and more like sulfur.

[tipsrfine]

AJ wrote, "The smell is present in all vents serviced by this AC unit."
Now AJ writes "...the problem only occurs related to the master shower hot water".

All I can take from this is that the smell is being distributed through all of the supply vents connected to the air handler in common with these vents. Not sure how you are connecting the smell with the master shower hot water.

[garychance]

Also are the water heaters, and furnace, in a enclosed space? As there may not be enough combustion air for all 3

[HomeownerAJ]

Also are the water heaters, and furnace, in a enclosed space? As there may not be enough combustion air for all 3

They are together in an unfinished space that represents about 1/4 of the basement space. There is plenty of room for storage so I expect enough air is present for combustion.

[HomeownerAJ]

A question relating to this problem is that while this may be a difficult thing to determine should I buy this house without knowing the source of the problem? How expensive could the fix be? How unsafe of a hazard is the root cause and sulfur smell? Is it solvable? How much worse might it get? What trade specialty can diagnose the problem and find the cause?

Do I walk away or own the problem and cost to resolve or consequence of not being able to resolve over the next 20 years my family will live in the house?