Sewer smell in bathroom ONLY when central a/c is on and kitchen sink is running water

Question

This is a weird one, and have come across multiple handymen and none of them can pinpoint the problem when I describe it. Though they have never been in the room when it happens.

Here is the scenario...

We leave the central A/C on all day because we have big dogs in the CA heat, so when we get home, the A/C is running. When we turn on the water faucet in the kitchen sink, immediately one of the bathrooms emits a sewer odor that lasts for about 10 minutes. This happens consistently so much that the first thing when I get home is to open the bathroom window before my wife starts using the sink.

The details...

I have placed my face in the toilet, bathroom sink, shower drain, and A/C vent in the bathroom and cannot pinpoint where the smell is coming from. The bathroom in question has a wall that faces the front of the house and that part of the house is where the main water supply line is. When the scenario with the smell is happening in the bathroom, if I go outside my house into that part of the front of the house, I can also smell the bad odor, but not as strongly as in the bathroom.

I once had a plumber here to see if I could create the smell. It was about 9am and I turned on the A/C for about 20 minutes then turned on the kitchen faucet, and OF COURSE it didn't smell, so he couldn't help pinpoint the situation. I guess a prerequisite is that the A/C be on for hours or all day.

In conclusion, the smell doesn't come in any other scenario. It's just the combination of the A/C and kitchen sink (no other sink in the house causes it). And the smell is only that bathroom. I have another bathroom and no problem there. I've had the HVAC unit for 3 years and it's really been happening for a year now.

Ask me any questions you may have and thank you in advance for reading this long story and for any input you may have.

Answer 1

This sounds like a similar problem I saw with a utility sink in a home.

You could see the gas bubble in the sink, so I made a larger trap about an inch deeper. It slowed the bubbling, but now another sink in a bathroom on the other side of the wall started doing the same.

When this was happening the house was developing slightly negative air pressure. I found the make up air duct (which is supposed to bring in fresh air from outside) had broken and was just pulling from the inside. A few air leaks in the attic and the home went negative and started sucking air through the P-traps.

I pulled the insulation that had been sucked into the duct, reconnected the flex pipe, and the problem was solved.

It sounds like you have a similar issue but it takes the additional water being added to the drain to cause the smell. I would add a make up air duct. They do waste a small amount of energy but make the home air healthier (even without the sewer issue). You may need the plumbing checked but if the house is positively pressurized this probably will quit happening.

That house had multiple sinks on the same vent that trapped air in the 2 back to back, this may be the same issue that a new vent or make up air would solve.