Hi,
I've been reading through previous posts and have found similar situations but nothing matching up to our problem exactly. We have a sewer odor in the house that seems to involve several variables. Here's as much info as I can give:

The house was built over 100 years ago. We rent the entire house (2 floors), with the exception of a first floor guest apartment that is rarely used (occupied for one week during the past year). Our bathroom, kitchen, and laundry room are all on the first floor in line with each other. The problem is that whenever a large amount of water is drained (usually while doing laundry or after long/multiple showers), a terrible sewer odor appears. The odor cannot be smelled outside. We moved in in June and did not have a problem until November. Also, the problem appeared just after the landlord had the septic tank pumped. It sometimes seems worse when it's colder outside (30s and below), but not always. Also, I did laundry yesterday and Saturday and there was no odor at all. However, we had VERY heavy rain all night last night and upon going downstairs this morning, the whole house smelled terribly. (Worse than ever.) My questions:

1. Could there be a dry trap in the guest apartment that's causing the odor to come through the walls into our house when we do laundry/shower? (the guest apartment bathroom shares a wall with a room in our house and the odor seems strongest in this room. This room is also in proximity to the laundry room and the bathroom).

2. Does pumping the septic tank cause sewer odors? (could something have been left open, or broken?)

3. The smell of laundry detergent can sometimes be smelled in the basement directly beneath where the washer is upstairs after doing a load of laundry. (Maybe not even related?)

4. How would the heavy amount of rain factor into all of this?

THANKS for any help you can provide!

1. Could there be a dry trap in the guest apartment that's causing the odor to come through the walls into our house when we do laundry/shower? (the guest apartment bathroom shares a wall with a room in our house and the odor seems strongest in this room. This room is also in proximity to the laundry room and the bathroom).

Yes a open trap is a direct connection to the sewer.

2. Does pumping the septic tank cause sewer odors? (could something have been left open, or broken?)

No, Pumping the tank does not affect the inside drainage.

3. The smell of laundry detergent can sometimes be smelled in the basement directly beneath where the washer is upstairs after doing a load of laundry. (Maybe not even related?)
Ahhhh! But I think it is related. (More later about this).

4. How would the heavy amount of rain factor into all of this?

A heavy rain could load up the drain field so that liquid would not drain out of the septic tank causing a back up.

First what's happening. Your washer pump discharges with much force and volume. If the discharge hits a partial blockage,(such as a septic tank that's too full or pipes that aren't large enough) it rebounds back sending a bubble of sewer gas ahead of it and out the trap that gives it the least resistance.
That's three ways the smell could have entered your home. The open trap in the unused apartment, a septic tank that so full of rainwater it won't accept the washer discharge and undersized pipes.
I've done all that I can do. The rest is up to you. Good luck, Tom

Thanks for the quick reply!

Another quick question:

The standpipe for our washer is too narrow for the amount of water the washing machine discharges. So, we were given a rubber seal with a clamp on it to create an airtight seal between drainage hose and standpipe. However, we found that the airtight seal caused water to siphon out of the washing machine, not allowing the tub to fill. (Result: water would continue to run endlessly as it drained out simultaneously.) Our solution is to loosent the clamp on the rubber seal as the washer fills up. When the washer is full, we tighten the clamp to create the seal (and prevent tons of water from flying out of the standpipe and onto our floor) and start the cycle.

1. Could this be related to the sewer odor problem? I tend to think not because the odor occurs after the shower runs or if it rains outside, regardless of if the washer is on.

2. Any idea how we could fix the siphoning problem (aside from installing a wider standpipe)?

Thanks!

How high is the standpipe over the washers flood rim? Tom

How high is the standpipe over the washers flood rim? Tom

The standpipe is about 6 inches above the flood rim.