The tl;dr:

Intermittent bad smell builds up in mech room of a brand new build on slope, tight home/ICF, ERV, no basement but has below-grade mech room, no gas heat (electric coil on blower only), heat pump. Septic system in front / downslope, with effluence pumped up to narrow back yard.

Our GC, plumbing/heating, and septic guys are all stumped.

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House detail:

My GC is helping me troubleshoot this 1yr old build, where last summer I had detected a hint of a sewer or sulphur like smell in the mech room. It went away after the summer, and then came back this spring/summer. Mostly smells of waste, or farts, general organic rot, or sometimes rotten eggs.

The house is a small 2500sq 500sq suite, separated & enveloped by 14-inch ICF. Mech room comes off the garage, below the suite. HVAC and all plumbing terminate there, and there's a floor drain. Suite has its own mini-split system, but the the main side uses a furnace/blower and regular HVAC that is in the mech room.

Air blower (w/backup coil) is connected to an out door heat pump / AC. No gas runs to the property. The mech room is underground on 3 sides, like a mini basement connecting to the garage. ERV in mech room has a condensate line joining to the furnace condensate line, and then run to the floor drain. Which, I was told was fine for electric heat systems condensate.

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The problem:

When the air blower is on (AC or heat), there's an obvious negative pressure situation in the mech room. After a time, the mech room builds up a mystery smell - sucking air from some unknown location. The rest of the house is fine, but if concentration gets high enough it starts to make its way into the large air return and then into the rest of the house; which can take days. But the mech room smell itself sometimes takes hours or even minutes.

I live in the small suite above the garage, and operate the larger side ("main house") as an AirBnB - where guest's occupancy seems to coincide with when the smell gets bad.

The smell is first detected coming in between joists in the corner of the mech room, from behind one of the walls. Behind that wall is just insulation, then concrete, then ground. No plumbing. Running a combustible gas detector near that area does indeed tick faster (but actual concentration unknown). The closest drains to that location is the guest bathroom directly above that wall - with tub, toilet, and sink. A whole chase/stack for the rest of the house is on the same wall, above, but at the opposite end (where the smell doesn't start).

The smell seems to only occur in the summer months, and only when the AC/blower is on. During busy/guest season. Off-season, I often can't even reproduce the issue when I try.

Troubleshooting:

- Repeated flushing of the mech room floor drain with hot water.

- Refilling the condensate p-traps.

- Checked all of the ducts, and the air returns.

- Checked home exterior/intakes for smell. No blockage of intake.

No smells or issues found in any of those. Doesn't matter if the ERV is on, or off. The only thing that seems to exacerbate the issue is if we have the suite bathroom exhaust fan on, and/or probably the main house bathroom exhaust fans.

When the AC/blower is on full, the negative pressure in the mech room is so great, you can feel air coming in around the door (to the garage) - which is a self-closing 1hr metal fire door, with seal intact. So, we know it's drawing air from elsewhere as well.

Plumbing/HVAC company (same people who did both) said there's an acceptable amount of negative pressure in that room, and suggests we try to find the source of the smell before changing anything related to the HVAC or plumbing.

The septic tank is quite a distance from the mech room, further down toward the road/driveway.

Theories:

The negative pressure in the mech room, combined with the ultra-tight home, is making up air by drawing it from other locations available within or around the mech room. That much is obvious, so the two theories I've got are:

ONE: It's simply backdrafting from the drain pipes within the house (not the septic tank). But that would mean the concentrated smell is coming solely from the drains - which doesn't seem plausible since the only drain pipe exposed to the mech room is the floor drain, and its p-trap is functional (visually verified). Plus, the smell issue doesn't change when I flush the floor drain.

BUT, this theory could explain why the smell evolves/changes so frequently. And, why the smell doesn't match the MAIN drain smell (we tested by removing the cap to the main drain, and did a smell test of the mech room vs. main drain).

TWO: Or, related to the above, maybe it's backdrafting from the septic tank. But this also doesn't make much sense that the smell would come from one corner of the mech room, from behind a wall that contains no plumbing. Just insulation, concrete, and ground level to that spot. And then interior walls above that spot.

Somehow, drainage activity is related (likely indirectly) along with the pressure. I just can't figure out the specific combination of events to create or prevent the smell consistently. If I have guests on the main side, I'll simply leave the mech room door open to the garage. The smell still builds up, but it's at least drawing more air from the garage (rather than the mystery location), PLUS it's diffusing some of the smell into the garage.

Not an ideal solution, but a good temporary workaround. Makes the car and everything eventually start smelling. And, after maybe four days of intense AC usage, the smell does start to build up in other places of the home.

The ONLY other theory (let's call it 2a) the septic pump's effluence, when the pump shuts off, drains back into the septic tank - my longshot theory is that this 40ft of liquid coming back into the tank, in combination with the AC vacuum effect, is pulling air from the septic tank at the same time that its being pushed (from the liquid draining back). But this would only happen once every 2 hours, as is the timer for the effluence pump.

So, I really don't know what to try next. This is complicated, and frustrating! Any out-of-the-box ideas?