[sheila20149]

We get a foul odor in the house when we use the plumbing fixtures and the forced-air furnace at the same time or around the same time. The drains don’t seem to have a smell and neither does the water. We’ve done several experiments to try to isolate the problem:
1. Turned on the furnace so it was actively blowing warm air, then took a shower in the master bath (on the second floor of a 2-story house with basement). There was no smell in the shower/toilet room, the smell was slight in main part of the bathroom, but the smell was very strong throughout the rest of the house except in the basement where we have shut the heating vents.
2. First showered in the master bath, then turned on the furnace. Same results as #1.
3. Turned on the furnace, then showered in the guest bath (on the second floor). Same results as #1.
4. Turned on the furnace, then ran the separate tub in the master bath. Same results as #1.
5. Turned on the furnace, then ran the dishwasher, washing machine and laundry room sink (on the first floor). The smell was throughout the house but not as strong as with experiments #1-4.
6. At various times turned on the furnace, then used the kitchen sink, bath sink, toilet, etc. No odor anywhere.
7. Again at various times, used the sinks, toilets, etc. then turned on the furnace. No odor anywhere.
8. Turned on the furnace, then showered in the basement bathroom, which drains into a sewer ejector pump and then out to the septic tank. No odor anywhere.

Another oddity, my husband showers every morning without the furnace on (and we don’t turn it on until a couple of hours or so later) and there is no smell. Today we turned on the furnace right after he showered, no smell.

The house is only 3 years old. There is a single plumbing vent stack through our roof. The furnace supply ducts for the second floor run through the attic, for the first floor and the basement they run under the floor. The return “ducts” are simply bare bays in between the wall studs and the floor joists.

Does the volume of water being used have anything to do with the problem? Do we have a plumbing vent issue? What our next steps? We’ve had two plumbing contractors come out and both are stumped. Any ideas are greatly appreciated.

[katoakita]

You may have a fresh air inlet pipe attached to the furnace that terminates close to your plumbing vent. Is it a gas furnace? - you may have a negative pressure in the home when the furnace is operating. You may have a leak in the plumbing vent piping. You may have a toilet with a cracked bowl allowing sewer gases to enter the home. You may have a floor drain in the lower floor that is close to a return air inlet to the furnace.