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Question DetailsAsked on 1/18/2018

What causes sewer odor when heat is on

I don't have gas heat, it's an electric heat pump system and have had the system checked twice. When the heat is on, a foul, sewage like smells comes through the vents. I've been told the smell is being circulated through the house through the vents but that the heater is not the problem. Have also changed wax seals on toilets and the smell remains. At this point my eyes are irrated and i get a soar throat when I'm home but not when I'm not. How can I find out the source and get the problem fixed?

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Two common causes of this - well, three actually - OK - shall we say four ? Do I hear five ? OK - finally got to 6 - OK, 7 - well- how about 8.

1) "Stinky Sock Syndrome" - called that for obvious reasons but with heavy growth can go septic and smell like a sewer, from heavy buildup of fungal growth and/or mold on the evaporator coil in the ducting, usually located a few feet "downflow" from the furnace - or in a separate air handler on the heat pump in a heating and cooling combined heat pump system like you evidently have.

2) System is pulling makeup air, when it is running, from dry floor drains or other unused (so evaporated out the water in the trap) from basement or guest room drains - commonly shower/tub drains. Make sure all drains, including floor drains, are filled with water )a bit of dish soap or baking soda and vinegar can help keep that water from going stagnant and swampy-smelling. Can also pull from a sump pump pit or from exposed pipe ends on a slab underdrain system. Like a furnace, a dual-purpose heating and cooling heat pump should also have a makeup air vent serving it.

3) Like above, is pulling air from a wet crawlspace or unfinished basement - or very rarely an attic or subfloor with fungal growth there. This "robin hooding" of air happens because the vent system normally puts a slight positive pressure in the rooms it is supplying, so you get leakage to the outside through the building skin - it then has to "make up" that air loss, which will happen predominately from sources close to the air intake for the air handler. The more enclosed and isolated the HVAC system is from the conditioned space" the more pronounced this effect.

4) rarely, fungal or heavy mold growth in the ducts themselves due to excess humidity in the incoming air or house, or due to short cycling of the system so it is not drying out the moisture in the ducts wach cycle. Almost exclusively an A/C season issue but I have seen it in cases where the HVAC system is in a cold or unheated space so the ducts are cold and condense moisture on the contained lint readily.


One check you can do - run the system on fan only, and if not too cold outside (below about 45-50) on air conditioning too, for some minutes while you are near an air supply vent. If you get a shot of that issue but it tapers off when running A/C or fan but not heat, then is likely a fungal growth - heat causes rapid release of chemicals from them. If exists with just fan or cooling also, then more likely mold in the ducts or on the evaporator coil - or coming from a source outside the HVAC system. If coming from inside the system should be lesser odor and effect on A/C - if from outside the system should be no significant difference between the three modes with fan running same speed.

When you run the system, check around the air inlet to the air handler (and to furnace if central air system, not a heat pump,or if you have a supplemental furnace to a heat pump system) - if the smell is coming from floor drains or crawlspace or such you should smell the smell being pulled to it across the open intervening space.

5) one other rare cause - drain tube from the evaporator coil is going into a sewer line without a water trap (or the trap wster has dried out in the winter, because the line will not drain water through it except on A/C mode - so it is pulling sewer gas out of the drain it is connected to because the air pressure in the duct (due to the movement of the air or orientation of the tube relative to the air) is lower than in the sewer line - drawing sewer gas in through convection and/or venturi effect. If that is the issue, you may need to have a trickle feed rigged to the trap it is feeding into, to keep a small trickle of wster going in to keep the trap filled.

6) rare - but if your system uses external water as a heat source (geothermal or rooftop solar water-filled panels) that water can commonly go stagnant or septic smelling (especially in closed panel systems) so a slow leak in the heat exchanger can release that smell - if the problem is in the heat exchanger smelling closely around it when the unit it off should detect it.

7) another rare cause - burning insulation or electronics or overheating motor or such in or near the heat exchanger, or slipping and burning drive belt for the rare belt drive unit.

8) very rare, but with the burning sensation maybe - leaking refrigerant from the heat pump, getting into the air flow from the heat exchanger and on to your breathing it in. A pressurte check on the system would detect this - if enough of a leak to cause this issue for days on end, the static system pressure should be dropping.

Here is a related link to a question with answer which might help too:

Contractor mode to track this down would be Heating and A/C - a firm that prides itself on diagnosis, not just swapping out parts. If the problem is duct mold (pretty rarely so, especially in winter) then Duct Cleaning is the Search the List category - some HVAC firms also do that. If you do get the ducts cleaned, be sure the system is pulling air out of the ducts to the outdoors - or for any indoor vacuuming of the ducts that the air exhaust from their system is equipped with a secondary filter (after the dust/dirt filter) which is a high-efficiency HEPA filter, to filter out any contaminants so your house does not get more of them in it. ALso, if that wss the problem, good idea after carpet is cleaned (with dust mask and gloves) to wash your walls, and have carpets wet cleaned because a lot of the dust and mold spores weill be all over the house, You will not get rid of all of them, but hopefully enough to eliminate your symptoms.

Answered 2 years ago by LCD

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