We have been fighting a sewage smell for months. When our heats comes on (we have a package unit) we get a sewage smell. Also, the smell only comes when it is so cold outside. If it is like 50 and above, the smell does not happen only when colder outside. We have had HVAC out 2, plumbers 3 times. They smoke tested our house--found no leaks, ran a camera down our ducts--nothing in the ducts, ran a camera line in our sewage line out to the street and no cracks or leaks. They are dumbfounded too. The plumbers say it is the package unit but the HVAC people say the unit is fine with no issues. We do not know what else to do except barely run our heat. The house was tested for carbon monoxide and that came but just fine. Just looking for some insight as I feel we have done all the tests we can. Also, we have a crawl space and that was all inspected and came back nothing in that that would be causing the smell.

you don't have a cold air return next to a toilet, do you??

Can you smell any sewer odor outdoors ? It may be originating from the plumbing vents on the roof.

Are there any HVAC drains (pump, etc.)that are connected to the sewer system?

...... that are IMPROPERLY connected to the sewer system

eg.

A/C coil condensate line should have AIR GAP above whatever trap it empties into

Condensing heat system pipe should have AIR GAP at discharge end

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

no, our toilets are not by this at all

Our condensate line just runs outside to the yard. The package unit sits on the ground and no smell around it

We have not done anything different to our packaged unit since it has been installed like 5 years ago. Only this was that we changed the heat exchanger and blower motor

There should not be anything in the sewer line because nothing has changed in 5 years and it just started to smell now.

You evidently have a heat pump.

If it only smells when it is REALLY cold outside, it would be indicative of the electric heat strips 'burning' or possibly 'breaking in' or possible a piece of insulation or debris inside the unit 'cooking' when the electric resistive strips 'kick in'.

Call your A/C guy back and ask him to visually inspect the heat strips.

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

We actually have a gas packaged unit.

Could it be possible there is gas leak in the system?