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Question DetailsAsked on 12/26/2017

Can the installation of a p trap under a furnace condensate line cause sudden health issues?

The p trap was installed below a furnace condensate line to replace a y that went directly in to the sewer line. Ever since then, when the furnace runs, eyes water, ears ring and toes and feet feel like they are injured or broken. Also after a long cold spell, occupants develop a bronchitis and sore throat. There is an internal trap on the the addition of a p trap proper?

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1 Answer


Potentially, if it is letting a lot of moldy, fungused air into the living space - though those symptoms do not sound to me like ones a moldy drain would cause. Also, would not generally cause instantaneous symptoms when the furnace runs - usually cause long-term general buildup of allergic or more commonly respiratory issues in the lungs due to mold spores getting in there.

Is the drain tubing connected to the trap or just sticking into the pipe - it should have a reducer on the trap that it fits into with a seal so sewer and trap gases do not get into the air.

Vinegar or vinegar and baking soda can also be effective in controlling mold and such in evaporator coil drain lines.

Another thing - while normal A/C in the cooling season should keep the trap filled, when the A/C season is over the trap will have to be kept filled with water manually - probably every month or few, otherwise sewer gases can seep through the trap into the house.

Another possibility is the trap is not a deep enough U, or was not properly connected to the drain pipe, so it is not actually properly retaining the depth of water needed to prevent gas blowback.

Because of the symptoms and the fact you said nothing about smelling sewer gases or hydrogen sulfide (sulfur or hard boiled egg smell), my guess is that Y (assuming it was open to the air) was providing makeup air to the furnace and it may now be starving for combustion air, putting carbon monoxide and nitrogen compounds into the air because it is not getting enough air. It could also then be pulling sewer gas into the room as it tries to pull combustion air.

Could also be totally coincidental timing and the furnace heat exchanger has gone out, letting combustion gases which should be going out the flue into the air ducts.

IMMEDIATELY have a Heating and A/C contractor come to check the unit out - be sure they have a carbon monoxide detection "sniffer". This is not something to wait on - whether sewer gases (which are normally explosive) or furnace flue gases (which can kill by asphyxiation) are deadly, and can also have cumulative serious health effects even in relatively low concentrations. Basically, if they are having symptoms like this (especially if more than one person) then the condition is dangerous.

If this is currently occurring, you could and should get out of the house (leave doors and windows closed so any gas stays in place and is detectable by them) and call the fire department and tell them you have a suspected carbon monoxide problem - they have sniffers to detect CO, and many also have sniffers which will detect hydrogen sulfide in sewer gases leaking into the house.

Answered 2 years ago by LCD

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