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Well-Known Member
Aug 9, 2020
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I'm having a healthy home problem with a "healthy home" HVAC system that is a little over 4 years old. The issue is more a matter of plumbing than HVAC, so I would like some input from those with plumbing experience for HVAC systems.

After being ill most of the time since Thanksgiving with a respiratory infection. I've been trying many methods of improving indoor air quality, such as replacing the additional room HEPA filter (system already has a MERV-16 filter), opening the east and west windows on good days for blow-through, burning candles, cleaning walls and bleaching floors, and some I have forgotten. Each one helped for a while. Then I decides to replace the MERV-16 filter at 5 months. That didn't help noticeably. Finally, I decided to replace the in-line humidity filter at 11 months of use (12 month schedule). I also wondered whether the temperature at the heat exchanger for the heat pump function wasn't as high as it was when the auxilliary fuel (propane) was being used, and that the interior side of the heat exchanger may have a layer of debris built up. So I turned up the thermostat to 77 for a couple of hours. This gave some improvement, and I repeated it a couple of times more.

Therein was revealed the answer: As I unlocked the Secret Chamber of Enlightenment in my troubleshooting maze, instead of glowing jewels, there was slime: a layer of tan slime covered at the top by a layer of pink slime. The filter was increasing the surface area for microbial organism to grow in a warm, moist aerobic environment. Last year when I had the clear vinyl drain tubing replaced with PVC, I thought those problems were over. The tubing had laid in a loop behind the unit effectively creating an S trap. I asked why there wasn't a P trap in that drain, given that water from the in-line humidifier was flowing constantly, but the arrangement appears to be dictated by the manufacturer. (btw, these in-line humidifiers can be purchased for other systems, and the one I used 20 years ago worked well)

So I removed the humidifier filter, cleaned the interior area of the housing with peroxide, reduced the water flow to a low level and closed the cover again. That night was good sleeping, but within four more nights, respiratory irritation was back. Many people may have dismissed this as an allergy.

I shut the water completely off and would have enjoyed a few more nights of sleep except for the very dry air that comes with heating in very cold weather. So I added a large-room humidifier, then a second, to get the indoor humidity up. (All I have to do is remember to add water three times per day to each one.) After a little while, I began to notice sewer gas odor in different rooms. The garage sink was beginning to drain slowly, so I expected that getting acquainted with the clean-outs was next on the list. Then I had another moment of enlightenment: the odor began not long after I shut off the water to the in-line humidifier. This is not supposed to be an essential element in the HVAC system, but with no P-trap in that setup, perhaps the drain line had dried out and sewer gas was coming into the HVAC system. I grudgingly opened the water line enough to keep the drain from going dry. No more sewer gas odor.

Whatever is growing in this damp, warm environment will require a visit from Stanley Steemer. Keeping that area dry and abandoning that device for indoor humidity control would prevent a future recurrence. Indoor humidity control is essential for health of people, dogs and a few plants. But the two room-size units will work until we can address this more permanently. Keeping sewer gas out is higher in the priorities. Does anyone with HVAC plumbing experience have any suggestions? Are these drains usually installed without P traps? Have you ever encountered this issue and found a way to resolve the issues?
We try to dissuade humidifiers unless it is an upflow/downflow using mister method.
Pad humidifier for sensitive persons: NOT recommended.
Of course, our climate on west coast is so different I cannot contribute.
Humidifier "drain" must never be connected directly to plumbing under any circumstances. That is basic plumbing 101.
Referred to as indirect drainage, must always have an air gap when dumping in to a p-trapped 'receptor'.