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Drip Pan Under the Attic Installed Air Conditioning Coil

Even if you don’t like rules, make that attic AC drip pan a part of your warm season house maintenance procedures before it will start leaking, and damage the ceiling.

The following industry standards apply to locations where accumulating / not draining properly condensate would damage building components, which includes the attic installed air conditioning coil. The rules listed below are based on 2006 IRC – International Residential Code [....], and 2006 UMC – Uniform Mechanical Code{….}:

Protection is required if condensate stoppage would damage building components[IRC] {UMC} :

•    Secondary drain to conspicuous point of disposal – you need to have a main drain from the coil tray located inside the coil compartment and a secondary drain in case the first one gets clogged [IRC] {UMC}.

Both AC condensate drains should discharge in different locations and that secondary drain to some easily accessible / conspicuous location so you can monitor it (code requirement). If you notice condensate dripping from the secondary discharge pipe, it means that the main condensate drain pipe stopped functioning for some reason.

The installers are sometimes tying together both – the primary and the secondary  drains, which installation negates the secondary drain purpose.

If the primary and the secondary pipe discharges in the attic, will you ever go there and check on them?

The areas that people install AC condensate secondary discharge pipe: above the kitchen window / through the roof soffit (not over or into the public way), sinks, toilet bowls (if you have some pictures and don’t mind emailing me them for posting online, please let me know).

• In case that auxiliary drip pan cannot be plumbed to some conspicuous location, it requires interlocked detector / cutout switch. The switch cuts off the power supply and stops air conditioning system operation as soon as the water starts accumulating inside the drip pan [IRC] – this is probably the most effective way of alerting the homeowner about the problem.

• There is an option you can use if there’s no drip pan or secondary drain installed under the air conditioning coil. In such case IRC calls for water level detection device with interlocked cutout installed in AC coil primary drip pan.

This particular requirement also applies to the down-flow type units (blower motor installed on top / blowing down) with no secondary drain and no means of installing auxiliary drip pan

• Primary AC condensate discharge pipe requires a trap installation (no traps on a drip pan secondary line )

For the attic installed air conditioning coil, a missing trap on the primary AC condensate discharge pipe compromises system efficiency, and might be responsible for condensate drainage problems, internal leaks, mold contamination of the coil compartment wall liner.

The next step is to find a place to drain the primary discharge line and the secondary drip pan under the attic installed air conditioning coil. This is often a challenge for HVAC contractors, especially when dealing with remodeling projects, additions, alterations of the existing property.

There are a few, constantly broken rules that apply to the condensate discharge pipe and its terminations;

• AC condensate discharge pipe must be at least ¾” in diameter and the minimum required slope towards the drain should be 1/8”. This applies to gravity / natural drain with no condensate pump assistance. Make sure that there are no reductions in pipe diameter along the entire run. There’s always some dust, lint, corrosion that will eventually cause blockages, especially if a smaller size condensate line is being used.
• Types of materials that are permitted to be used as a condensate drain pipe: PVC, cast iron, galvanized steel, copper, polybutylene, polyethylene, ABS and CPVC.
• AC condensate shall not discharge over or into the public way: the sidewalk, stairs, etc.  You should also avoid areas where condensate might affect / damage house finishes, cause site erosion, or penetrate foundation / cause foundation settlement.
• Dripping condensation will eventually stain / cause discoloration of concrete, stone, brick, siding (and other materials), and by keeping them moist promote mold growth on their surface. Also, you have to keep in mind that some of those AC condensate discharge pipes might be responsible for condensing / high efficiency furnace drainage. In such case, house exterior wall termination (and sometimes attic installation that is not protected / insulated) of the  discharge pipe might freeze up in a cold climate. This will compromise high efficiency furnace operation and flood the heat exchanger.
• Direct connections of the AC condensate discharge pipe to plumbing system waste or vent pipe are not permitted. This is the most (at least from my experience) abused installation requirement.  Because it is easy and much faster than trying to do it right (especially in the attic), many installers of the air conditioning system connect the condensate discharge pipe directly.

There are two very important concerns associated with a direct connection of the AC condensate discharge pipe to the plumbing system waste or vent pipe;

1. It makes difficult or in most cases impossible to early detect any blockages inside the “A” coil compartment.
2. There is a possibility of bacteria growing back up the condensate line and / or sewer gases contamination of the air conditioning system

AC condensate might drain indirectly into the waste pipe – an example would be the tub overflow, the sink tailpiece, laundry drain. However, in all those examples, discharge pipe must be connected above the p-trap on the sink side, bathtub overflow side, laundry machine discharge pipe connection side – such connection is called “indirect” waste pipe connection.

What is that “indirect liquid waste receptor” – it is a plumbing fixture (like a sink) with a p-trap installed on its drain line that separates it from the sewer / waste system, and makes the connection indirect.

• Sinks  – if it has a safety overflow, condensate discharge pipe must be at least 2” above it, without overflow hole – 2” above the sink edge

• Floor drains are acceptable for condensate discharge, but NOT directly into the ejector pump pit the floor drain is connected to

• Sump pump pit is also acceptable
• Trap in the attic attached to the vent stack – if you use it to drain the primary and the secondary condensate drain pipe, your local building inspector may not approve it.
• However, it might work if combined with the water level detection device / interlocked cutout switch.Also, such drain will perform properly only if you keep the trap filled with water. Prolonged periods of time without operating  HVAC system will cause water to evaporate and allow sewer gasses to discharge into the attic.

With attic installations you can try to use the plumbing vent chase or other vertical space between the attic and lowest level to drop the condensate carrying drain pipes down and discharge them to any of the listed locations. Just make sure that you seal the penetrations on both ends – at the attic floor and at the bottom – for example basement ceiling

Very bad choices for AC condensate discharge pipe terminations;

• Crawlspace – dumping any moisture into the crawlspace is always a bad idea, moisture attracts all kinds of insects, can contribute to differential settling and facilitate the growth of a variety of molds that can promote unhealthy conditions

• Discharging under the concrete slab – I see such installations sporadically, usually in older homes / basements, and I can’t even imagine the bacteria / mold growing under the concrete layer and contaminating condensate discharge pipe.

So how is your drip pan under the attic installed air conditioning coil doing…

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