How to prevent a clogged drain

In Houston your air conditioner doubles as a dehumidifier. On a humid afternoon a typical system can pull several gallons of water out of the air, and all of it is supposed to leave through a ¾-inch PVC condensate line. When that line clogs, water backs up into the drain pan. If you have a float switch, it shuts the cooling off. If you don't, the pan overflows into a ceiling, a closet or the attic.

Drain flies, musty air and a system that “randomly” shuts off in July are usually the same problem. There's standing water in a line that was never pitched right, never cleaned, or got tied into something it shouldn't have. Preventing it comes down to plumbing and habits. A spray won't do it.

Give the line proper pitch

Condensate has to fall. From the pan to wherever it ends, the line needs to run downhill the entire way. Figure ⅛ to ¼ inch of drop for every foot. Any flat stretch, sag or uphill bit is a spot where sludge collects and algae gets going, and that's your clog.

This is the installer mistake we run into most on service calls. A line can look fine because it eventually reaches a sink and still have a low spot up in the attic or in a joist bay. Water that doesn't move turns into a gel-like film, and the film turns into a clog. If your line was run level “to keep it neat,” it was run wrong.

When we clear a chronic clog, we check the pitch too. Clearing out the algae without fixing the belly just means you'll see us again in six weeks.

• A continuous downhill slope from the pan to the outlet, with no flat runs and no uphill hops
• Enough hangers that the pipe can't sag between them and hold standing water
• A condensate pump if the air handler sits below the sewer or too far away for gravity. Don't try to make gravity work with a long, flat line.
• A cleanout tee near the air handler. You can't maintain a line you can't get to.

Clean it with vinegar, on a schedule

White distilled vinegar is the homeowner tool we actually recommend. It breaks down early algae and biofilm without attacking PVC the way bleach and some chemical openers can. During cooling season, once a month is about right in Houston.

• Shut the system off at the thermostat.
• Look for the T-shaped access near the indoor coil or air handler. It's a PVC fitting with a cap on it.
• Pour in somewhere between ¼ and ½ cup of white distilled vinegar.
• Wait half an hour. Then chase it with a cup of warm water.
• Go see if water's coming out the outside end of the line, or at the sink.

What not to pour down the line

Don't make bleach a monthly habit. It's hard on some components, it doesn't do much to dissolve the slime that's actually clogging the pipe, and mixing leftover chemicals in a drain is a good way to make the problem worse. Enzyme cleaners are a reasonable second choice. When vinegar isn't enough anymore, we put a wet/dry vacuum on the outdoor end of the pipe.

Keep the sink the drain runs into clean

Plenty of Houston systems drain into a utility sink, laundry sink or floor drain instead of out the side of the house. That sink is part of the condensate system. If it's full of lint, soap scum or standing water, or the tailpiece drains slowly, the A/C line can't empty. Water backs up the pipe and into the pan, even when the PVC itself is “clear.”

Treat that sink like equipment. Keep the strainer clean. Every so often, run some water through it so the P-trap doesn't dry out and let sewer gas into the closet. Draining into a floor drain? Keep the cover and pit free of sludge. Otherwise the gunk you just flushed out of the line grows right back in.

Don't merge two drains into one

Two air handlers on one line. A furnace and coil teed together. An A/C line tied into a plumbing drain with no air gap. Every one of those is a setup for repeat clogs. The spot where two lines meet is almost never pitched right. Water from one unit backs up toward the other. Clog one side and both systems go down. Then try figuring out which unit is really the problem.

Your condensate shouldn't be sharing a pipe with the washer, the shower or another air handler. Give every coil a line of its own, pitched on its own, with its own outlet. Add an air gap so dirty sink water can't get siphoned back up into the pan.

• One evaporator coil gets one drain line and one outlet
• Never tee two condensate lines together on a flat or uphill run
• If two units have to share a sink or floor drain, they still need separate pipes and an air gap
• Don't dump condensate into a vent stack or a sealed plumbing line

Other habits that actually prevent clogs

Pitch, vinegar, a clean sink and separate lines cover most of it. The rest is keeping dirt and biology out of the pan to begin with.

• Change filters on time. A packed filter lets dirt slip by, and it ends up on the coil and in the pan.
• Don't let the outside end of the line get buried in dirt or mulch, or packed with bug nests. A pipe pushed into the soil is a plug waiting to happen.
• Put in a float switch, and test it. It will shut the system off before a clog turns into a ceiling stain.
• Have the pan and line professionally flushed at the spring tune-up. Vinegar keeps a clean line clean, but it won't replace a snake and a vacuum on a neglected one.
• Don't pour cooking grease, paint or “drain opener” into a sink that's also catching your A/C line.
• If you're seeing drain flies, standing water in the pan, or the system cutting out on the float switch, the line is already losing. Call before the attic gets wet.

When to call us

If the line clogs more than once a season, the pitch or the outlet is wrong. If two systems share a pipe, we'll separate them. If the sink is the choke point, we'll tell you that instead of trying to sell you a new air conditioner. A clogged drain is a maintenance and installation problem, and it's one of the cheapest fixes we make in Houston, as long as somebody actually looks at how the water is supposed to get out of the house. Already have water in the pan, or a system that shut off and won't cool? That's an AC repair call.

Drain calls come in from all over our area, and we see plenty of them in Cypress and Humble. If yours keeps backing up, the town pages list what else we handle there.