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Old 07-04-2016, 02:07 PM   #1
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Condensate drain drawing enough vacuum to prevent drainage


The setup :
3 ton 15 seer split, all electric
Air handler is in vertical installation ; in order from the bottom up is the return box, coil, heating element, and blower
HVAC was a later addition in the house (Ranch built in 1960) so the air handler is installed in a bedroom closet that's adjacent to the hallway on top of the return box with a 20 x 20 return grille in the hall wall just above the base board.

I actually installed this system 2-3 yrs ago to replace the ancient undersized system that came with the house when I bought it 6 yrs ago. I replaced everything except the duct work (thermostat, thermostat wiring, Romex back to the main line disconnects on both units, new copper service lines, everything )

The rub:
Condensate drain problems.
Had an issue last year, my wife tells me the floor is wet, open the return grille and it's a lake on the bottom of the return box. Shop vac out the water, disconnect the condensate drain line, blow it out with about 100 psi compressed air, no more problems.

Last week, same thing, wife says floor is wet, same procedure, no joy, water still flowing out of the unit in places other than than the drain line (places I can't see).

Stick my head in return box and look up, coil is absolutely filthy. So I decided to try something while I was cleaning the coil, I cut the drain line (pex) to about 18" and put a five gallon bucket under it. I had already decided to put in a new drain line (conventional foundation, 12 ft run, 10 min job) just to eliminate that variable, and figured with it cut and draining into the bucket, I can see exactly what's going on and I won't send all the crap I'm about to clean off the coil down a new drain line.

Shop vac the surface gunk off the coil, lots of hair (3 dogs), Spray coil cleaner, let sit for 30 min, rinse with pump sprayer, everything out the drain into the bucket just fine. Turned the fan back on and rinsed with pump sprayer again ( to suck the water through the coils to clean it, I think your supposed to spray from the outside in and let gravity do the work but I don't have good access to the outside of both sides of the coil).

That's when I noticed something funny. Water was still draining, but slower, then I put the filter on (brand new) and it all but stopped. It was literally slurping it back up with a drop escaping every 20-30 seconds. I put another filter in front of the first (to simulate a dirty filter) and it did stop. Put my finger on the bottom of the drain line to kill the air draw, immediately fills with water and when I remove finger rushes out.

It can't be inadequate return flow, because my return is literally wide open. The bottom opening of the air handler is around 17"x25" and it's 6" above the 20"x20" return grille.

So I come to 2 conclusions: 1. My coil is still dirty, 2. The 1/2" pex drain is inadaequate.

I went and got a coil comb and got every bit of every nasty whatever out the coil and cleaned it two more times, paying special attention to the bottom where it meets the drain pan as well. I can now see through it and it looks brand new.

Next, I replaced the 1/2" line with 2 3/4" drain lines. My drip pan had another hook up that was plugged about 2" to the right and 3/4" higher then the one I was using. I figure 2-3/4 vs 1-1/2 gives me 200% more volume in the drain line, allowing plenty of room for water and air to pass each other. Even better surely by time it's full enough to submerge the lower drain it'll breathe through the top and drain through the bottom (since the air draw/vacuum will then be broken to the bottom line, like when I put my finger on the end). Nope.

Left 2-3/4 pigtails into a bucket to validate my logic, did not pass. When I stack 2 filters, water flow stops. If I put my finger on the bottom of the lower drain line, the upper one dang near sucks like a shop vac and the lower fills with water till I release my finger.

I know the coil is clean, the return is wide open air flow, no obstructions, so??? Would a p-trap make a difference? Would that break the vacuum or would it just suck it back up and dry out the trap making it useless? Is this just normal and under normal operating conditions (clean coil and filter) the weight of the water always overcomes the air draw before it overflows the drain pan?

This is fairly big deal because where I'm at in W TN it's very hot and humid. Not counting what I collected while rinsing with the pump sprayer I almost completely filled a 5 gal bucket 3 times in about 18 hrs with the condensation out of the drain line. So, when it gets out of hand it does so quickly.

Any knowledgeable insight appreciated.
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Old 07-04-2016, 03:17 PM   #2
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Re: Condensate drain drawing enough vacuum to prevent drainage


Install a trap at the air handler with a t after the trap. Leave the t open. It's important that you install the trap next to the unit and the t after the trap. Also 20x20 is to small for a return for a three ton unit. Is there anyway you can install an additional return on another wall or a larger return. That is actually aggravating the problem
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Old 07-04-2016, 04:36 PM   #3
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So on the T is that with the middle to the top of the p trap with one leg down and the open leg straight up? And I'm not arguing, but help me understand, if the the bottom of the air handler is 17x25 (425 sqin) and the return is 20 x 20 (400 sqin) is that really enough to restrict it? I'll have to measure how much wall is left (it's in a pretty tight spot between an inside corner and a door) and then check common return sizes to see what I can fit. Would 26x20 (520 sqin) be sufficient?

I basically pulled out what was there and replaced with new components and I'm sure the system set up is incorrect
Everything original to the house is solid. I've replaced floors and the tar paper on top of the 1x subfloor looks like you rolled it out yesterday. But it had a couple of additions and what not before I got it and everything not original has been a nightmare that I've slowly removed/replaced/corrected. For example a 22' glue-lam beam for the carport coming through the osb sheathing of the garage,and sitting, not nailed, not strapped, sitting on top of a single stud. The power lines came through the roof decking of the addition to the original Weatherhead which is now "indoors". Shingles laying across the lines and all.
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Old 07-04-2016, 06:36 PM   #4
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Re: Condensate drain drawing enough vacuum to prevent drainage


Take a look at this link to help you understand how to hook up your drain
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Old 07-04-2016, 06:53 PM   #5
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Re: Condensate drain drawing enough vacuum to prevent drainage


You don't want to make your return air any smaller than you original opening. Where your drain is located you have negative air pressure and sometimes it is so bad that it will actually cause a drain to not work at all. Installing a trap should alleviate the problem. You will have to seal the overflow drain as well. When your coil is dirty you actually increase the negative pressure which will increase the problem that you have. A dirty air filter or restrictive air filter will do the same. Your actually increasing the suction by having to small a return as well. I bet when you install a filter with the unit running the filter slams hard. I would say a minimum of 20 x25 return air grill but larger would be better.
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Old 07-10-2016, 02:05 PM   #6
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Re: Condensate drain drawing enough vacuum to prevent drainage


Your current filter return is designed for a 2 ton unit (you need 200 sq in per ton). A 3 ton may or may not work (in your case, at least in part, it's causing the condensate not to drain).

Edit: a honeywell 4" pleated media filter (20x20) designed to fit in your return grill may help. What filters are you using? 1" Filtrete filters are a no-no - too much return static pressure. Use cheap fiberglass filters if you don't want to stump up for the 4" honeywell.

Last edited by sktn77a; 07-10-2016 at 02:09 PM.
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Old 07-10-2016, 04:26 PM   #7
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Re: Condensate drain drawing enough vacuum to prevent drainage


20x20 filter return grille is way to small for 3 tons. All negative pressure drains MUST have a trap in them to work.
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