this post was submitted on 11 Jun 2025
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The new AC in this house is suddenly producing a sour smell, but I don't have hundreds/thousands of dollars to call for repairs. What should I do? (hexbear.net)
submitted 3 weeks ago* (last edited 3 weeks ago) by Comrade_Mushroom@hexbear.net to c/diy@hexbear.net
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We miraculously managed to get a good deal that we could blow all our savings on to get this new HVAC unit installed about a year ago. In the last few days, it's suddenly started producing a sour smell, particularly in my room. Looking up the issue, all I see is people saying to call someone to get it cleaned because it's mold and/or bacteria build-up. I changed the filter, but the one that was there was barely dirty at all, so I don't think that was much of a fix.

I can't go into detail, but there's a lot going on right now and we can't afford much. We barely even have time to organize fixing something like this. But I don't want to take risks with our health.

Can I get some advice please? What would y'all do?

*also my god the youtube algorithm-fueled thumbnails that come up when searching for tutorials on what to do about this are truly horrendous, look at this shit, just straight up embarrassing

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[–] FishLake@lemmygrad.ml 27 points 3 weeks ago* (last edited 3 weeks ago) (2 children)

I’m your guy.

Couple of questions:

  1. Is your AC producing cold air?
  2. Is the filter wet?
  3. Do you have a heat pump system?

You might have a dirty drain pan and/or clogged drain pipe. You can check and clean these yourself. You’ll need a screw driver.

You can DM me if that’s easier.

Edit:

  1. Do you have a swamp cooler or does your HVAC system have a humidifier?
[–] Comrade_Mushroom@hexbear.net 8 points 3 weeks ago* (last edited 3 weeks ago) (2 children)

Thanks!

  1. Yes, cold air is happening
  2. The filter was not wet when I removed it.
  3. I don't think so?
  4. I also don't think we have a swamp cooler, I don't know about a humidifier. I don't wanna dox myself but we're on the north side of the midwest, so it's a pretty cold location, if that makes a difference.

It sounds like cleaning the drain pan/pipe might not be too risky to do myself, I suppose I can start by checking a video tutorial for that.

One thing I do notice, however, is that the smell is more intense whenever the unit is running during the day. Also, I'm not 100% sure, but there's a chance the smell is (either only or almost entirely) emanating from the vent in my room. I don't seem to smell it as much in other areas of the house, but I haven't gone sniffing enough to be sure if that's true.

[–] FishLake@lemmygrad.ml 4 points 3 weeks ago* (last edited 3 weeks ago)

I’m going to guess that you have a traditional central air system, not a heat pump. And I’m going to bet it’s a clogged drain pipe or dirty drain pan. Canonical_Warlock left some really great info about cleaning evaporative coils yourself. Also if one room is smelling you can also try to cleaning the vents in that room. If you’ve got pets or kids things can get spilled/stuck in those vents and grossness ensues. Those are probably the most easy explanations for the smell that you can reasonably fix by yourself over a weekend.

If neither of those work then there might be something else going on. The evaporative coils could be icing over. The system may have low refrigerant which could lead to the coils freezing, melting, and growing all kinds of gunk. You will need a licensed HVAC servicer to recharge your system. Additionally your ducts might need to be inspected and/or cleaned. You may have an unknown leak somewhere.

[–] Biddles@hexbear.net 4 points 3 weeks ago (1 children)

I don't think so?

Is it a minisplit or is it central air?

[–] Comrade_Mushroom@hexbear.net 5 points 3 weeks ago* (last edited 3 weeks ago) (2 children)

I watched this video to find out about cleaning the drain line, it seemed simple enough so I marched down to the interior AC... unit... machinery... giant grey boxy thing to find the PVC pipe to uncap, but I don't see anything like this coming out of the unit I have. The closest thing is a long tube that comes out and snakes down into a much thicker, larger PVC pipe. Not confident that's the same thing. Thankfully I had the foresight to check before pouring out a cup of vinegar, at least.

[–] FishLake@lemmygrad.ml 5 points 3 weeks ago (1 children)

Does that bigger PVC pipe go into the floor?

[–] Comrade_Mushroom@hexbear.net 4 points 3 weeks ago

For the sake of keeping my info private, I'll DM ya some pictures I took

[–] HexReplyBot@hexbear.net 1 points 3 weeks ago* (last edited 3 weeks ago)

I found a YouTube link in your comment. Here are links to the same video on alternative frontends that protect your privacy:

[–] came_apart_at_Kmart@hexbear.net 21 points 3 weeks ago (1 children)

legit I would call the guy that installed it to at least diagnose the problem and explain how it happened.

contractors can thrive or fall by word of mouth and something like that happening after just a year in a new install is not good.

[–] Comrade_Mushroom@hexbear.net 12 points 3 weeks ago (1 children)

Alright, I will give it a shot. I suppose it doesn't hurt to just ask.

[–] Mindfury@hexbear.net 10 points 3 weeks ago

yeah, you might have some warranty coverage or otherwise can at least ask the person what they think without moving to a quote

if you need to, you could put the screws to them as per above, but a friendly ask would be the first move

[–] Canonical_Warlock@lemmy.dbzer0.com 9 points 3 weeks ago* (last edited 3 weeks ago)

What kind of system is it? Is it a central system where there is one coil with ducts going to each room or is it a split system with a seperate AC in each room?

As far as cleaning it goes, normally I advise only having it professionally done because the chemicals used can damage things if used incorrectly but when it comes to the indoor coil that is less of an issue as the chemicals involved in cleaning that are much more mild. The biggest issue you'll have is getting access to the coil itself depending on what kind of system you have.

A cleaning shouldn't be horribly expensive to have done professionally if you want to go that route. I'm a commercial refrigeration mechanic so my prices are much higher than most residential HVAC contractors. A coil cleaning and system checkup on a regular central air style system would typically cost one of my customers about $300. But like I said, that's commercial pricing. A residential contractor should be cheaper. I would recommend having it done professionally because while they're in there the service tech should also be doing a quick check of other components to catch any major problems before they happen.

But if that isn't in the budget right now and you do want to do it yourself, basically the process to clean an indoor coil is just -

  1. Shut down the unit. If this is a central system then that means shutting off your airhandler or furnace. On a split system it means shutting off power to that indoor unit usually via the breaker panel. Use a non contact voltage detector (like $10 at any hardware store) to verify that the system has no power going to it before proceding.

  2. Get access to the coil. If you're lucky then this just means taking off an access panel on your air handler, if you're not then this mean pulling apart a section of duct or disassembling a split system head unit. This is usually the hardest step. Pay attention to how you're pulling everything apart to ensure you can put it back togeather again.

  3. Remove as much physical debris from the coil by hand as possible. Do not bend the fins or use any metal tools. You can use a coil cleaning brush or other long soft bristled brush to brush out the coil if need. Most of the debris however will becin the condensate pan under the coil and can just be removed with a shopvac.

  4. Spray down the coil with a coil cleaning spray. I frequently use Nu-Calgon Evap Foam No Rinse (lookup part number 4171-75). It comes in an can and you just shake it up and spray down the coil. The goal is to use the pressure of the can to spray in between the fins of the coil where it will foam up and push debris out of the coil. This stuff will also kill anything growing on the coil. Once applied you don't need to rinse it off because it's designed to be rinsed off just by the condensation that builds up on the coil when the system runs. This cleaner shouldn't hurt your skin or any surfaces but I highly recommend eye protection because it can spatter a bit while spraying it.

  5. Put everything back together and run the system. If the smell was coming from the coil then it should begin to get better. It may take a day for the condensate to fully rinse the cleaner off the coil though so give it time.

As far as preventing this problem in the future the best thing to do is to keep up on your filter changes and ensure that you are using the correct filters. Another thing that can be done is you can have a UVC bulb installed in the duct by the coil to regularly sterilize it and prevent anything from growing on it. However that really really should only be done by a professional because UVC lights are no joke and can cause severe damage to eyes, skin, and surfaces.

[–] chungusamonugs@hexbear.net 7 points 3 weeks ago (1 children)

If you can get to the actual blower, there should be a metal tray with a pipe dripping into it (or just a pipe sticking off the side if the blower is part of the outside part of the HVAC system). Especially on hot, humid days with the air conditioning running, it should be dripping pretty consistently. If it gets clogged (dirt, bugs, etc) the condensed water has nowhere to go and rests in the part thats blowing air around. If you think this may be it, you can try to free it with an unwound coat hangar (depends on how its installed as to how feasible this actually is to do). You'll know pretty quick if the smell is coming from that.

[–] Comrade_Mushroom@hexbear.net 5 points 3 weeks ago (2 children)

Alright, I will check for this. Do you have any good references for what the "blower" might look like?

[–] chungusamonugs@hexbear.net 3 points 3 weeks ago

Fishlake's picture is good. The heat exchanger (tower fan thing) is on the outside, but some units have the blower (hamster wheel that pushes the air through your vents) outside too. More often just the heat exchanger is outside and "air handler" (box with the blower and the thermostat and the furnace) is inside the house in a basement, crawl space, or attic.

If the HVAC outside is a giant rectangle thats wider than it is tall, its also got the blower, but if the HVAC outside is taller than it is wide, the blower is inside the house somewhere.

[–] FishLake@lemmygrad.ml 2 points 3 weeks ago

You’re not going to be able to see the blower unless you have x-ray vision. But if you removed the housing on your unit it would look something like this.

[–] miz@hexbear.net 1 points 3 weeks ago

your AC's humours are out of balance. add simple syrup to compensate