Residential Air Handler help (freezing)

dcg9381 · Apr 28, 2026

Man, I'm having a run on HVAC issues....
We've got a Rheem/Ruud RH1T3617STANJA:

3-ton, 16 SEER multi-position air handler designed for R-410A systems, featuring a constant torque (ECM) motor and 208/240V, 1-phase power. It offers versatile 4-way airflow (upflow, downflow, horizontal left/right) and a durable aluminum coil with a corrosion-resistant drain pan.

The issue I'm having is that the unit appears to be freezing and leaking down into the pan. Second time it happened. First time, I replaced the filter, just assumed it was my lack of filter maintenance. Turned the AC off, ran the fan on manual. Dried out the pan and it worked just fine for another 40 days.

All of the vents are open, not aware of any restriction. Do these things have a thermistor that detects a frozen coil?
I should probably open it up in back, should the pan extend all the way back there? Even with a frozen coil, seems to be leaking in an odd spot.

Drain line is clean and free. Some water is coming through the drain line.

PCustoms · Apr 28, 2026

Can't help with that, but what's with the yellow nm-b dangling?

Chuckster in NJ · Apr 28, 2026

Interior Pan can be rotting out and leaking.
Drain line clogged with bugs, dirt or debris.
A "short charge" or dirty filter may be the cause of a frozen coil.

FWIW: I am leaning toward a condensate pan failure…….. A common problem with older units and this can be repaired very easily.

danski0224 · Apr 28, 2026

Freezing coil can be an airflow problem or a refrigerant charge problem or both.

Looks like it's been leaking for a while.

Need to get it running properly first.

PoorUB · Apr 28, 2026

I agree with danski!

dcg9381 · Apr 28, 2026

PCustoms said:
Can't help with that, but what's with the yellow nm-b dangling?

The work of a licensed electrician. Not live or connected. Apparently he believes in leaving a little extra. Everywhere.

Chuckster in NJ said:
FWIW: I am leaning toward a condensate pan failure…….. A common problem with older units and this can be repaired very easily.

5 year old unit.. ish. So open the handler up and check the condensate pan? I can't see the from the filter side.

danski0224 said:
Looks like it's been leaking for a while.

It hasn't been. Drain line has been clogged a few times (very common in TX) - I'm installing a chlorine feed downstream to help with that.
Original drain was fed into the downstairs bath, which just serves to overflow the bathroom. I ran a new exterior line.

Drain pan was not properly sealed, fixed that, installed a float switch.

Most of the "rust" in the pan was caused by using some fairly caustic chemicals to clear the drain line one time. It was washed out with baking soda to deactivate...

Chuckster in NJ · Apr 28, 2026

Open up the unit and see how the interior pan looks, I am willing to bet it has some "pin holes" in it.
Here is the fix: Dry out the pan and pour some POR15 (rust preventer) in the pan and it will seal up the pin holes.…… This works well compared to other products like Flex Seal.

This is the stuff www.por15.com buy a quart and make sure you wear rubber gloves,

Ohmthis · Apr 28, 2026

As said above, get the system working correctly first. Is there a float switch on the interior pan? I install the condensate float switch in the pan drain plugs. There are two, one is higher than the other. That’s the one that I use to the shutoff switch. The whole unit is likely not level as well. I’ve seen ice slide into the pan and block the drain. Hope it gets figured out.

dcg9381 · Apr 28, 2026

Ohmthis said:
As said above, get the system working correctly first.

It's odd, because it seems to work fine for long periods. I cleaned it all up last time, replaced the filter, no issues for 30-45 days.

Ohmthis said:
Is there a float switch on the interior pan? I install the condensate float switch in the pan drain plugs.

Not as installed. I went back and added a float switch on (exterior) pan... Are you talking about a switch on the pan that's inside the duct work? Didn't know that was a thing.

I'll open it up. Suspect the interior pan too.

Ohmthis · Apr 28, 2026

I’ll post a picture. Essentially, if the pan fills up, it shuts the system down. I prefer to wire it in series of the condenser/heat pump control wires (yellow typically). This will shut down the outside unit and make you wonder why there isn’t any AC. It has been code for a long time.
This is a picture from the net, not one of my installs!

dcg9381 · Apr 28, 2026

Ohmthis said:
I’ll post a picture. Essentially, if the pan fills up, it shuts the system down. I prefer to wire it in series of the condenser/heat pump control wires (yellow typically).

Got it. Have those on the other installed units (vertical), not this one. Won't help if the interior pan is leaking... But agree, it's another way to catch things sooner. I can just put this in series with the other float switch I added.

How it is code to run HVAC condensate drains into downstairs baths baffles me...

Ohmthis · Apr 28, 2026

It may help depending on what the main issue is. Yes, wire it in series with the other one.

danski0224 · Apr 28, 2026

dcg9381 said:
I cleaned it all up last time, replaced the filter, no issues for 30-45 days.

It's taking that long for the filter to load up and reduce airflow.

BurtEggley · Apr 28, 2026

pan problems and freezing are two issues. 1. Make sure the pan drains and does not leak. Lots of things from algae to dirt and bugs can plug it as others have said. AC units always shed water 2. Check the filter and coil. Make sure both are clean. Lack of air flow can cause freezing, Check the indoor fan too. Sometimes the cage can get so dirty it no longer moves air 3. have some one check the refigerant pressures, subcooling and superheat. While they are there they can also clean the outdoor coil and check the fan.

danski0224 · Apr 28, 2026

dcg9381 said:
The issue I'm having is that the unit appears to be freezing and leaking down into the pan. Second time it happened. First time, I replaced the filter, just assumed it was my lack of filter maintenance.

How old is the equipment?

How long from installation to first known incident of coil freezing?

Pleated 1" filters?

Normally, a low refrigerant charge or low airflow problems will cause the evaporator coil to freeze up quickly and repeatedly.

However, I have seen a failing compressor run on the ragged edge of freezing the evap (low suction pressure) or just low enough to start freezing and then it thaws out with repeated on/off cycles.

I have seen many pieces of equipment running like this, and usually they will appear to cool "ok" from the perspective of a homeowner, until it gets to a design day (hot).

Typical gas and go guy will hook up the freezon gauges and dump in the freezon until the low side indicates whatever the "tech" feels is correct. Now it is waaaaaaaay overcharged.

The only way to diagnose it is to check not only pressures, but subcooling and superheat, along with compressor amp draw.

If I had to guess, I'd say that your equipment is done.

Inefficient compressor, or improper charge need to be checked with subcooling and superheat.

I also do not see a trap on your condensate line. Most air handlers that I have seen pull air through the coil, and require a trap, or else it (the condensate) will not drain properly. If anything, this is the source of the drip in the picture. The coil drain pan is full of water and overflowing, because it can't drain. Read the directions for details.

dcg9381 · Apr 28, 2026

BurtEggley said:
1. Make sure the pan drains and does not leak.

Which pan? Pan below the unit was a **** show and absolutely did leak. I fixed it. I added a float because once the pan over-flows it's going to get expensive. I have seen the internal "pans" - the ones that divert flow to drains go bad.

BurtEggley said:
Lots of things from algae to dirt and bugs can plug it as others have said. AC units always shed water

That's the "main" condensation drain. Totally agree. I'm actually in the process of adding a 1" PVC pipe where I'll "feed" it chlorine tablets. All the condensate drains in TX eventually get "slimed up". I've already done a substantial correction by putting this unit on it's own dedicated drain line.

BurtEggley said:
2. Check the filter and coil. Make sure both are clean. Lack of air flow can cause freezing, Check the indoor fan too. Sometimes the cage can get so dirty it no longer moves air

Coil looked good on the filter side. My initial assumption was that I didn't change the filter often enough. But 30-45 days in with a new filter, I'm back to having the unit problem.

BurtEggley said:
3. have some one check the refigerant pressures, subcooling and superheat. While they are there they can also clean the outdoor coil and check the fan.

I can clean the outdoor coil, I do so every 1-2 years, I can tell you that it doesn't "look bad", but I'll go through it.
When we start talking about refrigerant pressures, I'm gonna tap out, I have the tools, but don't have the time. I say that due to the fact that if the pressures were off and I needed to add refrigerant, I have a bigger problem.

dcg9381 · Apr 28, 2026

danski0224 said:
How old is the equipment?

4-5 years.

danski0224 said:
How long from installation to first known incident of coil freezing?

4 years.

danski0224 said:
Pleated 1" filters?

No. 4" filter.

danski0224 said:
I have seen many pieces of equipment running like this, and usually they will appear to cool "ok" from the perspective of a homeowner, until it gets to a design day (hot).

It's actually only happened recently during "cooler" weather. Cooler weather being < 90 degrees.

danski0224 said:
If I had to guess, I'd say that your equipment is done.

F-me. Warranty card is up there, I'll check it, but won't cover labor. New home construction, the HVAC guys that did it ONLY do new home, they don't do service.

danski0224 said:
I also do not see a trap on your condensate line. Most air handlers that I have seen pull air through the coil, and require a trap, or else it (the condensate) will not drain properly. If anything, this is the source of the drip in the picture. The coil drain pan is full of water and overflowing, because it can't drain. Read the directions for details.

There is a P-trap. The condensate line is always a problem in TX and I've fixed it. This is not a plugged condensation line. At least not downstream of the air handler.

danski0224 · Apr 28, 2026

dcg9381 said:
No. 4" filter.

Not good. Put in a plain 1" fiberglass filter. No pleated filters of any type.

dcg9381 said:
There is a P-trap. The condensate line is always a problem in TX and I've fixed it. This is not a plugged condensation line. At least not downstream of the air handler.

Does not look like there is a P trap in the picture in first post. I see an open stub sticking up about 8" out of a tee.

dcg9381 said:
F-me. Warranty card is up there, I'll check it, but won't cover labor. New home construction, the HVAC guys that did it ONLY do new home, they don't do service.

Needs to be checked to pass or fail it.

dcg9381 · Apr 28, 2026

danski0224 said:
Not good. Put in a plain 1" fiberglass filter. No pleated filters of any type.

I'm an engineer, not a doctor, but the concept of total surface area comes up. Do I stuff a 3" spacer in there and put in a 1" filter?

danski0224 said:
Does not look like there is a P trap in the picture in first post. I see an open stub sticking up about 8" out of a tee.

You'll have to trust me on the trap. If you don't, I'll get my *** up there and take a photo, but it looks like this:

danski0224 said:
Needs to be checked to pass or fail it.

Yep. Got it. I'll open the air handler up.

Ohmthis · Apr 28, 2026

danski0224 said:
Not good. Put in a plain 1" fiberglass filter. No pleated filters of any type.

Does not look like there is a P trap in the picture in first post. I see an open stub sticking up about 8" out of a tee.

Needs to be checked to pass or fail it.

Can you please explain the preference for a 1”fiberglass filter? While they flow good, they aren’t the best at filtering. Unless there are very bad duct leaks, smoking in the house, or a ton of pet hair (just naming a few things) that filter should last awhile. Although it is on the smaller size of air flow for a 3 ton system. 4” pleats provide a ton of surface area for filtration. Maybe the system needs More surface area.

OP, do you have a way to test the static pressure before the filter and afterwards? That would help figure the flow part out.

Ohmthis · Apr 28, 2026

Looking up that filter, it is spec’d to a maximum of 1200 cfm (exactly 3 tons) if the installer cranked the ECM up to 450 cfm per ton (happens a lot if the flow charts are extrapolated, they pick the one that exceeds the flow of 400 per ton). That can cause some flow issues.

brewchief · Apr 28, 2026

Trap like that is useless if the tee is open between it and the air handler.
I don't know if Rheem used an aluminum coil but if so avoid chlorine bleach and other chemicals as they can damage them.
Drain pan is plastic so unlikely to be leaking.

It will be an airflow or refrigerant problem most likely.

PoorUB · Apr 28, 2026

danski0224 said:
Not good. Put in a plain 1" fiberglass filter. No pleated filters of any type.

dcg9381 said:
I'm an engineer, not a doctor, but the concept of total surface area comes up. Do I stuff a 3" spacer in there and put in a 1" filter?

This I disagree with this, stay with the 4" filter. It should handle air flow up to around 5 tons. A fiberglass filter will pass a lot a light dust and the next "filter" in the system is the AC coil and the dust plugs it up.

Check the blower wheel to see if it is dirty. Even a dusty blower wheel can take about 10-15% off the air flow. A real dirty blower wheel can kill air flow completely. You might have to take the blower out and clean it.

Make certain the air filter is clean.

Also, make certain there are no duct issues that could block some air flow, crushed duct for example. Also, make certain all the registers are wide open. I have been in homes where the owners closed half the registers because they don't want to air condition part of the house. It kills air flow and freezes the indoor coil.

Beyond all that, it might be time to get a good HVAC tech out and have them test subcooloing and superheat. The system might be low on refrigerant.

PopcornSutton · Apr 29, 2026

I changed out my whole heatpump system after 13 years of use, new air handler new condenser new lines. I changed filters regularly. I was SHOCKED when I looked at the bottom side of the evaporator coil on the old unit, was very dirty and partially blocked. With the new system installed, I cut an access door in the side of the plenum (updraft) so I can periodically remove it and look at the underdside of the coil, just for piece of mind. Once a year I flush the evaporator coil with detergent then clean water from the top down and let it all drain out. I modified a 1 gallon pump up sprayer with a long wand with a 90 degree bend to reach all the coil depth and spray perpendicular to the fins so the flow will go through.

danski0224 · Apr 29, 2026

PoorUB said:
It should handle air flow up to around 5 tons.

It doesn't. Not even close.

American Locomotive · Apr 29, 2026

A 4" deep pleated filter like that should have more than enough filtering area for a 3 ton furnace. Also, the filter isn't the issue because he's gone 4 years without an issue. He had a self-admitted lack of maintenance for that duration, too.

It's almost certainly freezing up because it's low on charge or there's an issue with the TXV/system blockage. When a system is low on charge, part of the evaporator will get way colder than normal and start to ice up, while the other portion of it stays relatively warm. With a fresh filter, there is probably just enough air flow to keep it from freezing. Those "durable aluminum coils" loovveeee to leak.

danski0224 · Apr 29, 2026

American Locomotive said:
A 4" deep pleated filter like that should have more than enough filtering area for a 3 ton furnace.

But it doesn't.

Manufacturer specifies 0.28" wc pressure drop at 500 fpm, which is 1388 CFM at a full 16x25 opening. Net opening is less.

Spec is for a new filter.

I've looked at enough coil pressure drop specs to reasonably state that just the filter and evap coil will exceed rated fan capacity.

Air handlers usually include the evap coil, but the total allowed static pressure reflects that.

That filter alone likely exceeds the allowable TESP rating of the fan. Never mind the ductwork.

When it's clean.

And those pre bent running traps are garbage.

There's definitely something wrong with it. The filter isn't helping.

danski0224 · Apr 29, 2026

That vent pipe on the coil drain is bypassing your "p" trap.

dcg9381 · Apr 29, 2026

danski0224 said:
That vent pipe on the coil drain is bypassing your "p" trap.

I don't understand. The vent pipe is upstream of the p-trap.

danski0224 · Apr 29, 2026

The trap blocks air from being sucked up the condensate drain, which allows for it to drain.

The vent provides a bypass around the trap. Air being sucked in will prevent the condensate drain from draining, if air is being pulled through the coil (vs being pushed, like a furnace).

cvairwerks · Apr 29, 2026

At 4+ years, I'll bet you need the coil to be cleaned. I'm north of the DFW area and we're reaching the point that even with a filter change every 28 days or so, the coil still gets loaded up with dust and dirt. That loading reduces efficiency which leads to cold spots on on the coil and localized freeze ups. Those trap more dust and dirt getting thru the filter and the cycle keeps repeating, until it doesn't take much running to freeze the entire coil.

PoorUB · Apr 29, 2026

American Locomotive said:
A 4" deep pleated filter like that should have more than enough filtering area for a 3 ton furnace. Also, the filter isn't the issue because he's gone 4 years without an issue. He had a self-admitted lack of maintenance for that duration, too.

danski0224 said:
But it doesn't.

Manufacturer specifies 0.28" wc pressure drop at 500 fpm, which is 1388 CFM at a full 16x25 opening. Net opening is less.

When did 3 ton require almost 1400 CFM, when everyone else says 1200 CFM?

I also have checked pressure drop across a 4" filter and didn't ever find numbers like you claim. Plus, like I mentioned an fiberglass filter will pass a friggen whole cat through it. they are just one step above no filter. I would never run one on an 90+ furnace or a unit with AC as you will plug up the secondary heat exchanger, or the AC coil. 80% furnace and no AC? Fine, run a fiberglass.

danski0224 · Apr 29, 2026

PoorUB said:
When did 3 ton require almost 1400 CFM, when everyone else says 1200 CFM?

That's what the *provided* pressure drop number works out to.

PoorUB said:
I also have checked pressure drop across a 4" filter and didn't ever find numbers like you claim.

Ok.

PoorUB said:
Plus, like I mentioned an fiberglass filter will pass a friggen whole cat through it. they are just one step above no filter.

I have torn out my fair share of old systems, and have never come across coils plugged with hair like cart wheels.

I recall one, and it was REALLY bad, like so bad that I have no idea how it could have happened.

There are at least two issues that are the likely cause of dirt getting past the filter.

Open filter slot- no door. Most systems I see are like this, and if the fitment is particularly bad, a not so insignificant amount of unfiltered air will get past the filter.
Poorly constructed furnace cabinets where the bottom leaks air. Many cabinets have a bottom that can be easily removed after taking out a couple of screws, and sometimes the fit is not so great. I have not got one in a while, but Ameristar furnaces used to NOT come with a bottom- it had to be ordered separately, or the installer had to come up with something else. I have seen plenty of installs where there are noticeable gaps in the bottom of the cabinet and even around the filter drop connected to the furnace where air gets in, past the filter.

A more restrictive filter will cause more air to bypass it, at least with the above scenarios.

BurtEggley · May 1, 2026

if you installed a factory type 4" or 5" filter housing when the unit was put in you can use a 4" filter. If your system was designed for a 1" filter use the fiberglass filters. Make sure all the vents in the house are open. If you have lost refrigerant, it can be anything. A leaky coil, a leaking schraeder valve, a line that has chaffed thru. Sometimes even a plugged drier. Someone is going to need to put gauges on it and see what the charge is. If it is low they will need to sniff the system to find the leak, then you get to decide it it is worth fixing based on the cost.

dcg9381 · May 5, 2026

OK, so I called for professional backup and apparently I'm a *******. I had the PVC upstream of the trap uncapped. Tech told me that this creates a pressure problem in the air handler and simply adding the cap back solves the pressure issue.

I had no idea......

Ohmthis · May 5, 2026

While that aids in the condensate, what is the issue with it freezing up?

dcg9381 · May 5, 2026

Ohmthis said:
While that aids in the condensate, what is the issue with it freezing up?

He said it wasn't freezing up if it was cooling... So I got that assumption wrong.

I asked him about another issue with the Rheem compressor. Basically one of the outdoor units is load as **** when kicking on (only after running a while). Basically sounds like if you clapped to boards together.

He said that Rheem has "scroll compressors" and when they are hot the pressures rise and they all do that... Loud start up. Typical of compressor type.

danski0224 · May 5, 2026

dcg9381 said:
they all do that... Loud start up. Typical of compressor type.

Uh huh...

danski0224 · May 5, 2026

dcg9381 said:
He said it wasn't freezing up if it was cooling...

Uh huh...

The unit is cooling as it hypothetically freezes up. Usually, airflow will significantly reduce or stop when the coil freezes.

Without putting a set of gauges on it and checking it out, no one really knows.

BurtEggley · May 5, 2026

not a clue. The condensate drain is just there to catch water as the condensation drips off the coil. The coil is not supposed to get below freezing when the airflow and the refrigerant charge are correct. To my ears, this conversation is sort of like saying the fish aren't biting because the rooster and the hens aren't getting together. Maybe someone else can explain the relationship between a condensate drain that is plugged, and a coil that freezes up.

Residential Air Handler help (freezing)

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