When you clean the evaporator coils, also check the condensate drain line is not clogged.

My question is:

Why is this smell only evident when I turn on the AC? The heater has no bad smell when turned on and the airflow should be going over the same equipment. It is that the heater is blowing moist air over the coils and thus mildew and such is building up and when the coils are active in when the AC is on, that is when the smell gets permeated to the airflow?

Thanks in advance.

Rusted evaporator coil end plates are unfortunately common:
* Rusty evaporator…anything that can be done to slow?
* Excessive rust on evaporator coil?
* 14 month old Trane rusting

The above links are by HVAC-Talk.

Rusted end plates don’t necessarily mean the evaporator coils are leaking refrigerant, especially if the aluminum fins and copper tube packs are corrosion free.

Take a look at this slides 4 to 8 of this Evaporator Coil Assembly presentation for a general overview.

Better evaporator coils such as the Trane All-Aluminum Comfort™ Coil feature all aluminum coils, including aluminum end plates and tubes. The inner coil panels are painted to resist corrosion and can be removed to access the inside of the coils for cleaning. This article illustrates the difference between the Trane All-Aluminum versus copper/steel coils after a 500 hour salt spray test.

While I not owned a Trane All-Aluminum Comfort™ Coil unit, I had an old AC system at my former home in Florida replaced with a Trane XL 16i system and was very happy with it.

If you want to do something about the rust on your evaporator end plates, paint the end plates with COLD GALVANIZE Corrosion Inhibitor. COLD GALVANIZE is very high in zinc which acts as a sacrificial anode, protecting the steel from rust. Turn off the AC system, open up the air handler to expose the coils, wait until the end plates are completely dry, be careful to protect the air handler cabinet and coil fins from overspray and apply the COLD GALVANIZE Corrosion Inhibitor, allow it to dry, then close up the air handler. I would only do this if your system is past the manufacturer’s warranty or before installing a replacement coil at your own expense. If your system is still under warranty, hire a licensed HVAC technician to do it for you if he says it won’t void the factory warranty. However, you will only be able to access the outside face of the steel end plate so it’s only addressing part of the rust issue.

All said, I would let the rust be and buy a better designed system when the time comes.

Sir, you published a picture of your air handler/evaporator coils back in 2009 and claimed that the unit was 9 years old. You pointed out the rust on the frame and some discoloration on the copper tubing. I had a Carrier unit installed 2 years ago and after about a year and a half started noticing rusting condensate coming out from my pvc drains. Looking inside I was shocked to see my 2 year old air handler look almost as rusty as your 9 year old one. I am worried that this is premature deterioration but my A/C company does not want to pursue it and claims that my cleaning of my drains with bleach may be partially responsible. Any ideas or suggestions? The unit is in my garage in Florida but my prior 14 year old Trane unit did not appear significantly more rusted than what I see now.

> Jeff also wrote:
> This allows air to be sucked back up the pipe…
The blower motor on my system pushes air into the evaporator coils as illustrated in the 2nd photo of the project. This creates a positive pressure in the coil box that will not “suck [air] backup the pipe”, rather it pushes air and condensate down the drain line to the outdoors.

The condensate drain line is in the original configuration as the HVAC system was installed; I have not modified this.

=== Reply to Jeff’s 2nd comment ===
> Jeff wrote:
> One more thing. The incoming cold liquid line should also be insullated to retain the “chill”.

What you describe as the “incoming cold liquid line” is incorrect.

The refrigerant liquid line is very warm-to-hot as it leaves the outdoor compressor unit; a result of compressing a gas to a liquid. There is no “chill” on this side of the refrigeration cycle; place your hand on the copper tube and you’ll feel it’s quite warm. This is why it’s an uninsulated copper tube all the way from the outdoor compressor to the attic air handler because warm lines don’t cause condensation and sweating.

The refrigerant liquid line was installed this way when the home was built and is the standard method of doing so; I have made no modifications or given advice to make changes to the liquid line in the project.

BTW – the HVAC tech who installed my system neglected to install the foam refrigerant line gasket. I found my gasket in the plastic bag with the owner’s manuals. To limit the air leakage, I partially sealed the access panel opening with HVAC tape.

Here is a picture. If you zoom in, you can see the black seal. http://i89.photobucket.com/albums/k238/honeychildjamie/gasline.jpg

Thanks, Bob!
Jamie