What sealant should I use between AC gasket and roof?

Chris Bryant wrote:
The trouble with using a sealant is that the gaskets are expendable- they will need replacing at some point. If the gasket is sealed to the roof, you've added a huge amount of time to the job- getting the roof clean and smooth.

This I learned, but the old gaskets did last 20 years before the refrigerant blew out......

Installed the new AC without sealant although I will not last as long as the old gaskets did.

The trouble with using a sealant is that the gaskets are expendable- they will need replacing at some point. If the gasket is sealed to the roof, you've added a huge amount of time to the job- getting the roof clean and smooth.

sjholt wrote:
If it was mine, I would smear a light coat of RTV on the none sticking surface as additional insurance. I did say if it was mine.

It's your rig, you can do anything you want with it but no A/C manufacturer recommends the use of any sealant, just a properly torqued rubber seal affixed to the underside of the top unit. The only "additional insurance" required is periodically checking to make sure those four bolts securing the top unit in place are properly torqued, which I think is the message many in this discussion have been trying to relay to the OP. ;)

If it was mine, I would smear a light coat of RTV on the none sticking surface as additional insurance. I did say if it was mine.

X2. If you want to do the job right, grab a clicker torque wrench from Harbor Freight, then find the maker's torque recommendations and use those. Hard to go wrong that way.

Old-Biscuit wrote:
NOTHING!!

Clean roof area around opening, Clean A/C Unit pan where old gasket was.

Install new gasket and set A/C Unit.

Tighten the 4 hold down bolts finger tight in cross pattern.
Then slowly 'snug' them up until gasket has been compressed by half (1" thick compressed to 1/2")
DO NOT OVER TIGHTEN.........just hand snug (Can crush gasket, warp roof and/or A/C Pan and cause leaks)

X1

The Gasket IS the sealant. You do not use any other product. Just the gasket, That is what gaskets do, they seal.

Thanks for the replies.

I just removed my, at least 13 year old and likely 20 year old, Dometic Penguin AC unit. It was held very solidly to the roof with sealant.

I was surprised and disappointed in the extra two hours required to remove the old AC and clean the residual with a scraper and acetone.

Hence my question. I will put the new one on with no sealant!

No sealant required, just proper torque on bolts.

Golden_HVAC wrote:
I worked on the emergency vehicles at my work with rooftop A/C units that where leaking. The gasket had compressed itself while it was parked, and now the bolts where loose. Like I could turn them 3 turns before they made contact with the inside frame of the A/C/ Then I tightened with a nut driver.

You can use a socket wrench and extension to make it easy for you, but do not "Tighten" beyond finger tight and about 2-3 more turns. Then check it again in a month or so, and snug it down a bit more if required. After 2 years, check it again.

The bolt is probably a 3/8" X 18 threads per inch. So each turn is 1/18" compression on the gasket. 2 turns = about 1/8".

Good luck,

Fred.

Dometic lists an actual torque value when you originally compress the gasket from it's 1" to 1/2" size and that is approximately 40 to 50 inch-lbs so you might use that value to retorque a gasket that is older.

That mumber is from the 6-03 579 installation and operating instructions.

Larry