What is max service temperature of GE Silicone II sealant caulking?

[Sum Guy]

I'm looking for some high-temperature sealant / caulking to use on my furnace (instead of using duct tape or aluminized tape). This is for seams in the usual places where air tends to leak out of the duct work and where the furnace connects to the ducts. This is not for the flue.

I expect the max temperature to be around 200C, and mostly between 50C and 100C.

I was searching the net for specs on GE's various sealant products, but can't find very much about max service temperature.

Here's an example of what I can buy at home depot (Canada):

This might be different than what's available in the USA (perhaps only the label is different, or maybe the formulation is different - I don't know).

Home Depot in Canada doesn't sell a caulking / sealant product that's specifically labeled as "high temperature", or furnace, stove or HVAC duct use.

What I did find at HD is 3 different 3M caulk products, labelled variously as "Fire Barrier" Sealant IC 15WB (and two others that had a similar application). These are designed or marketed as sealants that are applied in drywall seams and around pipes and wires as they pass through walls (drywall, brick, cement, etc) and I guess they're meant to contain fire or smoke for maybe an hour or two. I did find on-line cheps for one of them - and it had a paltry max service temperature of

48C.

I imagine that a job-shop HVAC supplier would probably have high-temp caulking, but I'm just curious if something like the GE Silicone II would do just as well (200C really isin't high-temp).

[A. Baum]

Well considering your hight temp cut off is around 90c I would work with that figure. Any Silicone will easily handle furnace ductwork from the plenum outwards.

[Frank]

I used a clear silicone, think it was the type I, to seal around glass doors on fire place. Holds up well.

[Sum Guy]

There are seams that run close to where the flue exits the furnace that will quite likely exceed 100C for extended periods.

[Jim Yanik]

Sum Guy wrote in news: [e-mail]:

go to any auto parts store and buy the High-temp gasket stuff made by Permatex. It will tell you on the package it's max temp ratings.

[Sjouke Burry]

I have seen 400-500 degree celcius mentioned for silicon and you can sort of verify that with a soldering iron. That should not make it melt down. At higher temps it turns into a white powder, without melting, and starts smouldering..

[Joe]

Years ago GE made a silicone sealant with red iron oxide filler that was rated for high temperature service. IIRC, there are still versions of this material available in auto parts stores and perhaps industrial supply houses. Silicone II in my experience is not a very good product compared to the the original types.

Joe

[The Daring Dufas]

Industrial and commercial cooking equipment supply houses sell high temp silicone sealant. What I have used is red in color it will handle up to

500°F. Look at Dow Corning 736.

TDD

[ransley]

Buy the Duct oastic sealer made for that job, its in gallon tubs and costs 1/5th to 1/3rd what silicone costs. The work is done every day by pros and they dont use expensive silicone. This stuff you just use a putty knife and it goes fast.

[Sum Guy]

Is it available in Canada?

Is it sold at retail, over the counter, at a place that's open past 5 pm weekdays or open saturday?

Is it sold at only one place in my city, clear across town in industrial east end?

Or did you factor in any of that criteria?

[Sum Guy]

I'd still just like to know how the home-owner-grade silicone caulking sold at the big-box stores would stand up to extended exposure to high temperatures.

According to this:

Grainger is listing the temp range of that product as being -65f to

400f. It appears to be the same product as this:

But I would love to track down an actual GE technical spec sheet for it.

[ransley]

Why should I factor in anything, im trying you give you help, a proven method, but your just being an ass. Anyway, Here I can get it retail, over the counter, at a place open till 8 on even sunday, and its not to far from me, sure you have it in Canada, so have fun looking, jerk.

[Hot Soup]

Like ransley said, mastic is what you need. It's sold at Home Depot/Lowes/etc in the States...dunno about Canada. Look in the aisle with the ductwork pieces.

Silicone will work if you like paying more for less. Your call.

Supply temps for a forced air furnace aren't 'high temp' - maybe 120F. That's well within the range of silicones.

[Sum Guy]

It's not the ductwork that I want to seal. It's more like the furnace itself, where the plenum meets the furnace, even some places inside the furnace cabinet, places that run close to some very hot surfaces, places where it would be hard to get a trowel into, places where a nozzle could easily reach and inject material.

One tube of silicone for maybe $6 - $8 vs I-don't-know for a pail of mastic. It's not the $$$ that's the issue here.

[Evan]

"Places inside the furnace" which probably have no business being "sealed up" as they were not sealed at the factory...

You do realize that even if you use mastic to seal the joints and seams in your duct work that mastic would have to be reapplied every time someone opens up those ducts to do service...

Why do you feel the need to make your furnace unit more difficult to take apart by adding silicone caulking to it in areas where it doesn't need to have it applied...

The goal is to seal the duct work to prevent conditioned air (whether heated or cooled) from leaking out into unconditioned spaces whether that be your basement or attic... Attempting to apply sealant onto parts of the furnace unit where the heated or cooled air are not exposed to is a foolhardy endeavor which will not result in any energy savings whatsoever...

As to your questions in your first posting:

NO, the "fire barrier" sealant is not that kind of caulking... It fills gaps where wires, pipes, etc. penetrate a wall and will expand to seal tightly around those penetrations when it gets heated by a fire and such sealants are considered "intumescent" and used in passive fire protection in the manner described...

Using silicone caulking on duct work seems to be silly since there is another product which is used by competent professionals for the one of the situations you wish to apply the silicone caulking to...

Is what you need to find... I am sure that you should be able to find a source for that mastic in Canada -- it may have simply been a matter of NOT KNOWING WHAT YOU WERE LOOKING FOR...

~~ Evan

[ransley]

You told him Mastic, thats a mistake, now he will want you find a store that has it, to pick him up, drive him to a store that has it, buy him a burger, drive him home and show him how to use it, all for free. But this guy should be paying for your advice now

[Sum Guy]

It's a 30-year-old furnace. Lots of slop in how things fit in some places. Rust in other places have opened up some seams.

Hey, I'm not the one advocating mastic. But then again, I don't think that ducts need to be opened up very often to do servicing...

When there is air blowing through or being sucked through seams, I think that's a good indication that it probably shouldn't happen. Like when heated air is escaping through seams where the ductwork is connected to the furnace. These are seams that are really never taken apart as part of normal servicing. And if they ever are taken apart, it's probably to replace something pretty substantial (like the entire furnace). It's not like I'm going to silicone seal any service doors or panels (hmmm...)

And it's also to seal the ducts and passages that pull return air into the furnace, to prevent the furnace fan from creating a negative air pressure around the furnace that would lead to air being pulled down from the flue and creating a reverse air flow in the burner chamber that cause combustion flames to leap out of the combustion area and into the area where the thermostat and gas valves are located, and heating and melting any wiring in that area.

Inside the furnace itself is where you will find places where conditioned air can escape and where incoming return air passages are not contained against leakage.

Again, it's not the ductwork (distribution runs) that I want to seal with caulking. It's basicically any of the ductwork in the furnace room itself, of which there is not really any ductwork as much as the main distribution and collection plenums or manifolds, air-filter enclosure, where those structures tie into the furnace, and even some places on or in the furnace enclosure.

Again, that material might be fine and appropriate to apply during initial ductwork fabrication and installation. It is not the appropriate product to use for the situation I'm describing.

Interesting item listed in it's specifications:

-------------- Odor: Will not contaminate sweet butter

--------------

I'll remember that next time I have some sweet butter nearby.

Doesn't really say anywhere if it's hard or flexible when cured.

[Sum Guy]

ransley unnecessarily full-quoted:

No, because I don't want to use mastic.

I just want to know what the max service temperature that GE Silicone II can be exposed to.

But nobody can answer that question, so they go off on tangents about this mastic shit.

[DanG]

Pure silicone is not good about staying stuck to materials for the long haul. You would be better served to look into hard cast duct sealant. It is normally applied with a paint brush, roller, or putty knife. It is also offered in caulk tubes. Here is an example:

~~~~~~~~~~~~~~~~~~~~ DanG Keep the whole world singing . . .

[Sum Guy]

DanG top-poasted and unnecessarily full-quoted:

I doubt that consumer-grade sealants are pure silicone. And besides, the tub and bath and exterior door and window sealant product seem to stick pretty well for the long haul.

Again, all I want to know is the max temperature that GE Silicone II (door and window) can be continuously exposed to without breaking down, running or coming off.