RTV Cauking?

Dude, you cannot patch a heat exchange, it needs to be replaced. It's a deathtrap waiting to happen.

Time to replace it. CO poisoning rings bell to you and yours? Want to take chance? Tony

It has to be replaced unless you want to die.

I don't think you will find any RTV Silicone that will hold up under those conditions, but a muffler patching compound might. They sell auto muffler "bandages" that use glass cloth and a sodium silicate adhesive that I've used to patch a hole in kiln. A couple of layers might last for a while if the hole is not too large; it doesn't have a lot of strength.

What's interesting is that everyone here has said "you will die". I understand the concern. However without being patched the system is not giving off any CO through the ducts. This was measured by a HVAC individual. Additionally, Even if you put the probe right over the hole it registers pretty much nothing. I was told that for the time being I can use RTV heat resistant cauking (red) to patch the rusted area.

Obviously I have CO detectors in my house, and do plan on replacing the furnance eventually.

Several people died a few years ago in NC because of a leak in a heat exchanger. The people had complained several times to the public housing commission because of smells but nothing was done. After the deaths they found several others leaking in the same complex. CO will only form if the furnace is not burning with enuf oxygen so it should not really happen unless there is a malfunction. But I would rather gamble in las Vegas.

This is Turtle.

OK it's your life and family at stake here SOO.

Go down to auto parts house and get you some Liquid steel / Steel Putty / Steel Max or what every they call it and mold a plug of it over the hole and wait about 6 hours. then fire it back up. Try to get it molded over both sides if you can. Do change it out as soon as you can for CO reasons if the furnace burning ratio become messed up.

TURTLE