what is fill-trol expansion system for furnace?

Answer 1

Well...furnaces don't use them, but boilers do.
The water in the boiler, when it heats up, expands. If there is no place for the pressure to go, it will blow the pressure relief valve off. So you need something that will compress, like air. The "old style" used a tank, with one pipe to it that sat above the boiler. This tank was mostly air, so the pressure would compress the air and not over pressurize the system...but those tanks tended to eventually fill with water, and had to be drained periodically.
The other thing boilers need is an automatic fill system, that maintains a base pressure in the system, and if it falls below the set-point, adds water.
A "fill-trol" is a combination expansion tank and fill valve. The tank has a diaphram in it, so it won't become waterlogged like the old tanks. The water and air are in separate chambers. The fill valve is a part of the assembly. Also, it has an air separator, which is just a big fitting where the water flows through, slows down and any air bubbles in the water are diverted out to an automatic air bleeder.
You can have any of these devices installed separately, but the "fill-trol" puts them in one package.

Answer 2

It is a small tank mounted on top of the unit looks like a portable air tank inside of this tank is a rubber bladder that exspands and contracts when pressure builds typically around $30.00 bucks maybe a little more depends where you buy it.