80% vs 93% AFUE furnaces

What sort of PVC piping is needed for one of these more efficient models? Just curious..

Typically 3".

-- If I had something witty to say, this is where I'd say it.

These are condensing furnaces, that is the exhaust needs to be routed horizontally out of doors and a limestone/marble bed provided underneath the outlet. Acidic water is produced and must be effectively dealt with.

Well, it's your house.

But when you keep the air circulating when the compressor is off some of the condensation water ends up back into the house rather than being taken away. And, unfortunately, those 3 speed fans use a signficant amount of power.

True.

Slight OT: my (our) first house had a thermocouple powered gas valve for the furnace. (This is still standard for gas powered water heaters.)

Were the electric to go out the furnace would still light up as normal. When the insides got hot, the fan controller would still attempt to switch on the fan but without electric the fan would not come up and the insides of the furnace would get hotter and hotter until the over temperature cutoff on the controller opened. When the insides cooled (from convection) the overtemp switch would open again and the cycle would start over.

Needless to say, they don't do it that way anymore.

But it is "nice" to have hot water when the electric is gone.