I need experienced opinion on how to size a 2-stage gas furnace.
The concept of a 2-stage furnace seems very efficient, if properly sized. To me, it seems logical to target the high heat stage to be slightly above the heat loss when considering extreme design conditions. The low heat stage would target the more typical winter conditions. Is this stream of logic not valid?
We contacted 4 heating contractors and obtained proposals. All 4 briefly inspected the existing furnace and asked a few questions about the duct system layout. None of the contractors performed a detailed load calc - Manual J or other. All 4 contractors proposed units that are much greater capacity than the original unit. (proposed units ranged 60 - 75k BTU). The low heat stage on these proposed units ranged from the same as the original furnace to notable more than the original furnace.
The original (existing) furnace is a Goodman 80% unit. It's gross input rating is 45,000 BTU/Hr. The furnace seems adequate. Temperatures throughout the house have been reasonably consistent. Even on the coldest days (overnight below 20 deg F), the thermostat setpoint is maintained. The system seems to cycle ON-OFF a lot, with the OFF phase being longer than the ON phase.
As a point of reference, I performed a load calc based mostly on ASHRAE Fundamentals Handbook. I estimated a heat loss load of about 35,000 BTU/Hr. This supports the original equipment rated at 36,000 BTU/Hr output.
Is my line of reasoning valid for sizing a 2-stage furnace?
When referring to an over-sized heat output, what multiplier would be consider "definitely over-sized" vs possibly acceptable?
(one salesman suggested that 70% over the design heating load is within the normal range.)