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I just had an HVAC guy out to give me an estimate on a new Carrier Infinity series furnace and he said the second floor rooms will always be colder because the thermostat is on the first floor and not as much air is force up there when the blower is at low speed.
Is it true that a furnace with a variable speed blower is not suited for a two story house with the unit in the basement?
I hope the 90K is the output or it's truly undersized! Even so, it's quite common to oversize furnaces beyond the calculated heat loss by 20-30%. It's also possible the heat loss calculation is low, it's really just an estimate, and part of why the furnaces are oversized. Even if it is undersized, it would not significantly alter your energy costs. Costs are dependent mainly on the home's overall insulation and the furnace efficiency, the capacity and running time is not a big factor.
Changing ductwork will better distribute the heat, but if more heat is leaking out than the furnace can generate, it will not help. While a larger furnace will better heat your home, it will not help your energy costs. If your costs are out of line for your type and size of house in your area, then more heat is being lost than is typical. In the long run, you are better off preventing heat loss than generating more heat.
There are energy consultants in most areas that have the proper tools and equipment to advise you where most of the heat is being lost and what could be done to most effectively reduce heat loss.
According to the manufacturer's documentation, your furnace should have a 3 or 4 speed blower motor. The problem is, the furnace control module doesn't use a separate speed when the unit is running in continuous FAN mode.
When the R-G circuit is made, the blower motor operates on heating speed.
The control module only uses two speeds, one for COOL, and one for HEAT/FAN.
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You could run the blower at a slower speed during both HEAT and FAN calls. But you'd have to insure that the blower is maintaining adequate flow during heating, otherwise you will damage the system. This is not recommended, as you could damage the system, and void the warranty.
You might be able to swap out the control module, but you'd have to contact the manufacturer to determine if there's a compatible replacement.
The other option is to build your own control circuitry, but that's beyond the scope of this site. And will likely void the warranty on the unit.
Best Answer
Does the ratio of static pressures between two stories differ at different blower speeds? That's a very interesting question, but I believe it's a moot point. Any dramatic difference would mean your system isn't balanced correctly in the first place. (needs zoning, split systems, or an actual balancing)
Stack effect will cause plenty of heat to go upstairs, while the cold air will settle. I'd be more concerned that it won't cool right. Except that furnaces usually only run at high speed for AC because cold air is denser, making it harder to push. If your old system heats and cools just fine I don't foresee a problem.
Without being in the correct centralized location (which is somewhere on the first floor of a two story), the thermostat will not control the house properly. In low-fire mode, indeed less air will go upstairs and downstairs, per minute. Blowing softly is what makes it more comfortable. It will run longer and more often, reducing the intermittent temperature differential, keeping the house nice and even.
So no, the same amount of 'less' air will be coming out below as above. Your contractor either is trying to up-sell you into a split system (or knows more than I do) or convince you that zoning is required (a possibility, however this can be retrofitted later at your discretion). Either of these additions would dramatically increase the cost.
Unless your house has automatic zoning and was balanced by a genius, it will always be hotter on the second floor in the winter, and colder on the first in the summer, just like everyone else's.