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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?

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    What is he suggesting instead? – Tester101 Oct 9 '14 at 9:53
  • He suggested a 2 speed fan unit. – Mark Witczak Oct 12 '14 at 1:36
  • I talked to 3 more furnace people and none were concerned. They said the variable speed fan would work fine. One showed me how (and where) to adjust the duct baffles to balance heating/cooling airflow between the floors. – Mark Witczak Oct 12 '14 at 1:43
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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.

  • It's my understanding that the stack effect only works in an open airway. Since I have floors/ceilings between my first and second floor, the stack effect doesn't help move air throughout my house very effectively. Commonly the only place where air can move upward, is through a single stairway. The problem is that all the cool air upstairs that the warm air is to displace, has to also move through this same singe path. My upstairs is warmer in the summer, and cooler in the winter. I have a high efficiency, low blower speed furnace. – Tester101 Oct 9 '14 at 9:48
  • Ah now I get it. He's referring to the end of the run. If it were in the attic he'd say the same about the basement. I'd ask @tester101 if he plays with his registers bi-annually to adjust the temp difference of the floors or if it's a negligible factor. I've installed them but I don't have one myself and now that I think about it, most of them have been split systems. – Mazura Oct 9 '14 at 23:13
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Air ducts are designed for a specific cfm, as the thermostat starts reaching set point and throttling back the cfm the variable blower will not produce adequate air flow to the second floor to the registers at the end of the run. Most air ducts are not perfectly designed. Second floors need more air flow in the summer. Heat rises in the winter.

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You could tandem a thermostat upstairs, but you will always be wasting heat downstairs. The proper solution is two furnaces or space heaters upstairs. I have a separate zone upstairs with a thermostat, but because we mostly only use one room up there I heat that room with a space heater.

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