Single, 2 stage, vs variable speed furnace

sirzoltanMay 4, 2011

Hello folks!

I need some help from someone with experience out there please.

I am deciding on a new furnace/refrigerated AC system for my house.

1220 Square feet total need heated and cooled via _one_ main duct that is connected to 3 bedrooms, 1 bathroom, the living room and the kitchen. The furnace is located in the "middle" of the house between the 3 bedrooms with the return air located directly underneath the furnace.

I was told that I would probably NOT be happy with a variable speed furnace because it wouldn't distribute the heat well out to the walls of my house. Does this make any sense to those of you out there that know more about this stuff than I do? I think the main concern here was that on a lower setting there wouldn't be enough force to spread the heat out before it returns to the furnace.

To make a longer story shorter [I hope!], the main reason I wanted a variable speed furnace was for the reduced noise it offers. I have very sensitive hearing and a very [IMO at least] quiet furnace currently [although it is 37 years old].

If you have any comments at all, please don't hesitate to comment!

Many thanks in advance,


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I think the argument against variable speed is probably wrong but....

If this opinion is based on a thorough inspection of your ductwork, then it might be right. Obviously lots of us have variable speed blowers and don't have an issue (in much larger houses). Generally, a lower fan speed leads to more even heat distribution.

Do you have dampers? Do you have decent insulation?

In my new house, I have registers at the walls - which is a pretty old school idea. The idea is to heat at the walls since that is the coldest part of the house. Well - the reality is that it doesn't matter much on a new house. But in a/c season, it does probably help to be near windows since solar gain is what you are fighting.

It doesn't take much airflow to get heat away from the furnace...

    Bookmark   May 4, 2011 at 9:09AM
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You may be misunderstanding the terminology. The term "variable speed" refers to the blower in the furnace. So, it is possible to have a single STAGE furnace, with variable speed blower. Your typical (older) furnace may have a multi-speed blower. where the installer 'selects' the required (single) speed through electrical taps.

Further, in the above case, the speed of the blower is not variable (i.e., you don't have a choice of fan speed, while heating. As with any gas furnace, the fan speed must be selected, such that the air flow through the furnace will produce a temperature difference (known as temperature rise) of the return air to the outgoing air, within specified limits. What a VS blower WILL do is ramp up in speed if restrictions (clogging filter, poor duct system)prevent the selected airflow from flowing. It will (with proper thermostat) reduce airflow in cooling to increase dehumidification.

A 2-stage furnace (also available with VS blower)and proper thermostatic control, provides greater comfort from having longer running cycles. NOTE: A 2-stage unit provides greater comfort, not greater efficiency.

Carrier/Bryant produce a 3-stage furnace, which they refer to as 'modulating', but there are larger gaps in output between stages.

A modulating furnace provides many stages of heat and utilize variable speed blowers. Again, the ultimate in comfort, but not more efficient than a single stage unit of the same AFUE.

You should note that newer furnaces blow a lot more air than older units of the same size. Combine that with a VS blower, if the duct system is not sized for the airflow, the system will be noisy and efficiency may suffer. So, proper sizing and duct system analysis is essential.

On a personal note, I replaced my oversized SS furnace with a Rheem Modulating unit (13 stages). Before changeout, the upstairs was always cold; the downstairs would warm, but would chill before the unit would restart. I had to use a space heater in an upstairs bedroom (computer). Now, I have relegated the heater to the basement, and frequently sit at my computer in short sleeve shirts.

Insist on a manual J (heat calc) and duct analysis.


    Bookmark   May 4, 2011 at 1:33PM
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