View Full Version : Two stage AC unit with single stage blower
05-21-2009, 01:20 PM
I'm looking to replace my 30-year old AC system and I'm interested in the variable and/or dual compressors that I've seen from American Standard units such as Allegiance16, 18, or 20.
My furnace was replaced in 2002 with an American Standard 80 single stage unit. One contractor said that I will not get the variable compressor performance because of the limitation of my furnace blower. The other contractor said this is not an issue.
Can anybody comment on this situation? I'm not sure what to believe.
05-21-2009, 02:56 PM
You won't get the humidity control that you would with a VS blower.
Your current blower wont' slow down enough for the coil to get cold enough in first stage.
There are VS blower upgrade kits your contractor could install.
I'd be very concerned about gettitng the correct airflow(cfms) required for each stage,without VS .
05-21-2009, 10:19 PM
We were told by carrier if we make a NEWer 58clr120 into a variable speed they would not stand behind the heater.
05-22-2009, 12:30 AM
I talked to the contractor who installed my furnace (an American Standard AUD120C954, 5 ton) and he insists that the compressor in the American Standard Allegiance 16 unit (4A7A6036B)
will operate in the "two step" mode - independent of the fact that the furnace is single stage.
Is this contractor mistaken on this subject? He seems very sure of himself.
Each step/stage needs to have to correct cfms(air flow).
This can be accomplished using different speeds of the fan motor,but the duct system needs to be designed to do so,not likely the case.
05-22-2009, 06:12 AM
He right, the outdoor unit will run in either stage.
So when the outdoor unit is running in its first stage. Your furnace blower will be moving/trying to move 1200CFM through the indoor coil. When it should only have 800 to 900CFM.
So it will cool your house. But not remove moisture very well. And you can end up having to set your thermostat to a lower temp to be comfortable.
And use more electric then your 30 year old A/C did.
05-22-2009, 10:12 AM
Thanks for the responses, everybody - this is very educational. I think I'm getting a handle on the situation better now.
The topic of air flow came up with both contractors when I complained about the noise that my blower makes - it's very annoying in every room of my modest-sized house. They both said they could turn down the speed of the blower with some dip-switches in the furnace. My original contractor seemed to discourage the idea of upgrading the blower to variable speed after calling American Standard for upgrade possibilities. He quoted about $ and said that American Standard doesn't recommend this modication. Maybe American Standard just wants to sell a whole new unit?
I suppose I should inquire about the proper setting to accommodate the dual stages of the compressor. Presumably there is a compromise level of CFM that would be a tradeoff between the two stages of the new compressor, correct?
05-22-2009, 11:44 AM
A. No prices in post.
Exceptions. Electric, gas, oil rates, factory rebates, tax credits.
B. A compromise in CFM between the stages, is still poor dehumidification in first stage. And a possible higher electric bill.
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