Can you mix a single stage and two stage furnace?

JeffB123May 5, 2012

After going through the posts here, I am concerned that I am being taken for a ride...

The compressor shorted to ground on my 2.5 ton Rheem air conditioner, and since the equipment on my 30 year old, 2100 square foot two story house had never been changed, I started looking at a full system replacement.

Since the original AC was put in, there was a second Amana unit added for the upstairs and a second furnace. The Amana unit has been working fine.

The current units are:

Attic -

Rheem Criterion II - 80 afue (couldn't find the model number no matter how hard I looked)

Comfortmaker G114000075-12M

Rheem Classic X - RPKA030jaz

Amana Distinctions - CKL24-IL - 1.5 ton

The Rheem is the unit that failed. Proposed replacements are...

Trane XV80, 60,000 BTU two stage gas furnace with variable speed blower to replace the Rheem Criterion II

Trane XR15 1.5 ton to replace the Classic X

Trane XR80 60,000 BTU single stage 3-ton multi speed blower to replace the Comfortmaker

The Amana will stay.

I have a couple of questions.

1. is it a good idea to mix a two stage and single stage furnace?

2. is it a good idea to leave the working Amana unchanged?

I know the Comfortmaker is a dinosaur (the manual is actually a series of stone tablets), but it is in the garage ... should THAT be the two stage if I only have one two stage?



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Sorry... I live in Georgia. Air conditioning is a matter of life and death here. Literally.

    Bookmark   May 5, 2012 at 11:00AM
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Disclaimer: Not a PRO.

That being said, a proper thermostat can control a single stage a/c with a 2 stage furnace. For a/c, blower speed is set via DIP switches on the furnace board.

My 13 stage mod and stat controls a 2 stage a/c. Could just as well been a single stage unit.


    Bookmark   May 5, 2012 at 12:30PM
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Your post is a little confusing. It is not clear which equipment is for the first floor and which is the second floor. I think it is this:

1st floor:
Comfortmaker G114000075-12M
Rheem Classic X - RPKA030jaz

2nd floor:
Rheem Criterion II - 80 afue
Amana Distinctions - CKL24-IL - 1.5 ton

There is nothing wrong with having one furnace be a single stage, and the other furnace being a 2-stage. The multiple stages are for comfort. A variable speed furnace will allow you get a more efficient AC. This would be attractive for your hot summers. If you are going to have only one 2-stage furnace, then it should be the one you use the most. I would think that would be the first floor furnace.

You have mismatched equipment which means the efficiency is unknown. The proposed furnaces seem oversized. It could be these are the smallest sizes Rheem offers, or the smallest with the required blowers for the AC.

I think the Rheem Classic X - RPKA030jaz is a 2.5 ton heat pump. The proposal is to replace it with a 1.5 ton unit. Did the contractor do a Manual J calculation to confirm the sizes?

    Bookmark   May 5, 2012 at 1:58PM
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You are correct on all counts.

The original equipment is certainly mismatched... that is the reason I am replacing so much at the same time. I want to maximize the efficiency of the entire system. I know you can best do this through matched components.

The original equipment on the ground floor is hugely oversized. I believe that the second floor components were added about 10 years after the original equipment to bolster efficiency.

I believe the classic X was originally thought enough to cool the entire home. Now, it just overpowers everything to the point where I occasionally have inside condensation on the windows.

The contractor did do a manual J calculation to determine the total current need.

Thank you so much for your help.


    Bookmark   May 6, 2012 at 11:10AM
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thanks for the advice... the difference in adding a two stage rather than a single stage is around $900. It is good to know that a good thermostat saves that money...


    Bookmark   May 6, 2012 at 11:19AM
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