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Discussion Starter · #1 ·
Hoping someone can help. Current furnace is sitting on top of 17.5 in box plenum. The furnace is old and undersized. The a/c coil is newer and is 21 inch. Looking to purchase 21 in furnace. Newer furnace is shorter by about 6 in. Can I use the existing 17.5 in box stand and build a transition which would be 17.5 in on the bottom, 21 inch on top to accommodate new furnace? Thank you.
 

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Ok. since there are no takers. Another question... Is it possible to set 21 inch wide furnace on top of 17.5 inch return air plenum stand? Any issues with the code or air flow restriction?
 

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Without a picture it's difficult to "see" what you have existing. I would probably install a new filter box, easier than screwing around with transitions, furnace weight and filter rack.

The furnace sits on top of a plenum box. The upstairs return routes to side of the box and downstairs return connects to the back of the box. Filters are installed upstairs and downstairs inside house in the return registers. I prefer not to touch the box furnace currently sits on due to lack of space. 21 inch box will not fit. That's why I am trying to find out if I can place 21 in furnace on top of the existing 17.5 inch wide box...
 

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Can I use the existing 17.5 in box stand and build a transition which would be 17.5 in on the bottom, 21 inch on top to accommodate new furnace?
Yes you can, that said, we have no idea what size equipment you are dealing with. The existing return and supply ducting may not be adequate enough as is and could be the root problem. Increasing the capacity of the furnace may not give you the performance boost you are seeking and could very well be much noisier.
 

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Discussion Starter · #6 ·
Yes you can, that said, we have no idea what size equipment you are dealing with. The existing return and supply ducting may not be adequate enough as is and could be the root problem. Increasing the capacity of the furnace may not give you the performance boost you are seeking and could very well be much noisier.
Understood. I'm trying to stay within the same output BTU range/CFM. The problem is that the coil on top of the furnace is 21 inch wide sitting on top of 17.5 inch wide furnace. I was just trying to match the coil size with the new furnace. So essentially I have two options, keep the same configuration and get 17.5 inch wide furnace or get 21 inch wide and match the width with the coil but in this configuration, 21 inch furnace would be sitting on top if 17.5 return air cabinet.

Also, the new furnace I'm looking at will have variable speed. Current furnace is single stage, single speed. Is there anything I should consider when switching to variable speed? Would my current A/C coil work with it? I'm just trying to find out if a coil has to be variable speed ready if there is such a thing.
 

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I'm trying to stay within the same output BTU range/CFM.
You initially said the furnace was undersized, do you have more of an airflow problem than a capacity problem? Installing a variable speed furnace without knowing exactly what size you need and what your existing ducting can handle is a pretty big crap shoot. At this point I just don't have enough info to give you sound advice.
 
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