[roughneck]

An increase in size should never be considered an upgrade.
You definitely have airflow/ductwork issues if you need 5 tons for that little amount of space.
A variable speed unit is no excuse for oversizing.
A bigger unit is not better.

 

[roughneck]

Thank you.

The current furnace is 97,000 BTUs. Do you think a 4 ton furnace with 120,000 BTUs and an ECM variable speed motor would be better?

Equipment isn’t sized via square footage. You should really have a proper load calculation done and see what size equipment is required. And then calculate required airflow from there.
Bigger equipment isn’t better.

 

[roughneck]

Thanks for your feedback. I will look at 4 ton 2 stage compressor and furnaces. Each zone has its own supply plenum and all the ductwork coming from each supply plenum is 6" flex.

No, you could very well go smaller then 4 ton.
The flex duct should ideally be kept to runs of 5’ or less. Everything else should be hard duct.

 

[roughneck]

Unfortunately the majority of the flex is longer than 5' and changing it now would mean ripping open walls and floors.

What’s the static pressure and CFM readings on the existing system?
The overuse of flex/ductwork layout is likely why your performance is poor.
How many 6” ducts are there?
DO NOT just buy another 4 ton unit. You very well could go much smaller and get more cooling then you do with a larger unit.

 

[roughneck]

In a perfect world you have the supply duct capacity of 1500 CFM. I’m betting you’re likely far below that if the flex wasn’t installed really good. It’s usually not.
You likely don’t have enough supply air capacity for 4 tons. Let alone a zoned system.

 

[roughneck]

You’ll need to test static pressure with a manometer and really should calculate CFM.
Zoning is really hard to get right. A lot of hvac companies simply get it wrong. And many don’t care.

 

[roughneck]

Thanks.

Test the static pressure at each supply register with all zones open?

No, supply and return static and then TESP.

 

[roughneck]

Post photos of the installation you have now.

 

[roughneck]

Thanks. When I inspect the flex it looks like its in good condition and as straight as it could possibly be. Would it be beneficial to increase the readily accessible flex ducts to 7" or 8"?

The gains would be very small.

Flex MUST be pulled tight, well supported and when used in turns, the changes in direction must be long sweeping radiuses.
It can be crushed, pinched, or turned tightly. It also cannot lay bunched up.

 

[roughneck]

Thanks.

Of the 21 supplies I have (21 x 75 cfm = 1575 cfm) If i changed the 8 or 9 ones that were accessible (maybe 2 in each zone and the ones with the longest runs) to 8" flex, I would increase the flow to 2340 cfm so I am gaining a good amount of airflow.

That’s only if all the flex is installed well. This is where testing airflow with the current system comes in.

 

[roughneck]

Need to measure static as well. Newer ECM blower motors don’t like high static, and usually live a very short life when subjected to it.

 

[roughneck]

Hello.

There are 23 supply lines and all of them are 6" flex duct. They all vary in length with some being very short and others being very long (the ones that go through the attic). I was going to change 11 of them (the ones that go through the attic) to R8 insulated 8" flex duct and then fix all the other current 6" flex duct to make them straighter, reduce sag, etc.

It would be best if you replaced what you could get to with metal.
As said before flex isn’t intended to be the only material used to build a duct system. But it is as it’s cheap and companies can use low cost unskilled labor to install it.
Homes with all flex systems with airflow problems are all too common these days.