Reducing duct size in old home

[bazagee]

Hi All,

I've just purchased a circa 1939 4,000 ft custom built home and opening the walls to renovate.
The house has two 1989/90 condensing furnaces suppling the same Plenum. I see that most of the vertical ducting to the second floor is 12 x 3 1/2 or 8 x 3 1/2 (returns?). As I reno the house over time I'd like to replace all of this vertical ducting. Since I'm not likely to find defusers (currently wall mounted) I was thinking of replacing the vertical stacks with some more standard sizing like 10 x 3 1/2.

Most all of the rooms have two supplies with one return. I understand that at the time the house was build it was probably ideal to have as large a duct as possible probably due to the inefficient HVAC systems of the era? I'm wondering now if I could get away with the reduction in CFM circulation into the home? We live in NWOntario and with only one furnace running the house seems comfortable.

I know an HVAC company could come in the replicate what is there but it strikes me as being a more expensive option than something that was standard off the shelf?

What do you say?

[Admin]

I don't understand why 2 furnaces would be sharing the same supply?  Those duct sizes aren't familiar to me.  What's the BTU rating on the furnaces?

[bazagee]

I don't understand why 2 furnaces would be sharing the same supply?  Those duct sizes aren't familiar to me.  What's the BTU rating on the furnaces?

I'll have to check the BTU on each. Ducting is for sure custom. Unless they 'split' somehow internally I don't see two separate Plenums it all looks like one to me - but then I'm use to hotwater boilers not forced air.

[Admin]

If you have 2 thermostats, try turning the fan on one furnace and see if there's air coming out of all the vents.  Then switch the other furnace fan on and check again.  It's probably zoned, so each furnace supplies separate parts of the house.  You need close two 150, 000 btus.  You probably have to 75, 000 btu furnaces.

It might be worth the service call to have a sheet metal worker have a look at your options.