Enlarage vs. add 2nd zone?

[sngnguy]

We are adding approx. 1,000 sq. ft (500 with a raised ceiling) to our 2400 sq ft ranch-style house in the SF Bay area. The current forced-air furnace is a 5-ton unit that is only 4-years old, and since the addition is on the opposite side as the furnace/AC, we are being told we need to add a second furnace/AC. They did some calculations, and said it was borderline, they couldn't guarentee that by using dampers, etc that could make it comfortable as the duct run is very long.

When we bought the house, that end was always colder/hotter as the season dictated. We enlarged the ducting and bought a new furnace 4 years ago, and things have been great ever since. I had thought we had enough tonnage to handle an addition, but now it seems like it is not.

Guess I would like some experienced views on getting a larger furnace and expanding the trunk duct so we can get air to that side, versus adding a 2nd zone, which at first glance seems excessive.

Thanks in advance.

[florida pe]

With a house that size, you will definitely want to add another system as you will only have 1 thermostat to control the entire house (3400 sq ft). There is no way that everyroom will be comfortable at all times with one large system. It is almost always better to go with two smaller systems. Plus, if one goes down, at least a portion of your house would be heated/cooled. Also, you can't get anything larger than a 5-ton for residential. You can get a larger furnace, just not a larger air-condioner.

[dash]

I'd add a zoning system,but if you need morethen 5 tons,you need a second system.


Only a properly done load calculation like Manual J,from http://www.acca.org will tell you the system size that is needed.You good have the calc. done with greater attic insulation then you currently have ,to see if that would reduce the size needed.

[kayjh]

I am going through a similar process with a new A/C installation now. I have a 3,200 sq. ft. ranch style house. The first contractor specified a single 5 Ton unit. The other two contractors specified 2 units. Contractor "B" said one 2 ton for the three bedrooms and halway and one 3 Ton unit for the LR/DR/Kit/Den. Contractor "C" agreed with contractor "B", but specified 2 - 3 Ton units. Contractor "C" has done other plumbing and HVAC work for us (commercially) over the years and is a meticulous installer - so we are going with him.

The installer basically told us what you have been told. One unit will have a difficult time balancing the whole house. His question for me was: If you are entertaining a large number of guests in the LR/DR and also using the bedroom areas, where will the thermostat be? If it is in the bedroom, it will either be too cold there or too hot in the LR/DR. He advised that a damper system is often not as effective as a dual system.

We're going with the dual system. If you aren't sure, get some quotes from other established contractors in your area and get them to explain and justify their recommendations.