Size A/C system properly - DIY Chatroom

70stang · 03-20-2006, 10:30 PM

I have had my house looked at by 4 contractors and have received two "Manual J" reports.

The house is in CT. 2071 Sq. ft. single story ranch. Base board hotwater heat, and typical split system A/C in attic.

Current 3 ton system installed in 1971 when house was 1350 sq.ft. House was added to in 1996 and since then the current system cannot keep house cool (below 80) on hot summer days. I have to system turned low so its cold at night (66) to keep it livable during day.

R13 in walls
R38 in Attic
Anderson Low E glass windows installed in 1996.
Full Cellar with R19 in Floor

Contractor #1 in 15 minutes told me I needed a 4 ton unit.
Contractor #2 took 2 days send me a quote and recommended a 3.5 ton unit.
Contractor #3 took 2 weeks said he did a complete Manual J load calc. and recommended a 3 ton system.
Contractor #4 took a month mailed me a quote and his "Wrightsoft" Manual J load calc. report showing a result of 2.3 tons which he recommends installing a 2.5ton system.

Appears I have a large difference in size system.
Anyone have any advice?

Thanks

Aceinstaller · 03-26-2006, 09:22 PM

mustang,

something doesn't sound right with your estimated R-value.

hot in the day and cold in the night? with an R-value like your describing, this should not be the case.

Typical installation here in chicago for your home would be 3.5 ton. Your contractors need to take a good look at the sheetmetal to ensure that it is sized properly, and that the furnace is sized properly as well.

It sounds to me that you have built an addition without spending the money on properly changing the furnace and sheetmetal to accomodate your new load. even with a new properly sized A/C, you will still experience these problems until the whole system is properly sized to accomodate.

simply just tapping new runs off of the sheetmetal that was designed for the 1300 sq ft home will leave you with the exact problem that you are describing. an addition like that would have led you to replace the whole system, or add a new system with it's own unit for the new addition.

Ace