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Oversized or Just Right

Posted by schroads (My Page) on
Thu, Feb 16, 12 at 0:59

I have posted previous about comparing quotes and received some good advice. I have received 6 A/C quotes and am looking for a general sense that I am not being taken for a ride with sizing. Before we make a choice or narrow it down to two for a final comparison, I want to make sure nothing is outrageous.

Four of the quotes note a Manual J Calculation and I saw two of those calculations from the respective contractors. These four quotes all note a total central A/C (whether conventional or high velocity) capacity at 4.5 or 5 tons. The numbers were close to 4.5 or just over 4.5 for the ones I saw. The other two contractors did not discuss Manual J and sized me at 3.5 tons each.

I live in New Jersey in a 2 1/2 story, 1920 foursquare, stucco exterior that has radiator heat (water) with a natural gas triangle tube prestige excellence combi boiler for heat and hot water. There is a third floor family room (450 sq. feet), 4 bedroom second floor (950 sq. feet) and a three room first floor (1100 sq. feet). There is no insulation in the outside or interior plaster & lathe walls. I do have rigid foam insulation and air sealing in the basement, with blown insulation and air sealing in the third floor knee wall. I have 31 original, double hung, single pane windows with new storm windows. The back of the house has a southwest exposure with no shade so the suns beats down in it in afternoon.

I think that four contractors that used a Manual J all stating 4.5 ton or 5 ton means I need a 4.5 or 5 ton system. I just want to make sure that it not outrageous sizing for the description above.

As an aside, the two contractors with 3.5 ton quotes are the cheapest by far, so a quick review had me excited. However, I noted no Manual J discussion and the sizing discrepancy.

Thanks in advance,


Follow-Up Postings:

RE: Oversized or Just Right

In a conventional AC system with air handler, the sizing goes from a 4 ton nominal to a 5 ton nominal. No such thing as a 4.5 ton AC condenser.

Big difference between a 3.5 ton and a 5 ton system.

What size is existing system you are replacing and what if any were the comfort issues?


RE: Oversized or Just Right

No way do you need 4.5 to 5 tons in NJ (I'm in NJ).

I'd be more concerned with your ductwork's ability to get air up to the 3rd floor.

Have you looked into zoning?


RE: Oversized or Just Right

Thanks for the replies. I have no system currently and no ductwork, hence the lack of comparisons for my house. I thought 5 tons was a lot for NJ (close to Philly).

I have four contractors provide me with six quotes (some gave me a conventional AC quote and High Velocity quote). In all quotes, the contractors provide for two systems -- one in the attic for 3rd and 2nd floors and another system in the basement for the 1st floor. The quotes provide for a 2.5 to 3 ton unit (air handler and condenser) for the 3rd and 2nd floors with another 1.5 to 2.0 ton unit (condenser and air handler) for the 1st floor. So the total amount for both systems is between 4.5 tons and 5 tons, depending on which quote I use. All of these contractors took measures, performed a Manual J, and reviewed the calculation with with me on the phone or in person.

I have two other quotes that provide for a 1.5 ton unit in the basement for the 1st floor and a 2.0 ton unit in the attic for the 3rd and 2nd floors. Neither of these contractors did a detail measures or a Manual J. However, the 3.5 tons seems more appropriate at first glance.

I am trying to figure out if I am missing something because 5 tons seems so large for about 2500 sq. feet of house.



RE: Oversized or Just Right

I live in a 2500 SF 2 story home in Camden County. The heat gain is around 3.5 tons.

Since you have a 3rd floor, I'm thinking yours is an older home with (possibly) lesser quality windows and insulation.

Has any contractor evaluated your ductwork (sizing and quality)? Also, be aware that heat gain has 2 components; a sensible (temperature) load and a latent (humidity) load. Did the contractors break down their figures? What outdoor design temperature did they use?

This is important since depending on your sensible load, a given x ton unit may not provide enough cooling capacity to meet it.


RE: Oversized or Just Right

How many rooms do you have? Have you considered mini splits?

RE: Oversized or Just Right

a 2500sqft home with no insulation in the walls, single pane windows and a SW exposure with no shade. That might add up to needing more tonnage than the regular rule of thumb.

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