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Heating & Air Conditioning

How do I determine if I need a 3 ton or 4 ton air conditioner?

12 years ago

About 6 months ago I came to this forum and received some excellent information about HVAC systems.

I have two quotes and both vendors suggested the same furnace; The Infinity 96 Two-Stage Gas Furnace Model: 59TN6.

However, one vendor suggested a 4 ton Infinity 21 SEER air conditioner and the other vendor suggested a 3 ton 16 SEER air conditioner.

How do I make the determination on if I need a 3 ton or 4 ton system? I'm in Michigan and the air conditioner is primarily used during the summer months. My air conditioning bill typically peaks in July only.

The vendor that recommended the 3 ton system did take measurements of the house. I'm not sure if the other vendor did. Also, I currently have a 4 ton air conditioning unit which is probably the original. The home was built in 1967.

Please help. Thanks.

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Comments (8)

PRO
CJ Mechanical of North jersey llc.
12 years ago
A manual J load calculation is the only real way. This requires measureing windows and type, insulation,infiltration,ectt. Ask them to perform a FULL manual J and to provide a copy for you. explain to them that in this day and age you want the most efficient system and need to know what the heat gains are in the house. Rule of thumb or simply replace what you have is not advised with out the manual J
fsq4cw
12 years ago
Have you discussed the merits of a heat pump?
SR
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I like the fact that your dealer did a load calc--shows he cares. He's right that the 3 ton should do the job most days, but if the load calc is 100% accurate, 3 tons won't always do the job on the hottest days of the year. The output on the 3 ton setup is 35,600 btus. How much it "won't do the job" I can't tell you. Could just be talking about a few degrees warmer in the house, if you can live with that. But depending on the latent and sensible breakdowns of your calculated low, could be worse, since the capacity is derated as ambient temperatures increase. I don't think the 4 ton would short cycle, not on low stage. And with the TCONT900 thermostat and communicating system, controlling humidity should not be an issue with the 4 ton unit, since it can run longer cycles in low stage at a lower airflow. See what your contractor thinks. Explain the fact that the 3 ton system won't always be putting out 36,000 btu's.
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None of the quotes include a variable-speed indoor unit, which I would recommend (of course I can't tell for the Rheem). Brand isn't as important as finding a competent installer to do the work! The Amana also does not use the new refrigerant; it uses freon (R-22). I'd ask for an upgrade price to a variable-speed blower (Amana: AEPF series blower, Trane: 4TEE series blower, Carrier: FV4 series blower, and Rheem: RHKL series blower). Prices sound reasonable. Did the old 4-ton unit cool okay, since that's the replacement size? Things like new lineset, ductwork modifications, new thermostat, etc. would be helpful to know for each quote. Here are the efficiencies from what I could pull up: Amana: 12.2 EER, 14 SEER Trane: 11.2 EER, 13.5 SEER (due to no variable-speed blower) Carrier: 11.5 EER, 14 SEER
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You need to check the blower size rating on your furnace. Usually this can be indicated by the full model number found on the data plate. I find it difficult to believe a 4 ton is needed. And I definitely think you are asking for trouble if you install a 4 ton condenser with a 3 ton rated blower. Plus how bout the ductwork sizing to deliver the additional CFMs. Not a good decision at all. Start with a properly performed load calculation for air conditioning. IMO
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aprilmack
Original Author
12 years ago
All of the vendors I spoke with stated that a heat pump wasn't a good option here in Michigan.
fsq4cw
12 years ago
Did they explain why? What's your electrical rate per kWh? What's the gas rate?

SR
mike_home
12 years ago
Hi April,

In your past posts you stated the house has 2400 sq. ft. It is difficult for anyone on this forum to state what is the correct size for your air conditioner. The load calculation is the only way to do this properly. I would have much more faith in the contractor who took detailed measurements. It is likely the cooling load somewhere between 3 and 4 tons. One contractor may be rounding up, and the other down. The 2-stage Infinity AC would be a good choice. On the low stage it would be operating around 2.8 tons. You would be comfortable and do a great job of removing humidity.

How many days a year does the temperature go above 90 degrees in the summer? I personally think a 21 SEER system is overkill for your part of the country. It is unlikely you will ever see a pay back of the additional cost over the 16 SEER unit.
brickeyee
12 years ago
"Did they explain why?"

Physics limits the efficiency of heat pumps in very cold areas.

Frost formation on the outside coil starts hitting efficiency badly, ad there is simply not a lot of heat to easily extract at lower temperatures.
fsq4cw
12 years ago
Re: brickeyee

While what you say is true, I live in Montreal where MANY have heat pumps and use them down to at least 10F. So depending on the electrical rate, it may still be a useful option.

SR
brickeyee
12 years ago
"So depending on the electrical rate, it may still be a useful option. "

And the winter humidity level.

And how long you expect the system to last.

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