# Sizing A/C and Furnace

• posted on July 25, 2005, 2:17 am
I will replace my A/C, which is 25 years old and probably has a SEER around 6, with a new A/C with a SEER 10 rating. I only use A/C 200 hours a year (San Diego).
The old one is 3 ton and cools just right. Since the new A/C will be considerably more efficient, can I buy a smaller A/C, let's say with a 2.5 ton rating?
Or, do I need to retain the 3 ton capacity but will merely see lower operating costs due to higher efficiency?
The same logic would apply to my furnace: Can I reduce the size from 80,000 BTU to 65,000 BTU, because the new furnace will have 20% higher efficiency? Or will I merely save money operating a new 80,000 BTU furnace?
My logic is a bit challenged due to my lack of knowledge.
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Walter
www.rationality.net
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• posted on July 24, 2005, 7:50 pm

around
80,000
efficiency?
If you have a 3 ton load then a 3 ton it needs to be. To small and it runs to much. To big and it will not cool effectively. Get your contractor to do a Manual J calculation on the home then you will know that the unit is properly sized. Price difference between the sizes is not worth the risk.
Same applies to the furnace.
Yes the more efficient the less it costs to operate,,,,,,,, usually.
IMO installing anything less than a 12 is a mistake. Next year the minimum Seer for new construction will be 13. I base my thoughts on living in Phoenix, and cooling is ~ 2500 hours a year. For sure I would check to see what the cost difference is between your target and a 12 or 13. If your looking at the long term then check this site.
http://198.147.238.24/ac_calc/default.asp
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• posted on July 24, 2005, 10:06 pm
Walter R. wrote:

You would size according to the furnace's OUTPUT Btu/hr; ignore the INPUT.

I lived in San Diego for over 3 years and wonder why you need that big an A/C and furnace. The weather was rather mild the year round.
I would rather slightly undersize than oversize on the A/C unit. How big is your home? High ceilings? I have a little half ton 6,000-Btu/hr window unit cooling the entire first floor; over 920 sq ft. Today the HEAT INDEX is 112-F; 92-F with 70% Relative Humidity, and the little A/C has it perfectly comfortable in here. A floor fan circulates the air from the units evaporator coil. The floor area is 77-F and 58% RH and very comfortable all day.
http://www.udarrell.com/airconditioner_current_temperature_btuh_charting.html
Have someone with credentials run the numbers so you know what size A/C and furnace should be installed. (Don't guess!)
You won't get the SEER or furnace efficiency if you oversize. http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
- udarrell
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Optimizing Air-Conditioner Efficiency
http://www.udarrell.com/air-conditioning-efficiency-btuhr-evaporator-coil.html
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• posted on July 25, 2005, 4:24 am
I live about 10 miles east of San Diego. Temperatures are 10 degrees higher here than in the city. My house is 1650 sqft. I was only wondering, if I get a more efficient A/C, should get a smaller A/C unit?
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Walter
www.rationality.net
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• posted on July 24, 2005, 11:46 pm

No. The size you need is independent of the efficiency; a more efficient unit will simply cost less to run.
Wayne
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• posted on July 25, 2005, 12:24 pm
Walter R. wrote:

Absolutely, NO! By all means have a manual J done and see what tonnage it calls for.
If you oversize with short run cycles the unit will not deliver anywhere near the rated 10-SEER! You may end up getting only 8 SEER or less due to short cycling. Why not get the SEER you paid for by sizing it right? In fact the comfort levels should be better due to correct sizing. http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
The summer outdoor design for San Diego is 80 dry bulb and 69 wet bulb for around 57% relative humidity. If you are further from the ocean with average of 10 degrees higher temps then the humidity would be less than SD's.
You probably have a lot of days when the humidity is a problem with rather moderate outdoor temps; that calls for more run time to achieve the comfort zone with efficiency!
Don't oversize the system; 3-ton is 36,000-Btu/hr. I don't run the 5,000 Btu/hr upstairs window unit during the daytime and the hallway stairwell is wide open; therefore the little half-ton 6,000 unit also handles considerable latent load (humidity) and some heat from upstairs. I know it makes considerable difference ion the latent load on the downstairs unit, because when I run the other unit the Relative humidity will drop 2 to 4% in a short period of time. Without the upstairs unit running it will normally keep the humidity at 50% or lower, and we have a lot of humidity here.
I would also ask what you could do to decrease the heatload with insulation, etc.
Do it right! - udarrell
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Optimizing Air-Conditioner Efficiency
http://www.udarrell.com/air-conditioning-efficiency-btuhr-evaporator-coil.html
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• posted on July 25, 2005, 6:32 am

Maybe he has an 8,000 square foot house. Maybe he is just outside of SD where the temperature can be a bit more heated in summer, say closer to El Cajone where it can be 100 when the coast is 72.

I agree with that but since the 3 ton cools just right, that seems to be what is needed. Worth checking as the house may have changed over the years with additions or added insulation.
The original question is "can I get away with less capacity if it is more efficient?" and the answer is no. A Btu is a Btu no matter how efficient the delivery is.
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• posted on July 25, 2005, 5:30 am
Walter R. wrote:

Tons measures the amount of cooling and increased efficiency only means it takes less energy to do the cooling. If you are happy with the size you have, I would generally not change it. That said too big or too small is not good, but if it is really comfortable for you, I would stick with what you have.
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Joseph Meehan

Dia duit