btu vs tonnage

[rookie123]

if 12,000 BTU = 1 ton why would I need 80,000 BTU for the 3 ton system I am about to have installed? This will be going in upstairs, 1,600SF. Original upstairs system is 18 yrs old, 2.5 ton - 75,000 BTU. I live in the Atlanta area and have more need for cooling than for heat. Thanks again for your advice, great site.

[bergvärme]

Your numbers don't add up. Are you talking about a 2.5 ton air conditioner? 2.5 x 12K = 30,000 BTU/hr, not 75,000 BTU/hr.

If you're worried about the size of your air conditioner, you might want to get a heat load calculation performed. If you oversize your equipment you run the risk of over-cycling the equipment which leads to poor efficiency, and also cooling down your space too quickly, leaving your space cool but with damp air. Properly sized equipment will cool and dehumidify.

[coordinatesales]

You are probably in the situation we are here in OK. We have to size the furnace for the blower to match the a/c rather than the heat to match the load. 80k furnace would be common here since that furnace comes with a blower that matches the 3 ton a/c unit. Most houses here don't need that much heat but the smaller furnaces available don't have the blowers to match the high cooling load. 80k vs 75k is just manufacturer difference. Some use 4-20k burners others use 3-25k burners.

[rookie123]

Co-ordinatesales, thanks for responding;

"You are probably in the situation we are here in OK. We have to size the furnace for the blower to match the a/c rather than the heat to match the load. 80k furnace would be common here since that furnace comes with a blower that matches the 3 ton a/c unit. Most houses here don't need that much heat but the smaller furnaces available don't have the blowers to match the high cooling load. 80k vs 75k is just manufacturer difference. Some use 4-20k burners others use 3-25k burners"

This is exactly it, I have more need for cooling capacity than heat, thus the 3-ton coil and condensor for upstairs 1600SF. The existing furnace-coil is a closet width 14.5" attic setup married to a 2.5 ton outside condensor. Trane does have a 3-ton blower furnace rated for 47,000 BTU and that seems to make more sense for my situation.

Any thoughts on a 17.5" wide furnace-coil being better on the cooling efficiency?

[hankmcneil]

you're trying to reason your way through sizing based on the old system which is most likely the wrong size. Have a heat loss done b4 u make an expensive mistake.

[docholiday]

In a situation where the furnace will be over sized or the air conditioning will be undersized, the air conditioning wins. You have to have the cooling capcity to do the job. You may consider a multi stage furnace to get the run times, but thats not always the best solution. It might just ease the pain a little. Either way the duct system has to be sized for the larger load.

In your case, I agree with the last post, you really should have a proper load calculation done. It's easy and the dividends are huge.

[dan sw fl]

Rookie,

I assume you have ~ 13 windows in 1600 SF in a somewhat new residence so ...
you likely need a 3-ton 2-stage Heat Pump
to economically address your Heating and Cooling needs.

http://www.trane.com/residential/pro...mps/xl16i.aspx

or equal heat pump from other notable mfg.

[coordinatesales]

Regardless of wether you use a furnace or heat pump a load calculation should be done. If you are indeed in the situation we have here, the cooling load will require a blower that is larger than the one in a furnace that matches the heating load. While oversizing is never preferable, it is better to oversize the furnace for proper blower operation than the a/c when the situation demands it. Some models of a/c condensers are not available in half ton sizes and that may be the reason for the increase from 2.5 to 3 tons or perhaps that is what the load calc called for.

As far as equipment width, I'm not aware of any efficiency differences. I may be wrong on that though. We just had a winter storm so I'm a little busy to be sorting throught the ARI site to verify.

[climatecontrol]

1 Ton = 22,000 BTU Heating Units needed OUTPUT
2 Ton = 44,000 BTU Heating Units needed OUTPUT
3 Ton = 66,000 BTU Heating Units needed OUTPUT
4 Ton = 88,000 BTU Heating Units needed OUTPUT
5 Ton = 110,000 BTU Heating Units needed OUTPUT

Roughly the Cooling Capacity is half the Heating capacity, therefor the 12,000 btu's you mentioned are for the cooling side of your system is for 1 TON of cooling. And also if you are puting in an 80,000 BTU 80% furnace, it will output 64,000 BTU's. So your 3 ton system might not do the job. Now I dont know how drafty the windows are or how well the insulation is in your house but those are pretty accurate numbers.

[klabkebash]

Also another reason why you need more Heat BTU's than cooling Btu's is because during cooling more your at most trying to acheive a 20 deg temp drop to make the area comfortable.

During heating mode, the deg can vary if it's a cold day. If it's 30 deg's out and you would like it to be 75 indoors, your using more BTU/Hr. Thats a 45 deg temperature make up you're trying to acheive. Consider your in Atlanta there aren't too many below freezing days, in Northern states, we heat way more degree days.

[wyounger]

Note that the OP mentions that this is a second-floor system.

In Atlanta, most second-floor systems need more cooling capacity than heating capacity. Essentially all forced air furnaces can move enough air for three tons of cooling. A load calc is very much in order here, because it sounds like your existing furnace is vastly oversized and you're about to buy another one just the same. Oversized furnaces are less comfortable and they won't function at anywhere near their rated efficiency.

My hunch is that a 45,000 BTU furnace (the smallest common size) will still have plenty of capacity to meet your needs, but with better comfort and actual efficiency (even if the efficiency is the same on paper as the bigger furnace). There are big efficiency losses associated with running short cycles. They have to be sized correctly to work as the manufacturer intended. Unfortunately it is quite rare in practice that anyone bothers to do the math to calculate the right size equipment for a given application; they just guess and err on the large size to make sure they have enough capacity, ignoring the compromises that the oversizing entails.

The house I lived in with a second-floor system set up like that was miserable in heating season. The furnace was so big for the job that it could only run for four minutes before it fried you out. The temperature swings were so big that you were cold, wishing for the furnace to start, and then as soon as it ran for 60 seconds, you were already starting to wish it would turn off. Ideally, you size the system so it matches the size of the job it needs to do; that lets it run long, slow cycles.

And btw, a heat pump is far cheaper to operate in Atlanta than gas heat. The only question is if it should have gas backup or electric backup, which for a retrofit will depend more on what's convenient to install than anything else. You will use very little backup heat either way, so the efficiency of the backup heat source is not important.

[Edited by wyounger on 11-30-2006 at 02:26 PM]

[rookie123]

Guys,

Thanks so much for all the input. I think coordinatsales had it pretty much nailed about blowers vs heat BTU's. The furnace equip specs (Trane in my case) limits the blower-heat btu choices and all are rated in 1 ton increments. Given that I am more concerned about cooling-CFM requirements for my 2nd floor (1600SF) Atlanta house, I opted for the 3 ton rated blower and the lowest heat btu available.

After much reading on this site and looking at my options I went with the Trane XT80 furnace and the matching 3 ton coil. The XT80 is a single stage but does have the high efficiency blower motor head on it, one step below the variable speed 2-stage model. I opted for the lowest heat BTU I could get which was 60K input-47K output.

[rookie123]

wyounger,

Thanks for the response. I could not get any of the several bidders to perform a load calc unless I committed to them before I got the estimate; (typical comment was replace what you have or base cooling on 500SF+/- per ton). That's fine, I won't lose sleep over it. My desire would be the heat pump with Variable Speed AH setup but I opted for a gas single stage 80% with high efficiency blower (Trane XT80).

[kevinmac]

Your contractor did a load calc or just put his finger in the air? 3 ton 1600sq ft when it originally had a 2.5ton? Did the 2.5 ton work okay or are you just wanting it to get colder then the 2.5 ton can produce. Keep in mind the systems today with VS AH etc can actually dehumidify much better and you will feel like it's cooler in the house then what the temp actually shows. I keep mine at 79 in S Fla but since the system does so well with the humidity it actually feels 3 degrees cooler then what it really is. Oversizing will not remove the humidity and make you uncomfortable! Better question this whole thing?

[Edited by kevinmac on 11-30-2006 at 05:10 PM]

[rookie123]

All of the HVAC estimates were based on finger in the air as none would run the calc unless I committed to them before I got the final bid. For my 2nd floor 1600SF in hot Atlanta there is no way that I would go with more or less than 3 ton of cooling capacity. There are 16 single pane windows upstairs including a large palladium window. The house faces east was built in 1988 with little shade but recently blown R-38 attic insulation.

[dan sw fl]

Originally posted by rookie123
All of the HVAC estimates were based on finger in the air as none would run the calc unless I committed to them before I got the final bid. For my 2nd floor 1600SF in hot Atlanta there is no way that I would go with more or less than 3 ton of cooling capacity. There are 16 single pane windows upstairs including a large palladium window. The house faces east was built in 1988 with little shade but recently blown R-38 attic insulation.

~ 41,300 BTU heat loss

with U-value for windows of 0.83