Calculating cooling capacity

[Dennis Peacock]

I'm needing to learn more about calculating Btu for cooling purposes as well as what the industry standard is per ton (12,000 Btu) of cooling capacity is. I used to use 600 sq ft per ton, but now, everything I'm finding says 450 to 550 sq ft per ton.

I'm researching what all it will take to get my energy bill down to a more manageable amount. Between last month and this month? My electric power rate when up exactly 100%.

So, now I'm researching windows, doors, overall heat load (since we spend most of our time "cooling"), SEER ratings, more energy effecient refrigerator, and such as that. Big undertaking? You bet, but I don't like being a totally igmoramous consumer, so I'm gettin' smarter by research.

Ok...all you HVAC and Energy Conserving type dudes and dudettes. Speak up and let's hear what you can teach this ol' dawg.

[Ben Grunow]

Dennis- I'm no expert but I know that HVAC equipment is sized using a complicated set of calculations based on heat loads (gains and losses). Some might use a direct square foot analysis for jobs where the building is just standard construction (cases where the designers experience allows them to forego (sp?) the calcs) with regular size windows and up to code insulation. BUT, in the real world houses have 2 story foyers and huge windows that may or may not be insulated and cathedral ceilings etc. and these spaces require more heat and AC to remain comfortable.

That said, I think there is a computer program for performing this calculation and the info required is pretty easily obtained with a tape measure and a little research. I dont know if this program is available online or what but a good HVAC contractor should be able to walk you through this or maybe do the calcs for you (for a case of beer or something?)

I do know that oversizing systems is the worst thing you can do because it leads to what is called short cycling where boilers and condensers dont run long enough to develop any efficiency and wear out from starting and stopping. It's worth the trouble to do the math.

BTW, I just started another HVAC thread too, hope there is someone out there who really knows. Good luck.

[Matt Warfield]

My grandfather always told me that it was a 400 square foot per ton but that was back in the inefficient days. He also said that you never round up on the tonnage. The thermostat runs the AC based on temperature but it's the humidity that really factors in the most for comfort. Having too large of an AC unit will result in shorter cycle times which means less moisture removed from the air. A ton for every 600 square foot is most likely a safe bet for a comfortable space. Precise calculations are nice and expected from professionals but calculating a requirement of 2.58 or 2.92 is still going to result in a recommended 2.5 ton AC unit.

Also, check to make sure your thermostat is accurate. A couple of degrees off can make a huge difference on the electric bill. And check the cooling fins and fan on your AC unit. Just like detail sanding, it's the little things that make the biggest difference.

Matt

[W Craig Wilson]

Dennis,
This is one of those areas where common sense and real science diverge rather rapidly. The 'per sq ft' method of load calculation really looks like this:


Heat Gain (Btu/h) = HTM x Area



That innocent looking factor Heat Transfer Multiplier (HTM) is a whole stack of variables (location, orientation, insulation value, etc.) the really describe your exact situation.

Here is a link that might help you understand some of the variabilities.
http://www.proctoreng.com/articles/bigger.html

Ben is on the right track - find an expert with a calculator and let him do the numbers right. Most reputable HVAC shops will be glad to help, sometimes for a small ($50) fee.

By the way, you can buy a simplified version of the manual for $63.95 from these folk:
http://www.acca.org/tech/manualj/

[Ben Grunow]

Had another thought Dennis- If you have the land and the cash now there are great solar power options that pay for themselves in less than a lifetime and can really help to lower your bills (maybe wind power would be an option if you have lots of land and wind?). Some states offer tax credits for these types of things and some systems actually let you sell power back to the power co when your home's consumption is down. GOod luck.

[Al Willits]

BTU's per sq also depends on where you live, AC load on a house in Minnesota are less than what's in Arizona, also seer factors vary and are cost dependent on location, 12-13 seer rating is average for here in Minn, but low in hotter climates. (higher the seer. the more efficent the unit)

Over sizing an AC does more than short cycling, it doesn't let the unit remove moisture well, very important if you live in a humid climate.
Also electric motors use a lot of electricity just in starting.

While your looking for them exact numbers to size your AC, remember them units only come in a few sizes, usually 1.5-2-2.5-3 ton increments and most heat loss programs I've seen have a built in fudge factor of appox 10% also.
Been awhile since I've played with any of the computer programs though.

I'ts not rocket science and with a good comp program, it's actually fairly easy, prob worth the $60 (?) to get one, ya can always use it to figure heat/AC for the shop/garage too...:)

Convincing the neighbor he needs to borrow it for $25 bucks will help defray the cost...:)

OR....check your local library, they may have either the comp program or hard copy you can get.

Al

[Steve Clardy]

Firts thing I would do is check over your amount of insulation in your attic.

[Kyle Kraft]

Our current home was a spec house that the builder tried the "bigger is better" theory on A/C sizing. I could pull temperature down FAST but the humidity remained so the house felt clammy, etc. After some letter writing, second opinions from some HVAC experts, I finally got the installer to swap out the 3.5 ton unit for a 2.5 ton unit at no charge. He made me sign and notarize a form stating that he could not be held responsible for my uncomfortableness after he performs the work.

Well he swapped out the units and I have never been more comfy!! I'm pulling humidity and then temperature, the unit doesn't short cycle, and its quieter.

[Ken Garlock]

When we built and the HVAC contractor was sizing up our geothermal system, he wanted a copy of the plans, the R value of the walls, and ceiling. He also wanted the brand and sizes of all the windows along with their R and E specs. He then calculated a system that would maintain 76 degrees at 105 outside; that was a 4 ton unit. I told him that 76 was not reasonable and that I wanted a 5 ton unit. Now we can maintain 72 when it is in excess of 105, and yes the humidity is maintained.

Doing it right is not a 'windage and elevation' guess, it takes some real calculations. If some hotrod shows up and walks through your house and tells you you need X size, send him packing.

My $.01, my opinons are not worth .02.

[Carl Eyman]

But what can you expect from a dummy? I think I understand why over-sizing a system is a bad idea, but doesn't the ideal size of the system change every time the outdoor temperature changes? So why aren't compressors fitted with variable speed motprs that would keep the system running continuously for the current conditions? The hotter the outdoor temperature gets, the faster the compressor runs - up to a maximum speed, of course. Alternatively, would it be a good idea to design your system with two or more compressors so you could use as many as needed to meet the load? When I had my system replaced a few years ago my HVAC guy wanded to put in a two speed system, but they weren't made in an appropriate size for my condo.

[Lee Schierer]

One thing you might want to look into is a geothermal heatpump for your heating and A/C needs. The problem in your part of the world is that it gets hot outside just when you want it to be cool inside. There isn't a lot of efficiency in transfering heat to hot air. With a ground loop geothermal system, your transfer loop is buried in the ground (you would need more land than a subdivision lot) where the temp is always 50 degrees. This makes it significantly more efficient than a conventional air type heat exchanger. http://www.geoexchange.org/

We've heated our home with a geothermal heat pump for over 25 years and have had no problems.

[Michael Yates]

Dennis,
Have been wading around in this here creek for a while, but this is my first post. Only because I do know what I'm talking about here. Worked in the HVAC field for a few years, in the past,still have a license. When your HVAC system was installed the size of your a/c and heating unit were match along with the amount and size of your ductwork. The only way to know want will help you is meet with your HVAC tech and he can show you things that you can do to help offset these bills. Depending on how old your system is because of the seer rating a new unit, of the same size,may be cost effective for you. Only because any unit made after 1/1/06 has to be a minimum of 12 seer.

If I were you I would crawl up into the attic and not look for insulation but ventilation. Most homes built in the last 30-40 years don't have enough.
Having said all of this, Have you had you system serviced this year? Are you changing and or cleaning the filters? Is your outside unit clean?
Call your service man.
Michael.

[Dennis Peacock]

Mr. Yates.......another fellow Arkansan......Howdy and WELCOME to SMC!!!!

My unit was installed when the house was built in 1994. It's an American Standard unit. I didn't build the house but bought and moved in it March 2000. According to my personal standards, as I have built a house or two, this home has been a real "piece of work"!!!! I guess many things pass an inspection here that normally shouldn't pass. My house is a 2100 sq ft brick rancher.

I have not found a reputible HVAC contractor here that doesn't have a history of ripping you off and still not fixing the problem. I can tell you who NOT to get.!!! :rolleyes: :D I have attic ventilation, may not be enough, but I have it. I got lots of insulation as well.


Dennis,
Have been wading around in this here creek for a while, but this is my first post. Only because I do know what I'm talking about here. Worked in the HVAC field for a few years, in the past,still have a license. When your HVAC system was installed the size of your a/c and heating unit were match along with the amount and size of your ductwork. The only way to know want will help you is meet with your HVAC tech and he can show you things that you can do to help offset these bills. Depending on how old your system is because of the seer rating a new unit, of the same size,may be cost effective for you. Only because any unit made after 1/1/06 has to be a minimum of 12 seer.

If I were you I would crawl up into the attic and not look for insulation but ventilation. Most homes built in the last 30-40 years don't have enough.
Having said all of this, Have you had you system serviced this year? Are you changing and or cleaning the filters? Is your outside unit clean?
Call your service man.
Michael.

[Eddie Watkins]

Dennis,
One item I changed out was my thermostat for a programmable model. Nobody is home during the day so it is set at 85°. The controls go back to 76° about 30 minutes before we get home. It has worked really well last winter on heating and was doing good this summer until yesterday. Normally, the tempature inside only gets up to a little over 80°. With the temp at 104° it evidently got warmer, when we got there it was still at 80° and having a difficult time cooling it. It was 6:30 before it cooled it to the normal temp. I did drop my utilities by at least 25% each month, though.
A couple of years ago the difference between a 12 SEER and 13 SEER was significant and would have required several years to pay back, if at all. That may have changed. I would be interested in what you find out.
Geothermal sure looked like a good solution for anybody with the land to install it. That would be something I will check out when I need a new unit.
I built a pergola over the a set of 8'x8' sliding glass doors and a 6'x6' set of windows that face West. That seemed to help also and it gave us a patio cover to boot without impeding airflow.
We also had window film put up that is supposed to reduce heat transfer by 50% and UV by 99%. Seems like it was only about $50 a window but that was several years ago. It is probably higher now. We had Altamere, a national car window treatment company, do it. No affiliation, just a place to start. I can put my hand up to the window and not feel a lot of heat coming through and it is less than I felt before I put the stuff up so I believe it helps some. I can't tell you if it is cost effective or not. THe wood blinds are not fading or cracking anymore, either.
Our climates are not that different so some of this is probably relevant for you. Keep us posted of what you find.

Eddie

Kepp us informed of what you find

[Lee Schierer]

Maybe a couple of these would help. When I installed a pair on the house we owned in South Carolina, there was a noticeable drop in attic temp the day we put them in.


http://www.dryroofing.com/turbine%20g.gif

[W Craig Wilson]

So why aren't compressors fitted with variable speed motors that would keep the system running continuously for the current conditions? The hotter the outdoor temperature gets, the faster the compressor runs - up to a maximum speed, of course. Alternatively, would it be a good idea to design your system with two or more compressors so you could use as many as needed to meet the load?
Carl,
Commercial installations do just that - multiple compressors to stage up or down as the load demands. Probably not cost effective for most homes, but there has been a trend in upscale homes to go to multiple units, each serving different sections of the home. These 'zones' also can run on completely different schedules. Probably big savings in electricity, if you can afford the upfront cost.

[Randy Denby]

Several manufactures have 2 speed compressors now.....actually most are using the new Copeland scroll 2 speed compressor. Trane had a Direct current variable speed compressor back about 12 years ago. It worked similiar to a rheostat light switch. It dimmed down when load dropped. Had a massive heat sink that was cooled by its on refigerant....and most developed massive freon leaks that was very expensive to repair.
Make sure the evaporator coil and blower wheel is clean. Look at the coil from the fan side, not backside. I've seen what looked like carpet on the coils from women using hair spray, then any hair, animal hair, carpet fuzz will just glue together and stop airflow. Could be a new glue for us woodworkers? Check plenums, duct for any air leaks. Make sure unit is charged to spec If you have electric heat, make sure no heat sequencers contacts are welded shut enabling a heat strip to short cycle. Found this a couple of times in the last 30 years. If its gas...turn off the pilot light.ALso, people have been convinced to go with these very restrictive air filters....not good. Filters work too good actually and can cut airflow by at least 20%. If you use these types, by all means double the size of the return air filter and grill.Also, make sure condensor coil is clean and pressures running to spec.
Here is an easy heatload calculator. If it just for your home, they only charge around 40 to download it.
http://hvaccomputer.com/talkref.asp

This other one is free, but mostly for garage/single room applications

http://www.consumerreports.org/cro/appliances/air-conditioners/reports/sizing-worksheet/index.htm

Randy