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Sizing a cooling system

Posted Friday, April 22, 2011, at 12:11 PM

Most central cooling systems that are installed in today's homes are oversized. By some estimates, as many as 80% of air conditioners and heat pumps that are in use today are too large in their capacity for the homes in which they have been put into use.

To the home owner this presents several undesirable conditions. Not only are the larger units more expensive to purchase but they require (and waste) more costly energy. In addition to that, "short cycling" (when the cooling unit operates for only short periods) will reduce the life of the unit and will fail to remove humidity from the home that is being cooled. Reducing the temperature in the home without sufficiently removing humidity can be conducive to mold growth.

There are certain formulas that trained heating and cooling specialists use for calculating the specific needs for a home and while the amount of the conditioned area in square feet is an important factor, it is only a part of what goes into those formulas.

When determining the proper size for the cooling unit the type and placement of the distribution ducts and vents, the placement and number of return air vents, the amount of insulation in walls and ceiling, the direction in which the home is facing, the types and numbers of windows, the color of the roof, attic venting, the amount of shading from nearby trees and many other variables also need to be considered. Failure to take all of these factors into consideration will result in waste, discomfort or both, and it is almost impossible to calculate these needs without taking the time to acquire the required data and model the home with the assistance of a computer program.

Select a heating, ventilating and air conditioning professional who uses "Manual J" and/or "Manual F" calculations to determine the proper cooling system for your home. You will be assured of increased comfort for the least expense while paying for less energy to cool your home this summer.


Comments
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What a pleasure to read! Mr. Bushart, you really coved a lot of good points. As a heating and cooling contractor in Northwest Arkansas, I can't stress this same information to my clients enough. Thank you for explaining how important a heat load is. I wish Manual J's were mandatory for any new installation and equipment replacement. If not to protect the home owner, to educate heating and cooling contractors to what they are doing. So many systems are oversized. I don't think I have ever changed out a system that was the correct size. I would think that in your area, a Manual J would be the last line of defense for a home owner, since there are no state license requirements.

Like you pointed out, oversized air conditioners do contribute to moisture levels in a home, promoting the growth of mold and mildew. These types of bacteria growth only need two things to thrive, food (moisture) and an environment (darkness). And just like any living entity, the more you feed it the faster and bigger it grows. In my experience the problem always boils down to ignorance or negligence and generally on the part of the person who installed the system. The true concern is that many insurance policies have omitted mold and mildew damage in their coverage. The only course of recouping damages is lengthy litigation.

This is why I always recommend to use contractors who perform a proper Manual J, and not just volunteer to show you the results, but also an EPA certification for handling refrigerant, general liability and workman's compensation insurances. And most definitely get references, and more than just one, ask for three! While all of these may not be required, they are good signs of faith and dedication to ensuring your health, safety and comfort.

Joshua Liljenberg

1st Choice Home Comfort

josh@1stchoicehomecomfort.com

-- Posted by josh@1stchoicehomecomfort.com on Tue, Apr 26, 2011, at 4:37 PM

Very good points, Josh. Thanks for the valuable input.

-- Posted by Jim Bushart on Wed, Apr 27, 2011, at 12:24 AM

Very good post. Your HVAC is one of the most important things that go into your home.

The importance of a properly sized system can not be stressed enough.

Home owners also need to be informed by contractors about the importance of the structure itself. Especially in new construction.

Many new houses are build with no regard to the HVAC until the structure is stood. In my opinion the hvac contractor should be contacted before construction begins. That way they can be informed of the many options available for construction materials.

Things like radiant barriers, foam insulation, low e windows can all help the homeowner out to have a more efficient environment when the job is finished. These materials that go into the house are much more cost effective than the HVAC system itself. The more efficient the house, the less capacity HVAC system you need. Thus costing less money to install and less money to operate.

Also, don't be fooled by salesmen. Either HVAC salesmen or construction material salesmen. There are countless times I have met with homeowners and heard that the window guy or the insulation guy has said his product will be "like" this in R-value when completed. Any competent contractor should know that R-value is exactly that, and should not be confused with infiltration values or other properties of materials used. A good contractor will know the true benefit of the materials or be willing to do the research to find the value of them. HVAC salesmen are the same way. Many of them work off of commission. I have a real problem with the whole aspect. They will come into your house and try to sell you the most efficient system at the highest dollar, because that increases their pay. Most don't give any consideration to the end result and what is best for the homeowner. Any HVAC system regardless of the brand and efficiency will do a good job if properly sized and installed.

Other materials should also be evaluated when having a HVAC system installed. While materials like flexible ducting and duct board are not against code. The warranty on flexible ducting is five years straight out of the box. This does not seem to me to be a good material to use when installing a system of such importance to your home. I believe a properly sized, sealed and installed metal duct system with the proper R-value of insulation and proper radiant barrier is the only way to go.

My Manual J program I use is worth it's weight in gold, because it gives me a true picture of the house and the loads on it from different directions and different things in the house.

I keep it updated constantly, because the building materials are constantly changing and being added to.

The only thing in Jim's post that I don't necessarily agree with is the consideration of shading that is not part of the structure. The surrounding landscape really should not be considered, as it could change at any time either by the homeowner to decide to remove a tree or possibly from storm damage.

Another consideration is the placement of the ducts. On a 100 deg day you can expect your attic space to be from 140 to 150 deg unless a radiant barrier is used. Even if a radiant barrier is used the attic temp can still be from 120 to 130 deg. This puts a tremendous load on you duct system. The duct has to be cooled before the house can be cooled. Placing your duct in the crawl or even possibly in conditioned space makes a huge difference on the system and the way it operates.

It is a sad thing with Missouri. To be a HVAC contractor it seems all you have to do is say "I am a heat and air guy" and that's it. Missouri really needs to get more involved in the inspection and enforcement of national codes.

In the state of Arkansas, we have a state inspector, a county inspector and city inspectors. Every job we do gets inspected. That's the way it should be. They require a Manual J load calculation be done on every new structure and submitted to them when a permit is pulled. They also make sure that other aspects of the house are correct. I see many houses built in Missouri where even the attic insulation is not up to code. The current National Mechanical Code calls for a minimum of R-38 attic.

We as a contractor adhere strictly to the national mechanical code whether we are working in Arkansas or Missouri. It is a lot easier for me to teach my guys the correct way to do things rather than let them fudge because there is no code or inspections for Missouri.

Thanks, Jeff Handley

Handley Heat & Air AR License #0163775

Handley Commercial HVAC AR License #0215970412

-- Posted by Jeff Handley on Fri, Apr 29, 2011, at 8:15 AM

Jeff Handley is a highly skilled and respected specialist in his field and while he does not agree with me on the shading issue, which I stand by as an important consideration for upgrading existing systems, I have personally recommended him to many of my energy consulting clients due to the outstanding work that he and his crew did with the heating and air conditioning system in our home. I appreciate hjis valuable and precise input.

My personal experience with testing the effectiveness and energy efficiency levels of HVAC equipment in Missouri has taught me to recognize that there is the design load for the HVAC system ... being the "ideal" load calculation based upon how the home is designed prior to construction --- and then there is the manipulated load calculation based upon the conditions that actually exist.

In this regard, the design of a building will include calculations for the HVAC system based upon the air tightness that is presumed based upon the construction plans, the R-value of the insulation as it is called for in the plans, the window specifications called for by the designer, as well as external and internal shading included with the design.

After the building is constructed and (particularly) after the passing of time, there are very wide variances in these factors that must be considered when sizing a new system.

As Jeff and Josh both point out, a whole new calculation must be made on the existing factors that they both pointed out.

Home owners are very likely to be cheated out of efficiency, comfort and lower utility costs when their old equipment is simply "swapped out" with newer and more expensive equipment of the same size without considering and addressing other issues. The old "rule of thumb" using the square footage as the determining factor simply no longer applies.

-- Posted by Jim Bushart on Fri, Apr 29, 2011, at 12:15 PM

Great post about "right sizing" A/C compressors. In many "very well insulated" I am finding that the local code officials (as well as the understrained HVAC people) oversize (OK, based upon codes, but not taking into account the level and efficiency of the insulation) the A/C.

That is why it is very important that your HVAC guy be well trained and factory certified. Around here (greater Chicagoland area) there are no real "requirements" for HVAC, usually just a business license. Because of this, anyone who can buy the meters (regardless if they know how to use them) considers themselves to be an HVAC expert. And I have seen the crappy work to prove it.

Remember, you always get what you pay for, but you will NEVER get more than youy pay for.

Hope this helps;

-- Posted by willlong on Fri, Apr 29, 2011, at 8:15 PM

Great info, Will. Considering the extreme weather conditions in the Chicago area one would think that there would be more stringent requirements...particularly with fossil fuel heating systems and their potential to do harm when not properly installed.

Thanks for adding your expertise to the conversation.

-- Posted by Jim Bushart on Fri, Apr 29, 2011, at 9:41 PM

Chosing a cntractor is the most critical aspect of any decision-making process, because it affects everything from price to performance to follow-on services, warranties, etc. It's no longer enough to use the nice guy. Sure, the nice guy may provide you a nice price, or smile at the kids and pet the dog. But, the nice guy may be beind the curve with regard to current technology, trends, and best practices regarding scientific calculations. An informed consumer is the qualified contractor's best chance at business. An informed consumer also has the best chance of finding a contractor who has the knowledge and experience to size and install the system that meets the price and performance standard within the consumer's budget.

-- Posted by jfarsetta on Sun, May 1, 2011, at 10:28 AM

It is great to have so many professionals adding their thoughts to this very important topic. Thank you very much.

There is much that the home owner can do on his own to keep his heating and cooling equipment in top working order (keeping filters changed, keeping grass and weeds from blocking air at the compressor, keeping all vents open throughout the house, etc) but these measures are there to keep a well planned and properly sized unit working at maximum performance. When the unit is improperly sized, installed and/or ducted...as our pros have pointed out...nothing else really matters, much.

For the best results in terms of comfort and efficiency, consumers should always ask their contractor to show them the "Manual J" calculations to see how the sizing and placements were arrived at.

-- Posted by Jim Bushart on Tue, May 3, 2011, at 11:42 AM

Anyone who wants to save money in Missouri should hire Jim Bushart, a Certified Building Analyst who has years of knowledge in this profession.

Regardless of air conditioning calculations the public must understand hiring a professional like yourself to find their energy loss in a house or commercial building is the most important aspect for them to save money regardless of what type of conditioning system they have.

Older buildings as well as new need to have an audit performed, the cost of energy is ever increasing. Its sad that most of the general public doesn't know this service is available at affordable prices compared to the saving they receive if they follow the procedures a highly qualified energy auditor such as yourself recommend.

-- Posted by Inspects on Sun, May 8, 2011, at 4:37 PM

Thank you for the kind words. Thank you, also, for pointing out the energy audit as a means of helping people to increase their comfort and decrease their energy bills while saving energy.

Taking care to ensure the proper sizing of the next heating and cooling system is a great first step.

-- Posted by Jim Bushart on Sun, May 8, 2011, at 10:40 PM


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James H. Bushart is a Building Analyst who is certified by the Building Performance Institute (BPI) and is also a Certified Missouri Home Energy Auditor by the State of Missouri Department of Natural Resources. He has performed home performance analyses (aka "energy audits") for homes of all types throughout the states of Missouri and Arkansas. He has also performed over a thousand inspections of Missouri homes and commercial buildings and holds various certifications and licenses in residential building inspections.
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