As the Quality Assurance Designee for our HERS rating providership, I do something that isn't healthful. I know it's probably driving up my blood pressure and taking years off my life. But I do it because I have to. It's part of a QAD's job. What is it, you ask? I check a lot of Manual J cooling load reports. It's already caused my head to explode twice this year, once when I wrote, Why Won't the HVAC Industry Do Things Right? Then a couple of months ago, what was perhaps the worst Manual J report I've ever checked sent me over the edge and Oversized AC, Screwed-up Manual J, ENERGY STAR HVAC Tirade! Yeah, this has been an 'issue.' The reason is that any builder who wants to get their home qualified for the ENERGY STAR label is required to size their air conditioning systems using the results of a cooling load calculation. The rule is that the air conditioner has to be no larger than 115% of the Manual J cooling load. It could be more than that if that number falls between sizes, but you get the idea.

ENERGY STAR Version 2 doesn't require much in the way of checking Manual J reports, but I go above and beyond when I look at them. One of the first things I do is to apply my rule of thumb. Understand here that the standard practice among HVAC contractors sizing air conditioners for new homes is to use a rule of thumb. It's often in the neighborhood of 1 ton of air conditioning capacity for each 600 square feet of conditioned floor area, usually abbreviated 600 sf/ton.how to size a portable ac unit So, when I get a Manual J, I find the capacity and the conditioned floor area and calculate that number. portable free standing ac unitI find out what the square feet per ton is for the house. ac-dc power supply meaningIf this is supposed to be an ENERGY STAR home, it shouldn't be 600 sf/ton.

It should be more like 1000 sf/ton or higher. That's my rule of thumb. Every time I look at a Manual J report that comes in at around 600 sf/ton, I know that all I have to do is look further into the details, and I can find mistakes that led to their oversizing. Some common ones are: Too much window area Wrong window types (U-value and Solar Heat Gain Coefficients too high) Ducts too leaky or in wrong location HVAC contractors doing their own load calculations are afraid to come out with a cooling load that's too low. They're afraid of call-backs from clients who can't keep their homes cool. They're also used to having to oversize cooling systems to overcome the problems of weak building envelopes and crappy duct systems. If they want to do ENERGY STAR new homes, though, and their Manual J reports are coming to me, they should know that I've got my own rule of thumb - 1000 square feet per ton. When that 2000 square foot house comes to me with a 3 ton air conditioner, I have no choice but to look further because they didn't pass my rule-of-thumb test.

As I said, checking Manual J reports is stressful, but somehow I cope. A few years ago I saw an interesting billboard for a liquor store in a semi-rural Georgia county (actually, the county I used to live in and where I built a high performance home). It said, "If you don't drink, don't start. If you do drink, buy your booze at Alcohol Alley." I was going to try to extend the metaphor to checking Manual J reports, but it's not worth it. I think I'm just gonna go have a drink.Air conditioner BTU requirements: this article provides an air conditioner BTU chart shows how to choose a room air conditioner for window or through-wall mounting. We provide room air conditioner or window air conditioner BTU sizing & choice charts. We also show how to actually calculate how much BTU cooling capacity you need based on building area or square feet, and we warn about dehumidification problems if you buy an air conditioner that is too big for the space you are cooling. Portable, window, or through-wall air conditioners are typically described by their manufacturer as suited for:

The table below gives recommended air conditioning BTU's necessary to cool a single room. The data in the table assumes that the ceiling over the room is insulated and that the room is not over or is not itself a special heat-producing area such as a kitchen or boiler room. Table 1: Base BTUs - Recommended Air Conditioner BTUs Calculating BTU Requirements: instead of using the table above, and particularly where factors may make the table inadequate (examples are given just below) you may want to CALCULATE the BTU COOLING REQUIREMENT using the procedure given below in this article. Ceiling height variations: BTU capacity tables for air conditioner selection typically assume typical 8 foot ceiling heights in residential spaces. If your ceilings are significantly higher, say 14 feet or more, you may want to use the next larger room area size when selecting the BTUh capacity needed for your air conditioner, particularly if your building is located in a hot climate with higher heating loads.

Building heat gain rate variations: in locations of high heat gain or high solar gain such as a significant exposure to direct sunlight or many sun-facing windows, or for buildings with little insulation, you may need to select a higher-capacity air conditioner than given by the table.of a typical home can be cooled per ton of cooling capacity: that is, one ton (or 12,000 btuh) of air conditioning can cool about 500 sq.ft. But the real answer is, it depends. Some of the factors that affect the ability of an air conditioner to cool a space need to be considered besides just the number of square feet. These include at least the following questions about air conditioning load and cooling requirements: Watch out: Do not buy an air conditioner which is oversized (too many BTUh) for the area you need to cool. You may think that bigger is better, but not in the case To make a room comfortable the air conditioner needs to both cool the room air AND dehumidify the room air.