Oversized AC, Screwed-up Manual J, ENERGY STAR HVAC Tirade!
I was checking some REM/Rate files for our HERS raters yesterday, mostly submitted for ENERGY STAR homes. I've come to expect Manual J heating and cooling load calculations submitted along with the files to be less than perfect. Mostly, I accept them because they're close enough.
Yesterday, though, I received a Manual J to go along with a file for an ENERGY STAR home that was beyond the pale. It was egregiously horrific. It was spectacularly sordid. It did come close to meeting the ENERGY STAR Version 2 requirements for Manual J (tight or semi-tight infiltration and correct design temperatures), but whoever put this one together was singularly devious in his efforts to justify the oversized air conditioning systems he wanted to install.
Yeah, he did the usual things to fabricate extra cooling load, but when that wasn't enough, he resorted to one trick that's not used nearly as often as it might be. Keep reading, my friend, and I'll let you in on his secret.
One of the first things I do when checking to see if a cooling system might be oversized is to look at the ratio of conditioned floor area (in square feet) to the cooling capacity (in tons). ENERGY STAR and other high performance homes usually come in at about 1000 square feet per ton or more. The house I built was about 2000 square feet per ton.
A lot of HVAC contractors, though, don't do Manual J sizing calculations but instead rely on rules of thumb. Mostly they use 500 to 600 square feet per ton. This house came in at 368 square feet per ton! That's ridiculous, especially for a house in Charlotte, NC.
When I went into the reports, here are the problems I found that are typical of bad Manual J's:
- They put 6 people in the calculation when this house should have had 4. (It should be the number of bedrooms plus one.)
- The HERS rater calculated that the house had 184 square feet of window area; the Manual J had 383 sf.
- The HERS rater used a window U-value of 0.32; the Manual J had 0.53. (Lower is better.)
Those three items alone inflated the cooling load sigificantly. Not enough for this contractor, though. Evidently he really wanted to install a 2.5 ton air conditioner for the upstairs zone, yet after all those shenanigans, the Manual J result was only 1.5 tons. So, what did he do to get that extra ton to show up in the Manual J? He could have gone in changed wall insulation or duct leakage or any number of other parameters, but there was an easier way.
Manual J calculates the sensible and latent loads separately and adds them together for the total load in Btu/hour. The sensible load is how much cooling you need to do to bring the temperature down, and the latent load is how much cooling you have to do to bring the humidity down. If you take the sensible load and divide it by the total load (stick with me here - we're almost there), you get what's called the Sensible Heat Ratio, or SHR.
The Manual J report often submitted shows the total load (sensible plus latent), but it also shows what they call the required total capacity of the equipment at a particular SHR. Whoever does the Manual J can override the default SHR of 0.75, and that changes the required capacity. Most air conditioning equipment comes with an SHR in the 0.7 to 0.75 range.
The crafty calculator who completed this Manual J figured out that by adusting the SHR, he could get the required capacity to equal what he wanted to install. In this case, he needed 0.53 SHR to get his 2.5 tons. Can you even get an air conditioner with 0.53 SHR?
Come on, HVAC guys! Do it right! If you can't do this for ENERGY STAR Version 2, you don't have a chance with ENERGY STAR Version 3, which is much harder.