I’ve been designing a system for a home here in Dallas and noticed a couple of interesting (and disturbing!) things about the equipment’s advertised capacity vs. the actual capacity at design conditions. 

Equipment is rated at AHRI conditions (95 degrees outside and 80-degree dry bulb, 67-degree wet bulb inside). These conditions make equipment performance LOOK much better than it will actually perform in the field because, let’s be honest, how many customers actually set their house to 80 in the summer? Plus, Manual J assumes a 75-degree indoor dry bulb and a 63-degree indoor wet bulb.

To find out how a system will actually perform, you have to interpolate the data. You need more than one data point, and there are tools to help you do this. For this design, I used the Mitsubishi Diamond System builder. The data from the product guide says this system shows pints/hr of dehumidification and the SHF (or SHR). 

However, once I put these model numbers for the 1-ton system into the system builder and accounted for design conditions, my results were quite different.

The graphic above shows the total capacity at 10,847 and the sensible at 10,425. This gives us an SHR of 0.96, much higher than the product guide sheet shows of 0.89. What gives? Again, the rating system for equipment is not an apples-to-apples comparison of your design conditions. The product sheet even says this in the fine print at the bottom. My design conditions here in Dallas are a 99-degree outdoor dry bulb, and I’m using the Manual J standard indoor conditions.

Be careful when selecting equipment to fully understand AHRI ratings vs actual design conditions. It can sometimes be hard to find the actual data that you need, but keep digging until you find it. Once you’ve found out how your manufacturer tracks and identifies this data, bookmark it for future reference. If you don’t and just assume everything will work out, you could be in a world of trouble down the road when the unit you put in has trouble dehumidifying like you thought it would.

—Matt Bruner