The Size Of Your HVAC System

Sizing HVAC equipment can be a challenge. Is it because its difficult to do? No, not really. Its because different Pioneer Valley homeowners have different expectations.

If you were to ask several HVAC contractors what temperature to expect inside your home during very hot and very cold days, you may get a variety of answers. Manual J is pretty clear about this: it should be 70F indoors during design conditions (different around the country of course, but in the Greater Northampton area most installers are using Zero degrees F. as their design. For cooling installations, it would be 75F indoors at anywhere from 87F all the way up to 92F outside design condition.

If you could be a fly on the wall at the local HVAC wholesaler, would be in for a real treat. You would hear all sorts of things about equipment sizing. You would hear, "I don't have a 75,000 BTU boiler, Ill sell you the 100,000 BTU boiler at the same price." You would hear, "I just don't believe that 2 tons of cooling will be enough, let's go with a two and a half." The moral of the story is simply this: No contractor wants a call when the weather breaks with a customer on the phone saying the system you installed is too small. Frankly, the average contractor would rather have you smack his big toe with a ball-peen hammer than get that phone call.

Several years ago we did an installation for a rather educated individual. After spending a number of hours with Wrightsoft's Manual J program, we determined that house would need 4 tons of cooling (there would be two systems). Well, as you already have figured out, when the weather got hot we got the call. After some discussion, it was finally learned that the home owner insisted on 68F during the hottest summer days. Our company was stunned; this was completely unheard of. At the end of the day, even though our agreement said sized according to Manual J (which states that indoor air will be 75F), we wanted to give the customer what he wanted, and recovered some of the costs for the added work. Needless to say we changed the way our agreements read to keep this from happening again.

While 68F may not seem outrageous, it really is very cold. Here are some concerns that should be mentioned:

The human body generally doesn't enjoy such huge spreads in temperature. Now, if you're going to be a coach potato, then this does not apply.
The AC equipment is approaching a point where the indoor coil will be very cold, even close to freezing. If the condensate freezes on the coil, that's a big problem. Were also concerned with liquid refrigerant slugging the compressor.
Most importantly, the humidity levels in the home may not be dealt with on normal summer days. Because the system is so over-sized, the system will run at shorter cycles, which makes it impossible to deal with moisture removal. The result is a cool and clammy feeling, which is less desirable than a slightly warmer but much drier feeling.

There are actually new utility-backed programs out there that encourage installing smaller equipment. This new fad is called Right Sizing. It basically identifies everything we've discussed here on this page, yet takes it a step further by offering a financial incentive to have a smaller AC system (there are currently no incentive programs available for heating systems, but expect to see this in the not-too-distant future) installed when replacing your current system. What makes this appealing to the utility companies is they can stretch out their power distribution on very hot days. What makes this appealing to the performance contractor is were doing just that: offering our customers a better performing AC system. The average HVAC contractor sometimes will be reluctant to down size, in fear of the call. However, when proper diagnostics and planning are considered, and with careful Manual J modeling, down sizing, or Right Sizing, will become the norm, especially for HVAC contractors at the top of their game. Learn more about the Right Sizing Incentive