Properly Sized HVAC: Case-In-Point, My Friend Michele

Some women’s girlfriends ask their advice on hair, make-up, clothing; I get asked about HVAC (heating, ventilating and air conditioning systems). And I’m fine with that because if I can help any one of them avoid some of the issues I’ve had, it makes this whole green building experience more worthwhile.

So, I was delighted a few weeks ago when my old high school pal, Michele, called to ask my advice on replacing her HVAC system. She and her husband had already spoken to one HVAC contractor, but she thought she’d check with me to see who I would recommend.

“Did the contractor run a Manual J?” I asked.

“Huh?” she replied.

I proceeded to give her my spiel on the epidemic problem of oversized HVAC systems, explaining that contractors are notorious for over-sizing equipment, which not only leads to inefficiency but creates indoor humidity issues during the cooling season. (If you haven’t read Part I of my blog on Oversized HVAC do it right now!)

I admit I get pretty passionate about this subject; it’s a running joke that I am obsessed with size. The gist of my impassioned spiel is that homeowners should make sure an HVAC contractor does a Manual J before installing any new heating and cooling system. This is a fairly detailed load calculation that a contractor should perform in order to appropriately size a system for your home. Historically, many (probably most) contractors have used the “rule of thumb” method based on square footage. Consequently, this is why most HVAC systems these days are oversized.

Anyway, suffice to say this contractor had not. He did, however, come back to my friend with a quote for a 3½ Ton electric heat pump system. (Her existing system was 3 Tons.) This sounded fishy to me, so I gave her the name of my contractor, whom she also called.

She later reported that the contractor I recommended performed what seemed to be a fairly exhaustive examination of the home, taking “all kinds of measurements.” This contractor (who always performs Manual J load calculations) came back with a quote on a 2½ Ton system.

First, I should explain that “Ton” does not refer the actual weight of the heat pump. Rather one ton of refrigeration can remove 12,000 B.T.U.’s of heat in one hour. The B.T.U. is the amount of heat needed to raise 1 lb. of pure water 1 deg F. So it is a measurement of capacity, not weight.

So, let’s think about this. The existing system was 3 Tons. One contractor tells my friend she needs a new 3½ Tons heat pump. Another tells her she only needs a 2 ½ Ton heat pump. If she didn’t know better (which she didn’t at the time) she would probably assume that if the pricing for the total installation was comparable that she should go with the larger system. More for the money, right? WRONG!

Let me put it to you this way: Would you by a size 10 boot for your size 6 foot because there is more leather?

It makes just about as much sense to pay good money for an oversized HVAC system because its never going to operate properly in your home and will undoubtedly end up costing you more in the long run.



Leave a Response