Most new HVAC systems operate at about 40% lower efficiency than their rating.

Have you ever heard this question: “Where is my energy savings?” I know that I have! Contractors are busily selling homeowners on the idea of replacing their old furnaces and A/Cs for new high-efficiency systems, pointing out that many times the replacement costs can be paid for by energy savings and deferred repair costs. However, after the systems are installed, the gas and electric bills often remain about the same. Why is this?

Oh, I’ve heard the explanations: “Weather is a little hotter (or colder) than usual. And the rate you are paying for your gas and/or electricity has probably gone up, so what can you expect?” But people are complaining that they don’t see any savings the first few months after the systems have been installed, so there appears to be something wrong here.

I was discussing this problem recently with an old magazine technical-writer friend a while back (Dominick Guarino of National Comfort Institute Inc.), and he offered me some interesting insights about what he has found in the classes that his company conducts nationally.

According to Guarino, most new HVAC systems that they check are operating at about 40% lower efficiency than their rating, and he attributes this to the complexity and the narrow operating “bell curve” of the equipment.

He says that manufacturers, in an attempt to reach the higher mandated efficiency ratings, have made installation design tolerances more critical, and few contractors are meeting these tolerances to achieve the rated efficiencies. So, efficiencies slide down the narrow curve at a faster and greater descent than with the old equipment. But he says that contractors, with the proper training and equipment, can bring those efficiencies up to within 5% of the rating.

Part of the lost-efficiency problem is caused by such known things as duct leaks and/or poor insulation. But he points out that something critical to high-efficiency-system top performance is that the ductwork must be a bit oversized. And when high-efficiency air filters aren’t considered in the design of the overall system, they shouldn’t be used!

With high-efficiency gas furnaces in particular, he claims that most regulators come set too low from manufacturers, resulting in insufficient radiant contact with the heat exchangers, so much of the higher efficiency is sucked up the vent pipe. In fact, he says that he has seen several furnaces where the orifices were undersized! The reason for this? His guess was that manufacturers, knowing that ductwork is commonly undersized, have set the heat output a little low to keep the furnaces from tripping out on high-temperature limits.

With air conditioners, the proper refrigerant charge, proper location of the condensing unit, proper design of the plenum and duct takeoffs, and proper air volume are all critical to high-efficiency performance!

So, what does he suggest?
  • Slight over-sizing of the ductwork.
  • Finding and repairing all duct leaks and poor insulation.
  • Checking and ensuring manufacturer-specified discharge air flow.
  • Checking and adjusting all furnaces for proper heat output and operating efficiency (yes, flue-gas analyzers are back in style).
  • And, of course, sending their people to classes and purchasing the proper diagnostic tools so that systems can be fine-tuned to reach peak efficiency.