Which is Most Important: The Efficiency of Equipment, Ducts, or the System?

Which is Most Important: The Efficiency of Equipment, Ducts, or the System?

Equipment efficiency typically deteriorates when installed in the field. The question is how much?

Most proposals feature increased equipment efficiency ratings as the most critical component of the sale. But is this correct? What if you could rate the performance of the installed system and prove something very different? Let’s take a look at what would happen if you would offer a 30% increase in verified system efficiency compared to a typical equipment efficiency increase of only 5%.

Equipment Rated Efficiency
Equipment-rated efficiency is a good thing. Every contractor, technician, and consumer needs a set standard that can be used to indicate potential energy consumption of the equipment portion of the HVAC system. We can thank AHRI for a set of standards and conditions that each model is rated under.

But understand -- this rated efficiency is not how the equipment will likely operate in the field. Rated capacity and efficiency are the maximum potential of the box in laboratory conditions.

The secret is in the manufacturer’s specifications. Any installation condition that fails to allow the equipment to function as specified reduces capacity and efficiency.

That little yellow sticker pasted on the equipment declares what happened in the lab. How well the equipment operates in the field is a different matter and is dependent on how well you install and commission it.

Equipment efficiency typically deteriorates when installed in the field. The question is how much? The secret is in the manufacturer’s specifications. Any installation condition that fails to allow the equipment to function as specified reduces capacity and efficiency.

Some examples: if fan speed is not adjusted to meet design conditions, equipment performance can take a dive. Also, if refrigerant charge is not adjusted to compensate for the length of the line set, efficiency decreases. If the rated Btu of a furnace is not being delivered, gas pressure and combustion air must be adjusted to assure the equipment is operating as designed.

The truth is that installation defects cannot be identified by a simple checklist or good engineering alone. The installed operating properties of the system must be measured and compared to the specifications of the equipment, industry standards, and best practices.

Efficiency and performance are carefully built into a system, not shipped in the box.

One indicator of duct loss is to measure the average air temperature entering the return grilles and subtract that temperature from the temperature entering the air handling equipment.

The fact is that the delivered capacity of almost any piece of equipment can be measured in the field. If the equipment is rated at 60,000 Btuh, it should deliver very close to 60,000 Btuh.

Is the equipment you install meeting the manufacturer’s specifications? How would you know if it wasn’t?

Duct System Efficiency
When most equipment is installed, it is attached to a duct system. Ideally, the heating or cooling generated by the equipment is expected to make it into the building. If you install air conditioning equipment rated for three tons of cooling with a furnace rated for 60,000 Btuh output, you would expect three tons of cooling or 60,000 Btus of heating going into the building, right?

Infrared cameras such as this can show air leakage not only of the entire house, but of duct systems as well.

In simple heating terms, let’s say the duct system loses 15,000 Btus of heat as the air moves through the supply and return duct system. This is due to the effects of poor duct installation and design practices. Think of it this way: 15,000 Btus of heat lost, divided by 60,000 Btus of equipment rated heating capacity equals 25% of lost heating. Or, a 90% AFUE furnace quickly becomes a 68% AFUE system.

These duct losses are due to Btu losses through inadequate insulation, restriction of undersized ducts and fittings, or transitions with excessive resistance to airflow. Some Btus are lost through supply duct leaks through which hot or cold air is pulled into return duct systems from unconditioned spaces.

If 25% of the equipment rated Btus are lost through the duct system, this means the duct system is only 75% efficient at best.

One indicator of duct loss is to measure the average air temperature entering the return grilles and subtract that temperature from the temperature entering the air handling equipment.

Example: If the average return grille temperature measures 70F and the air entering the air handling equipment is 60 F, because the return duct is leaking and located in a cold attic, the return duct system lost 10 F.

If the equipment temperature rise is 40 F, you just lost 25% of the equipment’s capacity (10º divided by 40º equals 25%).

Do you see the potential of including duct system efficiency testing in the services you offer customers?

Verified System Efficiency
When you measure the performance of the equipment and duct systems you design, install, or service you are measuring and verifying system efficiency.

Unfortunately, the typical HVAC system only delivers efficiency averages less than 60% of its rated capacity in the United States.

This is bad news for the HVAC industry because it is evidence we have lost focus on our primary services of comfort, safety, and efficiency.

This is good news for you because it opens the door of opportunity to anyone willing to discover how poorly their HVAC system is operating. Every customer -- after learning their system is operating at 60% or less -- wants it to perform better. Those contractors who are able to deliver better comfort, safety, and efficiency by improving system performance have an unlimited inventory waiting for them to show up and claim.

If successful, this article may have sparked some questions. Do your systems really operate as well as you tell customers? Only time will tell what you are willing to do about it.

Rob “Doc” Falke serves the industry as president of National Comfort Institute an HVAC-based training company and membership organization. If you're an HVAC contractor or technician interested in a free procedure on how to verify HVAC system performance, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI’s website at nationalcomfortinstitute.com for free information, articles, and downloads.

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