HVAC Residential Market / Furnaces / Air Conditioners / Ventilation

Duct Dynasty: Why That Oversized System Works Great

Show Consumers Why Bigger Isn’t Always Better

February 3, 2014
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One of the things I live for when training is to see that moment in time when a student in the class just gets it. That moment in time when something I say, or that they read, clicks and the floodgates open. This occurred very loudly during a private training session for an HVAC company I recently conducted.

After one of the salesmen in the training event was introduced to the concept of measuring HVAC system performance and delivered Btu, he had a life-changing realization. At that time he exclaimed, “So that’s why those oversized HVAC systems work great in the homeowner’s eyes.” In this article I’m going to look at what brought him to that realization. Hopefully it excites you too.

The Woes of an HVAC Salesperson

One of the common frustrations an HVAC salesperson has is dealing with a homeowner who won’t downsize his HVAC equipment when needed. No matter how grossly oversized the equipment is found to be, the homeowner simply wants to replace size for size. No amount of persuasion or Manual J output seems to budge him from this decision.

What are you to do? Are all such homeowners really this stubborn or could their reasons go much deeper?

One thing a homeowner understands is the comfort of his home. If his current HVAC system is keeping him comfortable, it is natural for him to assume he needs the same size of equipment as a replacement. This is where the communication breakdown begins between the salesperson and homeowner. The salesperson knows the equipment is oversized for the home, yet the homeowner is happy and says it heats and cools just fine. How can the homeowner be comfortable with a piece of equipment that would condition twice the amount of house? The answer lies in the true performance of the HVAC system.

Duct Losses and Gains

Duct loss and gain of conditioned air as it travels through the duct system is a huge factor in the performance of an HVAC system. This system impact is unfortunately overlooked by many HVAC salespeople and is rarely measured. When these factors are accounted for, they may be severely underestimated if the ducts are in a conditioned space as these values are typically assumed.

One of the most powerful tools a good salesperson can use to show a customer the impacts of the duct system on equipment capacity is to measure duct system losses and gains. This simple measurement will allow the homeowner to see how these aspects of a duct system are impacting the performance of the HVAC system. The cool thing is you can get a pretty good idea about radiant losses just by utilizing a dry bulb thermometer.

The formula to measure supply duct loss using a dry bulb thermometer is:

(Supply plenum temperature – average supply register temperature) ÷ temperature change (ΔT) through the equipment = percent of Btu loss through the supply duct system

The first step in this process is to obtain the temperature change or (ΔT), across the equipment. Let’s say you have a furnace with a 55?F ΔT. This would be found by measuring the return and supply plenum temperatures and then subtracting them. In this example, let’s say we have a return temperature of 70? and a supply temperature of 125?.

Next, an average supply register temperature will need to be obtained. Typically three registers that represent an average of the duct system will work for this. In this scenario, let’s say we have an average of 97? being delivered into the home. This is found by taking temperature readings from three supply registers, which we will say measure 100?, 95?, and 95?. These three readings are all added together and then divided by three, as this was the number of temperature readings taken.

The formula filled in with the above numbers would look like this:

(125? supply plenum temperature – 97? average supply register temperature)

÷ 55? ΔT through the equipment (125?- 97?) ÷ 55? = 28 ÷ 55? = 51 percent Btu loss through the supply duct system

Did you just have that aha moment? Do you see why that oversized HVAC system might be working great?

When you account for the duct losses, that oversized HVAC equipment is actually delivering the Btu needed for the building. The homeowner doesn’t know this. It’s your job to show him. Once he sees he is only getting half of what he is paying for, the lightbulb tends to go off. When this occurs, the ability to address the oversized equipment shifts significantly in your favor.

Once the risk is removed, the homeowner may be willing to listen to your advice. If all you have to offer him for advice is that his equipment is oversized, he’ll probably kick you to the curb. But what if he does listen? What if he takes your advice for the replacement of the proper size equipment and you don’t address the existing duct losses? You will now boast a very angry customer who put his trust in you. Ouch.

The cool thing about that formula is it works for cooling, too. Just switch the plenum and register temperatures. Ever get a call that a bigger cooling system is needed and it happens to be located in an attic? This formula will help you find answers beyond that of your competition.

Delivered Btu

The aha realization the salesman in that class had once he learned about delivered Btu and system performance was amazing. His eyes were opened and he knew that’s how the oversized HVAC system worked great. Only half of the equipment’s output capacity was making it into the conditioned space.

Duct systems that pass a leakage test and are designed by industry standards are assumed to be well-performing duct systems. The rubber meets the road when the field-installed performance is measured. No design can truly be guaranteed until the installation has been tested and verified. Performance can only be assumed when the measurements that are essential to verify true performance are not taken.

Duct leakage, in and of itself, is a very important measurement when it comes to HVAC system performance, but it is not the only element of system performance. Some of the other elements include proper sizing, insulation, and airflow delivery. These other parameters all deserve an equal share of attention because a duct system can only work properly when all four aspects fall into place. In order for these parameters to be verified, additional measurements need to be taken.

National Average Performance: 57 Percent

This type of problem is walked past on a daily basis as the performance of an HVAC system is often assumed instead of actually being measured. According to data that National Comfort Institute (NCI) has compiled over the past decade, the national average for an HVAC system’s performance comes in at a meager 57 percent of the equipment’s rated capacity.

By measuring system performance and delivered Btu you can help your customers understand why that oversized furnace seems to be working great. They will also begin to understand why they are wasting so much as well. By measuring and communicating clearly you can create a win-win outcome for both parties.

Publication date: 2/3/2014 

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