Sizing Is a ‘Must'

June 18, 2007
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[Editor’s Note: This is the first of three articles by Ken Summers about the three most important items an HVAC contractor should know.]

What are the three most important things a good HVAC contractor should know and understand today? Glad you asked. We’ll explore all three “musts” in this series of articles.

First of all, consider sizing equipment using a load calculation program one of those three “musts.” It is both essential and critical. Oversized equipment is bad for the client. It doesn’t remove summer humidity, contributes to uneven temperatures, costs more to buy, more to run, more to repair, and dies sooner.

HVAC systems have changed over the past 30 years. Bigger, high-efficiency indoor coils are now a fact of life. In order to get both efficiency and humidity removal, it is now more important than ever to put in the right-sized piece of equipment.

In the past, a contractor who installed a particular piece of equipment based on a load calculation and it did not maintain temperature usually received a nasty phone call from the customer. To avoid this, contractors often fudged on the design temperatures by 5°-10°F, both winter and summer. Other common fudge factors are to assume the highest infiltration rates, ignore window shading, and round everything up.

Why could contractors not trust Manual J when done “by the book”? In the end, contractors were (and many still are) short cutting and guessing at infiltration rates. They are not looking at duct leakage. And, they are totally ignoring the thermal envelope.

Infiltration rates, duct leakage, and the thermal envelope all factor into the sizing equation.



INFILTRATION RATES

When measuring a home for a load calculation, most good contractors take the time to measure the house within an inch. However, when it comes time to calculate the infiltration rate, here is where the guessing begins. The choices range from leaky to tight, but this can change the infiltration rates dramatically - up to 30 percent of the load.

Let’s take a 2,562-square-foot home in Nashville, Tenn. The winter outdoor design temperature is 14°, the summer outdoor design temperature is 94°, and the grain difference is 30. The difference between a leaky house and a tight one is alarming.

The leaky infiltration rate in winter shows 598 cubic feet per minute (cfm). In summer, it’s 273 cfm. However, when changed to tight with tight ducts, the infiltration drops to 102 cfm winter and 51 cfm summer.

In this example, the loose home has an infiltration load of 36,167 Btu in winter and 11,171 Btu in summer. However, the tight home winter gain is only 6,169 Btu. In summer it drops to 2,087 Btu. So if you assume it is tight, but it is really leaky, you end up with a system that doesn’t keep up. But if you assume leaky, and it is really tight, you have a system that is way oversized.

Today a contractor can actually measure a home’s infiltration rate with a blower door and use the actual cfm rather than guessing. In fact, Manual J now has inputs for using blower door test results.

DUCT LEAKAGE

Contractors also need to understand duct leakage, as this impacts the load calculation even greater than natural infiltration. As researchers started understanding ducts and the impact they have on a home’s infiltration, they found that having a 1-square-inch hole in the duct that leaks air to the outside of the home would be like having 30-square-inch hole in an outside wall, simply because there is a large fan which creates a driving force.

So the big question is: “How much duct leakage should we have?” The answer is simple: None. Reality is a different matter, though. It is impossible to obtain, even in the best-designed and installed duct system.

Therefore, standards have been developed and adopted by many authorities and jurisdictions across the country. “Allowable leakage” is typically 5 percent of the air conditioning flow at a test pressure of 25 Pascals (0.10 inches). This means that a 3-ton air conditioner is allowed to have no more than 60 cfm of leakage, which converts to an 11-square-inch hole in the ducts. This goal is obtainable.

But what happens if the duct leakage is greater? The average home across the country has 50 square inches of leakage, which is like having five 30-inch wide windows open 10 inches apiece. You can see that impact on the ducts can very easily affect equipment size. In many cases, when a system sized to Manual J can’t keep up, duct leakage has been found to be the hidden problem. While Manual J version 8 now has inputs for measured duct leakage, the best solution is to test for and seal the leakage, not to model it.



THERMAL ENVELOPE

Last, HVAC contractors have to look at the thermal envelope. While many believe this is not an HVAC contractor’s problem, Manual J says differently. This is why it asks for insulation levels, window U-values, and infiltration rates. Unfortunately, Manual J does not ask how many wide-open gaping holes the home may have nor does it ask if there is missing insulation in the home.

The reason Manual J does not ask this is because a home is not supposed to have any such issues. Gaping holes and missing insulation can, of course, affect heat gain and cooling losses. Imagine on the hottest days of summer or the coldest days in winter opening windows and doors in a home and expecting the equipment to work properly. Unfortunately, this is exactly what is happening 24 hours a day, 52 weeks per year.

With diagnostic equipment today, such as blower doors and infrared cameras, a contractor can easily determine the integrity of the home and let the homeowner know up front what needs to be done to make the system run properly.

This way a contractor is able to guarantee what he/she is putting in will work and provide the customer with the ultimate indoor comfort and lowest utility bills. While some HVAC contractors offer thermal envelope repairs in-house, the most common approach is to develop a good relationship with an insulator.

Bottom line: A contractor should aggressively size equipment by performance testing the ducts and home and doing a load calculation. Do so accordingly and you will not get those nasty phone calls from customers complaining that the equipment is not working properly.

Publication Date: 06/18/2007

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Praise

Terry Boone
June 24, 2007
Ken, very good article.

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