Super-Size That System?

January 17, 2003
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It’s an oversimplification to believe that the increased energy efficiency of HVAC systems is in some way responsible for reduced moisture control. However, even though efficiencies have gone up, residential HVAC moisture-removal (latent) capability has not improved, explained Glenn Hourahan, a professional engineer and recently appointed vice president of Research and Technology for the Air Conditioning Contractors of America (ACCA).

He believes that in many cases efficiency and moisture control complement each other in HVAC systems. He maintains that better moisture control can be achieved with a truly higher efficiency system.

However, the way systems are typically sold and installed, they are really being sized larger than is needed based on the home’s size and other factors (insulation, windows, etc.). These A/C systems are too big to remove enough moisture. They are designed to lower temperature instead of humidity.

Key Factors

There are actually four factors to consider, Hourahan said:

1. Capacity;

2. Cost;

3. Efficiency; and

4. Moisture removal.

Cost and capacity are more likely to reduce moisture-removal ability than higher efficiency is, Hourahan said. “It’s really a question of capacity versus moisture removal.”

Cost is involved because the low-bid contractor is more likely to put in the same-size unit as existed before. “He’ll ask, ‘Were you satisfied with your old air conditioner’s performance?’” Hourahan said. “The customer will answer yes, so he’ll put the same size in without running a load calculation or asking any questions about new windows or insulation.”

Regarding capacity, “The public’s wish to buy a larger capacity A/C system is part of our belief that in all things, bigger is better — the mentality that made super-sized fries popular,” Hourahan explained. Many people believe that larger capacity A/C must be better. They think it must be able to cool faster and “better” than a smaller system, he noted.

However, in order to handle that latent load, Hourahan said, smaller is actually better.

Load Calculations

This leads to the importance of running a load calculation. In order to do this properly, ask the homeowner about new energy-saving features that may have been added, such as insulation and new windows.

This can result in a lower A/C load than was originally installed, especially if the first system was oversized, Hourahan pointed out.

Buildings are tighter, and many have added ventilation from bathroom fans; commercial and light commercial buildings have added ventilation to meet cfm minimums. This all adds up to an increase of humid air drawn in during cooling season in climates with hot and humid air. It probably was not figured into the load if the contractor swaps in like for like, Hourahan said.

“What you install should be within 15 percent of the load you calculate,” he said. “Ten percent under is better than 10 percent over.” Since cost plays a role with consumers, this could work to the contractor’s advantage.

At night, when there is more humidity than heat, a slight undersizing allows for more moisture removal because the unit runs longer.

Ductwork

Ductwork problems are one area where contractors can help solve a customer’s moisture and efficiency problems.

In unconditioned spaces, leaks can add to the load and also draw in still more humid air. If the cooling system is oversized, it won’t remove enough moisture from the incoming air; it reaches the temperature required by the thermostat and then shuts off.

Ductwork can also have too much friction drop — more energy is needed to blow air to overcome it, Hourahan pointed out.

If the ductwork is repaired where necessary, the load is decreased and a still smaller, lower cost A/C unit can be bid, he added. “Efficiency and moisture removal are working hand in hand, not against each other.”

Go in and do your load calculation using Manual J, Version 8, Hourahan advised contractors. Then figure out how many grains of moisture need to be removed.

Make sure you have the right coil sizes. “If you just match the efficiency ratings, you’ll match the efficiency, but you also need to size for the latent load,” he pointed out.

Errors

The biggest error is basing a replacement bid on swapping out like-for-like equipment. Don’t just ask, “Were you satisfied with the system’s performance?” Look further.

Contractors also need to become comfortable running Version 8 load calculations. “If they do a load calculation, they may put in a smaller unit, but they may make wrong assumptions about some building parameters, such as window overhang, and whether the insulation is R-7 or R-9,” Hourahan said. “The new load calculation methods in Manual J Version 8 are more sophisticated and can factor in more building parameters, figuring loads room by room as well as for the whole house.” It is very good for multiple-zone homes — those most affected by ANSI/ASHRAE/IESNA Standard 90.1-2001. (For more on Standard 90.1, see page 14.)

“You want to learn and understand the implications of variables,” Hourahan said.

ACCA’s new spreadsheet can be used as a training module in its own right, he continued. It gets contractors used to looking for key parameters. He said ACCA advises running a few load calculations by hand first, to get a feel for the variables involved, then using a computer program.

“Do it right.”

Hourahan will be one of the speakers featured at ASHRAE’s upcoming Winter Meeting Public Session on the impact of humidity on residences, to be held Tuesday afternoon, Jan. 28, at McCormick Place in Chicago.

Publication date: 01/20/2003

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