Can You Solve the Muirbrook Case?

Sept. 1, 2006
This article is quite different from those Doc usually writes. This is a short story about an air diagnostic test on a home. The story includes a service

This article is quite different from those Doc usually writes. This is a short story about an air diagnostic test on a home. The story includes a service manager, an HVAC contractor, and a customer, the Muirbrooks. It also includes the history of the system and the air diagnostic test data collected during the air diagnostic sales call.

Your assignment, should you decide to accept it, is to evaluate the system from the data provided and prescribe the best solution for the Muirbrook’s system. Are you up to the challenge?

It was late summer and the cooler mornings were a welcomed relief for the service department at Abby Mechanical. The summer had been a scorcher and everyone was ready for a few days off. Several times during the summer Sherry Muirbrook had called Yvonne, Abby Mechanical’s customer service representative, seeking relief from the heat in her home. Sherry was sure the cooling system wasn’t working as well as it did three summers ago. Yvonne agreed to look a little deeper into the situation before once again sending over a service tech.

Perry, Abby’s service manger, swallowed hard as he reviewed the summer-long Muirbrook project in the database. The four service calls added up to nearly $1,000. Refrigerant was added on every call. The condensing unit and coil were replaced three years earlier with top-notch 5-ton, 16 SEER equipment and a “high-efficiency” filter at a cost of over $8,000. And that was after a sizable rebate from the local utility company, which hoped to reduce peak demand and promised reduced utility bills.

Perry read where Carl Muirbrook, Sherry’s husband, had called with concern that his utility bill had shown a serious increase the summer after the new equipment had been installed.

Perry printed off the Muirbrook history and called Brad, Abby’s president, on his cell. “I’m almost back to the shop; I’ll meet you in my office in five minutes,” replied Brad.

The two reviewed the Muirbrook history together and decided it was time to look past the boxes and get serious by checking the performance of the whole system. The service records looked pretty typical, but after attending an air balancing seminar last month, both of them were beginning to see there was a whole lot more to a system than just checking the equipment. Besides, they were both anxious to get out in the field and play with the new air balancing hood and other air testing instruments the company acquired after the training.

Yvonne called Sherry and asked if she could send out “the big guns” at 2:00 the next afternoon. “We’re trying out some new technology for we’ve acquired. We’re anxious to solve your problem,” Yvonne explained. Sherry agreed.

Brad dug out the ACCA manual J and D calculations he had completed three years before when the equipment was replaced. Fortunately, it had a room-by-room load specifying airflow for each room. Brad filled in the design portion of the air diagnostic reports and Perry printed out the equipment specs and engineering data from the manufacturer’s web site. With air diagnostic reports prepared and new instruments loaded, they arrived at the Muirbrook’s home at the appointed time, ready to take a new look at system performance, not just equipment operation.

Sherry greeted Perry and Brad at the front door and invited them in. The home was beautifully decorated and looked like a magazine. Sherry joked that if they could get the place comfortable it would be “perfect” and then commented how Martha Stewart should offer a how-to episode on solving indoor comfort problems.

Brad explained that they had come to measure the performance of the system. “Although we’ve serviced your equipment since the house was new 10 years ago, we’ve never looked at the airflow and delivered cooling capacity of your system. Using this new technology, we hope to learn how to make your system work as well as we thought it has for the last 10 years. All this time we have assumed the duct system worked as it should. Recently, we’ve learned that this usually isn’t the case. If you’d like, you can join us and see what we find. It’s interesting and educational. An hour from now, you’ll know more about your system performance that most air conditioning guys.”

“Whatever it takes, I’m game” said Sherry.

While Perry set up the air balancing hood, Brad reviewed the air diagnostic report with Sherry and showed her how much airflow each room needed. Brad explained that they would also measure the system’s temperature and blood pressure and then do a few calculations and rate the system’s performance.

For the next five minutes, the trio sat at the kitchen table and reviewed a comfort survey. Sherry’s home office was the most uncomfortable, and that’s where she spent most of her time during the heat of the day. The utility bills were way too high and the system seemed to work fine during the summer mornings, but never shut off when it was hot and the house got ugly and sticky each warm afternoon. Carl would often have to turn up the TV when the fan came on and Sherry had to go in the hall bathroom to put on her makeup because the master bathroom mirror stayed steamed up for a half an hour after she showered in the mornings.

“This air conditioning system is a mess,” Sherry said, summarizing the comfort survey. “I see what you mean when you say that you’ve been checking our equipment, but missing the big picture.”

“Pretty sad,” admitted Brad. “We’re a really good company compared to industry standards, but by using this new technology, we’re hoping to move a decade ahead of where our industry sits today. It’s time to start testing airflow. Let’s begin with your two problem areas, your office and the master bathroom exhaust fan.”

Perry grabbed the balancing hood while Brad checked the data he’d entered from the ACCA Manual J and calculated required air changes for the master bathroom exhaust fan. Sherry agreed to be the project recorder and write down all the test results on the air diagnostic report.

“The office needs 175 buckets of air per minute,” Brad explained. “It’s the air that carries the cooling into the room, if the air is low, the cooling you need doesn’t make it into the room. Got it?”

“I’m an airhead now,” Sherry joked.

Perry covered the grille and mashed the button. The hood read 55 cfm. The three looked at each other and Sherry interrupted the momentary silence. “Cool tool, I see I have a problem,” Sherry whispered. “Can you fix that?”

All Brad could say was “Awesome, it works. This is really cool stuff. We can finally fix all these problems and make tons of money.” Fortunately the words didn’t make it out of his mouth in front of his customer.

“Sure we can fix this,” said Perry. “Let’s check your bathroom exhaust fan next, it needs 80 cfm.”

Perry placed the hood on the exhaust fan. It measured 32 cfm. Brad quickly added, “How about a new exhaust fan, Sherry? I learned the ones they use in most new homes only move about 25% the air you need, and this proves it.”

“After 10 years of trips to the hall bath every morning, that would be a welcome addition to our home,” Sherry said with a sigh of relief.

The crew continued to measure supply and return grilles in the rest of the home and found the 5 ton system delivering only 1,640 supply cfm and pulling only 1,455 through the return grilles. Compared to the required airflow of 2,000, these numbers were pretty shocking.

Each reading was recorded on the air diagnostic report. The family room return grille was extremely noisy, and Perry measured velocity through the grille at 1,120 feet per minute.

While Perry and Sherry measured airflow, Brad had been taking wet bulb and dry bulb temperature readings at several of the supply registers and return grilles. The average supply grille temperature was 60.4F degrees and the return averaged 76.2F. While the equipment temperature drop had looked pretty good since the equipment was replaced, Perry had noted that all the readings had been taken in the mornings. The system temperature change of only 15.8F was pretty sorry compared the usual 20F techs usually found in this arid part of the country.

The wet bulb readings averaged 58.4F on at the supply registers and only 64.3F through the return grilles. Brad converted his wet bulb readings to enthalpy and found an enthalpy change through the system of only 4.15. He was pretty sure Perry and Sherry were finding total system airflow well under 400 cfm per ton, so that added to his concern that he enthalpy change was so far under the 6.3 or so that he was looking for.

Sherry totaled the airflow measurements and announced total airflow through the supply grilles was far less than the 2,000 they were looking for. “It’s no wonder his system hasn’t ever kept us comfortable. And I’m starting to see why our utility bills have been so high in this home,” she said.

Perry raised his eyebrows and tilted his head. He was pretty amazed at what he was learning as well, although his pride was a little hurt at the performance of this equipment his department had been servicing for a decade. He had never imagined the equipment could appear to be working, but the system could be performing so badly.

Everyone met at the indoor equipment. While Perry drilled the holes to measure total external static pressure and filter and coil pressure drops, Brad and Sherry started to pull the numbers off of the air diagnostic report and calculate system efficiency.

Perry attached the tubing to the Magnehelic® gauge and slipped on the static pressure tips. He checked the nameplate and found the fan was rated at a maximum total external static pressure of 0.50-in. w.c. He leveled the gauge and started testing pressures. The pressure entering the furnace just after the high efficiency filter was 0.42-in. The pressure before the filter was 0.09-in. As he measured the pressures leaving the air handler he found 0.36-in., and just past the coil the pressure measured 0.05-in.

Perry was curious to see the temperature change through the equipment and compare that to what Brad had found at the registers and grilles. Entering the furnace he measured 82.2F dry bulb and 66.9F wet bulb. Exiting the equipment he discovered 61.4F dry bulb and 57.5F wet bulb.

Just over an hour had passed since Sherry had greeted Abby Mechanicals two leaders at her front door. With the air diagnostics completed it was time to review their findings, make the final equipment calculations and draw up a proposal specifying the work required to bring the system into top operating condition and finally solve the Muirbrook’s comfort problems.

Your assignment, should you decide to accept it, is to 1) use the air diagnostic testing test data to diagnose the Muirbrook Project and draw your own system performance conclusions 2) write a proposal describing the work required to renovate the Muirbrook system so that it performs sufficient to deliver a minimum of 90% of the equipment rated BTUs into the their home. E-mail your diagnosis and proposal to Doc at [email protected] or call him at 800/633-7058.

The solution to Muirbrook’s Comfort and efficiency issues will be published in Doc’s next Contracting Business Hotmail article.

Rob “Doc” Falke serves the industry as president of National Comfort Institute a training company specializing in measuring, rating, improving and verifying HVAC system performance. If you're an HVAC contractor or technician interested a Extreme Weather Duct Diagnostic Test Procedure, contact Doc at [email protected] or call him at 800/633-7058. Go to NCI’s website at for free information, technical articles and downloads.