The Mysterious Muirbrook Case is Solved

Sept. 1, 2006
This article is a continuation of a short story published in the last newsletter (Hotmail 10/3) where Doc described an air diagnostic sales call and challenged

This article is a continuation of a short story published in the last newsletter (Hotmail 10/3) where Doc described an air diagnostic sales call and challenged readers to diagnose the system and send in a proposal describing how to repair the system.

To review part one, click here. Or, to check your air diagnostic aptitude, read on…

When we left the Muirbrook project, Brad and Perry, owner and service manager of Abby Mechanical had just complete testing the cooling system and they were beginning to make the final calculations and rate the efficiency of the installed system. Their next step will be to prepare a proposal describing the scope of work needed 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.

Sherry’s Office
During the comfort survey, we learned Sherry’s home office was the most uncomfortable room in the house.

Diagnosis: The required airflow was 175 CFM, when airflow was measured with the hood, only 55 CFM was being delivered.

Recommendation: Repair or replace the supply duct to the office. If adequate air cannot be delivered, perhaps add an additional supply duct run from the equipment.

The Master Bathroom Exhaust Fan
A steamed up mirror had hampered Sherry’s makeup application in the master bathroom for 10 years.

Diagnosis: Brad had calculated required exhaust airflow at 80 CFM and the real bathroom problem was revealed when airflow was measured at 32 CFM. Perhaps she could actually use her master bath for makeup for the first time if the problem could be remedied.

Recommendation: Replace the existing exhaust fan with a 100 CFM fan rated at a minimum of .30-in. static pressure. Include a 2 1/2-in. x 10-in. rectangular metal exhaust duct with a good quality back draft damper and a termination cap. Also offer a two-stage timer designed to pull the full 100 CFM for the first 15 min. of a shower and then ramp the airflow down to 50 CFM to keep the bathroom ventilated during makeup time. The fan will shut off after 30 min. of operation.

Total Airflow
As Perry measured the registers and grilles in the house, Perry totaled the airflow measurements. In addition to Sherry’s office, several other rooms had airflow far too low or much too high.

Diagnosis: Since the cooling equipment was rated at five tons, 2000 CFM supply and return airflow were required for the system to operate at peak performance according to the manufacturer’s engineering data that Perry had pulled off the Internet. With only 1640 Supply CFM and 1455 return airflow, BTU delivery and ventilation prospects looked pretty bleak.

With supply airflow exceeding return airflow by 185 CFM it looked like a return duct leak was a sure bet. This was confirmed by Sherry’s observation that the equipment cooled fine in the morning, but wasn’t able to keep up in the afternoon when the 130F attic air would be pulled into the system offsetting a major portion of the cooling equipment’s capacity and adding serious humidity to the home.

Recommendation: Nothing short of a duct renovation will make this system work at it should. A duct renovation is a series of repairs corresponding to the test data gathered in the air diagnostic report. Ducts with airflow too high need balancing dampers. If they already exist, they need to be adjusted.

Ducts with low airflow need to be repaired or replaced. Some sections of duct have been damaged and must be repaired, many can use added suspension. Typically 20% of the ducts need to be replaced with larger ducts. Some registers may need to be replaced also. Temperature and pressure testing may identify more duct renovation work that may be necessary.

System Temperature Testing
Taking wet and dry bulb readings offers additional evidence to help diagnose the system performance. Since we tested in cooling mode, both wetbulb and dry bulb readings were taken over the system.

Dry Bulb Diagnosis: The average supply grille temperature was 60.4F and the return averaged 76.2F. Dry bulb testing showed a temperature change through the system of 15.8F. This is nearly 25% lower than the expected 20F temperature drop required for this dry region.

But Perry also took dry bulb readings the equipment. With temperature entering the furnace measuring 82.2F and 61.4F exiting the coil, the loss through the equipment was 20.8F.

The difference between the system temperature change and the equipment temperature change is duct loss. The duct loss in the system is 6F. One additional calculation is to divide the duct loss of 6F into the temperature drop over the equipment to find 29% of the BTUs removed from the coil are being lost through the duct system.

Wet Bulb Diagnosis: The supply register average wet bulb readings temperature was 58.4F and the 64.3F was measured at the return grilles. Brad converted the wet bulb readings to enthalpy and found an enthalpy change through the system of only 4.15. A typical enthalpy change through a typical system at 400 CFM per ton is 6.3 to 6.5.

Perry measured the wet bulb change through the equipment and found an entering wetbulb of 66.9F and an exiting wetbulb of 57.5F. This equals an entering enthalpy of 31.54 and an exiting enthalpy of 24.76 leaving an enthalpy change through the system of 6.78.

Recommendation: The primary defect identified by temperature testing was the difference between the return grille and equipment entering temperatures. This 6F gain is evidence of a serious return duct leak pulling air in from the attic that may run as hot as 130F in the summer.

While duct renovation will take care of most of this problem, additional duct insulation will be needed on longer duct runs in the attic area to protect from further thermal gains from the attic.

Static Pressure Solutions
Static pressure is a quick diagnostic measurement that can be used to plot the airflow moving through a fan when the fan speed is known. By measuring the pressure drops over system components and comparing the to industry averages, we can determine if the filter and the coil are correctly selected. When compared to rated fan static pressure, an ideal filter pressure drop should not exceed 20% and an ideal coil pressure drop should not exceed 40%.

Diagnostics: When a fan is rated at .50-in. total external static pressure and the measures static pressure is at .78” airflow is typically far below what is required for maximum system performance. Perry found the manufacturer’s fan data in the installation instructions and found the fan could only move 1752 CFM. By subtracting 1455 return air from the fan airflow we find a 300 CFM return duct leak. This is why the system keeps up in the mornings, but falls behind in the afternoon. It is also one of the reasons for the excessive utility bills.

The filter pressure drop over the found by subtracting the pressure before the filter of .09-in. and after the filter of .42-in. revealing a .31-in. filter pressure drop. Compare this to the ideal filter pressure drop .10-in.

Recommendation: Reduce filter pressure drop by installing an additional matching high efficiency filter on the opposite side of the furnace intake. In addition, add an additional 14-in. return duct and a 14-in. x 24-in. grille from the hallway into the new filter housing to further relieve total external static pressure. Clean the indoor coil also to reduce pressure drop, and clean the outdoor coil as well to increase heat transfer.

BTU Delivery
With the test data available on the system, we can now pour the numbers into system efficiency formulas to calculate the delivered BTU of the system and compare it to the rated BTU removal capacity of the equipment.

Diagnosis: Brad takes the measured delivered supply CFM of 1640 times the measured enthalpy change of 4.15 times the total BTU constant of 4.5 to find the system delivered BTU of 30,627.

He then checked the manufacturer’s engineering data for identify under the current operating conditions of 76 entering wetbulb and 85 degree drybulb outdoor that the equipment is rated for 56,200 BTU. The CSER (Cooling System Efficiency Ratio) came in at 54%.

Sherry, Brad, and Perry were all surprised that this system that appeared to be working all along was only functioning at 54% of rated capacity. Brad quickly added that 57.3% was the national average according to a recent NCI study, so perhaps the system wasn’t so bad after all.

Recommendation: In addition to the repairs listed above Brad added the final adjustments needed to assure the system was operating to the max after the work was completed. This included testing, adjusting and balancing the system once the work was complete. And then recharging the refrigerant circuit in the system, and finally calculating the final CSER of the system to assure the delivered capacity exceeded 90% of the equipment rated BTU system delivery.

All existing equipment was in good condition and would function properly once the renovated duct system allowed it to.
The price for the system renovation came to $3940.00. Not bad considering the system efficiency will be raised from 54% to somewhere over 90%.

“This is just like replacing your current equipment operating like a 3-ton system at 8.5 SEER with 5-ton 15 SEER equipment for half the going price,” said Perry.

“Does that include my new exhaust fan,” asked Sherry.

“This price includes all labor, material, and tax for the system renovation work, the exhaust fan and the system balancing when we’re done,” said Brad.

While Brad completed the proposal and Perry gathered up the tools and took them out to his truck, Sherry called her husband, Carl. After a five minute phone conversation, Sherry approved the proposal and the work was scheduled for the following Tuesday.

Happy Ending
When the work was completed the final stats looked like this:

  • Supply Airflow 1922 CFM
  • Return Airflow 1877 CFM
  • Total External Static Pressure .52-in.
  • Delivered BTU 53,932
  • Final CSER Rating 96%
  • Bathroom Exhaust Fan 102 CFM
  • Cost of Labor and Material $866.00
  • Gross Profit Margin 78%
  • Abby Mechanical Very Happy
  • The Muirbrooks Thrilled

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.