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Tech talk

April 10, 2000
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In the March 13 installment of TECH TALK, we posed the problem of a homeowner in southern Ontario, Canada, who recently had an air conditioner and high-efficiency, variable-speed gas furnace installed, and later discovered that the air conditioner never shut off, “because the cold air isn’t forced to rise above floor level and the thermostat temperature doesn’t lower.”

After many visits from a technician, these homeowners were finally told that the furnace is doing what it’s supposed to do.

A quick recap

Location: 950-sq-ft home in Ontario Equipment:
  • Brand-new, noninsulated ductwork in good condition per visual inspection (replaced existing ductwork from older gas system);
  • 60,000-Btu gas-fired furnace;
  • 1.5-ton, split-system air conditioner;
  • Electronic air cleaner and bypass humidifier.
Repairs: Within one week of installation, the furnace’s inducer motor had to be replaced. Maintenance: Homeowner states that they have cleaned the filter every two months since the system was installed. Also, “We make sure that outside coils do not get dirty.” Fan setting: Installation contractor recommended to keep the fan on at all times, in order to ensure that the electronic air cleaner is working. Temp measurements: With the outside temperature 75°F and the thermostat set at 63°, the inside temperature was 73°. According to the homeowner, “Two hours later there was no change in the inside temperature.” In summation: “The contractor now repeats … that the air conditioner should have an output at 600 cfm, which is the same output as fan mode,” says the homeowner. “However, when we were sold the furnace and air conditioner, we were told it would output at a high speed.”

Tech heads talk

Kenneth Dyson, general manger of B&B A/C & Heating Service Co., Inc., Annapolis, MD, replied that “It is very possible that it is only a minor wiring problem, allowing the indoor fan to run at a low speed.

“On the older furnace circuit boards, the ‘Y’ terminal was a dummy terminal for the thermostat wires. Many times the ‘Y’ wire was just wire nutted together at the furnace to connect ‘Y’ to the outdoor unit.

“Many furnaces now use a lower speed when only ‘G’ is energized on the circuit board, and once the thermostat closes ‘R’ to ‘Y,’ the fan will then increase to the correct speed for cooling. (Many heat pump air handlers now do the same thing.)

“The solution for this furnace in Ontario may be just to connect ‘Y’ to the furnace circuit board.”

Rick Ream, residential service manager for Refrigeration Sales Corp., Cleveland, OH, responded with the following:

“Is this a communication problem between the tech, the contractor, and the homeowner? Yes. Do the contractor and/or homeowner not fully understand the system’s operation? Yes.”

Here’s how Ream said he would handle this service call:

“First, I would explain to the homeowner and contractor that new, variable-speed furnaces are equipped to handle a large range of cooling capacities. The furnace cfm for cooling should match the required cfm for the air conditioning unit. The industry standard is approximately 400 cfm per ton, plus or minus 50 cfm.

“Second, I would go to the t-stat and make sure it brings the a/c unit and blower on.

“Third, I would check the a/c unit for proper operation —

“1. Check superheat or subcooling.

“2. Measure the temperature drop across the coil.

“3. Inspect the duct system.

“4. Check register and grille location.

“Finally, after the unit is working to the manufacturer’s specification, I would explain to the homeowner that I am sorry for his inconvenience, and everything is now working properly.”

John Mills writes that “I firmly believe in the 400-cfm per ton setting for proper moisture control.

“Where are the supplies? My guess would be low-wall with low-wall returns. Cranking up the blower probably would help little in this case. Adding high returns or ceiling fans would solve this problem.

“I have a variable-speed furnace in a similarly sized house. I have floor supplies and ceiling returns. My blower runs at under 400 cfm per ton for the first 8 min before going up to full setting. I can maintain a 25°F difference in/out with a 2-ton a/c unit.

“There was a communications problem in telling the homeowner that the blower would use ‘high’ for cooling. Too many installers do this and it is not always right. It may be a poorly trained salesman here; sounds like the installers did it right.”

Dennis G. Morgan, of Modern Aire, Inc., in Havre, MT, asked, “Is there a damper (clearly marked Open Winter/Close Summer) on the bypass duct of the humidifier? And was the homeowner instructed as to its use?

“If the answer to either of the above questions is no, I think that is the problem. When the a/c is running, supply air (approximately 100 cfm) is being ‘short circuited’ directly back into the return air and reducing the total supply air quantity accordingly. And depending on other conditions, the evaporator coil could be freezing up, causing airflow to be further reduced and compounding the problem.”

According to service tech Larry Dailey with the Blake Medical Center, Bradenton, FL, this case “does not appear to be a communication problem. It’s a system problem.”

His solution: “It’s difficult to see the t-stat set for 63° with the unit running continuous for 2 hrs, with an outside temperature of 75° and temperature in the home of 73°, in a 950-sq-ft home, and the temperature doesn’t drop at all.

“I would attempt to change the speed control of the fan motor on the cooling cycle to high speed. This would mix the air and solve the problem of cool temperature at floors and warm ceilings by mixing the air better.

“And, hopefully, it would solve the problem of a dissatisfied customer in need of help.”

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