Glancing Back: At Your Service

September 14, 2001
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1950: THE ABC'S OF THE TXV One of the objectives of The News has been to help those servicing equipment to better understand the parts and equipment in order to diagnose and troubleshoot problems more effectively. In the September 11, 1950 issue, the function of thermostatic expansion valves (TXVs) was the topic under consideration in an article by Paul Reed.

"It is perhaps unfortunate that the thermostatic expansion valve is called a 'thermostatic' expansion valve," he wrote. "It gives the impression that the valve is to be used to control temperatures in somewhat the same way that a thermostat does. This is not true."

Actually, thermostatic expansion valve is a "constant superheat valve," Reed stated, because it lets the vapor in the last part of the evaporator to warm up, or superheat, the same amount of degrees all the time, wrote Reed.

"If we allow the vapor to warm up 20o, the amount of evaporator required to do the warming up of 20o will have to be more than if we allow 10? for the vapor to warm up. Consequently, less of the evaporator is left for active refrigerant if the superheating is 20o than if it is 10o. So the fewer number of degrees allowed for superheating, the more of the evaporator is active."

There are a number of reasons why the evaporator may not be operating correctly, but don't begin by adjusting the TXV,

he said. "The trouble is more likely something else, such as low on charge.... Check everything else first, but don't leave the job that way. Readjust the TXV if necessary."


In the September 18, 1995 News, Brad Nielsen of Research Products Corp. wrote that his company implemented some yearly procedures to keep humidifiers up to snuff. He provided some recommendations.

"When conducting a visual check of a humidifier with an evaporative water panel, always remove the evaporative panel from the unit for inspection," Neilsen stated. If the unit has an evaporative pad, it should be checked to see if it has a buildup of minerals. If the front, back, or both sides have mineral deposits on them, it should be replaced, he said. The distribution tray or other method for distributing water should also be examined to make sure it is free of lime.

"If the unit utilizes a bypass damper for summer shutdown, make sure the damper is open for the heating season," he said.

"A humidifier that looks too clean is probably not operating. A check of the water supply, humidistat, and power supply (if it is a power humidifier) should reveal the problem."

If the homeowner is unhappy with the humidification level, Nielsen said to check:

  • If the humidistat is set at the proper level;
  • To see if sufficient room for humidified air to escape the house exists or if a window has been left open; and
  • Whether the furnace is running for enough time. "It may be necessary to extend the blower operating time," said Nielsen.

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