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Are there any simple things I can do myself to determine how my heating and cooling system is working?

Schaible's Plumbing - Friday, November 30, 2012

Question: I’ve been told that my heating and cooling system is old and I should replace it. Are there any simple things I can do myself to determine how well it’s working?

Article originally published in “The Connections” (

-       -- By Allan Pincus

Answer: Bill, that’s a great question and the answer is “yes.” If you can read a thermometer and humidity gauge, then you’re already there.

The tips below are useful for a basic system check. Consult with a local expert for a detailed analysis.

During Heating Season

To test if your equipment is heating properly, get yourself a pocket thermometer such as the Taylor model 5989N. This item costs around $5.

During a cold winter’s day and with the system running for at least 15 minutes, insert the system of the thermometer into various supply registers around your home and record the temperatures. Your registers closest to the furnace should read from 110-115 degrees F. The most distant registers should read no less than 100-105 degrees F.

If many of your readings are below 100 degrees, your furnace may be generating too little heat for your home, there may be excessive cold air infiltration into the ductwork, or the fan blower speed may be set too high for the heating mode. The equipment itself might be fine.

If many of your readings are over 115 degrees F, your furnace may be oversized for your home, may have the gas inlet set too high, or there may be too little air flow through the furnace, possibly due to a clogged filter.

If all of your registers are between 105-115 degrees F, but the house remains uncomfortable, it may not be the equipment malfunctioning, but rather excessive air infiltration around the door, windows, and heat loss through a leaky attic or basement. Sealing the house against outside air infiltration and beefing up insulation may resolve the issue. A consultation with an energy efficiency expert may help.

During Cooling Season

During the summer, high humidity is the bane of the cooling system. Temperature, humidity, and air flow through the equipment all affect system performance.

Air conditioners must condense moisture vapor (humidity) from the air before it can be cooled, consuming a large percentage of its capacity. The more humid the air, the more moisture must condense out before it cools, reducing how much the air temperature is lowered.

Many people presume their older equipment isn’t working simply because the system struggles to cool the house during the summer. In reality, new equipment may not perform any better in the home unless the home is properly analyzed for its heating load, air tightness, and duct sizing.

With just a little know-how, you can make some basic measurements to determine whether or not your equipment is working.

All that is required are two instruments. The first is the pocket thermometer. The second is a hygrometer (a.k.a. relative humidity gauge). Exo Terra’s model PT2466 retails for about $7.

First, determine the relative humidity in the home. Set your thermostat to 75 degrees and place the hygrometer near it. Humidity levels change very slowly, so it may take some time to get a stable reading. Allow the house to cool to 75 degrees.

50% relative humidity is considered comfortable at 75 degrees F. If your system is working properly, your hygrometer should be reading between 45-55% relative humidity.

If the humidity remains above 55%, or if the house never reaches 75 degrees F, the problem may not lie with your equipment. There may be an excessive amount of air infiltration into the home from the outside. Sealing leaks at windows and doors might be all you need to do. A consultation with an expert is a good idea.

In our region, when the temperature outside exceeds 90 degrees F, it is not uncommon for cooling equipment to run continuously throughout the day.

Once at 75 degrees F and while the system is still running, measure several register temperatures using the pocket thermometer. Next, compute the temperature difference between 75 degrees F and the average temperature of the registers. This is known as the “split temperature difference” across the system.

If the difference is less than 17 degrees, there may be a problem but not necessarily with the equipment. Several causes for low split temperature include: Air moving too quickly through the evaporator coil, incorrect level of refrigerant charge, and excessive heat being absorbed in the ductwork. Correcting one or more of these problems may resolve the low split. Speaking with an expert would make sense.

If the split is between 18-23 degrees, your system is functioning properly and replacement is likely unnecessary. Your home should feel comfortable, even if the equipment is running continuously, especially if it is very hot outside.

If the split exceeds 23 degrees F, your system may be oversized for your home. Being oversized is not an equipment problem unto itself, but it does contribute to higher energy bills, hot and cold spots within the home, and can contribute to mold growth within the ductwork. Again, an expert’s opinion might be helpful.

Like taking one’s blood pressure and temperature measurements to assess one’s health, the measurements described above can indicate the basic operation of your heating and cooling equipment before jumping to the conclusion that it’s time to replace it, regardless of its age.

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