Duct Cleaning

By Ed Taylor
Via E-Mail

I am responsible for the mechanical system that serves a lecture hall and auditorium that is 30 years old. The owners are looking to replace the entire system with new equipment, lined ductwork, grilles, etc.

About 80 percent of the system is located under the building in a crawl space. I'm looking for an alternate way to solve the problem. Would it be possible to clean the ductwork?

From Steve Spielmann
Technical Manager
Goodway Technologies

Your question is not easy to answer given the limited amount of information. It would be necessary to know the construction of the ductwork involved as well as its condition. After 30 years in a crawl space that may be subject to high levels of humidity, the ductwork may not be worth trying to save. It may also be to the property owner's advantage to explore the many options that modern HVAC technology may offer.

Heat Rejection

From John West
Chesterfield, Va.

My question involves the proper heat rejection of a residential condenser. I have been told that 15 degrees to 20 degrees F across the condenser would be acceptable. What would you suggest would be better, the lower or higher number? Should the heat rejection be exactly equal to the heat pickup?

By Dan Kramer, P.E.
Specialist Grade Member of RSES

The air temperature rise across a residential condenser may well be 20 degrees to 30 degrees F. The air temperature rise across the condenser is a rudimentary measure of the system performance.

Hot air coming off the condenser as a result of a high temperature rise can tell you that lots of heat is being rejected at the condenser. However, hot air off the condenser confirmed by a high air temperature rise could also tell you:

  • The airflow is too low because the condenser coil is plugged with dirt.

  • The fans are not working right.

  • The fan pitch is too low.

  • The fan is loose on the shaft.

  • Or, the motor is running too slow.

    Hot air off the condenser could also tell you that air recirculation is causing the warmer air discharged by the condenser to be recirculated back to the condenser air inlet. This should immediately be suspected if the condenser or the high side is in a confined space. Indoor unit air recirculation is a major cause of reduced system capacity, long running times, and excessive power consumption.

    Of course, a small air temperature rise could also indicate a compressor not working efficiently. This could be caused by a faulty compressor, plugged suction filter, or low system charge. Cool air off the condenser (small air temperature rise) could be caused by a poorly performing evaporator. For instance, low evaporator airflow - a major problem - could be generated by an iced evaporator coil or one plugged with dirt because the owner had removed the plugged filter "intending" to replace it tomorrow, but never got around to it. It could be caused by an evaporator blower running at low speed, or dirty blower vanes. So, if the condenser and fans are clean and the motor is up to speed, air temperature rise is a pretty good measure of system performance.

    While condenser air temperature rise is a rough indication of system performance, only the condenser "TD" measures condenser performance. That is, the temperature difference will tell you whether the condenser is working right. Most residential air-cooled air conditioning condensers are sized by the manufacturer to have a 30 degree to 40 degree TD at about 45 degrees suction. In this context, TD is the difference between the temperature of the air entering the condenser and the temperature corresponding to the inlet condenser pressure (or, more conveniently, the head pressure).

    To determine the TD, you would observe the head pressure on your gauge and read the entering air temperature to the condenser at the same time. You would immediately write down these numbers. You would look up the temperature corresponding to the observed pressure on your P/T chart for the refrigerant in the system. Write it down. Then you would simply subtract the entering air temperature from the corresponding temperature you read from the chart (or your special calculator). That's the TD. It is directly affected by the load, by overcharge, by noncondensables, dirty fins, and fan performance, as well as the size of the condenser related to the load.

    You could not directly compare the air temperature rise across the condenser with the air temperature drop across the evaporator. That's because the condenser airflow is closer to 1,000 cfm per ton while the evaporator airflow is more like 400 cfm per ton. Also, remember that the heat rejected at the condenser is the sum of the heat picked up at the evaporator and the heat put in by the compressor motor that is split between compressor work and motor winding losses. If you have measured the air temperature rise across the condenser and can measure or estimate the airflow, you can (roughly) calculate the heat rejected at the condenser by the following:

    CFM x Air Temp Rise x 1.085 = Btuh

    I hope that helps.

    If you have a technical question, fax it to 847-622-7266 or on our Web site click on The Hotline.

    Publication date: 01/12/2004