Tough to diagnose problems in HVAC systems

Hey there, time traveller!
This article was published 17/06/2007 (6795 days ago), so information in it may no longer be current.

QUESTION — Recently, we had a new 92 per cent efficient Lennox furnace and a new Lennox air conditioning unit installed. For myself, the furnace not using the chimney is something new but the intake/outlet vents installed, basically at ground level, work very well.

There is an odd situation, however, with the new air conditioner. The heating contractor who installed the unit has looked at it several times and after changing the control board, which didn’t correct this situation, the best guess is that it’s a thermostat issue. I will probably be getting a new thermostat this summer for our single-storey house. The furnace fan is a four-speed operation and we run the low speed continuously to maintain some air movement in the house. When the thermostat calls for cooling, in the summer, the furnace fan switches to high speed, Stage 4, and the outside unit starts running. Approximately every 20 minutes, the furnace fan completely shuts off, very briefly for two or three seconds, but the outside unit keeps running. After that brief pause the furnace fan goes straight back to Stage 4, high speed.

Are you familiar with this situation? Do you agree that it’s a thermostat issue?

–Garry Parker, e-mail

ANSWER — The recent, short spell of very hot, humid weather offers a good opportunity to answer this question as many homeowners have switched their thermostats to the cooling mode for the first time this year. To answer your first question directly, I am not familiar with that type of situation happening in the normal operation of a residential air conditioner. Having said that, I will offer some suggestions for possibilities and answer your second question in the process.

Most modern thermostats, electronic or mechanical, are designed to seamlessly switch from the heating to cooling modes with a simple adjustment of the controls. This normally requires moving a switch or pressing a button on the thermostat and lowering the temperature setting to engage the previously dormant air conditioner. With many older units, the power to the exterior condenser unit should be turned on a minimum of 48 hours prior to use to prevent damage. Newer models may not require this precaution, but it is always a good idea to flip on the safety shut-off switch near the condenser, or the circuit breaker in the electrical panel, well in advance of lowering the thermostat. After this time has elapsed, simple adjustments on the thermostat should put the air conditioning system into full use.

Thermostats for heating, ventilation, and air conditioning (HVAC) systems work by sensing the air temperature in the conditioned space to be heated or cooled. If this air temperature is above that setting while in the cooling mode, the thermostat will send a signal to the controls in the furnace to engage the air conditioner. Under normal operation, the air conditioner will turn on and the furnace fan will begin to blow air over the inside coil until the thermostat is satisfied that the temperature has reached the desired setting. At that point the thermostat will “tell” the HVAC controls to turn off the condenser unit and then the furnace fan, in a predetermined sequence. In your case, there seems to be a miscommunication between the thermostat and the furnace fan controls, for a brief period during the normal cycle.

There may indeed be a malfunction in the thermostat, but it is just as likely that there is a problem with the furnace controls or other safety features. There may be one main electronic circuit board in your new furnace, but there may also be other sensors and safety components that are causing this malfunction. There may be a high temperature sensor in the furnace fan motor or other component that is malfunctioning. The fact that the condenser unit continues to operate, even after the furnace fan motor briefly shuts down, may give this scenario more credibility. If the thermostat were at fault, I would expect that both the condenser and the fan would shut off, but that is only speculation, based on my limited knowledge of your particular system.

Whatever the cause of this issue you are seeing, the main consideration is whether this defect is causing a problem in the overall operation of the air conditioning system. If the furnace fan shuts off for an extended period of time, while the condenser unit is still operating, it is possible that the inside coil will freeze up due to a lack of air passing through. If the fan is only down for a few seconds, it is unlikely that would be a major problem. Checking to see that the condensate drain hose is clear and draining while the air conditioner is operating should eliminate this concern.

Modern HVAC systems have complex, high-tech electronic controls. These may malfunction, but are normally very reliable for a long period of time. Diagnosing problems with these electronic components may be beyond the knowledge of many HVAC technicians during a brief service call, so replacement of suspected defective components is often the most cost-effective way of determining where the problem lies.

If you have an older mechanical thermostat, upgrading to a programmable electronic thermostat is a wise investment for energy savings. Installation of a new unit of this type may indeed be the answer, but will be a relatively low-cost upgrade even if the problem is elsewhere.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors – Manitoba (www.cahpi.mb.ca). Questions can be e-mailed or sent to: Ask The Inspector, P. O. Box 69021, #110-2025 Corydon Ave., Winnipeg, MB. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com