The residential HVAC industry has experienced many changes and subsequent growing pains over the past few years. From the self-powered standing pilot ignition and gravity heating systems of the 1950s to today’s electronic ignition and variable DC blower fan circulation systems, these changes have been accepted by most technicians – some begrudgingly, some readily and some are still back in the dark ages. For years, continuous air circulation has been considered the ideal for a forced air system. In the January, 1957 Manual 6 of the National Environmental Systems Contractors Association, it is suggested:
- “This manual provides the essential information needed to properly adjust a year-round system for comfort air circulation – CAC - CAC means healthful and comfortably uniform conditions from floor to ceiling and room to room – hour after hour – day after day – the whole year through.”
What’s old is new
Here we are, over fifty years later, and what was old is now new again! CAC was accomplished originally with optional two-speed fractional motors mounted on belt driven blower assemblies; this method was quite effective for the time. It did require regular maintenance be performed to replace belts and, occasionally, blower bearings. The next generation of product saw the addition of shaded pole or PSC (permanent split capacitor) motors mounted in the blower wheels as a “direct drive” configuration. This configuration saved the consumers money in the initial equipment cost and reduced maintenance expenses as belts, pulleys and blower bearings were now removed. Some minor control changes to the appliance provided up to four selectable fan speeds in one package. This design change did require an increase in motor capacity; ½ and ¾ horsepower blower motors became the standard. Along with the increase in horsepower came a jump in electricity use to start and run the direct-coupled assembly.
Slow start for DC
DC motors have been around for quite a few years now; initially they were ignored as a temporary product in the marketplace that would fade away due to high initial cost. They were deemed too expensive not only by homeowners, but also contractors. In some cases the contractors where afraid to install them because of the presumed time necessary to set them up. However, marketing people did a superb job in praising the DC products and pushing sales in the marketplace. At the same time, homeowners had more leisure time and the internet showed them all the new products available to reduce their operating costs. Unfortunately, many HVAC techs did not take the time to learn of these advantages. In some cases the customer was more informed than the professional he had called for pricing a replacement unit! Their misunderstanding of these new products, combined with the lack of the tools needed to set them up, compounded the issue. It didn’t help that diagnostic training was either extremely vague (Is there power to the motor – Yes or No?) or seriously complicated with no middle ground. (Usually a fifteen-line procedure to get the same question answered!)
New tools, training
This has now improved with simple diagnostic tools available for reasonable cost. The manufacturers have provided information charts that help the technician determine which component has failed. Local technical service advisers offer courses throughout the year. It has always been a good idea to brush up from time to time; this will keep you caught up on the new systems. Information on the variables is golden due to the costs that homeowners can pay out for a system being misdiagnosed and the wrong parts being replaced. Selling the customer an extended warranty may also help their peace of mind. Now with the various government agencies’ throwing rebate and incentive money on efficiency programs, DC products are the new “in thing” and the market has swung to favour the concept.
The theory of using a variable speed furnace in a residential application is, when set up correctly, to move air around the home, at a gentle flow throughout. This flow should not bother the homeowners, making it feel cool or creating drafts. During the off-cycle of the heating or cooling mode, the low speed constant operation will pick up air through the return air grills in the home. This mixed air is sent into the home and eventually this equalizes out the temperature from room to room and finally floor to floor. One of the added benefits is the constant air movement through the furnace’s air filtration system for better comfort and indoor air quality. Running the DC furnace motor twenty-four hours a day, seven days a week, there is minimal additional cost as these products will run as low as 65 watts compared to up to 500 watts for a PSC motor of similar capacity.
Setting up the furnace
Variable speed operation is based on rpm or torque and cfm. Therefore, they do require set up by adjusting the dip switches on a tap selection board. For example, today’s furnaces will have as many as six different continuous speed selections, eight different heating speed selections and another eight or more speeds to select for cooling operations. Within the installation manual for the furnace, the speed selection dipswitch positions are well marked for adjustments. In addition, these switch settings can be referred to on the airflow and static pressure charts provided in the manuals. This will help find the determined set-up and the actual CFM the equipment is running. These new static pressures and values should be recorded in the set-up guide for future servicing. The air conditioning set-up of the system is separate from the low speed constant and the heating set-up; the dipswitches come from most factories set to the highest speed. Most often, it will be necessary to adjust, the cooling switches as well. The requirements of the a/c evaporator’s correct airflow are referenced within the install manuals. Static pressures and airflow rates are needed to maintain the correct superheat for the performance of the system. The static drop through the coil is measured and when the appropriated changes are met, they should be recorded in the guide for future servicing. With the proper set-up, in accordance with the manufacturer’s instructions, a variable speed system will function as designed and do its part in saving energy and providing comfort for the homeowner.
Robert (Bob) Bettles in lead technical service adviser and trainer for Wolseley HVAC/R Group Canada. Brian Guttormson is technical service advisor for Trent Metals Ltd. (Supply). Bob can be reached at Bob.Bettles@wolseleyinc.ca while Brian is at firstname.lastname@example.org
Final setup is done with dipswitches on the programming board.
Better tools, like this GE troubleshooting assistant, have solved many of the issues technicians had with DC motors.
Click here to access videos of interest to Canada's mechanical contractors, from trade show highlights to new product introductions, expert interviews and contractor profiles.