Correctly Sizing a Capacitor I don't know how many times a technician has said that they installed a part based on what was on their service truck.  I have heard of technicians wasting money over-sizing contactors, cutting down air filters, and even using controlled substances to clear condensate drains!  Of course, these scenarios all get the job done, but I would argue the many reasons why not to do these.  The one thing that gets to me is when a technician doesn't verify they are installing the correct size dual run capacitor.  Believe it or not, there is a simple method to figuring the correct size capacitor, without waiting on hold for the distributor's guru.  Of course, you could use a multimeter that reads microfarads (uf), but this will only tell you if the existing capacitor is weak - not the correct size! Testing Volts / Amps With the condensing unit operating under a load, you will need to measure the total volts between the HERM and COMMON terminals on the run capacitor (i.e. 345 VAC). 

Next, measure the amperage on the wire leading from HERM to START on the Compressor (i.e. 4 amps). Use the equation below to verify the size of the capacitor. The resulting microfarad (uf) should match the size of the installed capacitor. An over or under-sized capacitor will cause an imbalance in the magnetic field of the motor.  This hesitation when operating will cause noisy operation, an increase in power consumption, a drop in motor performance and eventually overheating or overloading motors like compressors.  The run capacitor should have the exact microfarad (uf) that the motor is rated for.  Capacitors rated above 70uf are considered Start Capacitors and are generally removed from the circuit electrically during operation.  This is where the rule of +/- 10% of the rating came from, for Start Capacitors ONLY!  The voltage rating should be no less than the listed amount for the motor, for central heat pumps and air-conditioners this is usually a minimum of 370VAC. 

Most new condensing units are specified for 440VAC capacitors, and are more durable during fluctuations in power supply.  I have seen some universal type dual run capacitors rated for as much as 700VAC, as this voltage rating does not impact uf performance.  The changes in uf will impact amperage draw and reflect on kilowatt hour usage though.how to size ac unit for room When completing your early season maintenance this year for your customers, do them a service by testing capacitors and verifying their size.  wall vs window air conditionerYou could just increase your service ticket total and save some all important power.  compare prices on ac unitsYou may even prevent a call-back during the next heat wave! 0 members like this

< Previous Post Next Post >Expert article on how to properly size a whole-house air conditioner for your home. Included are tips for reducing the load on your air conditioning unit. Manufacturers produce whole-house central air conditioners in a range of sizes, which raises one of the most important questions you’ll have to answer when shopping for one: How big should it be? With air conditioners, the term “size” has nothing to do with a unit’s physical dimensions but, rather, its ability to produce cooled air. Both room-size and whole-house air conditioners are manufactured in a variety of capacities, so it’s helpful to understand how they are measured. An air conditioner’s ability to produce cooled air is measured in BTU (British Thermal Unit) per hour and in tons. A BTU measures heat output. One BTU equals the amount of energy needed to raise 1 pound of water by 1 degree F. One ton is the equivalent of 12,000 BTUs per hour, or the amount of energy it takes to melt 1 ton of ice in a day.

For one thing, the larger the capacity, the more a unit will cost since output is directly related to cost. Also, it’s important to pick a size that is appropriate for your house. An air conditioner that is too small can’t keep up with load requirements on a particularly hot day. One that is too large will cycle off and on too frequently, doing a poor job of dehumidifying the air, which degrades comfort. In fact, it’s better to slightly undersize an air conditioner than to oversize it. Also, the air flow into and out of rooms must be carefully balanced to ensure efficient operation of the system. These factors, in addition to how well a house is insulated, how it’s used by your family, your climate, and more must be taken into account when selecting and designing your system. Though you should consult a qualified air-conditioning contractor before making any purchase, you can estimate your requirements by figuring you will need about 1 ton of capacity for every 400 square feet of living space in a marginally insulated house.

For example, a 2,000-square-foot house would normally require about a 5-ton air-conditioning system; a newer, well insulated house can often be served by a smaller system. Featured Resource: Get a Pre-Screened Local Air Conditioning Contractor These simple measures will help you reduce your air-conditioning needs and save a bundle. When the weather gets hot, an air conditioner must work hard—that is, run frequently and for long periods—to keep a home at a comfortable temperature. Considering the energy cost of running an air conditioner, the net result of this is high utility bills. If you’re able to minimize the temperature in your home by other means, you can cut down on the work your air conditioner must do. In fact, you can reduce the load on your central air-conditioning system by taking just a few simple measures. For starters, clean or replace your furnace or air handler’s filters. Clogged filters restrict the movement of air through the system, which reduces efficiency.

Then consider the many improvements that can reduce your home’s heat gain such as utilizing heat-reflecting film on windows that face the sun, installing a foil radiant barrier in the attic, and improving your home’s insulation. Also to reduce heat gain you can simply pull shades or window coverings over sun-facing windows. Stir up breezes using a whole-house fan or ceiling fans, or circulate air using the “Fan Only” setting on the thermostat or equipment that controls your air handler’s blower. Also, when possible, minimize midday activities that add humidity to the air, such as washing clothes, showering, and cooking. Humidity makes the temperature feel warmer, and the air conditioner must work harder to remove humidity from the air. If you would rather hire an air-conditioning pro to inspect and maintain your AC equipment, this FREE service will help you find a qualified local AC professional. For more helpful tips, see 9 Ways to Cool Your House for Less.