Network with Industry Professionals Science and Technology for the Built Environment Supplier Directory & Sponsored White Papers Citation and Abstract Services Publication Updates and Errata Standards, Research & Technology Standards Forms & Procedures Purchase Standards & Guidelines Resilience Activities in ASHRAE Advanced Energy Design Guides HVAC Design and Operation Training 2017 Las Vegas Winter Conference Courses ASHRAE Courses at Industry Events Self-Directed or Group Learning Rules of the Board ASHRAE Associate Society Alliance Young Engineers in ASHRAEResources & Publications | Choosing the Right System for Your Home Buyers Beware: Correctly Sized Air Conditioners Save Money! When you buy a new air conditioning or heating unit to replace your old one -- or if a completely new system is being installed in your new home -- the installer needs to know how big or, more importantly, how small the unit should be.

In order to know exactly how big (or small) it should be, the contractor needs to calculate how much cooling and heating capacity it must have to keep the occupants comfortable. This is called the load of the home. The load of the home is partly dependant upon the home’s square footage. However, a true load calculation goes beyond that. An accurate load determination includes building construction; orientation to the sun; low profile rv rooftop ac units“R” value of the insulation; air conditioning repair cost estimatenumber, size, and placement of rooms; ac unit for 1300 square foot housenumber, size, and placement of windows and doors; types of windows and doors (thermal efficiency); number and arrangement of floors;

Residential load calculations make use of mathematical formulas that take all these variables into consideration. They have been computerized, so they’re not as time-consuming as they were in the past. Determining the load by using rules of thumb almost always leads to an over-sized heating and cooling system, resulting in an increased initial cost, increased monthly utility bills, increased maintenance, and shortened equipment life because the equipment cycles off and on too frequently. Who Does the Load Calculation? The contractor you choose to install your new system should be able to perform these calculations. You may ask him for this service. If he doesn’t want to do it, or says it isn’t necessary, you may want to look for a different contractor. In some areas, the local utility will perform a load calculation – check with your local utility. How Big Is Big? Cooling size is given in tons of cooling; 1 ton = 12,000 Btu/h. 'SEER' (Seasonal Energy Efficiency Ratio) indicates the efficiency of the air conditioning unit, not its capacity.

Why it’s Important to Have the Right Sized System Having the wrong-size heating-cooling system can lead to multiple problems. Over-sizing your cooling system is not a good thing. Without getting into too much detail, your cooling load consists of two parts: We’ve all heard the saying, “It’s not the heat; A cooling system that is too big cools down the temperature very quickly, but it does not run long enough to remove sufficient moisture or “humidity” from the air. The result is you feel cool but clammy. Many people then turn down the thermostat to make the air conditioner run longer, increasing the utility bill. In some cases, the insufficient moisture removal resulting from over-sized cooling equipment can lead to mold growth and other types of moisture-related damage. A correctly sized cooling system runs long enough to “wring” moisture out of the air. This allows you to be comfortable at a slightly higher thermostat setting and allows you to save money on your utility bill.

It also takes care of the “mugginess” without making you feel too cold. Air conditioning systems are sized in “tons,” which is a measure of the rate at which they deliver cooling, not how much they weigh. One ton of cooling is equal to 12,000 Btu/h (British Thermal Units per hour), the rate of cooling required to freeze one ton of ice at 32°F in one day. It is better to have a system that supplies a little less than the required tonnage of cooling, rather than a system that supplies more than the required tonnage of cooling. Heating and Cooling Equipment Efficiency Ratings The table below shows efficiency ratings for good, better and much better heating and cooling equipment. SEER means Seasonal Energy Efficiency Ratio and provides an annual measure of the efficiency of the air conditioner. Higher numbers use less energy. AFUE means Annual Fuel Utilization Efficiency. HSPF means Heating Seasonal Performance Factor. For existing homes, the contractor should also check the condition of your ducts.

Ducts deliver heated or cooled air to your rooms. Ducts that excessively leak air or that do not deliver the right amount of air to each room can increase operational cost from a few percent to over 50%. AC Unit Maximum Breaker Re: AC Unit Maximum Breaker Is this new construction? Because it is only 5 amps I would look closely at the wire and insure that it is rated for the higher amperage under an inductive load. It may be wrong IAW the Mfg. but an over current device is there to protect the wire on the branch not the appliance. If the wire to the unit is rated for the breaker size at least you won't have a fire hazzard to the unit. Also a concern is if the breaker is rated for HVAC use. Also consider that the condenser may have internal wiring matched to the Max Fuse/Breaker Rating indicated on the label, and an older oversized breaker may not trip if the compressor motor locks up ... which could complete fry the equipment ... IMO report it as a defect needing evaluation/repair and call it a day ...

every HVAC guy and electrician I speak to say it is alright to go to the next 5 amp size. They want to see that the wire size is consistent with the higher size. None of them will change the breaker when called out to confirm this. At ITA I remember them saying that the wire size can be smaller if matched to the minimum ampacity rating on the dataplate. Being an HVAC tech for a long time that is what would happen. The tech will say no problem. As long as the breaker is correct for the wire size 5 amps isn't a big deal. Start up amps can be in the 60-amp range. I've seen units under this same scenario (25 amp nameplate) that would cause nuisance trips when a 25-amp breaker was installed and when the 30-amp breaker was put in the trips stopped and there were never any problems. It is over fused if a breaker larger than the manufacturer's label calls for is installed. Here is a good explanation I picked up somewhere: A/C condensers contain a hermetically sealed compressor motor as well as a fan to circulate air across the coils.

The rules for protection to motor circuits are different than for circuits with simple resistive loads. When a motor first starts , it draws a much higher amount of current than it does after it is running. The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. The inrush current lasts typically only about 6 electrical cycles, or 1/10th of a second - less time than it would take to damage the wire or its insulation. However, if the overcurrent device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start. Motor circuits get around this problem by dividing the two separate functions of an overcurrent protection device. "Overloads" are currents that can damage a circuit if allowed to continue for a sufficient time, whereas "short circuits" and "ground faults" are high currents that can cause immediate damage. Large motors and air conditioners separate these overcurrent functions. The breaker or fuse ahead of the air conditioner only protects against short circuits and ground faults.

Overload protection is built into the compressor itself. The wire to a condensing unit must be large enough to allow the equipment to start. The device is therefore NOT sized to protect the wire against overloads. It is not uncommon to see a 50 Amp breaker on a #10 wire. The wire is protected against overloading by a separate thermal protection inside the compressor. How do we know what the motor requires for wire size and proper fuse/breaker protection? It's all on the mfg. label attached to the appliance. If the label calls for fuses, there MUST be a fuse in the disconnecting means, not a circuit breaker. If the label calls for a "HACR" type breaker (Heating, Air Conditioning and Refrigeration), then that is the type of breaker that must be used. If the label simply states "maximum sized over current protection" then any form of overcurrent protection may be used. The HACR designation is not always visible on a circuit breaker once it is installed inside a panel or disconnect. Note on the label taken from a Lennox A/C data plate:

FUSE - 25 UL (CSA is a Canadian standard. We use UL in the U. S.)UL - Not allowed in U. S. All this means is the MINIMUM wire size required is #12 (solid copper); the MAXIMUM rating of a fuse (if a fuse is used) must be 25 amps and the MAXIMUM rating of a circuit breaker (if used) must be 25 amps AND the breaker must be a HACR type.However, if the over current device is sized to protect the wire against overloads, the device might trip, and the machine would not be able to start. "it draws a much higher amount of current than it does after it is running." This is called an inductive load. "The high "inrush" current can exceed the rating of a breaker or fuse sized to protect the wire. " This is why HVAC breakers must be rated for HVAC use. The wire, breaker (to protect the wire), and type of breaker must be correct and is reflected in the "minimum circuit data". Michael MerinoMerino's Home Inspection & Education Inc." I understand that the circuit is over-fused, but the appliance is connected to that circuit.