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HVAC Condensing Units Page 1

Air Conditioning Units & Heat Pumps Condensing Unit Basics

Condensing UnitsAir Conditioning & Heat Pump condensing unit is a simple yet technical piece of HVAC equipment. It sits out in the back (or side) of your house and kicks on and off almost by itself. At least it seems that way to most people. In this box made of sheet metal, is the heart of your HVAC cooling system. Or for those with heat pumps, it is the heart of your HVAC heating and cooling. A heat pump condensing unit will look similar to an air conditioning condenser. There are differences inside the the heat pump condenser and air conditioning condenser equipment. A heat pump condenser has a reversing valve and an air conditioner condenser does not have a reversing valve. A heat pump will provide heating and cooling and an air conditioner will only cool your home or business. Both systems are common in the fact that both types of condensers have condenser coils, a compressor, controls, and a condenser fan motor. Unless you have a water to air heat pump system you probably do not have a heat pump if you live North of the Mid-Atlantic region (North of Maryland).

The Heart of an Air Conditioner

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The air conditioning & heat pump condensing unit houses the HVAC compressor (the heart of your system). It is the pump that is moving heat to the outside and bringing the refrigerant (which absorbs the heat) to the inside of your home. Vice versa for heat pumps in the winter. For those of us inside the HVAC heating and cooling business who work on condensing unit compressors, we understand them to be the transferors of heat. This transferor of heat, the compressor (inside the condensing unit), is hermetically sealed and non-serviceable. There is not much you can do with a burned up compressor except replace it with a new one. However, there is much you can do to maintain the equipment to give it a longer than average life and keep it running as smoothly and efficiently as the day it was new. Other components inside the air conditioning and heat pump condensing unit include the condenser coils, the condenser fan motor, and several condensing unit or heat pump controls.

Checking the Air Conditioning Units Condenser Fan Motor

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In late winter or early spring it is a good practice to check the condenser fan motor to make sure it turns. Unplug or turn off the condensing unit at the electrical disconnect box and then set the thermostat to cool. Go back outside and restore power to the condensing unit. Watch the air conditioning units & heat pump condenser fan motor to make sure it turns. The air conditioning & heat pump condenser fan motor should be blowing plenty of air up. If the HVAC condenser fan motor fails to start it is recommended that it be replaced. There is probably a bearing going bad in the HVAC condenser fan motor. Whatever the reason there is no need to take chances having the HVAC condenser fan motor fail and cause problems or damage the compressor which will require compressor troubleshooting to determine if the compressor is burned up. If the air conditioning & heat pump condenser fan motor fails on a hot day, the unit stops cooling and the pressures in the condenser rises until a high-pressure switch (not all units are equipped with high pressure switches) trips or the compressor overload shuts the compressor down. There is a possibility, with a failed HVAC condenser fan motor, that the compressor fails for good never to run again. An HVAC condenser fan motor is a lot cheaper than a compressor. Make sure the condenser fan motor is turning or running before the hot weather arrives.

Basic Condenser Maintenance & Upkeep

The question you are probably asking now is “What can I do to keep the condensing unit running smoothly and efficiently?” First you can keep the air conditioning & heat pump condenser coils clean of grass, dirt, and mud. Over time, these things build up inside the HVAC condenser coils and block the condenser coils. This accumulation of debris reduces the designed surface area of the air conditioning & heat pump condenser coils. Reducing that surface area of the condenser causes the compressor to work harder because there is less heat being exchanged from the HVAC condensing unit coils to the atmosphere. The less heat being displaced to the outside air the more heat that stays inside the condensing unit and the refrigerant. This causes the condensing unit pressures to rise inside the condensing unit. For the typical*air conditioner (AC) or heat pump the pressures should not exceed 300 P.S.I.G. on the hottest day of the year. *(Recent HFC refrigerants out on the market operate at higher design pressures.) If your condensing unit has dirty condensing coils and the thermometer outside is above 70 degrees then your condensing unit is most likely running at a higher pressure than it is designed to run at. The solution is to clean the HVAC condensing unit coils.

Condensing Units - Cleaning the Air Conditioning & Heat Pump Condensing Unit Coils

Before you drag the water hose over to the condensing unit and start spraying, you’ll want to secure the condensing unit. This begins at the HVAC thermostat and ends at the condenser electrical disconnect box located at the condensing unit. Turn the thermostat to the off position and pull the plug inside the condenser electrical disconnect box. Some condenser electrical disconnects have a switch like a circuit breaker located inside them. Turn the power off. After the power is off and the air conditioning & heat pump condensing unit is secure, break out the water hose with a good nozzle that will allow you to spray water at a high pressure. A little soap will help clean the dirt and other debris off the HVAC condenser coils also. Apply the soap and let it soak for a few minutes. Then spray the condenser coils. Be careful not to use too much pressure as you may bend some of the heat exchange fins that surround the air conditioning & heat pump condenser coils. For best results, it will help if you spray the water from the inside of the condensing unit coils out. This may require you to take the top of the condensing unit off. If you are not mechanically inclined do not attempt this procedure. Simply spray all the dirt and debris off the HVAC condensing unit coils as you can possibly spray off. Among the things to be aware of around the air conditioning & heat pump condensing unit are:
  • Weed Eaters around the electrical wires especially the thermostat wire.
  • branches from trees falling into the fan blade.
  • Insects getting inside the condensing unit unit and inside critical electrical components.
  • Children playing around the condensing unit (toys like balls can bend the heat exchange fins on the condensing unit reducing air flow).
  • plastic children’s pools, tarps, and other objects which can be picked up by the wind and blown on top of the condensing unit. There should not be any objects blocking air flow over the condensing unit and at least 2 feet of space around the sides of the condensing unit.

Condensing Units - Loose Electrical Connections in HVAC Systems

A loose electrical connection can cause big problems and safety issues with HVAC equipment. A good maintenance check of any HVAC system will include checking for loose electrical connections or wires that are corroded or frayed in some way. The picture to the right is a picture I took after repairing a large rooftop unit where a loose electrical connection almost started a fire in a commercial building. You will see a large fuse and two smaller fuses that have an indication of smoke on them. There is also a fuse block with the middle part of the fuse block melted completely away. Aluminum melts at approximately 1200 degrees Fahrenheit and the top center lug melted. Loose connections cause very high temperatures and the possibility of a fire. The smaller fuses are 100 amp fuses and the bigger fuse is 250 amps. I replaced two large fuses and all three small fuses along with the fuse block and a contactor that was damaged by the dripping hot aluminum falling into the coil of the contactor. Typically, on start up of new equipment the first thing the start up technician checks is to make sure all the electrical connections are tight. This is an important check on any regular maintenance schedule so that this problem does not happen to you.

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Condensing Units