Air conditioner & heat pump controls & switches:
This article explains the function, location, identification & use of all air conditioning & heat pump system operating controls. Photos and text help you to find & recognize each of these controls and the text explains what the control does.
We include links to detailed diagnosis & repair articles related to the various HVACR controls & swtiches. We also review the basic air conditioning safety switches, contactors, relays, refrigerant metering devices, motor overolad switches, relays, resets.
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This chapter is part of our extensive air conditioning inspection, diagnosis, & repair document which describes the inspection, diagnosis, and repair of residential air conditioning systems (A/C systems) for home buyers, owners, and home inspectors.
If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.
See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building.
Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Safety disconnects should installed outside next to the compressor/condenser unit and are often also installed next to or mounted on the air handler/blower unit.
If you cannot find an outside electrical disconnect at your compressor/condenser unit, one should be installed. These controls are recommended for safety to reduce the temptation to open the cabinet and work on the equipment with power on.
Working on electrically "live" cooling equipment risks both shock and mechanical injury such as being cut by the fan if the motor starts unexpectedly. Safety shutoffs are required for new equipment.
See A/C - HEAT PUMP CONTROLS & SWITCHES for details.
Our photograph of a modern circuit breaker panel (left) shows where your search for the air conditioning or heat pump system main circuit breakers would typically begin. Look for two control circuits for the air conditioner or heat pump system that will typically include:
The safety switch on newer equipment may be a simple pull-out fuse-block type power disconnect, leaving circuit protection to be provided only at the circuit breaker or fuse for the A/C circuit where it originates in the electrical panel.
Where the actual overcurrent protection is provided (at older circuit breakers used as auxiliary safety disconnects at the equipment, and at the main panel at the origin of the cooling circuit for the compressor/condenser unit) electrical overload protection size (circuit breaker or fuse amperage rating) for modern A/C equipment is specified by the manufacturer.
The Maximum Fuse or HACR type Breaker: specifies the maximum overcurrent protection or MOP to be used to protect the equipment. The permitted ampacity of the equipment electrical circuit protection (fuse or circuit breaker amps) expressed as MOP or Maximum Overcurrent Protection. If MOP is specified, the breaker or fuse protecting the equipment should match this number.
As we explained at the beginning of this document, a hermetic compressor draws varying amounts of current (measured in amps) as its internal pressure changes during operation. We said that current draw is higher when starting the motor than when the system is in steady state operation.
Current draw is highest if the motor is starting against its highest back pressure such as if the air conditioning system has been turned off and then back on in the middle of operation. Because fusing an air conditioning compressor at the minimum level can result in blown fuses or tripped breakers during these intervals of heavy current draw during compressor startup, compressors are either protected by a slow-blow fuse or a somewhat larger than minimum circuit breaker.]
On some older equipment MOP is not specified. Only when MOP has not been specified can the overcurrent protection required be determined by alternative means such as [RLA OR BCSC whichever is greater x 175%], or if the compressor keeps tripping that device or blowing that fuse, RLA x 225% might be used.
The National Electrical Code (NEC) specifies the degree to which a breaker or fuse may exceed the RLA. [For example, if the MOP or fuse size is specified by the manufacturer to be 40 amps, then a 40 amp breaker must be installed with no increase or change in that rating.]
Multiple switches are often present on cooling systems. As we reminded in the previous chapter, if the air conditioning system won't run, before requesting a service call check all of the switches as well as the thermostat for proper settings.
When the air conditioning system is running, if you measured the amperage, it would be roughly 80% of the RLA. The breaker size is typically about 125% of the total of the compressor RLA and the condenser fan FLA (full load amperage).
We are referring here to the main circuit breaker that controls the air conditioner compressor/condenser unit - a switch that is typically located in the main electrical panel or in a sub-panel serving the air conditioning or heat pump equipment.
Our photo at left shows a different switch: an outside service switch that incorporates a circuit breaker next to the compressor/condenser.
This circuit is for use by the service technician and because it is downstream of the wire bringing power to the compressor/condenser unit, it is not protecting that wire from an overcurrent. While both of these circuit circuit breakers must be properly served, don't confuse their role nor their location.
The rationale is that the circuit breaker protecting the air conditioner compressor unit should trip in the event of a locked rotor [the revolving axle of a compressor motor, for example] or some significant electrical event, but should not trip during start up loads which, as we know can be significantly higher than the RLA momentarily [as the compressor motor draws higher amperage to get itself started].
An air conditioning electrical circuit is different than a general household circuit in that we have a known current load.
[There is only one device connected to the air conditioning electrical circuit, and we can read its operating characteristics.] We are not worried about an overload situation where people plug several appliances into receptacles on a single circuit. Generally speaking, the amperage draw is fine or is way too big.
Code Citatin: Section E3602.10 of the IRC says,
Branch circuits for air conditioning and heat pump equipment. The ampacity of the conductors supplying a multi motor and combination load equipment shall not be less than the minimum circuit and capacity marked on the equipment.
The branch-circuit overcurrent device rating shall be the size and type marked on the appliance and shall be listed for the specific purpose." In short, do what it says on the data plate.
Thanks to Alan Carson for these details.
Continue reading at THERMOSTATS, HEATING / COOLING or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
Or see AIR HANDLER / BLOWER UNITS - home
Or see COMPRESSOR / CONDENSER REPAIR - home
Or see COOLING & HEATING CONTROL & SWITCH INDEX - index to all controls
Or see DIAGNOSTIC GUIDES A/C / HEAT PUMP - suggestions for diagnosing compressor or condenser fan and coil problems that can mean intermittent or totally lost cooling capacity of your system.
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(Jan 27, 2014) george said:
I am wiring a bryant 113a,21/2 ton ac unit. calles for 14 wire 46' run with a 25 amp breaker. My new run is 55'. Should I increse wire size or breaker size.
(Jan 27, 2014) (mod) said:
It's always safer to use a heavier gauge wire than to put in a larger circuit breaker. For HVAC equipment, to handle the start-up surge in current draw, it is often permitted to install a one-step oversized breaker but before doing so take a look at the guidance in the installation instructions for your equipment and on the equipment data tag. And see
(May 4, 2014) Gary Althoff said:
Circuit boards trouble codes
Gary I am guessing your succinct query asks for a guide to the trouble codes displayed by some HVAC circuit boards. I agree that such would be useful information. I've attempted to get at this question before for several readers and I find the subject too broad and grand for a single table or guide. For obvious reasons the various manufacturers do not use standard indicator codes nor even methods of displaying them. We see individual colored lights, blinking light codes, digital codes, alphameric codes etc.
What we can do is dig up the error code guide for a specific brand and model of equipment.
5/20.2014 Jennifer McCullough said:
A couple of weeks ago we found that our upstairs A/C unit was leaking water into our garage. At that point everything worked just as it should for heat and air. That is a small problem, but not the pressing issue here.
A few days later, an A/C repair truck showed up at our house saying he was here to fix the A/C and my kids let the guy in to look at it. The problem was that we hadn't called anyone. The kids tried to contact me, but I was in a bad service area on my way home from work and wasn't aware he had even been there until I got home and he had already left. Apparently he fiddled around with the downstairs unit in the garage for about 20 minutes and supposedly called "me" and found out he was at the wrong address and left.
We didn't know there was a problem until later that night when it became icy cold in the house. Even when we turned the thermostat to off, it kept blowing cold air. We saw a switch on the actual unit in the garage and tried turning that off. It turned off the fan, but then the A/C wouldn't come back on unless we turned the switch back on.
Then we had a couple of days down in the low 40s and needed heat, so we turned the thermostat to heat and flipped the switch back up. I never felt warm air come out of the vent, and just like with the A/C, it wouldn't turn off. The actual thermostat still clicks as though it has reached a temperature to turn on or off, but the fan only blows according to the switch position. The upstairs heat and A/C work fine.
Do you think this misguided gentleman simply disconnected the thermostat from the unit or what, and how do we correct it without calling a service person to our house and spending who knows what? The kids only saw green letters with A/C and Cooling Repair on the white truck...no company name or anything and the guy didn't give them a card. It will be near 90 later in the week and a working downstairs A/C would be great to have.
Jennifer, as the Poughkeepsie police sergeant said to Anna M., a woman who complained about the night time noise made by train engineers blowing their whistle at crossings, Well that's one I haven't heard before.
Since the repair person actually did something to your A/C system and didn't take away anything from the home I'll assume it was a somewhat light-headed repair person at the wrong address. Indeed I once entered and began inspecting conditions in the wrong house. I was joined by a termite inspector - we'd both been given a wrong address of a supposedly unoccupied home - by a realtor. Since two of us had come to the same address we never imagined we were in the wrong house. Luckily, soon and onlyt by noticing the names on mail lying out on a table did we figure out our mistake.
If there are only a few A/C repair companies in your area you could call around and in your least threatening tone (so as to get cooperation not defense) ask them to check service call records to see who was at your home on the date in question. Then if you can identify the company speak with the service manager and ask if he'd be kind enough to send over a senior technician to unfoul whatever was fouled up.
It sounds as if someone jury-rigged wiring or subverted a control.
(May 20, 2014) Jennifer McCullough said:
Thanks. Unfortunately I'm in metro Atlanta/Marietta, GA and there are hundreds of companies, so I don't even know where to begin. I sent an email to a place that advertises around here to see if they recognize the truck. Beyond that I don't know what to do. We could probably redo the wiring if we knew where to start because it just seems like the thermostat isn't running the unit any more.
Jennifer, if you don't have an idea who was there you're left with having to bring in an independent repair technician. Sorry.
If you want to check wiring of the thermostat, just disconnect the thermostat wires entirely - preferably at the A/C unit as that will exclude the chance of wires shorted together anywhere enroute from the thermostat to the unit.
With the thermostat disconnected you can be certain you're not calling for cooling. If the system still runs a control or control board has been snafu'd.
(June 20, 2014) Frank said:
my ac is working fine except that the compressor does not shut off. I have to turn off the breaker. Whats wrong.
Thermostat set below a level of cooling that the AC system can achieve, a short in thermostat wires, or a failed control or relay.
21 April 2015 mark S. said:
my a/c unit comes on while my oil furnance is running any ideas?
Are they running on the same thermostat? If so the TT or its wiring or a control board is defective.
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