HVAC system control & switch Questions & Answers:

This article series explains where to find and how to use the switches and controls for air conditioning and heat pump systems.

Air Conditioner Switch & Control FAQs set 3

These questions and answers abour air conditioner and heat pump controls and switches were posted originally

at CONTROLS & SWITCHES, A/C - HEAT PUMP where you will find a discussion of thermostats, zone dampers, and circuit breakers on air conditioners and heat pump systems.

Service tech says we need an A/C shutoff switch for our attic heat pump - What's code?

I'm having problems with an attic heat pump system. This is a new home(2yrs).

When the tech went into the attic he mentioned that he thought a shut off switch should also be in the attic (one is outside).

He said he had to pull the back off the unit to shut it off. He thinks this may be a code violation. Where can I verify this in the state of Maryland? On 2011-06-03 by Tammy

by (mod) - Standard National Electrical Code will require a service switch at the air conditioner or heat pump

Standard National Electrical Code, of which Maryland has adopted a version, will require a service switch at the equipment.

Watch out: this requirement for local A/C equpment shutoff switches is a safety concern, not just convenience. The reasoning is that it is dangerous for the service technician to be invited to work on equipment for which there is no local shutoff switch.

Your Tech is correct. You can verify this with your local or state electrical inspector.

What's the right size and cost for an electrical disconnect for my A/C attic unit?

I'm trying to sell my place and the inspector is saying Attic air handler AC system Electrical Disconnect: Breaker disconnect Over-sized breaker, recommend installing breaker rated for feeder size. Does anyone know what something like this will cost.

I had AC air handler replaced in Feb. 2003. On 2011-06-13 by Judy

by (mod) -

RE: "the inspector is saying Attic air handler AC system Electrical Disconnect: Breaker disconnect Over-sized breaker, recommend installing breaker rated for feeder size."

Judy:

I'm not sure that your inspector is correct. Because air conditioning systems can draw high amps just during motor startup, and to avoid nuisance tripping of a circuit breaker or blowing of fuses, it's common for the circuit to include a breaker or fuse that is one size higher in amp rating than would be selected based just on the size of the wiring itself.

You can easily resolve this question by taking a look at the data tags on your air conditioner.

Most units, and all units sold in the last few years, include a data tag that gives the recommended overcurrent protection or breaker or fuse size for the unit.

If your fuse or breaker size is the same as recommended by the manufacturer you should be OK.

An exception would be if the installer routed an undersized *wire* to the unit. For example if the RLA (running load amps) of a system was 20 Amps that would use a #12 copper wire circuit, and the installer might have installed a 30A breaker if the data tag permitted it.

Now if the installer of that system used #14 wire (rated for just 15 amps) then the wrong wire was used and the problem is not the breaker size it's the wire size.

IN sum, take a look at the unit data tag to see what breaker size is recommended.

The cost to add a local service switch, parts and labor, should be under $500.

 

Air conditioner only runs if I hold down the reset button

My air conditioner is not working. I pushed the reset button on the contractor relay switch and the compressor/condenser started to work. When I released the button the unit stopped working. Any suggestions? On 2011-07-19 by mitch

by (mod) -

Mitch I'd sure like to see some sharp photos of the relay switch you are describing.

See our CONTACT link at page top or bottom for an email address.

That said, when pushing a reset button starts a piece of equipment but the equipment stops as soon as you take your finger off the reset button, then the conditions that caused the reset to trip to "off" are still present - in other words there is a problem to find and fix such as a short, seized motor, or less often, a bad relay itself.

Watch out: what you are doing is dangerous, risking an electrical fire or a dangerous shock. Stop it.

 

A/C unit stops working, turns on after I turn power off and back on

My AC unit stopped working 3 days ago.I checked breakers, fuses, and switches and don't know what it is. I was advised to pull out the fuse from a box by the unit, hanging on the side of the house, then put it back and the unit would reset itself.

It took about 6 hours then the unit started again only to stop working 12 hours later. After repeating the process I got the same result. What could cause this? On 2011-07-18 by Nick

by (mod) -

Nick there could be a simpler problem such as a bad control or switch or relay, but a system that resets itself but then goes off on reset again sounds like a motor overheating - maybe seizing? Check the starter relay;

A/C Won't Re-Start: Can I Avoid Replacing the Failing condenser unit Contactor Relay witch?

II have a 1994 2 1/2 ton ac split unit. Problem is room gets to proper temp compressor shuts down (but compressor fan stays on cold air).

Go upstairs raise thermostat till click then lower till activates compressor. Within minutes the room is cool.

I have replaced hard start capacitor,run capacitor, tried a spp6 hard start on run capacitor(foolish).

I really don't want to change out the RLY1097 start relay. If some one has a solution please let me know. The AC unit works perfectly when running. On 2012-12-16 by Dave

by Cool Air

the reason that you condenser wants to shut down is that your contactor relay points are stuck! On 2013-01-25

by mike e

I have a York 4 Ton 3 Phase Heat pump unit thats giving me problems. The board is getting 24v.

The High pressure switchs are fine. When I short the test switch together the contactor relay just for the fan buzzes for a few seconds(like its going in and out) then stops and nothing starts up. I can push in the compressor contactor manually and both the fan and compressor come on.

The board is not sending 24v to the compressor contactor when i short the test switch together. Any ideas on this would be helpful?

I've already changed the 5X440 capacitor, The fan relay. The low pressure and high pressure switches are closed the thermostat is calling for cool, the fan relay is getting 24v and the reversing valve is not stuck. On 2012-08-18

by pedro c.

It sounds like the contactor is going out, look at the copper where the button pulls and if the contactor is burned and pitted then it is time to replace the devivce. On 2012-12-26

 

A/C Condenser Unit Won't Shut Off unless I flip the breaker

I have an outside unit that wont shut off without tripping the breaker. A friend suspected the thermostat so I replaced it, but it didnt solve the problem. Any suggestions?

The inside fan kicks on and off within the set temp range, the outside motor just keeps going. Thanks for any help. On 2012-07-17 by Tim

by Bobcat

To: Tim B

Is this a heat Pump or Straight AC? A heat Pump uses a circut board to control the outside fan, and a straight AC should operate the fan when ever the compressor runs( cooling ).I will try to help On 2012-07-18

by (mod) -

Bobcat, thanks for helping out.

Tim:

regarding "I have an outside unit that wont shut off without tripping the breaker. "

- if the indoor air temp never reaches the set temp on the thermostat the system will just keep running - so if that's the case we need to determine why the indoor cooling isn't satisfying the thermostat.

- if you turn the thermostat setting "up" to be sure that the indoor air temp is below the set temp on the cooling thermostat - that is, the thermostat is satisfied, and then

if the equipment still never stops running, I suspect a bad control board or relay switch - and it's certainly time to call an HVAC/R service tech.

 

Power blinked, now the A/C blower fan won't run at proper speed

Power blinked Now at 100+ ambient cond fan blows 75 and dropping. Ceiling vents 120+ on cool.

Higher on heat. American standard heat pump w/new comp fan mtr, & capacitor .

Works backwards can give gauge readings if needed. I have 20+ years heavy Trk repair and a/c repair.

Some home exp. But not heat pump. Have meters also.
HELP !! 69 days of 100+ degrees in Texas. Gonna be workin all nite. Cooler outside than in house. On 2011-08-24 by Michael Baughman

Reply by (mod) -

Michael:

If by "Power blinked" you mean you had a power surge or outage, I'd check for a bad capacitor (can make a motor run backwards or fail to start) as well as for a burned up relay switch or control board.

 

Air conditioner condenser unit stopped working after we turned the A/C off

A couple of days ago my wife turned the ac off so we could open the windows and when she tried to turn the unit back on, the outdoor ac fan would not come on, the furnace fan would constantly run even if the thermostat was switched to the off position, and i can't even get the heat to work

I've tried most if not all the suggestions you have offered, but to no avail. (i'm not a HVAC tech so some things i didn't feel i should do.) we had our system repaired in the spring, and surprise it isn't covered under warranty. any help or additional advice would be greatly appreciated.

P.S. I have replaced the thermostat and the same problem is happening. On 2011-08-17 by nick

by (mod) -

Nick, perhaps the diagnostics at DIAGNOSTIC GUIDE A/C or HEAT PUMP  will help you a bit if you haven't seen all of those steps.

If you're nevertheless not getting anywhere after all of those more obvious things are checked (like a condensate pan overflow switch that can leave a whole A/C or heat pump system shut down), presuming everything is turned on and there is voltage, etc. the direction of my guesses would be a bad start capacitor or control relay or board.

Why does my air conditioner keep blowing fuses?

My air conditioner keeps blowing fuses according to the repairman, but he never does anything to stop the problem. Where are the fuses in the unit, and why would it keep blowing fuses and/or thermostats. - On 2011-08-13 by LaTonya

by (mod) -

LaTonya:

Watch out: If your A/C is blowing fuses there is a problem, unsafe wiring or equipment that is drawing too much current, which is a sign that it needs diagnosis and repair. Could be costly - a bad compressor - or not so bad - like a device that helps your A/C compressor motor get started.

Tell your repairman you'll call him to the fire when the problem he hasn't fixed finally causes an electrical fire.
Or find another repairman.

The text in the article above on this page lists where to find the controls on your A/C system and includes some photos of what they look like. Your fuses may be in your main electrical panel or the repairman may be referring to local fuses at or even inside the equipment - I don't know.

 

Outside A/C unit won't turn on, lights dim and the motor hums.

My Bryant a/c outside unit does not kick in even though there is a draw on electricity (light dim slightly for a second) I went outside and heard the motor humming and it felt hot to the touch.

The house HVAC interior fan is running ok and the thermostat is sending the right message.

Is it safe to open the outside unit and try and locate the reset button on the motor? There is an outside fuse box that I can disconnect first. Thanks On 2011-07-17 by don

by (mod) -

Don: dimming lights when the A/C tries to turn on followed by A/C not working sounds like a seizing compressor - you need a service call.

by A Gandhi

DanJoe,
I have a very similar situation like Don. We felt power went off for couple of seconds in the afternoon. But we didn't realize until couple of hours later that our A/C is not working. Circuit breaker was not tripped in the main electrical box.

Blower is working fine. Replaced batteries in Thermostat. I tripped the circuit breaker, turned off the switch near blower unit, waited for few hours. It made no difference. Thanks.

by (mod) -

A Gandhi:

Try replacing the starter relay; be sure to mount it in the same position as the old one.

by A Gandhi

Upon further investigation, it seems my starter relay was blown. I can see black mark on the relay. Component part number: BAYKSKT257 (If I am not wrong). But it never tripped the Circuit breaker...

Does it seem possible?

by (mod) - HVAC starter relay could fail and stop working without necessarily tripping a circuit breaker

A Ghandi

Sure an HVAC starter relay could fail and stop working without necessarily tripping a circuit breaker; the breaker trips on overcurrent; if a relay burns up and stops switching it does not necessarily pull an overcurrent nor trip a breaker. It just stays "off".

 

Lennox A/C fan won't stop; keeps clicking

The fan wouldn't stop running in our Lennox a/c unit. We replaced the contactor relay, but now it won't run at all and it makes a loud clicking noice. What is wrong now? On 2011-07-10 by monique

by (mod) -

Monique:

is it the fan that's clicking or just the relay? Does the compressor motor itself start and run? Did you wire the new contactor relay exactly as the old one?

Air Handler Runs but Compressor/Condenser unit Doesn't Turn On

I have a split unit Heat pump system. Last night I noticed that the house heated up to 82 degrees when the thermosat was set at 75. Th air handler fan was running continuously when set at the auto position on the thermostat. The outside compressor unit was not running.

I checked the condenser pan float switch and the condenser pan was dry. I checked and reset all of the circut breakers for the unit and I cycled the thermostat controls on an off.

The air handler unit fan came on but the compressor did not cycle on. The compressor fan also was not running.

This was at 5:00 am. I then shut the entire system down because the air handler fan is an serious energy hog and there was no need for that to be running if the compressor wasn't working. When I woke this morning at 6:30, I turned the unit back on at the thermostat and went outside to see if maybe the motor contactor wasn't being made.

Miraculously the compressor was now working. My question is , is the an overload condition that would cause the compressor motor to shut down during operation and be able to retart after it has cooled down and how do i check the amperage for that? - On 2011-08-09 by Mark

by (mod) -

Good question, Mark. If the compressor motor has experienced hard-starting, such as trying to restart against head pressure and being unable to do so, it might nevertheless be able to start some time later when you've left the system turned off for an hour or more. During that time the refrigerant pressure on the high and low sides of the system would equalize, falling to a low number, making it easier for the compressor to start.

If that's the problem the motor may be nearing end of life but might get some extra use life if the service tech installs a hard start capacitor to give the motor some extra starting energy.

Another possibility is a control (like a refrigerant metering device) that malfunctioned.

Your service tech can check the start and run current draw on the motor to see if it's within normal bounds per the data tag on the equipment.

My new thermostat doesn't include a control for air conditioning, just heat

My thermostat has been replaced with one that only has an on and off, and only has heater controls. there is no mention of the airconditioner on it . could that be the reason why the a/c does not work On 2011-08-07 by frank

by (mod) -

Frank: sure, if your prior thermostat controlled both heating and cooling the replacement unit needs the same wiring and connections to work properly. But if the TT you had before was ONLY controlling cooling, the new one can usually be made to work.

Philip: if your room temperature is below the thermostat set temperature the A/C system should stop running. Including the fan if the fan is set at AUTO. A fan might nevertheless continue running if at the air handler you have a fan limit switch whose control has been set for continuous fan operation - that might be the case if your air handler also is used for warm air heating.

But if the compressor never turns off and the room is cool and actual temperature is at or below the set temp, then there is a control problem or a thermostat problem. I'd check the TT wiring and if it's correct try swapping out the TT.

 

After I turned off my air conditioner to change the filter it won't turn back on

I turned off my American Standard air conditioning unit to replace the filter. I unplugged the electrostatic metal frame and changed the filter (like I have for approximately 5 years). When I plugged into the frame, the small indicator light did not come back on and the digital wall thermostat does not come on

I have checked the circuit breaker in the garage. No problems.

I checked the electrical socket where it is plugged in to see if that worked...no problems. I have called several repairmen and they are not able to help until Monday....It's 112 degrees today and I need help. On 2011-08-06 by Jenny

by (mod) - check for a blower door interlock switch that's open

Jenny

"...wall thermostat does not come on...." means you've lost power to the equipment.

Your thermostat is powered by a low-voltage transformer, usually near or even inside of the HVAC equipment itself. If you nipped one of those wires your TT would be dead.

A second thing to look at is whether you've mounted the filter assembly exactly correctly and that the blower compartment doors are fully in place, closed, and latched.

Often there is an electrical safety interlock switch on HVAC equipment compartment doors and covers to keep someone from being injured by moving parts when a cover is removed. Check all of your covers for neat and square installation and check that no one knocked-off a door sensor switch.

 

Air conditioning works for hours then shuts down

My A/C works fine for several hours or days and then shuts off. I can switch it off for a while and then it will come back on and work fine for another few hours or days. It has been really hot the past few weeks - 95+ and think my unit is overheating and may have some type of auto shut off.

I have never had to mess with a reset button. Any ideas on how to help or should i just call a tech? On 2011-06-03 by Duke

by (mod) - when your A/C keeps going off on reset

Duke when your A/C keeps going off on reset and will restart after a cool-down period it sounds as if a motor is overheating. I'd call a service tech. Let us know what you're told as it may help someone else.

Also check that at the outside compressor/condenser the airflow through the unit is not blocked.

My A/C wont' run, I'm burning up in GA

Just tried to use the AC yesterday for the first time this year. Won't do anything. Checked the thermostat by jumping the y to r. Nothing.

Appears that the insulation is stripped from the two small gauge wires going to the compressor outside. Where the wires go through the wall to the compressor, something has eaten or stripped the insulation.

Checked the breaker for the ac, and put the thermostat in fan only mode...still nothing. Any ideas?
Thanks On 2011-06-02 by Shadowfax419,

Reply by (mod) - AC won't start no matter what

RE: Burning up in GA: if your AC won't start at all and you tried jumping the thermostat wires, and considering that you saw insulation stripped off of those wires at the wall, I'd check for voltage at the thermostat wires - if there is a wiring short circuit it's possible that the low voltage thermostat transformer has failed.

See THERMOSTATS (Found in INDEX to RELATED ARTICLES at our Continue reading suggestions at the end of this article)

Also of course check that all switches and breakers are on and that there is voltage at the equipment.

If your equipment has no electrical power I suspect a tripped circuit breaker or a blown fuse.

More by Burning up in Ga

I checked the transformer for voltage at all terminals...none. I checked the low voltage wire and it appears to be ok where the insulation is stripped off.

I checked with my ohmeter and there doesn't appear to be either a break in the wires, or a short. I am not familiar with the LV transformer. Is there any way to check that, and if so, where is it? (probably a dumb question :})

There is a box on the compressor unit from the local EMC which controls use during peak hours. Could that be the problem? p.s. I also checked the black and white lugs on the contactor at the compressor and have voltage.
Thanks for your help.

More again by Burning up in GA (update)

Well...I noticed on the circuit board for the blower that there was no blinking status light. I pulled the automotive type fuse on the board and it's blown. Did a little research and it's supposed to be a 3a. My question is what if anything special do I need to do prior to installing the fuse. (turning off gas, electrical etc)

Solution by NOT Burning up in GA - blown fuse!

Followup Just for everyone's info. I turned off the breakers for both the AC and the furnace, popped the fuse in, turned on the breakers and voila, cool air.

Guess my advice is to check the board on the air handler for a status light and fuse. Great forum and my thanks for the moderators who volunteer their time and knowledge in order to educate us dummies. :)

by (mod) -

Not burning up in GA - thanks for the nice follow-up note. None of us knows it all; working together and exchanging information and citing sources helps all of us.

Jan: I think you need a service call. From what you write, when you got the blower fan to run it was by forcing it by setting the thermostat to the "FAN ON" position.

That tells us that the fan is capable of starting and running normally. The fact that the fan never comes on when you are in A/C cooling mode even though your outside compressor/condenser is running suggests that there is a control problem. Or someone has messed with your thermostat.

 

A/C lost after tornado: how do I turn it back on?

I have been out power for a few days after a tornado. finally have powere restored but the ac fan will not kick on. the outside compressor fan is working but not the blower. and the system is crawl space. On 2011-05-29 by earl

by (mod) -

A/C lost after tornado:

Earl, it's possible that some equipment, motor, control, or switch was flooded - did your crawl space flood?

Watch out: if your A/C equipment, wiring, switches, controls, or electrical panel were soaked or flooded, leaver power off for safety; don't try turning things back on before you get help from an electrician. The risk is fatal electric shock or a house fire.

Can you find a reset switch on the fan motor - look for a red button that popped out.

Check circuit breakers and fuses for a tripped breaker or blown fuse.

 

Evaporator unit fan motor is overheating and seizing when I switch from heat to Air Conditioning

I have a crazy question. I have fan, heat but when I switch to AC the 5a fuse pops right away. This is my start up for the season.

Nothing has changed. all connectio are solid and fan runs fine. Any help would be appreciated. Thanks

My evap fan has stopped working properly. It will start but runs slow, and will run for about a minute then shuts off. It feels really hot to the touch.

I have replaced the capacitor and did not help. Any suggestions on what else it might be? I'm thinking the motor is going out but not sure. On 2011-05-26 by Anonymous / Jason

by (mod) -

If switching from heat to A/C blows a fuse your A/C system needs diagnostic service by an expert: it could be anything from a short or bad control board or switch to a seized motor.

Jason: it sounds as if your evaporator unit fan motor is overheating and seizing. You need a service call from a trained service tech.

 

My Air Conditioner blower won't turn on - do I replace the motor?

I'm at my air conditioner unit and I have 242 v incoming to the unit and 121v going to each terminal. The thermostat is set to cool and 60 degrees and the blower isn't kicking on. I have voltage and nothing works.

I also turned the unit to on and I have no air flow. I checked the voltage at the circuit card and it has 121 v at the main terminal on the control card/ Does this mean the ckt card is bad or is it a bad relay?

... I just heard the Coolant kick in The cooling element is cold and I can feel the cold temp radiating from the cooling element and I can smell the freon.

I think this may be a bad blower or is it something else. On 2011-05-22 by Keith

by (mod) -

Sounds as if you need a service call but first see that the blower fan motor is not off on reset on it's own motor thermal reset button, and if a belt drives the blower be sure it's in place

Watch out, the blower compartment is a dangerous area for shock, and from moving parts, so turn power off before looking in.

Also, Keith, freon is odorless so if you smell something it's probably something else, maybe a burned up component

by Keith

OK Dan, you are right about the freon. I didn't see a reset button anywhere on the motor. I also looked on the Control board and there was nothing there for resetting the unit. I can hear an audible hum when the unit is set to on and the cooling element is getting frosty.

I'm wondering if I order the motor and just replace it, would that fix the problem?

I also checked the capacitor and it has 123v. Dan (or anyone) if that motor is bad what should I look for? Do you think this is the case?

Reply by mod:

I prefer not to order expensive replacement parts before we're confident that we've diagnosed the problem. For example, replacing a costly motor only to find that the problem wasn't the motor but was a blower door switch always makes me feel foolish.

 

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Continue reading at A/C CONTROLS & SWITCHES, A/C - HEAT PUMP - topic home, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• A/C - HEAT PUMP CONTROLS & SWITCHES - home
  • OUTSIDE HVAC SWITCHES & CONTROLS
  • INSIDE SWITCHES & CONTROLS
  • OPERATING CONTROLS, A/C & HEAT PUMP
• ELECTRICAL POWER SWITCH FOR HEAT
• LOST COOLING CAPACITY

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