My air conditioner when left on never seems to shut off. Even during the night when it substantially cools down it still seems to run all night.

I live in Ottawa, Ontario and for example tonight the temperature is down to about 18 degrees celcius and want even a very warm day and it's been running for at least an hour straight and its been turned on (I don't know if it's been running) all day and set to a reasonable temperature of 25 degrees celcius.

Any trouble shooting tips or ideas?

  • Does the system blow cold air the whole time? Is the fan switch on the thermostat set to ON, or AUTO? Does the outdoor unit, indoor unit, or both run continuously? Does the indoor temperature ever hit the thermostat set point (i.e. is the home cooling down enough)? – Tester101 Jul 23 '15 at 9:50
  • The air blows the whole time but it doesn't seem very cold. I think the fan is set to auto but I'll have confirm tonight. It just seems like it's the outdoor unit that is running all night but I'm not too sure what the indoor unit is. I sat up for a long period and it does hit the temperature set on the thermostat. At that point it seems to shut off for maybe 15 to 20 mins and then run again for upwards to an hour. – Dan King Jul 23 '15 at 14:27
  • You are taking about a central A/C system, right? – Tester101 Jul 23 '15 at 15:06
  • Yes a central A/C – Dan King Jul 24 '15 at 4:35

I think that your coils are frozen. Ironically, I had the same issue with my HVAC system last week, although I am a few hundred miles south of you. Had an A/C man come out and he taught me about how units freeze up. Here are some things to check:

1) Are your air filters clean? Dirty air filters can suffocate the system and reduce the amount of efficiency. They could also cause the unit to freeze up (Which I will get to in a moment). If your system has been run without filters or with dirty filters for an extended period, the coils could also be very dirty and you will need a professional to clean them.

2) Is your thermostat set to a reasonable cool temperature? Do you have it on AUTO or ON? Sometimes leaving it set to "ON" 24/7 can be detrimental to your systems health.

3) Would you say that your vents feel under-powered? If so, this may be another sign of the coils freezing.

4) Is your A/C Drain line free? The drain line is a small 3/4" PVC pipe that comes out of your unit and drains outside. If it gets clogged, then the natural condensation that your A/C unit generates can't be drained, and could cause the system to operate incorrectly. You can try vacuuming it with a wet/dry vac. It will let out somewhere outside your home, which is where you want to connect the vacuum. Don't use a regular household vacuum! On my unit (See picture below) there is a T joint and a cleanout, so I didn't have to cut my line. I was able to pour water through it with a funnel to verify that it was not clogged.

enter image description here

  1. Touch the insulated refrigerant line. You should see it uncovered right next to the indoor unit). It should be very cool to the touch. If it is only slightly cool after several minutes of running the outdoor unit, then you are probably low on refrigerant.

  2. Is the air sufficiently cooled? Get a thermometer and measure the temperature of the air as close to the unit as you can. In my case there is an intake a few feet from it, so I measured the temp in the duct. Then, measure the temperature in a outgoing vent. The difference should be about 20 degrees (F). If it's less (it was about 11 for me), then you may not have enough refrigerant in your lines.

So here is something to try: Turn OFF your A/C, and run the fan only. So turn your thermostat to OFF and the fan to ON (not auto). This will help defrost the coils. After about 4 hours, cut on the air again. Does the air feel significantly cooler and at a better air flow rate? This may indicate that the coils did in fact freeze.

Thanks again google images!

When your coils freeze, it reduces airflow. The outside unit keeps pumping cold refrigerant through the system, thus making the problem worse. Sometimes the ice will make the air feel very humid, but because only a small amount of air can go through, the house will not cool.

It's okay if your coils freeze up once in a blue moon, (once or twice a year). But, if you are having to defrost every day like I was, then you need to call a repairman. You likely have a refrigerant leak. This is nothing to panic over - it's not like Carbon Monoxide or anything, but, it means that your system is burning electricity like no tomorrow ($$$) and you are still cooking in your own castle.

The repairman will assess if you have a leak and what your pressure is and should be at. He will refill your lines (and replace them if he needs to). While the bill will be high, I ended up saving $200 a month on my power bill after he came out (House went from 4000kwh to 1200kwh).

Note: Don't try to recharge the A/C refrigerant with an OTC kit, there are multiple kinds of refrigerant, and you should never mix them. A professional can properly assess what kind of refrigerant you need, if there is a leak, and what your options are to fix it.

Good Luck!

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  • @Teser101 The pictures is of the inside unit, and the insulated line is cold(er) than the other line. Are you sure? – DarthCaniac Jul 23 '15 at 16:39
  • Yes. The compressor (outside) compresses the refrigerant into a hot high pressure gas, which condenses in the condenser (outside) into a hot high pressure liquid. The liquid then travels inside via the liquid (smaller) pipe, to the expansion device (inside). When the refrigerant passes through the metering device (expansion device), it changes from a hot high pressure liquid, to a cold low pressure liquid. The heat from your home is blown across the coils of the evaporator (inside), where the heat can be absorbed by the refrigerant. (CONTINUED). – Tester101 Jul 23 '15 at 16:45
  • (CONTINUED) As the refrigerant gains energy (heat), if flashes (boils) into a low temperature low pressure gas. The gas then flows back to the compressor, where it can start the refrigeration cycle again. – Tester101 Jul 23 '15 at 16:46
  • @Tester101 Makes sense, but, how come the larger line is insulated and cooler than the smaller line? Does the expansion take place in the unit itself? – DarthCaniac Jul 23 '15 at 16:49
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    I know it seems counter intuitive, but trust me, the A/C works by pumping hot high pressure liquid refrigerant into your house, and taking cool low pressure gaseous refrigerant out. – Tester101 Jul 23 '15 at 16:49

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