Need a simple low temperature A/C lock-out when outside temps drop below 60

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  #1  
Old 06-28-13, 06:21 AM
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Need a simple low temperature A/C lock-out when outside temps drop below 60

I live in Colorado and the temperature swing from high to low is easily 30+ degrees. To prevent damage to my compressor, I want to cut the electrical circuit whenever the outdoor temps drop below around 60 degrees overnight (my compressor's manual says 55 degrees is the minimum safe operating temp).

I figure there must be a simple and cheap outdoor thermostat of some sort that I can wire into the 24v trigger wire that runs from the furnace control board and tells the compressor to turn on. If it's below 60 degrees, I just want to interrupt the circuit to prevent the compressor from coming on when it gets the signal from the indoor thermostat.

I always leave my fan on the "On" position on the indoor thermostat, so I figure the system will simply think it's cooling but it actually won't be if the outdoor thermostat breaks the electrical circuit due to the low ambient outdoor temperature. Will this work? What type of a device do I need to purchase and how would I wire it? I can't seem to find what I'm looking for after a few google searches. Is there a better way to do what I'm trying to achieve?

Thanks!
 
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Old 06-28-13, 07:29 AM
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A remote bulb thermostat is probably what you'd like. I left a link to one on amazon. You can find them in many places...... Grainger, HVAC supply houses, eBay, etc.

Make sure you get one that has a SPDT (single pole double throw) switch so that you can wire it that when temperature drops....the switch opens.

This unit can go inside and you can run the bulb part outside or you could put the whole unit in the compressor.

Amazon. HONEYWELL T6031A1136 REMOTE BULB CONTROLS - T6031:
 
  #3  
Old 06-28-13, 08:13 AM
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Why would your A/C be running when it's 55 degrees outside?
 
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Old 06-28-13, 08:28 AM
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Something seems off on how your system is working. The compressor will only come on when it calls for cooling...if it's only 55-60 degrees outside...why would it ever call for cooling?

I don't think I've ever heard the minimum safe operating temp thing before...but I'm not a HVAC tech. Since compressors have heaters in them....why would below 55 operation be an issue?

There's also quite a bit of debate on whether running the fan 24/7 is a good idea. Is it humid in the summer months there? If so, when the compressor shuts off, you will be getting quite a bit of residual moisture off your coils being re-introduced to the house, making it feel warmer than it is. It also uses a lot of energy over time.

Also, is the fan ON speed the correct speed for your A/C? Seems like ON runs it at high speed and A/C normally runs at medium? I may be wrong on this. Could be heat that runs at the slower speed.
 
  #5  
Old 06-28-13, 08:35 AM
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Thanks PJMax. I'll take a look!

tomf63 & GunGuy45,

It's entirely possible that the A/C might try to come on if it's below 60 degrees since the temperature drops pretty quickly at night here (daytime high of 85 = nighttime low of 50-55). As a result, the interior of the house may not have cooled to below the set-point of the thermostat by the time the outdoor temperature drops below 60. For example, I set the thermostat to 72 degrees around 9pm to cool the house down nice and cold for bedtime. The A/C will cycle on and off a few times to maintain a 72 degree temperature during the 9pm-12am period. After midnight, it's not likely to come on again since the outdoor temp has usually dropped low enough by this time that no additional heat is gained until it warms up the next day. However, my thermostat does not have a programmable swing range and seems to try and maintain the temperature with +/- .5 degrees or so of the set point. So it's entirely possible that the outdoor temp could have dropped to below 60 while the indoor temp rises to 72.5 and the A/C would kick on. No, it wouldn't happen often, but it's certainly possible in this climate.
 
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Old 06-28-13, 08:44 AM
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GunGuy45,

See previous response above re: why the A/C could come on when it's below 60 outside.

It's super dry in Colorado and even drier where my house sits at 7000 ft. Outdoor humidity is almost always less than 20% and indoor humidity hovers around 30% according to my 2 hygrometers. Leaving the fan on 24x7 allows this moisture to recirculate into the house just like you mentioned which is actually a GOOD thing in this super dry climate. Additionally, I have a 3-story townhouse and if I don't leave the fan running continually, I end up with large temperature variances between the floors. Leaving the fan running greatly helps level out the temperature differences.

My system does not have a dedicated "On" speed for the fan motor - it just runs on low I believe (same as heat). Cool runs on high speed.

As for why you're not supposed to run the compressor at low ambient temps below about 55-60, I know that it can damage the compressor, but I'm not 100% sure as to why. I believe it has something to do with the refrigerant turning from a gas into a liquid at lower temps and also the increased possibility of the indoor coils freezing up at lower temps. Like I said, I'm not 100% sure as to why, but I've read that it's bad.
 

Last edited by jkozlow3; 06-28-13 at 09:13 AM.
  #7  
Old 06-28-13, 08:53 AM
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PJmax,

Do you know if they make these for use with low voltage, 24v? I was hoping to interrupt the low voltage trigger from the thermostat/furnace control board, but this one says "line voltage" which I'm assuming means I'd need to interrupt the 220v line going to the A/C compressor? (which I'd rather not touch!)

Also, now that I think about it, if I was going to place this thermostat in the outdoor condenser itself, couldn't I just use a regular old 24v thermostat (without a remote bulb) and set it to "Cool" and "60" degrees. This would complete the circuit to the condenser 24v trigger any time the ambient temp was above 60 and interrupt it whenever it was below 60.

Maybe I'm wrong. Are regular 24v thermostats even rated for outdoor use?

Thanks!
 

Last edited by jkozlow3; 06-28-13 at 09:11 AM.
  #8  
Old 06-28-13, 09:11 AM
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A crank case heater installed on the compressor would be a cost effective and simple way to protect the liquid settling that you are concerned about.
 
  #9  
Old 06-28-13, 09:12 AM
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I think your being over cautious here. I don't think you'll harm the compressor the few times it may run when the outside temp is a bit low. Here in NJ the evenings can get down into the upper 50's after it's been 80 during the daytime. Our bedroom is on the 2nd floor and we frequently run the a/c at night to cool things down upstairs, even in cooler evenings. The unit has survived 39 years being operated like this.

I think the point to the 55 or lower operation is geared more towards a cold start when the oil may be a bit heavy to initially pump. If the unit's been running all day, the compressor will be pretty warm inside and shouldn't have any issues.
 
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Old 06-28-13, 08:59 PM
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Just to answer your questions directly.....indoor wall thermostats won't last long outside.

The remote bulb thermostat I left the link for can work with any voltage. It's just a simple switch. It's rated high voltage so that it can also be used on high voltage.
 
  #11  
Old 06-29-13, 06:34 AM
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Perfect - thanks again PJmax!
 
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Old 06-29-13, 06:49 AM
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Seems like a lot of work when you can just bump the stat up a few degrees at bed time...or program it (if it's programmable) to bump itself up at midnight.
 
  #13  
Old 06-30-13, 07:59 PM
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The reason why it's a bad idea to run an a/c at low ambient temps is because the outdoor temperature has an impact on the refrigerant pressure in the condenser coil.

The lower the head pressure is (high side, feeding in indoor coil), the less refrigerant to be fed into the indoor coil.

When there isn't enough refrigerant entering the indoor coil, the evaporator pressure (indoor coil) can get too low -> that in turn causes the refrigerant boiling point to drop below 32F, resulting in freezing.

The boiling point of all substances is proportional to pressure.

Now, it's possible to get a system to run properly in cool weather by having a low ambient kit (which cycles the outdoor fan on and off to prevent the pressure from dropping below a specific threshold), but a simple outdoor t-stat will work in your application.

They make them for heatpumps - it's just a matter of finding one which can be set to 55F.
 
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