Hello people hopefully smarter than I:
My wife insists that if we have the A/C on, and it becomes cooler outside than it is in, that the A/C will run longer and more often.
I’m of the opinion that it will only run longer and more often if it’s warmer outside, thereby making it work harder to keep the indoors cool.
I’m not saying I only want reply’s from HVAC repairmen and Climate Control Engineers, but please don’t reply with opinions - I’m looking for facts.
Thanks,
TBone82
That doesn’t really make any sense. I just can’t see any reasonable way that having the outside temp being cooler than the inside temp would cause the AC to run longer. IANAHVAC expert, but I do know a few things. Most home AC units have a sensor somewhere in the house (usally near a return vent). When the temp. reading at the sensor equals the number set at the control pad the unit shuts off outside temp. doesn’t affect that.
Only in the case of a long cold spell and it is in heat pump mode. Otherwise the statment is off the wall!
Like in a long hot summer.
The A/C responds to the thermostat. A modest change in outside air temperature takes quit a while to affect the indoor temperature and require more or less A/C.
OTOH an anticipating temperature sensor in ouside air could speed up the response to changes in outside air temp.
Pedigree on request if the above is not clear.
I am not an HVAC guy but I know something about physics and I cannot imagine what your wife has been smoking. It is possible for the interior of a house to be warmer than the outside temperature due to two effects: radiant heat from the sun at the exterior, and “greenhouse” effects in the interior.
If it’s 68[sup]o[/sup]F outdoors but a calm, sunny day, the sunshine will heat up the exterior of your house and conduction will heat the interior, subject to how good your insulation is. Sunlight through your windows will also heat the interior. These effects can cause your A/C to run to keep the interior down to 72[sup]o[/sup].
BUT BUT BUT if it’s hotter than 72[sup]o[/sup] outside, convection kicks in on top of all of that, and the interior takes on even more heat.
Look at it this way. If we extend your wife’s claim, the A/C would be running constantly when it gets below freezing, and if it went into the 90’s the A/C wouldn’t run at all.
Before bending over backwards to refute her claim you might ask her to provide some rationale first 
What exactly happens in that case? Wouldn’t a heat pump work more efficiently if the exterior temperature is lower?
TBone82 said:
I’m not any smarter than you, however I am an HVAC repairmen… 
If you ran your residential A/C with a setting that was higher than the outside temperature, the thermostat would [most often] become satisfied and the air conditioning would be turned off, (by the t-stat) not run longer.
IOW, consider that the primary load on a house is the surrounding ambient temperatures outside. Through infiltration,—through the roof, windows, door openings, walls etc----- heat enters the house. The thermostat ‘senses’ the heat buildup in the house and calls on the A/C system to remove the heat and discharge it into the backyard. (usually) This infiltration of heat we call a “load” that is being imposed on the house, that the A/C system has to respond to. Other loads (that produce heat) are humans, computers, dryers, stoves,lighting etc etc. But the primary load on a house is outside ambient temperatures.
So… your wife is [basically] wrong. The lower the temps outside, the lower the infiltration (of heat), the lower the calls on the A/C, the shorter the run times.
So…in your example if the thermostat is set at 72°, and the outside temperature is 68° the amount of load [imposed on the house] is either non-existent or severely diminished. (Direct sunlight, other loads and heat build-up may still produce a load that the thermstat may need to respond to) the A/C will run shorter and less often.
That is basically true…however there are other variables that produce a ‘load.’
It may be 65° outside, but if the wedding reception is at your house this weekend, expect the 100 guests to impose a load on the A/C system.
If your roof is dark, poorly insulated w/ little or no shade, with single pane glass in your windows, no storms or weather stripping, poorly fit doors, leaky poorly insulated ductwork etc-------
and your neighbor’s house suffers from none of these sins, his system will run a lot less, even though the temperature outside is the same for both of you.
The point is this—it’s one thing to have the load outside. It’s another to keep it from entering the house. (read: insulation etc) A well insulated house does a better job of reducing the rate of ‘heat transfer’ and thereby reduces infiltration and run times, no matter the load outside.
Not only will insulation in roofs/walls, windows etc influence run times, the setpoint on your t-stat will have direct impact on A/C run times. Are you comfortable at 72°? Try raising it one degree—1°F—next May. Wait 2 weeks and raise it one more degree. You may be able to raise it 2-4 degrees—and if you do it progressively you may never notice it.
And once I quite the voices in my head I will become a repairman. :dubious:
This is the right answer.
If you’re tring to pump heat out of the house (as in AC mode) a lower exterior temperature makes the pumping more efficient. If you’re trying to pump heat into the house (as in heat pump mode), you’d like the exterior temperature to be higher.
Realted to this is the increasing popularity of ground water heat pump systems. With these, you pump heat from a source (ground water) that during heating season is considerably warmer than the air, yielding much better efficiency.
Also, not mentioned anywhere else in this thread, one of the biggest sources of heat in a home is electricity usage. Every watt (watt-hr, to be technical) utilized by your fridge, TV, computer, TV, lighting, etc., becomes heat that your well-insulated home retains and must be pumped out by your A/C unit.
Of course, a modest change in temp will take quite a while to affect a change at the thermostat in any home.
What is quite a while?
The rate of heat transfer is relative to the insulation and infiltration in the house. So, two houses sitting side by side will experience different rates of heat transfer. *(no matter the temp outside) * Therefore one of the houses may need the A/C to run more often and for longer times than the adjacent house because of less insulation and other factors. (“R” values etc)
In any event, a modest change in temp will have a less pronounced effect on either house. But insulation (and other strategies) goes along way in reducing run times and utility $.
Thanks to everyone who viewed and responded. My wife admitted that she was going by what she thought someone said years ago, but she’s a logical person and everyone did a good job showing her the way.