Setting AC in 100 plus degree weather
August 1, 2018 7:45 PM   Subscribe

If I set my AC to 84 when it's 105 degrees am I conserving the system or making it work harder because I'm allowing it to get too hot?

I live in the southern cali LA "valley" area. I have a very old central ac unit. I recently had the fan/motor replaced as it died when we reached 115 degrees a couple weeks ago. I also recently installed a programmable thermastat which I have change the temp through out the day. I keep it at around 79-80 in the morning and evening and allow it to eventually work it's way up to 84 in the late afternoon when it's hottest. When the AC begins it's decent from 84 degrees to 79...usally around 8pm at night it does take some time to get there. My overall question would it harder to maintain 84 degrees then it is to maintain 80 degrees because it's set so high and so much warm air is getting in? Any thoughts here are appreciated as well as suggestions for thermostat settings when experience these high temperature days.
posted by ljs30 to Home & Garden (7 answers total) 1 user marked this as a favorite
I think this sounds reasonable. But efficiency (and waste) depend a lot on the details. See this related recent question and my response.
Generally, you’re also right that the system running hard is not a sign of great efficiency. But I bet it would run harder if you kept it pegged to 79, compared to letting it warm up a bit.

Since you can’t clone the Earth to do a controlled experiment, your best be is to try to compare the power used during two comparable months of weather using different strategies. Or even just try a few similar days and log on/off times by hand.

On balance, consider a smart thermostat like Nest etc. this is exactly the problem they are made to solve.
posted by SaltySalticid at 8:00 PM on August 1, 2018

I'm a mechanical engineering degree holder and your question hits on, oh, about 3 or 4 Jr or Sr level course topics, none of which is trivial to utilize in the theoretical realm, let alone the practical realm.

Don't get me wrong, you're asking a valid/cogent question and I applaud your thinking but it's not going to be an answer with any sort of validity for your practical/real world application due to a heap of variables, mostly the fact that your AC is very old and we know nothing about the tightness of the air envelope of your home, among many other factors.

The topics that would have to be considered here range from failure analysis (based upon your mention of replacing parts being an issue/concern) to theoretical heat engine efficiency to the electrical side of things when we get into startup capacitors and how motors work. I'm not going to go into all that but I'm sure if you google around you could find someone who did the math and/or said more useful things regarding consumption ideals but their assumptions / givens are not your givens so it's a bit of a wash if you ask me on the logical/academic side. That is to say that your basic question of "run it constantly (or at least more frequently) to maintain a higher temperature vs run it less to keep the home at a higher temperature until you get home at which time you'll drop the temp down to comfortable at the price of your AC running hard for a bit".

Here's my two cents on the salient points that I do think bear considering more than others:

1) If you keep your house at Y degrees and the outside is at a given Z degrees. The heat transfer between one to the other is, insofar as we're talking about variables you can control, dependant upon the difference from Y to Z. Higher differences (say you set the temperature at X instead of Y where X is lower than Y) means faster changes, or heat transfer I should say. You can see that here with this calculator if you'd like. Faster changes means your HVAC works harder to maintain a given temperature. This is a point for "keep that temp delta as small as possible for as long as you can". A thought experiment would be to consider if you had the delta value of Zero, aka you set your thermostat to equal outdoor temp. Your energy usage/wear and tear for the duration would be zero.

2) It's generally accepted that it's more efficient/less wear to utilize a system that is sized properly to run constantly rather than a system that has to start and stop. There's both failure modeling reasons (like in the shafts of your motors/fans/compressors) and electrical engineering reasons (startup energy surges). I'm not going to go into the weeds here but machines like being in constant states where strain / surges are minimized. This is, in a way, a case for either of your hypothetical use cases I think.

3) The only thing I'd say that would be worth noting is if your AC is really struggling and ends up running constantly for very extended periods of time. I don't know what that time period looks like for a household [old] AC unit because it's not defined anywhere... but what I'm saying is that if ends up running constantly/hard for hours on end or if you're having problems with condensate (water) buildup or you're burning through fuses/breakers then you need to alter whatever you're doing to find a better solution.

4) Keep your filters clean and get heavy curtains for windows. Things like that will likely make more of a difference in wear/tear and energy bills than this line of questioning. Not that I'm judging because, again, I get it, I'm just pretty clear that your ROI times on curtains / maintenance are the low hanging fruit of this sort of stuff.
posted by RolandOfEld at 8:51 PM on August 1, 2018 [12 favorites]

ljs30: " My overall question would it harder to maintain 84 degrees then it is to maintain 80 degrees because it's set so high and so much warm air is getting in?"

From a HVAC technician point of view: when dealing with small systems, say less than six tons which means practically all but the largest of home systems, and assuming the system was properly sized in the first place, generally your best approach is to minimize the differential between set point and outside temperature regardless of how much the system is running or what duty cycle ends up looking like. IE: keep the temperature as high as possible while still being comfortable and increase the set point or turn off the system entirely when the space isn't occupied for long periods. There are going to be a few edge cases where that isn't the case but those are a vanishingly small percentage of installed residential systems.

Shading South, West and East facing windows (in that order) is generally the most cost effective thing (both wear and electrical) you can do by reducing solar load. Shading condensing units (thereby lower head pressures) being careful to not impede air flow is also of benefit and often fairly cheap and easy.
posted by Mitheral at 9:36 PM on August 1, 2018 [8 favorites]

ljs30: " I recently had the fan/motor replaced as it died when we reached 115 degrees a couple weeks ago."

Also this is almost certainly 99% coincidence. The fan motor most likely would have failed in the next couple of weeks regardless of ambient; the high ambient being the teensy, tiny straw that broke the camel's back.

Also 115F ambient isn't really all that hot for condensing equipment. See for example roof top heat/cool units which routinely see that sort of ambient temperature and higher. I've seen refrigerant pressures in my service set that indicated shade temperatures of 135F on a roof. The fan motors would of course been even hotter than that.
posted by Mitheral at 9:45 PM on August 1, 2018 [3 favorites]

I can't favorite Mitheral's replies enough. And I need to go off and build a shade panel for my condenser, thanks for that reminder.
posted by RolandOfEld at 6:19 AM on August 2, 2018

Again, most home AC systems, especially if yours is older, don't run 'harder' when it is hotter. They are on or they are off. It doesn't matter if the temperature differential between current and requested is 1 degree or 20. All you are determining is the length of time they are going to run. If you are trying to save money, then less time running is better. If you are trying to maintain a constant temperature, then more time running is better.

The default program (unless you changed it) on your programmable thermostat is the Energy Star recommendation to run the A/C the least which saves the most money.
posted by The_Vegetables at 8:55 AM on August 2, 2018 [1 favorite]

As an aside, it’s just The Valley.
posted by sideshow at 5:48 PM on August 2, 2018

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