With the end goal being to lower the current (staggering) electric bill, here are some facts:
Hot climate – no snow in winter
2 story rental, 1650 sq. ft. living space
35-40 y/o air-conditioning unit – (landlord won’t swap 'til it conks.)
3 ceiling fans - 1 down - 2 up
4 desk-top fans - 1 down - 3 up – one easily moveable
Most ‘living’ happens up
Which configuration of usage, of the following, would be cheaper on a monthly basis?
A/C set to 75 degrees farenheit at all times, no fan usage.
A/C set to 78 degrees farenheit at all times, fans used all the time.
A/C set to 78 during the day and 75 at night (for sleep), fans used as needed.
Obviously the occupants of this home don’t agree thus, the teeming millions are consulted. Informed opinions only, please, and many thanks in advance for those.
It depends on the specifics of your equipment. Since the residents don’t agree, why not experiment? Take three months and try each option, and see which bill is cheapest.
Check the wattage on the AC unit and the fans and see what percentage the AC operates at each setting. Then it is just a simple computation. Why argue when you can test? My guess is that fans use way less power than the AC, so option 2 would be the cheapest.
The ancient AC unit makes the argument almost moot. It probably draws so much power and converts it to cool with such low efficiency that the only different scenario is to not use it at all.
Fans, of course, don’t provide any real cooling at all - they cool humans through sweat evaporation alone, and contribute negatively to the heat in a house from motor heat. All of them together probably don’t use 1/10 as much power as the AC, maybe 1/3 if you try to keep the AC off and run the fans a lot.
I’d look into one or two window AC units and keep the bedroom and living rooms comfortable with them, using fans as needed to boost cooling when you’re actually in front of/under them.
ETA: Running the AC unit only part of the time will probably compound your cost problem. AC is most efficient when it runs more or less full time - that is, when it’s allowed to find a stable point in temp/operation and hold it. If the unit is decades old, I’d guess the insulation etc. is, too. It’s too costly to run full time, and won’t save a lot running at max for a few hours every evening or whatever. I’d try to get it out of the equation altogether if you can.
I’ll throw some numbers at you. Assuming you have a 3ton Central Air, it’s going to draw a minimum of 25 Amps, but it’s 40 years old, so let’s bump that up to an even 30. That’s 3,500 watts when it’s running
Assume 50 Watts per ceiling fan, 25 watts per desk fan, that’s 150+100 = 250 watts when they are all running full blast.
The fans use about 7% of the electricity that the A/C uses.
Assuming an outside temp of 90, when you increase the thermostat from 75 yo 78,you’re reducing the temperature difference from 15 to 12 degrees, that’s 20% less work for your AC. That savings percentage goes up when the outside temp is below 90, and down when it’s over 90.
I’d say going to 78 will save you 20% of your A/C usage at the cost of 7%, a net benefit of 13%
The final choice is being at 78 half of the time, which would give you a base 10% savings, already less than the choice #2, and you still will use fans.
Not to be redundant on this, but it’s really important to remember that fans provide comfort, not cooling. I’ve seen too many people with a houseful of fans running - a dozen, sometimes - with only one person in the house, and wondering why it kept getting hotter.
Fans only do three things besides spin and look pretty:
Move air over your body, contributing to convection and evaporation cooling.
Help distribute air within a room to even out the temperature.
Add heat from their motors (and a tiny fraction from friction).
Has the ancient AC unit at least been checked that it has the right amount of refrigerant, and everything involved is clean?
The two main things you can do are give the outside condenser unit a once-over to see that the coils aren’t clogged with junk, and that the filter(s) inside the house is clean. If it’s more than three months old, it’s probably not clean.
Insulation is also going to be a big deal. If the house was not well insulated cooling will be lost quickly. You can try a swamp cooler (see youtube) to get by but given the old inefficient nature of your AC there is no low cost solution other than using the fans and toughing it out.
Reasonably efficient window units are cheap. Here’s one I found while shopping for my own use recently. EER of 11.2. Most efficient air-source AC money can buy has an EER of 18, for a comparison point. (the Fujitsu RLS3 also costs about $2500 installed while you can install that window unit for under $200 including the materials to mount it)
Also, go to your local hardware store and buy some solar screen. Mine sells it by the foot, but you may have to buy it by the roll. You can tack it over the windows or make a kind of curtain, but get it over the windows somehow. It’s made a big difference in my west facing bedroom in Texas.
Absolutely true, except in some cases where return airflow is not good (say, a master bedroom at the end of a hall, with the return back at the beginning) where leaving a window ajar for outflow will help in cooling that room. Especially if the door is kept closed for privacy. It’s lower in efficiency, but will cool better.
Well, not untrue, but you could say the same thing about a '75 Caprice and a '15 Prius. Residential AC in particular was no efficiency champ before about 2000, and does not have the lifespan of commercial gear.
Thanks so much for the thoughtful responses. The idea of a window unit is being discussed. We already have solar screens and no windows are open. The a/c unit is professionally maintained every spring. I suspect it’ll be here longer than us. But, that electric bill is going down… Thanks again for the advice and suggestions.
At 35 years age an a/c system may need a replacement radiator, or perhaps something as simple as a flush out.
I have only worked on large industrial a/c, with proper cooling towers rather than the small domestic systems, however the principles are the same, economics may well be different.
It may not be economic to replace a radiator, but radiators are not terribly special things, all you need is a matrix and some means to couple it to the system. Domestic systems will be single phase change - so you won’t have more than one heat conducting medium or multiple heat exchangers.
All you have on a domestic system is a compressor, gar reserve, and fluid circulation system - some energy recovery system might be involved, perhaps a really clever one will link in to some sort of water pre-heating system but as long as the major components are ok there really won’t be too much difference in efficiency, the main losses will be the degradation of items that can be readily replaced - so that’s fluids, pipework and radiator.
Its a lot to shell out for an entire system if you can make 90% of the improvements for less than 10% of that cost through a little bit of maintenance.
What **casdave **says is true… but only in his admitted range of experience. Commercial AC systems and chillers are huge installations, with lots of expensive fixed parts (from pads to ducting to blowers to zone control dampers and on). They are built a lot tougher than residential units, and are built to be replaced and upgraded in pieces as the years go by.
Most residential units just plain aren’t. Aren’t built that well, aren’t that durable, aren’t made to repair and upgrade one piece at a time. At least, not cost-effectively, either in machinery or efficiency costs.
Might it be effective to have the central A/C set to fan all the time? Most newer systems can be set so that the household fan is running even when the compressor etc. aren’t - supposed to be better at keeping the temperatures more even.
The landlord certainly has no incentive to replace the unit until it dies. Units from that time were built to LAST - the one my parents installed in the early 1970s was still in place when we sold the house in 2005. A replacement is money out of his pocket, and won’t benefit him at all except some tax savings because it’s for a rental property. If the tenants can make a case for a newer unit helping to preserve the house in some way, you might be able to persuade him to go forward with a replacement.
Newer units (as noted in a thread I started last week) simply don’t last. The one we’re looking at replacing is 11 or 12 years old.
I think you have to draw a distinction between “functional” as “still makes noise but doesn’t emit smoke when I turn the switch on” and “functional” as “does a good job of what it’s supposed to do.”
No, because fans don’t make the air cooler. They make people feel cooler by pushing air over them. If there are no people in the room, a fan is a waste of energy.
My landlord bought new ACs, a new roof, solar screens, programmable thermostats, etc. as part of an incentive package and loan from the city electric company. He gets a tax break and a fifty year old complex that’s always full. We get electric bills under $75. In the summer. In Austin.
