Mr. Electricity answers questions about cooling/air conditioning

We have window units throughout the house.  Should we get rid of them and install central? -- Spiney Norman

With a central air conditioning system, will I save energy by closing the register in an unused room?  Our electric company says that I won't.  Would it harm the central unit in any way to do this? -- K. Barrera

Like many people, I used to think it was a good idea to close registers in unused rooms to save energy.  But when you close registers you promote duct leakage in the system, which winds up costing you money. (source)  And yes, you can damage your system, besides the ducts.  If you're routinely using only part of your house, consider using window units or a mini-split system.

How much electricity does the fan switch on a central AC use? -- McBroom Family

The fan on an HVAC system in an average size house uses about 325 to 425 watts. (The former is from BC Hydro, the latter is from my own measurement of a 1995 system.)  Some newer units have fancy variable-speed DC motors that use only around 23 watts. Either way, it's a lot lower than running the AC, which uses around 3500 watts (including the 325-425 to run the fan).

For those who have been puzzled by the fan switch, here's an explanation: If the AC is off, then turning the fan on will simply recirculate the inside air without cooling it. (It draws in through the intake and blows out through the ceiling vents as it normally does.) There's not much advantage to this, because it doesn't make the house any cooler, but it can help keep the air "fresh" since it's being drawn through the filter and it's being moved around a lot.  When the AC is on, the air's already being circulated, so in that case there's no difference whether the fan switch is on or off.

Does leaving the AC set to auto save electricity?  Or its the same as turning it on and off myself?-- Anonymous

It's the same as turning it on and off yourself -- assuming that you turn it on and off exactly when the auto switch would have done so. If you don't get around to turning off the AC for 30 minutes after it would have shut off automatically, then you'll pay for another 30 minutes' worth of electricity. Unless you're diligent about turning the AC off as soon as it gets cool enough, the auto switch will save you money because it turns off the AC right away.

Good question. Fans don't actually lower the temperature of the room, so it doesn't help to leave them on when you're not home. I added a detailed explanation and a picture about this.

As for how using fans will impact your electric bill, the How much your appliances use page shows that a typical ceiling fan uses between 55-90 watts,while a 2.5-ton central AC system uses 3500 watts. If you have the fan on when you're home instead of using central AC, you'll save a ton of electricity. If you run the fan when you're NOT home instead of running nothing, then you'll be wasting 55-90 watt-hours every hour the fan is on.

The way you design the home is much more important than which AC unit you stick in it after it's built. Since you're building homes from scratch you have a great opportunity to design homes for energy efficiency. This is called green building and it's become especially popular over the last decade. The City of Austin has a special green building program with resources for homebuilders and homeowners.

No matter what kind of home you build, the first step in selecting the right AC unit is to get one that's sized appropriately for the house it's intended to cool. Units that are too big or too small won't cool efficiently. Your AC vendor can help you with this.  There's more about the process on the Department of Energy's website. The second step is to look at the SEER rating, which is a measure of how efficient an AC system is compared to other systems of the same size. For maximum efficiency go with the unit with the highest SEER rating.

Your ceiling fan advice is all wrong. summer direction should be up not down. cold air drops and is at the floor. you want to bring the cool air upward.-- Steve

Absolutely not. No matter which way you spin the fan, all the air in the room is going to get mixed. The point is that by blowing the air directly down, it washes over your body and cools you. If you blow the air up then you don't get the draft, and the draft is the whole point of using a fan for cooling.

This is ridiculously easy to test: Just sit under the fan with the air blowing up, then switch the fan so the air blows down, and feel the dramatic difference. Clearly you've never done such a comparison. You might want to do so before insisting that I (and everyone else) is wrong about something that has universal, unanimous agreement.

What do you think of the $25 homemade air conditioner that a Canadian engineering student devised by running water through copper tubing coiled around a standard floor fan?-- Nick Gressle

I think Geoff Milburn is a quite clever young lad and has a fine career ahead of him in engineering.  But even though his idea works it won't save money on cooling costs.   You're much better off with a window unit AC.

In Geoff's open system, the cost of electricity to make ice in your freezer for the water bucket will approach what you would have spent by just running a window unit air conditioner.  In his closed system, the cost of water can easily exceed what you would have spent to run a window unit AC. Finally, the homemade AC not only doesn't save money, it doesn't cool as well as a window unit AC would in the first place.

At right is a comparison of Geoff's homemade AC to a traditional small window unit AC.  I used the lowest possible rates for water in Austin.  With Austin rates if you use more than 2,000 gallons of water per month, then you're charged a higher rate and the cost of the homemade AC is even higher: 25¢/hr. -- or more than four times what it would cost to run an easy, simple window unit AC, which would cool more than twice as well anyway.

Of course you need to use your own local rates for water and electricity to get a more accurate comparison for your area, but you get the idea.

1. Keeping the AC on constantly.
2. Having the AC on during the day, then in the evening when it's cool turning it off and opening the windows instead, then turning the AC back on when we go to bed.

The reason I ask is that my roommates think it takes more electricity to turn the AC off and then turn it back on later.-- Julia Van Voorhies

Number 2 is cheaper, but maybe not for the reason you think:  With the windows open you let warm air out of the house.  Your home's walls absorbed energy during the day, and then in the evening that energy is released both to the inside and outside of the home.  Also, heat gets added to your home from lighting, cooking, computers, and people.  With the windows closed that heat can't escape, and so your AC has to remove it.  But with the windows open instead, you let the heat get out.

If I put a fan in the window, should I have it blowing the cooler air in,or the hot air out? -- Bill, Middletown, CA

It doesn't really matter. One way vs. the other may be slightly better for your particular circumstances, but there's no easy way to figure out which way that is other than to try both ways and see which works better for you.  Even so, there probably won't be that much difference between the better way and the other way. What will make a lot more difference is making sure you have at least one other window or door open to provide ventilation, and making sure the fan is sealed in the window (no gaps on the left or right side of it).

For our daily activities downstairs I have the thermostat set at78 and the upstairs one at 80.  (When it gets warm upstairs, I use the fan in the office room too.)  I am concerned about the load on the downstairs thermostat given that I can't "seal off"the upstairs area. We live in the coastal area of south Texas, where it can get quite hot and humid!  Help! -- Jaya S. Goswami, Kingsville, Texas, June2004

I'm having a hard time understanding exactly what you're asking. If you're asking whether it's okay to run just the downstairs AC and not the upstairs AC when there's nobody upstairs, then yes, that's fine. If you're not using the upstairs then there's little reason to cool it. Remember, hot air rises. I wouldn't worry about the downstairs unit being overworked unless it's unable to keep the downstairs cool when the upstairs AC is off.

If you're asking about what temperature you should set each thermostat at, only you can answer that because only you know your own comfort level.  Just set each thermostat for your comfort level, remembering that the lower you set either thermostat the more energy you'll use.  That's really all there is to it.  If you want the downstairs to be cooler then it's the downstairs thermometer you'll adjust, not the upstairs thermometer.  Same story for cooling the upstairs.  There's no magic here—just set each thermostat for your comfort level for that part of the house, remembering that the lower you set either thermostat, the more energy you'll use.

Why did you install AC downstairs if the downstairs was already cool enough without AC?  Maybe you already had a central heating system so you just installed an HVAC unit to operate through the existing ducts?

No, you shouldn't close registers, because that promotes leakage in the ducts (and possible damage to the system), which will cost you money overall. (Lawrence Berkeley Labs)

The solution to this problem will require an AC professional.  Most likely they'll be adjusting the dampers.  Alternatively, they might seal off a register downstairs and cut in a new one upstairs.

As for how to use ceiling fans, just turn them on and make sure they're spinning in the correct direction, usually counterclockwise. Good luck!

It doesn't matter what you set your thermostat to (before or after you get home) or how hot it is outside — you'll always save by keeping the AC off when you're not home.  When you run the AC during the day, your house becomes a magnet for heat because it's cooler inside than it is outside.  So your AC has to continually remove all that heat that's coming in.  But with the AC off, your home heats up once and then the heat stops coming in, because your house is not a heat magnet.  If you keep the AC on all day, then the penalty will be less at 85° than 75°, but the penalty's still there.

Because so many people wonder about whether to run the AC while they're at work or not, I have a special page devoted entirely to that topic.

By the way, if your fiancee isn't comfortable unless the temperature is at a meat-locker-like75 degrees, then you should probably marry somebody else. Thanks for writing!

How can I determine if my bedroom circuit is capable of handling a window unit air conditioner? It has a 20-amp breaker. -- Nick Sibilia

First you need to find out how many amps your air conditioner draws. It's probably listed on the AC unit somewhere, probably where the plug goes in.

If the amps aren't listed, then it will at least tell you the number of watts, and you can divide the watts by 120 to get the amps. (For example, if the AC draws 1500 watts, then 1500/120 = 12.5 amps. This assumes that your AC is a normal 120-volt AC. If it has a normal plug that goes into a normal outlet, it's a 120-volt Ac.)

Next, add up the amps of everything else on your bedroom circuit, especially the lights. (If you have a 100-watt bulb, that's 0.83 amps: 100/120 = 0.83.) (How to find out what's on the same circuit.)

As long as you don't go over 20, you should be fine.

Make sure you don't miss anything else that might be on that same circuit. For example, that same circuit might be powering TWO different rooms.

Our new home has a split-level A/C system (one unit for upstairs and one unit for downstairs), each with a programmable thermostat. We're downstairs during the day, so during the day I set the thermostats to keep the downstairs cool during the day and the upstairs cool at night. The downstairs AC, which is bigger, runs 3-4 hrs/day while the upstairs unit runs 7-10hrs/day! The AC system has been checked for leaks/fan malfunction etc. and it was given a clean bill. Also, could I turn off the first floor unit at nights instead of just setting at a higher temperature? -- Aprash, Houston TX

The second floor unit is probably running more because (a) it's smaller and less powerful than the downstairs unit, (b) hot air rises, so the second floor unit has more work to do, (c) the second floor is closer to the hot roof, or (d) some combination of the above.

If your downstairs unit is larger than the upstairs, that could have been a mistake made during installation. It might make sense to have them swapped; check with an AC professional to have them analyze your situation. Another possibility is to locate the air return for the downstairs unit closer to the second floor. That way you shift some of the responsibility for cooling the second floor to the downstairs unit.

You could certainly turn off the first floor unit at night, but it would be more convenient to just set a temperature so high that it wouldn't kick in. That will save you the hassle of having to turn it on/off every day.