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A/C running constantly, does not shut off when set temp reached.
March 9, 2009 3:19 AM   Subscribe

Hi all, thanks in advance for the assist: This weekend I started running the A/C in the house (we put in a programmable thermostat last summer). the prob. we are having is that it runs constantly no matter what temp. you set it to (my wife set it to 90 last night as we were going to bed and it continued running until I gave in and turned the whole thing off). the previous night I turned it on, with the temp. setting high (mid 80's) and went to sleep and we woke up freezing in the AM. the fan is set to Auto, and nothing about our "setup" has changed since last summer. Except that it doesn't stop running when it gets to the set temp. Thanks again for any help or insight the community might have.
posted by rxbert to Home & Garden (11 answers total)
 
Disclaimer: I don't have central air, so I'm unfamiliar with exactly how they work.

Did this same thermostat work properly last summer? If it did then it seems fairly obvious that something has broken (as opposed to the tstat having been incorrectly wired at installation). I suspect it's the tstat itself that is broken, but I'm wondering whether the AC has a magnetic switch that might be sticking.
posted by jon1270 at 3:48 AM on March 9, 2009


What type of thermostat do you have? If it's the old 'dial' type, pop the cover off, and you may see a mercury switch (glass bulb with a small ball of mercury inside). That's attached to a bimetallic strip (coil of wire), when it tilts enough, the mercury rolls to the other side of the switch, making contact. This turns your 'appliance' on or off (heating, cooling). You should be able to activate this by hand (gently push it). Can you see something wrong here?
posted by defcom1 at 4:23 AM on March 9, 2009


Some programmable tstats have reset buttons under the cover. Try resetting. Otherwise, if everything worked last summer, buy another tstat and re-install it. They're cheap. If that doesn't work, you might need an electrician. Do you have an HVAC service contract? It might be a good idea to sign up now. If you're turning on your AC in early March, you're in for a long season of trouble.
posted by birdwatcher at 4:31 AM on March 9, 2009


Your thermostat might need to be replaced, but there are a few things you can check before you do that, the most significant of which is ventilation. If air isn't circulating through your house well enough, the cold air put out by the A/C might not be reaching the thermostat, meaning it will keep your unit running forever.

Make sure your vents are open, not clogged, etc., and make sure that if you have returns, the top ones are open and the bottom ones are closed. If your thermostat is in a room that can be closed off, open the doors.

You could also just try closing the vents in your bedroom and let it cool from the rest of the house.

I'd try all of these things before doing any significant investigation of the thermostat, because if there's a problem here it won't matter how many times you replace it.
posted by valkyryn at 4:58 AM on March 9, 2009


Normally thermostats are fairly straightforward contraptions, as defcom1 describes. I would think the problem is a bad connection from the lines connecting the two, and not from the thermostat itself. If you know how to use an Ohm meter (use the Google if you don't) you can test to see if the connection is good. My money says it isn't. Either the line is frayed somewhere (deep within the bowls of your drywall) or, if you're very lucky, a bad solder connection at the terminals (Google again).
posted by Civil_Disobedient at 5:14 AM on March 9, 2009


Is it possible that you somehow have the "heat" and "cool" parts of the thermostat crossed?
posted by ArgentCorvid at 6:01 AM on March 9, 2009 [1 favorite]


Does the thermostat show the current temp and the set temp? If yes, when it runs too long, does it show the current temp being less than the set temp? If yes, you've got a problem with the thermostat. Check for an override or hold setting that may be turned on. Or you may have a defective thermostat.

If the current temp shows greater than the set temp (or it doesn't show the current temp), but you feel like the AC is running too long, it may be that the AC needs to cool off the whole house, especially the area around the thermostat. Keep in mind, you're not just cooling the air, you're also cooling the contents etc. That takes time. If you're cold in one room, close the vent a little while the rest of the house reaches the desired temp.
posted by Simon Barclay at 6:03 AM on March 9, 2009


Y'all, he's in Texas! It's already summer down here! (My AC's on in New Orleans.)

Anyway, we had a problem like that recently - the programmable thermostat kept setting itself to 70. We'd put it up to 74 or so, it would reset to 70 and blow and blow and blow until the house was freezing. Turned out that the batteries in the thermostat were dead. (When there is a hurricane and you have no power, it uses some of the battery, which for some reason, waited a few months to completely die and freak out.)

Try changing the batteries in the thermostat. Yours may freak out in a totally different way.
posted by artychoke at 6:32 AM on March 9, 2009


If you tell us the brand of thermostat, maybe we can search for this problem, manuals, etc.
posted by cashman at 7:33 AM on March 9, 2009


If you bought it last summer, the unit should be replaceable under warranty. I had a Honeywell thermostat that did the same thing, the unit was defective and replaced under warranty.
posted by torquemaniac at 10:13 AM on March 9, 2009


Careful with it staying on for extended periods of time. There is a good chance the entire unit could freeze and do some expensive damage. Sounds like it should be covered (if not manufacturer's maybe homeowners?) so it might be wise if the resetting doesn't work just to get someone out. Good thing though we're only in the 80's till tomorrow ;-)
posted by doorsfan at 11:47 AM on March 9, 2009


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