Baby, it's COLD inside...

December 7, 2009 1:51 PM

Thermostat seems to work fine. Except at night. When it gets cold.

Quick background info: We bought this house a year ago. Yay! Ran into this same problem almost immediately. The heat works fine. Sometimes.

Last night my wife came home after I'd already gotten in bed. She said "it's freezing in here!" and I suddenly realized it was pretty cold. Temp had been falling steadily since I'd gone to bed at ten or so. So, rough night and two grumpy toddlers later, we were trying to figure out what happened. Then the heat (the glorious HEAT!) came on. And all was good. Last night it got cold again.

The thermostat seems to be the problem. Says the temp is 66 degrees. Says it SHOULD be 74. Fan is on AUTO and blows continuously, cool to (cold!) room temp air. It seems like it's TRYING to heat, but it has no heat to give. The next day, all is good again. What is it about cold winter nights (we had this problem last year right after we bought the house and muddled through the rest of the winter after the thing seemed to auto-correct itself.)

If I didn't know better, I'd think we had accidently programmed the thing to cool down to 66 at night. But we checked the programs. Daily and weekly. The temporary override feature also says it's working to bring the temp up, but it stubbornly stays at 66. I mean, a SOLID 66. Which is why I think the thermostat is to blame.

When it does heat, it blazes like the noonday sun, and does a great job. But at night, it's a different story. Anyone have an explanation for this? Similar problem that you fixed? We thought about calling a technician, but we're afraid he'll just come and charge us $150 to show us how to program our thermostat. But we're tired of shivering under the covers at night.

Here is the manual for the thermostat. It's the 5000 model.

posted by Spyder's Game to home & garden (16 answers total)

My guess would be the same as yours -- there's a program set. Many people set their heat low at night when they should be snuggled down under their warm down comforters. We set ours to 64! It seems to put the cats in stasis which is a nice side benefit.

The fact that it's still blowing air is problematic. I would guess you've got either a faulty thermostat (you can get pretty cheap ones for around $20 at home depot if you want to try just swapping out your old one and seeing if it makes a difference). Or you've got something wrong at the furnace level or the sensor that is attached to the furnace.

I'm sure a visit from the HVAC guy is in order if you don't think you've got a thermostat malfunction.
posted by amanda at 2:03 PM on December 7, 2009

Check the furnace itself (I'm assuming it is gas). If it is relatively modern, it has a bunch of sensors that can sense (correctly or not) conditions that will prevent the furnace from operating. It could be that the cold winter nights are causing the furnace to work harder and this causes some sensor to read something out of spec. Check the manual for the furnace to find out how it indicates errors and then check when it isn't working to see if there is an error. The furnace is a far more likely culprit than the thermostat.
posted by ssg at 2:05 PM on December 7, 2009

Possible simple explanation that you can rule out or confirm instantly - Do you have a nightlight or other device that comes on around sunset in the vicinity of your thermostat (in particular, under it)? Or related, any highish wattage devices in the same room that periodically activate? For example, a computer waking up to run a nightly AV check or defrag could easily warm a small room by a few degrees.

Anyway, one cheap (free?) diagnostic to consider here - Does a regular thermometer show the same temperature (or have the same difference throughout the day)? If not, $20 will get you a new programmable thermostat; If so, just find the source of nighttime heat and move or disable it.
posted by pla at 2:24 PM on December 7, 2009

Seconding that it sounds more like the furnace than the thermostat. If the thermostat says it's calling for heat, it probably is; the thermostat seems to agree with your version of reality (that is, "it's 66 but we wish it was 74"), in my experience when thermostats have trouble, they disagree with you.

We had a pretty similar problem a while back (right down to the 66 degrees), where the thermostat was calling for heat but none was coming. Investigating in the basement, we discovered the blower was going but the flame would only light for about thirty seconds at a time. Turns out the trouble was a dirty flame sensor: the furnace has a sensor that looks for a flame (apparently the flame itself conducts a small electric charge and can be detected that way); if no flame is found, the gas is shut off (in order to avoid filling the home with natural gas, which is no one's idea of a good time). If the sensor is dirty, it doesn't recognize that there really is a flame, and shuts down until ten or fifteen minutes later, when the thermostat says it's still cold - and the process begins again.

Sounds like you've got something pretty similar going on. I'd say to go ahead and call an HVAC tech (or your gas utility, if they offer such services). Even if the tech tries at first to show you how to program the thermostat, if you believe the thermostat is busted, or that there's an issue with the furnace, you can insist that he check both things out before leaving (and a responsible one will do so anyway. And if it was doing this last year and you didn't have someone look at things, I'd hazard a guess that your furnace could use a tune-up anyway.

Oh, and if it's a dirty flame sensor - or anything else with the furnace, frankly - it's really not something you want to try and fix yourself.
posted by nickmark at 2:34 PM on December 7, 2009

Heat pump systems have an auxiliary "stage two" heater that kicks in when it gets below 40 degrees outside. It sounds like that's what's not working for you. Either the unit itself is broken or the thermostat is not telling it to turn on.
posted by bgrebs at 2:48 PM on December 7, 2009

Seconding nickmark's advice. If the blower is blowing cold air, then most likely the problem is with the furnace. You should call in a technician. Depending on the kind of heat you have, you should have the system serviced every 1-2 years anyway. I have an oil burning boiler and I try to have it serviced every year.
posted by brianogilvie at 2:50 PM on December 7, 2009

We had a similar problem not too long ago. When the thermostat program told the furnace to bring up the temperature in the morning, nothing would happen. This only occurred on the really cold nights. Brrr. We even replaced the thermostat and it didn't fix the problem.

What did fix the problem, though, was not programming the thermostat to go so low at night, especially on very cold nights. It seems that the temperature around the furnace (which, incidentally, is in its own little room in the garage) was confusing the sensors so the furnace wouldn't kick on and warm up the house in the morning.

But I'm definitely nthing the suggestion to get someone out to check over your furnace.
posted by DrGail at 3:00 PM on December 7, 2009

Something like this happened to us recently. It turned out that we had an inadequate gas supply and the furnace wasn't always getting enough gas. In our case, the problem was an ancient gas regulator installed by the energy company 40 years ago, which is no longer needed because modern meters have them built in; they came out and removed the ancient regulator and things have been fine. I don't have any idea why you'd be colder at night, although it took me a few days to realize our furnace wasn't working all the time because the house was warming up enough during the day from big south-facing windows that I didn't realize it wasn't working. If there's something else coming on at night that draws on the gas supply, that might have something to do with it.

We had also had intermittent problems with burners on our gas stove not lighting on the first try, but didn't think anything of it.
posted by not that girl at 3:49 PM on December 7, 2009

Is it possible that the thermostat's connection to your heating system is miswired? I'd pull the thermostat and check that the right wires to the furnace are going to the right connectors on the thermostat.

Some thermostats can control both an AC/fan and a heating system. I would imagine that, were the wires for the two systems crossed, you might get a fan going on cold nights, and heat blasting on warm mornings.
posted by zippy at 3:51 PM on December 7, 2009

You haven't told us enough to properly answer your question. Please note that the people above are guessing that you have a gas furnace or a heat pump or....? Do you know what you have? If not, let us know if you have a gas heater in a closet, one in the basement or a combined unit sitting just outside the house. The answer to your problem is highly dependent on what kind of furnace you have.

Second, where do you live? Minnesota or Florida? Makes a difference.

Last, what was the outside low temperature last night?

Give us the answers and we'll take it from there.
posted by Old Geezer at 7:12 PM on December 7, 2009

Thanks for the replies so far all! Some more clarification:

Pretty sure we have a gas...something. Pretty sure it's one giant unit on top of the house that runs the heat and A/C. My lack of knowledge about this stuff should be fairly clear by now. We don't have a furnace or any other unit inside the house. Must be gas, too, because we don't get a monthly shipment of fuel oil or anything either.

We live in California. Burbank to be exact. Tonight's average temp looks to be about 52 degrees. Where would I find out what last night's low temp was in Burbank?
posted by Spyder's Game at 7:19 PM on December 7, 2009

OK, last night's low was 47 degrees. That is not cold enough to affect the efficiency of a heat pump, although it is near the borderline. Since the unit is entirely on the roof, I'm 80% sure that you have a heat pump. Since newer units would not be roof mounted and older units are subject to the pilot light blowing out, I don't think you have gas. People don't like to climb on the roof after every windstorm to re-light a pilot. For this reason, gas units are usually two-part with one part in a closet indoors. Newer combined units are usually installed at ground level and have an electric starting mechanism.

There could be a couple of non-thermostat-related issues. First, there is a solenoid valve in the heat pump that reverses the flow of coolant. It goes in one direction to take heat out of the air and send it into the house. It goes in the other direction to take heat out of the house (cooling). If it got stuck during the summer, it might be trying to cool the house even when asked for heat. When the auxiliary heat comes on, it fights with the heat pump and grudgingly sends some heat to the house. It might happen to get unstuck after the aux heat is on and you would then get full heat. It is a $10 part that will take a $100 service call, especially because it is up on the roof.

The second problem is that a heat pump has a built-in delay to keep the unit from coming on until the pressures inside the unit equalize. If you set the thermostat for a certain temperature and the heater does its job getting you to that temp, and you push the thermostat up within ten minutes, the unit will not run. It will just take its time to equalize and then come on. Try setting the temp at about three degrees above the temperature in the house. Let the heater come on and do its thing. If it heats OK, wait ten minutes after the heater shuts off and raise the thermostat temp. If it comes on OK, that was the issue. If it doesn't, call for service.

Good luck.
posted by Old Geezer at 8:09 PM on December 7, 2009

1- Change the time on the thermostat to something like 8 hours off. See if the problem occurs at the same time or not. You will learn whether the problem is thermostat or elsewhere.

2- You are in California? Is it electric heat? Maybe your utility has one of those rolling blackout-prevention devices on the furnace somewhere. The theory is that it shuts down some of the load on the electrical system when it gets too close to maximum capacity so that they don't have to shut down a whole neighborhood to save the system. I think I read somewhere that December is one of the peak electricity months for CA, because of the auxiliary heating and the Christmas lights and the short days.

3- And or, there is an outside sensor that senses the outside air and modulates the heat output based on that. Normally, it should operate the opposite- the colder it is, the hotter it heats. I think it's called an economizer. Might be broken.

4- Something ethereal is kinda-broken like the flame sensor. When it's warmer out, it's just warm enough for the exhaust to flow out OK. When it gets a little colder, it freaks out.

5- There is a light switch somewhere that accidentally controls something about the heater- turning off the laundry room light for the evening shuts off the heater. Or something.

6- You are backing the car up too close to the vent and blocking it?

7- Now I'm just making stuff up.
posted by gjc at 9:02 PM on December 7, 2009

(2b- Electric heat could be heat pump or resistive. I seem to remember that these things specifically just shut off the compressor, but not the fan.)
posted by gjc at 9:05 PM on December 7, 2009

Our furnace had a similar sounding issue that turned out to be a cold solder joint on the furnace controller board - the furnace would only turn on if the air around the board was warm, so we had lots of unexplained cold in the house. Unfortunately, ours worked for many years longer than the voluntary recall on the controller was active, so we had to pay half price for its replacement.
posted by rfs at 11:52 AM on December 8, 2009

Thanks for the advice, all! HVAC guys came and determined that it was a faulty circuit board in our rooftop heating unit. Said circuit board was $500 alone - urk! But now that it's replace, everything is heating very nicely. Thanks for your invaluable input!!
posted by Spyder's Game at 11:34 AM on December 14, 2009