Programmable Thermostats

I have a programmable thermostat in my house and have heard millions (not really but a lot) of pros and cons. Anybody have an opinion on whether or not it saves or wastes money using it?

Mine saves me money. It’s pretty hectic around my house at bedtime and many are the times I’ve forgotten to turn down the thermostat at night. I’ll have $300+ electric bills during the winter if I don’t keep that thermostat down. Having a programmable one keeps my bills to < $200 a month during the winter. I can live with that.


How could a programmable thermostat cost you money? The obvious use for it is to turn the temperature down when you’re not at home or asleep. I can’t see how that could cost money.

My experience is that it was great in the winter, but in summer it doesn’t help much.

Apparantly it is easier to heat quickly than it is to cool quickly, so you endu using the same BTU’s to cool the house back down as you would have to kept it cool all along. I once had an apt that was expenses paid, and I asked the mgr about turning down the A.C. when out and he said it would actually end up costing more $. Don’t know if he was right, but that is what I’ve heard.

As for heat, when I had gas heat it was great. Now that I have a heat pump, I doubt it would do as well especially if it is really cold. That gas heat could warm the whole house in 10 minutes.

Well, just my humble experience. But I went through one winter in a new house with an old-fashioned “set it and forget it” type thermostat (which is exactly what we did)… Then installed a programmable and have seen heating bills 30% less this winter, despite it being colder and nastier. Think I paid $60 for the thermostat, so saved that back in LESS THAN one month.

Also, not necessarily “programmable” issue, but the old one had a dial and the new one is digital. So now if I want 68 I get 68. Before, it was dialing on a pretty rough indicator calibrated in 5 degree increments… So setting it at “about” 68 (i.e. a nudge below 70) mighta been 70, 69, 67 or even 68… But who knows? A digital thermostat at least lets you hit the number you want on the nose – even if it’s not programmable (though I don’t recall seeing a digital-but-not-programmable model available)

There are programmable thermostats designed to work with heat pumps. Not sure what the difference is, but my programmable thermostat says not to use it with heat pumps(multi-stage systems).

Warning! From here on it’s pure speculation on my part on how these work.

It may be that the ones designed to work with heat pumps have longer cycle times between when they request a temperature change from the heating/cooling system and when they check to see if the temperature has actually changed. If a gas furnace can heat a place in ten minutes then a thermostat should poll the temperature pretty often to tell if the furnace should stay on or not. A thermostat designed with a heat pump in mind may only check every twenty to thirty minutes or so and cut in/out with tighter tolerances. BUT because it takes longer to make temperature adjustments it may trigger a “start” event at a lower number of degrees from optimal. So it may actually poll MORE frequently than a gas-based furnace and cut on and off more frequently because each cycle affects less change in the temperature.

That probably wasn’t very clear. Let’s try that again. Imagine a hypothetical situation where a 5° change can be made by a gas furnace in ten minutes. Let’s say a heat pump takes twenty minutes to affect a 5° change. Both test the temperature of the house every five minutes. They’re both set for 68°. At time 0 minutes the temperature in both houses is 68°. Five minutes later it’s time to check temperature and turn on if necessary. At time 5 minutes the temperature is 65° in both houses. The house with the gas furnace says “no big deal, it’s only 3°, my furnace can overcome that difference in no time. No need to turn on and spin up my fan and fire up my heating units for such a small amount of time, it’s not worth the wear and tear to make such a small adjustment” However, the house with the heat pump says “Hmm, my furnace takes a little while to correct this temperature, I better get working on it.” So it kicks on. Five minutes after that, at 10 minutes from our start time, both thermostats check the air again. In the gas house it’s now 63° and in the heat pump house it’s 66°(The heat pump has stopped any further cooling and raised the temp a bit, but not all the way because the heat pump takes longer to heat). The gas thermostat says “ok, time to turn on” and it fires up the furnace and the fans. The heat pump thermostat says “hmm, not bad, we should keep going though, we’re not quite there”. At fifteen minutes the temperature in both houses is back up to 68° and both furnaces shut off. The heat pump worked longer, but it’s higher efficiency could well have saved you money and your temperature didn’t vary as much as the gas furnace would have. Also the heat pump(because of its slower change in temperature) doesn’t have as much of a chance of heating over the specified temperature(which is wasteful) within a single cycle.

So, the programmable thermostats designed for heat pumps probably work differently in at least one of a few ways(there may be others that I didn’t think of, in fact there probably are) than other programmable thermostats. I’d still expect them to have a nice ROI over conventional(bimetallic strip) thermostats.


Now, this is what I call a GREAT debate…

Having said that, I think these gadgets are the greatest thing since sliced bread. I hate having heat on at all at night (I live in CA and it never gets THAT cold), but sure love to have the hearter kick on about 1/2 hr before I get up.

Set it and FORGET IT! Wow, that’d be great for an infomercial…

[Moderator Hat: ON]

I was going to move this as soon as I saw the thread, but left it in case there was actually the possibility of some debate. Seeing none, I’m moving it to IMHO, since pretty much people are talking about their opinions of it.

David B, SDMB Great Debates Moderator

[Moderator Hat: OFF]

Pretty sure mine has some kind of hysteresis function; it doesn’t switch on the moment that the temperature drops one degree below the setting (i.e. a small, cold draught from a briefly-opened external door may not trigger it)

All thermostats have hysteresis. If they didn’t, the furnace would just switch on and off rapidly until it damaged itself.

On even cheap thermostats, the hysteresis is usually adjustable by turning a little wheel inside the thermostat.

Dr. Lizardo:

Digital does not necessarily mean more accurate. The true accuracy of what you are seeing is dependent on the technology used to measure the temperature. For example, there are ‘digital’ scales that simply use a variable resistor attached to the spring plate on the scale. They are no more accurate than dial scales, although they give you a ‘precise’ number. On the other hand, other digital scales use strain gauges, and those actually can measure more accurately.

When digital watches first came out, if you asked the time from someone wearing one, they’d often respond, “The time is EXACTLY 3:52 and twenty seconds”. Never mind the fact that their watch is off by four minutes…

Mtgman, I believe you’re over-thinking the problem. The reason that a programmable thermostat for a heat pump has to work differently is simply due to the fact that most heat pumps, if the actual room temp is more than, say, 5 degrees below the “set” temp, kick on their backup heat, which is usually resistive electrical heat, the most expensive kind.

So, let’s say you’ve programmed your thermostat to keep the house at 55 during the day, then bring it up to 67 about 15 minutes before you get home. If you’ve got gas heat, that’s no problem: the furnace fires up, and runs until the temp gets up to 67. If you’ve got a heat pump, however, it’ll turn on its backup heating mode for a while, then, as the temp rises to 62, it’ll start using its compressor to do the job. This is wildly inefficient.

I’m not sure how heat pump friendly programmable thermostats deal with this problem, but I believe this is why they need to operate differently.

I love the programmable thermostat! Nice to have the heat come up just before it is time to get up and also nice to not have to remember to turn it down later. Ours has separate programming for during the week and weekends. I could probably set every day differently but not sure. It also works just fine with our AC in the summer… I tell it I want the house to stay at 75 and it does :slight_smile: