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While I can set my house to be a particular temperature, I still have to manually switch it between "Heat" and "Cool" mode, and this seems to be true of every thermostat (programmable or not) that I've seen. Is there a reason for this?

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It's not every thermostat, but the ones without an automatic switch are certainly in the minority: homedepot.com/… – BMitch♦ Oct 3 '11 at 21:49
@BMitch: Interesting find; I hadn't seen one before. Nevertheless, I'm curious why it's such a rare thing. – Adam Robinson Oct 3 '11 at 21:52
Hm.. mine does. It's optional, though and has to be explicitly enabled. – tylerl Jun 30 '12 at 3:12

4 Answers

up vote 12 down vote accepted

The logic involved is much more complex.

Right now the logic is simple: 

if AC Mode and it's too hot, turn on AC, else turn off AC
if Furnace mode and its too cold turn on furnace, else turn off furnace

But the determination of when to switch between heating and cooling modes is much more complex - otherwise the ac would kick on, and it would get too cold, so hte furnace would kick on, making it too hot, and a cycle would begin that would never end.

So the logic must be more like:

if the average ambient temperature has remained below a certain threshhold 
    for a given period of time 
    and the air conditioner hasn't been on in quite a while, change to furnace mode 
but if average ambient temperature has remained ABOVE a certain threshhold
    for a given period of time 
and the furnace hasn't been on in quite a while, change to a/c,

then if we're in furnace mode and it's cold, turn on the furnace, 
but if we're in a/c mode, and it's hot, turn on the a/c.

The circuitry involved is much more complex, so the price is higher and the market share obviously will be smaller.

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9  
You also have to worry about the outside temp. If you had a fireplace lit, and that caused the ambient temp around the thermostat to be too high, it could kick on the a/c. If it were freezing outside, it could end up damaging your a/c system. – James Van Huis Oct 3 '11 at 22:49
2  
Thermostats that switch automatically generally have separate heat and cool setpoints, and most of the time you'd have them some distance apart (i.e., heat at 70 and cool at 78), so the chances of bouncing back and forth would be fairly minimal. That said, we don't use the auto mode. If we have a particularly warm day in winter, there's no way I want to kick the A/C on, even if the temp goes above 78 or 80 for a short time. – TomG Oct 4 '11 at 3:24
I appreciate what you're saying, TomG, but you're talking about a manual set -stat, while I'm talking programmable. – The Evil Greebo Oct 4 '11 at 10:25
To offer a counter point, I can see a huge market for automatically switching thermostats in second homes, where you want the range to be something like 55-90F while you're away. – BMitch♦ Oct 5 '11 at 1:16
1  
A market, yes. A huge one? ... How many people actually own 2nd homes? – The Evil Greebo Oct 5 '11 at 2:11
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up vote 5 down vote

There are programmable thermostats that automatically switch from heat to a/c and to heat again. Some Honeywell thermostats, such as the 8000 Series, have 'auto' setting where you can program the Heat temperature and the cool temperature and the thermostat will automatically switch from heat to cool. I don't know how long they've been on the market though.

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up vote 2 down vote

Some do automatically switch. I remember getting a new thermostat for free with my new A/C unit. The manual said that the model XXX-A auto switched. In small text it said something like "This feature is only needed in cities like Phoenix where it can be very hot during the day and very cool at night." I literally said D'Oh! I live in Phoenix, and they would not give me the XXX-A version. Oh well...

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up vote 2 down vote

Check out www.nest.com for the Learning programmable thermostat.

It isn't stated right up front but it has an auto switching mode and does so very intelligently. It also uses local weather forecasts (via wifi) and time to temp (the time it takes for your system to heat and cool the home) to help determine what mode it should be in and when.

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Comment from an anonymous user: Nest can be in cool, heat, or cool/heat mode. It will not switch automatically between those three. In cool/heat you have a min/max set point. It kicks on heater/ac as needed in that mode to keep house within that range. – BMitch♦ Oct 28 '12 at 10:33

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