Installing Digital Thermostats

I installed two digital “set back” thermostats this week in our house: they automatically turn the temperature in the house down at night and up in the morning. The packaging claims 30% energy savings are possible, so it seemed like a good investment, to say nothing of a way of using less oil.

One confusing thing: the instructions, which are quite complete, talk at some length about the need to “label the existing wires with included labels” before disconnecting the existing thermostat to ensure the wires can properly be routed to the new one.

My problem was that the existing two wires didn’t have labels. So I didn’t know where to route them on the new thermostat. Fortunately, in the Professional Reference Guide for the new thermostat, I found the following:

When the thermostats relay (switch) connects RH to W, the heat comes on. If the heating system already has a thermostat connected, use the same two wires that were on the old thermostat. When there are just two wires, there is no polarity, i.e., either wire can go to RH or W.

And in the manual for the old one, I found that, indeed, I had two wires, one RH and one W.

So I just wired one wire to the RH, and one to the W. And that worked.

We’re now 16.5 Celsius at night, and 19 Celsius in the morning.


Steven Garrity's picture
Steven Garrity on November 9, 2004 - 01:22 Permalink

I installed a Honeywell digital thermostat in my house last year. I couldn’t identity which wires were which, but I tried it one way and it didn’t work (I think the heat came on no matter what the setting), then I switched the wires and all was well.

One heads-up — I was a bit over-zealous and originally set mine to 15-deg at night. I had to turn it up a few degrees, because I was waking up at night freezing.

Peter Rukavina's picture
Peter Rukavina on November 9, 2004 - 01:25 Permalink

What I want a thermodynamicist to tell me is “how far can I turn the heat down at night before I end up using too much energy in the morning to bring the temperature back up.” I tried asking the heating expert on Maritime Noon about this, but didn’t really get a good answer.

Steven Garrity's picture
Steven Garrity on November 9, 2004 - 01:52 Permalink

Peter, I’ve wondered about that before, and I think the conclusion I’ve come to (don’t ask how) is that our houses lose so much heat all the time, and hold so little of it, that you could turn off all of the heat at night, and turn it on during the days, and it would be cheaper then keeping any heat on at night (does that make any sense?).

I could always post the question to the seeting masses online like I did for my rain walking/running question — but that will only turn up 100 people like you and I — and no one with a clear answer.

oliver's picture
oliver on November 9, 2004 - 03:23 Permalink

It was a long time ago I took thermo, but I have PhD _in science_ so you’ll just have to believe anything I say, even should an english major who knows better should show up. My reflexive reaction is that Steven is on to something. As further food for thought I’ll offer that the rate of heat loss will be linearly proportional to the temperature difference between inside and outside, which has the implication (for what it’s worth) that a) the cooling rate is exponential and diminishes asymptotically to nothing as the inside approaches the temperature of the outside b) with a pencil and a memory for how to do integrals this would be a no-brainer, and c) in the winter when it’s -40 outside then subtracting five degrees on the thermostat from your ideal probably isn’t worth that sacrifice, because that’s only a small fraction of the inside-outside difference. I predict that’s more or less what you heard on the radio. I remember a related conundrum back in school about whether it’s better to pour the refrigerated milk into your coffee before or after you leave the cup to stand around and cool.

oliver's picture
oliver on November 9, 2004 - 03:34 Permalink

Also “how much energy you use to heat the house up in the morning” will depend a lot on when in the morning you’re to doing the heating and whether it’s time of year when the outside air and sun are helping you. If we were to assume that it’s just as cold in the early morning when you want to heat it back up as when you turned it down to save energy, that would make the problem more straightforward….but still I don’t know the answer.

oliver's picture
oliver on November 9, 2004 - 03:36 Permalink

Then there’s also the heat capacity of the house to reckon with….

Charles's picture
Charles on November 9, 2004 - 08:00 Permalink

I think Steven hit a good point. The amount of heat you lose has less to do with the difference between the outside and inside temperature, and more to do with how well insulated your house is. A well insulated house will lose less heat, no matter how large the temperature differential is.

My place is about 30 years old, has very poor insulation, and has forced air heating to boot (which is bad, because my living room is on the other side of the house as my furness, so the room I spend the most time in gets the least heat.) I’ve been thinking about getting someone to come out and give me an estimate on improving the insulation in my place (re-seal the windows, more insulation in the attic, etc..) but I’m at a loss about who to contact. Schurman’s maybe? Any fellow islanders got some pointers?

Brian's picture
Brian on November 9, 2004 - 11:31 Permalink

Charles, if you’re interested in improved home energy efficiency, check out the Natural Resources Canada Energy Efficiency site, particularly the residential section. There are advisors and contractor lists, as well as info on a program which can fund part of the costs of any energy related home improvements.

John Boylan's picture
John Boylan on November 9, 2004 - 14:57 Permalink

It’s interesting to read what people are posting on this one. I’ve always been told that keeping a constant temperature is more energy efficient than cycling high/low, but I don’t know if it’s true or not.

People with old house might be interested in “Conserving Energy in Historic Buildings”. It’s published by the Technical Preservation Services of the National Park Service in the States. The focus is on preserving the character of historic homes and being more energy-wise. It is dated (1978), but if the NPS is still posting it it should be good stuff. 1978. Does anyone remember the first energy crisis? The url is…
The NPS puts out great technical briefs on everything from historic roofing systems to how to repair historic masonry.

Cameraguy's picture
Cameraguy on November 10, 2004 - 01:07 Permalink

Thanks for the great topic… I was afraid of getting a digital thermostat, for fear I would not be able to connect the thing… Now, I’ll give it an asymptotically uneducated attempt. I’m in a new house (less than one year), and the thermostats are a joke.. right for hotter, left for colder. And a few marks inbetween.

Marcy Kaplan's picture
Marcy Kaplan on October 2, 2006 - 04:10 Permalink

For all the posts that you all made, it made me understand nothing at all. This is such a waste of time in attracting people like me who wanted helpful information on installing digital thermostat. I hope people like you would refrain from messing the information highway with cra**y little things that farts out of your empty skull. Thanks for nothing

woot's picture
woot on October 28, 2006 - 14:22 Permalink

I agree with all of your logic, but all i have to say is woot

Programmable Thermostat's picture
Programmable Th... on January 5, 2009 - 18:16 Permalink

I just recently got a programmable thermostat and I love it! I highly recommend getting one.

jon's picture
jon on January 16, 2009 - 03:59 Permalink

I think that your logic is great, except for one thing. all electronic switching circuits like to be located on the ground side of a circuit. reason being because on the ground side the electricity has already passed through the load (or in your furnaces case the heating relay) on the circuit and there is very little voltage left (direct current situation, alternating doesnt really matter,FYI your heater control has low DC volts). this allows the circuit to close and open without high voltage arc because by this point in the path of the electrons the voltage has mostly being used up, the amperage through the circuit is always the same but the voltage drop across the load has reduced the arc at the switch (digital thermostat) so when i read my instructions and it said the wire that has the heating relay on it goes to the “w” terminal i paid attention so as not to shorten the life of my “switch” or digital thermostat with unneccesary high voltage. the things they teach canadian mechanics at college.