Replacing thermostat that support outdoor temp sensors with 1 that does not

I am wanting to replace my thermostat with a wireless thermostat. In my cu rrent lennox thermostat, it has 2 wires going to 2 different T terminals wh ich are for outdoor temperature sensors. The new thermostat I'm purchasing doesn't have connections for those wires. What am I losing by not reconnec ting my outdoor temperature sensor wires? Should I even move forward with t he replacement? The model I'm purchasing is a Honeywell RTH9580. The model I'm relacing is a Lennox 51M33.
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On Friday, August 16, 2013 8:50:08 AM UTC-5, Travis Dietz wrote:

which are for outdoor temperature sensors. The new thermostat I'm purchasi ng doesn't have connections for those wires. What am I losing by not reconn ecting my outdoor temperature sensor wires? Should I even move forward with the replacement? The model I'm purchasing is a Honeywell RTH9580. The mode l I'm relacing is a Lennox 51M33.
This is heat pump system by the way. I guess another question is what will work differently in my system when 1 is installed versus the other?
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On 8/16/2013 8:33 AM, Travis Dietz wrote:

doesn't have >> connections for those wires. What am I losing by not reconnecting my outdoor >> temperature sensor wires? Should I even move forward with the replacement? The

If you provide links to the t-stat info I might read it. Hard to say elsewise.
The outside connections may be used to provide more heat/cooling when it is cold/hot outside. Heat pumps may have 'multi stage'. Heat-cool change over may be different.
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On Friday, August 16, 2013 11:23:44 AM UTC-4, bud-- wrote:

http://www.lennox.com/pdfs/manuals/Lennox_Merit_Programmable_Thermostats_Ma nual.pdf
See page 7. They talk about the outside temp being used to optionally set a "balance point". The explanation of what that does, as is so common, isn't very good and it makes no sense to me.......
SETTINGS - Balance Point NOTE - The balance point is only available with the outdoor sensor. When the outdoor sensor (X2658) is connected to the 51M42 thermostat, balance point adjustment is available. The balance point feature allows the measured outdoor temperature to govern operation of the heat pump and backup heat source.
NOTE - The balance point feature allows the outdoor temperature to rise or fall 3°F above or below the balance point. This prevents excessive cycling of the equipment when the outdoor temperature is near the balance point. For example, if the balance point is 40°F and the actual outdoor temperature is 35°F, the outdoor temperature must rise to 43°F before equipment adjustment occurs. Conversely, if the balance point is 40°F and the actual outdoor temperature is 45°F, the outdoor temperature must drop to 37°F before equipment adjustment occurs
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Travis Dietz wrote:

If you want outdoor temp. read out from the 'stat and auto adjusting humidifier setting, you need out door temp. sensing unit. When I automated humidifier setting it did not work well to our liking.
I have a Davis wireless weather station out in the front yard which gives all kinda info on weather condition real time. It is connected to the 'net feeding my local info to the NOAA network.
I believe Lennox 'stat is made by Honeywell. I am using Honeywell wireless 'stat w/o internet connection option module. It is YTH6320 series. This thing can have all kinda option modules like outdoor temp. sensor. I am a fan of Honeywell stuff being retired Honeyweller.
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Travis Dietz wrote:

Wireless thermostat and WiFi thermostat is two different thing.
Wireless ==== there is no wires between HVAC system and the thermostat. WiFi ==== You can access and control your 'stat via Internet from any where. 'stat will have it's own IP address.
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Tony Hwang wrote:

Don't know what the OP wants, but I don't care what is the outside temp. I'm just concerned about the indoor temp.
--
Bill
In Hamptonburgh, NY
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willshak wrote:

But they are related. For one, if outside is cold, furnace has to run more to maintain indoor temp.
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You may not care, but you might find your house more comfortable if your furnace cares about the outside temp.
I'm not all that well versed on the outdoor sensors, but I understood how they worked when I read about them when my dad was shopping for a new furnace a few years ago. I forget the details, but basically, by monitoring the outside temperature, they can adjust the run time of your furnace so it doesn't cycle on and off as often.
I'm sure someone (probably nestork) will post a few paragraphs with more details.
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Here is what my dad and I were researching when he was shopping for a new boiler. He ended up getting the outdoor reset unit because of the rebate that was being offered.
http://www.heat-timer.com/En/EducationDetail.aspx?Id=3
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On Friday, August 16, 2013 8:56:01 PM UTC-4, DerbyDad03 wrote:

I don't see why the thermostat needs the outside temp information. It knows what percentage of the time the system needs to run to maintain the desired temp. If it's cold outside, it's going to be running more. If it's warm, it's going to be running less. So, if the idea is to achieve some energy efficiency by lowering the boiler temp, if the system is only running say 10% of the time, then the thermostat knows what it needs to know, ie that it can lower the water temp. And if it's running 50% of the time, then it knows it's cold out.
I suspect the same idea is at play here with the heat pump, but Lennox BS doesn't say what it actually does.
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Either I'm missing your point, or you're wrong, and I mean that in the nicest way. ;-) I'm not claiming to be an expert, this is just my understanding of the difference between the thermostat knowing the outside temp and not.
Without Outside Reset
It's 40F outside and the boiler heats the water to full temp. The house reaches the set temperature rather quickly and the boiler turns off. This happens a few times, perhaps that's the 10% cycle time you spoke of. As far as I understand it, that type of cycle time is not very efficient and it's going to take some cycles before the stat recognizes the trend. Therefore, you are running an inefficient system while the stat learns the most current trend. OK, so then the stat learns the trend and lowers the temperature of the water. However, in the meantime, a cold front blows through, the outside temp drops significantly and the not so hot water takes an inordinate amount of time to heat the house. What's the stat supposed to do? Will it recognize the extra long run time immediately and raise the water temp next time? (I don't know, that's a real question.) Let's say it does, so the next time it comes on it runs at full temp. Wait...what happened to that cold front? It's gone, the house hits up quickly and the boiler shuts off not only being inefficient but also confusing the crap out of the poor thermostat.
With Outside Reset
Well, they explain it pretty clearly at that website.
The starting point for most systems is the 1.00 (OD):1.00 (SYS) (Outdoor Temperature : Heating Water Temperature) ratio. This means that for every degree the outdoor temperature drops, the temperature of the heating water will increase one degree. The starting point of the curves is adjustable, but comes factory selected at 70°F Outdoor Temperature and 100°F Water Temperature. For example with a 1.00 (OD):1.00 (SYS) ratio, if the outdoor temperature is 50°F, this means the temperature has fallen 20° from the starting point of 70°F. Therefore, the heating water temperature will increase 20° to 120°F."
No need for the stat to learn anything. No inefficient boiler use. No long or short run times resulting in uneven room temps. There's an immediate adjustment to the water temp based on the current outdoor temp. The hottest water when the cold front blows through, cooler water as soon as the sun comes out.
Sounds like a good idea to me, assuming I understand it correctly.
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On 8/16/2013 9:24 PM, DerbyDad03 wrote:

As you describe, an outdoor-reset controls the circulating water temperature in a hot water system. It looks at the outside temp and the water temp. It increases the circulating water temperature as the outside temp falls. I believe they improve the efficiency of a hot water system.
I presume a heat pump system is hot air. You could do something similar with hot air, but the description does not say there is a discharge-air-temperature sensor which I think would be needed. Outdoor temperature could be useful for a setback thermostat to determine how far in advance to start heating the house.
For a heat pump, the heat pump will shut down when it is too cold outside and the system will run on resistance heat. Could be what the outside air temp is used for, but that doesn't seem to be what is in the description trader provided, which I agree is far from clear. It does talk about backup heat source, but 40 degrees sounds too high to switch to all resistance heat. Maybe resistance heat and heat pump are used together below the "balance point"? The few systems I have seen, a t-stat at the compressor shuts down the compressor when it is too cold out and a 2-stage-heat thermostat runs the resistance heat on the 2nd stage.
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On Friday, August 16, 2013 11:24:10 PM UTC-4, DerbyDad03 wrote:

The thermostat should be able to recognize the trend quickly. The temperature outside doesn't typically change 40F in just an hour. The thermostat knows how long the system runs each time it starts up, how long it's been between start-ups, how fast the temp inside has been dropping, etc. It also knows the time of day and typically temp outside is going to be colder at night, warmer in the day. Typically the run times are going to change gradually. System knows that it's been running 10 mins, 3 times an hour. This time it just ran 9 mins. And instead of reaching the turn on point in 10 mins, it just took 12 mins. Conclusion: Less heat is required.
Therefore,

OK, so then the stat learns the trend and lowers the

That would depend on if it's a totally dumb thermostat that doesn't make any adjustment or a smart one. No reason a smart one would not recognize that it's most likely gotten colder outside on the first cycle where the system needs to start sooner than the last cycle and now takes longer to bring the temp up to the set-point.

Yes, if the outside temp swung around wildly, then having the outside temp sensor would give the thermostat better info. But I still think the info the thermostat already has is a pretty good proxy. What's the thermal mass of one of today's typical home boilers? They aren't the huge, honking things from the 50's. It's not like the thing is firing up something with enough excess heat to heat the house up a whole lot more, no matter what happens.
And how many systems actually have an outside temp sensor at all?

That explanation is for a boiler system. The OP has a heat pump system and the Lennox explanation makes no sense to me. Also, I believe the OP said his thermostat had two outside temp sensors connected?

I'd like to see a reference that has run actual measurements on what difference in fuel usage you get by doing this. And what the difference in efficiency you'd get by having the thermostat estimate the outside temp based on it's run time/temp change history instead. Even with two stage furnaces, either the thermostat makes the call based on the recent history and how large the temp change required is, or it's done by the furnace. Without the 2 stage thermostat, the furnace always starts at low stage and runs at that for a fixed amount of time, usually ~10mins. If the system is still running at that point, then it kicks in to high stage. The difference in efficiency isn't all that great either, maybe 1 or 2%? For a typical home boiler, IDK what the difference in efficieny might be.
No long

My grandparents had a 50s vintage boiler system with a plain old mercury thermostat. I don't recall uneven room temps. And apparently an outside temp sensor isn't that essential today either, because I'll bet only a small percentage of systems of any kind have them. The exception being when there is a humidifier. Then having the humidifier run by a controller that is aware of the outside temp is a benefit. But even that has obvious limitations.
There's an immediate

I don't think it's a bad idea. I just question how much difference it really makes versus what a smart thermostat can figure out without it.
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Without going over this point by point, I'll just toss out a few comments...
I was specifically talking about an outdoor reset system for a boiler. I know squat about heat pumps, so I couldn't discuss whether an outdoor temp sensor makes any sense at all. However, in general, I wouldn't want my boiler thermostat deciding what temp the water should be based on the time of day or any recent trends - especially based on time of day. That just makes no sense. Sure, it's "usually" colder at night, but not always and not always with the same variability. Given the choice between the stat making an assumption and the system making a decision based on time, indoor temp and outdoor temp, I'll take a decision based on facts every time.
As far as how many systems offer an outdoor reset option, all I know is that every boiler quote that my dad got (4?) included the outdoor reset as an option. Each quote was for a different brand of boiler, so they seemed to be pretty common in the boiler world - or perhaps it was just my dad's New England location.
Regarding the savings achieved, this 2004 study might give actual savings numbers somewhere in the 25 page report, but I'll just quote the conclusion reached:
"The primary conclusion of this work is that condensing boilers can achieve energy efficiency benefit even when used in homes with common baseboard radiators by incorporating a reset control which modulates the water temperature supplied to the baseboards with outdoor temperature."
http://www.heatinghelp.com/files/posts/770/Condensing%20Boiler%20and%20Baseboard.pdf
I also found a 1984 study that quotes the savings as 10-20% in apartment buildings after outdoor resets were installed, but a 30 year old study of apartment building boiler systems might not be relevant. While the theory behind the savings stills hold, the efficiency of more modern system probably cuts into the percentage somewhat.
http://www.heatinghelp.com/files/posts/9700/outdoor_reset.pdf
Bottom line, at least as far as I can tell, is that basing the water temp on the outside temp - with a properly calibrated outdoor reset system - just makes sense. The only real question is, similar to the situation that occurs when sizing a house for a new heating system, is whether or not all the factors required to accurate calibrate the outdoor reset unit so that it works properly in a given house were taken into account.
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On Saturday, August 17, 2013 7:08:11 PM UTC-4, DerbyDad03 wrote:

You don't make the decision just based on if it's 9AM or 1AM. That indeed would make no sense. But you can use that as one factor to regulate the water temp the boiler is supplying. Again, if the system has been running 25% of the time from 1 AM to 7 AM, then it ran 22% of the time from 7AM to 9AM, that combined with the fact that it's now daytime, suggests that it's OK to move to a cooler water temp. It tries that and sees what happens. Within minutes, it knows how fast the temp in the house is responding to the new lower water temp, giving it more information. In an hour, it knows what percent of the time the system ran and can again adjust.
In fact, I could argue that basing the decision on that, vs outside temp, might produce better results. Let's say it's 25F outside. Does the house need the same amount of heat when it's 25F at 3AM at night, with a 25MPH wind as it does when it's full sun on a still day at noon? The thermostat knows how much heat the system has supplied over the past hour and I'd say that might be more relevant than just the outside temperature.
I did a bit of googling and it looks like this whole idea of outdoor reset has some holes and some controversy in it. For one thing, what happens with setback, which can save a lot of energy use? Presumably, if the outdoor setback is linked in to a smart thermostat in say a single family home, then the smart thermostat can accommodate that. You set the temp back to 60F at night, but in the morning, it knows it has to get the temp up to 70F fast, so it fires at the higher water temp, But if it's not linked in, then what? If it's 40F outside, and the reset controller limits the boiler temp to the lowest, it's going to take a long time to raise the temp 10F, So? Don't set it back? Set it back but have it start going back to 70F at 4AM instead of 6AM? Either of those negates the savings of setback.
And then what about multi family or apartment buildings? You can probably kiss nighttime setback goodbye, because then there isn't one smart thermostat that could solve the above. So, which saves more energy? Setback or squeezing a little more efficiency out of the boiler with outdoor reset? IDK
Given the choice between the stat

The thermostat knows that with the water temp set to 110F, the system ran 20% of the time the last hour, 22% the hour before, it's now 8AM, that right now the system has run for 8 mins and the inside temp has risen .4F. Those aren't facts?

From googling it appears that this is a govt mandate now on new boilers and it's a minimum reqt.

I'm surprised you're not interested enough to look for the actual results. First, you're conflating two things:
1 - Getting higher efficiency via reducing the boiler water temperature when possible
2 - That using outdoor temperature is the only way to do that
I said from the start that I agree you can get slightly higher efficiency by going to a lower water temperature. It makes sense, because you're going to extract a little more of the combustion heat. I said I'd guess you get an extra 1 or 2% efficiency. Look at the above study you provided, page 9. It shows that by varying the water temp from lowest to highest, the boiler efficiency goes from 91.6% to 94%, with the highest efficiency being with a return water temp of 93F. So, the difference in efficiency was at most 2.4%. And then consider that the most energy burned is going to be when it's coldest outside and that is when the boiler is going to be told to fire at the hottest temps, so you don't get 2.4% more efficiency during the periods when most of the energy is used, you get 0 or maybe 1%. How much difference is that going to make in an energy bill?
There is another interesting aspect to that whole experiment. On page 23 they show the efficiency curves for the condensing boiler they used for the test and for the conventional, plain old chimney vented boiler that was alternately used. The difference in efficiency between the two is only about 5%. That would seem to be so shocking, that you're left wondering if something is radically wrong with the whole test procedure. It's like testing some new device on a Porsche Turbo and on a Smart Car and finding that the device produced a 1.5% difference in fuel economy for both cars. Reporting that as a conclusion and then saying, by the way, the Porsche got 28 mpg and the Smart Car 30 mpg.

I skimmed through that. They basically took some apartment building that had a disaster for a heating system. They talk about over heated hallways, residents opening windows because the apartments were too hot, etc. That sure sounds like there are serious other problems that need to be fixed and should be fixed, by other means. I think even in low income housing, people don't open the windows instead of turning down the thermostat. It's a sign of a malfunctioning or extremely bad install. So, yes, I can see how an outdoor reset would make a difference in energy usage under those circumstances. But it doesn't seem like a fair comparison test to me and we also don't see any discussion about if there were complaints from other residents that now were not getting enough heat either. With what it sounds like was going on in those buildings, I would not be surprised if that was a side result.

I'd say the real question is if you can do pretty much the same thing, perhaps even better, eg avoiding the setback problems, with a smart thermostat without using an outdoor sensor. Here's a company with a product that seems to agree:
http://www.exqheat.com/thermostatsandthelaw.html
"Reset Controls are Required for Hot Water Boilers Manufactured after September 2012. Outdoor Reset Control vs.... Indoor Real Time Reset Control for Boilers
Are we heating the outdoors? Exquisite Heat adjusts boiler temperature eve ry hour of the day from indoor real time heat loss data, taking all buildin g factors of heat loss from windows, wind, sunlight activity, insulation, l ighting, occupancy, and individual comfort needs into consideration. "
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