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.
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?
>> connections for those wires. What am I losing by not reconnecting my
>> temperature sensor wires? Should I even move forward with the
If you provide links to the t-stat info I might read it. Hard to say
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.
On Friday, August 16, 2013 11:23:44 AM UTC-4, bud-- wrote:
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
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
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.
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.
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
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.
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.
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
Sounds like a good idea to me, assuming I understand it correctly.
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
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
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.
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
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
And how many systems actually have an outside temp sensor at
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.
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.
Without going over this point by point, I'll just toss out a few
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
"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."
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.
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.
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:
"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. "
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.