Replace thermostat without turning off furnace electricity?

[Sukhoi]

I want to install a basic Honeywell programmable thermostat: http://yourhome.honeywell.com/home/Products/Thermostats/7-Day-Programmable/RTHL2510C.htm

The furnace is an ancient in-wall gas unit, no blower, with just R and W wires to a very old barely functioning thermostat. It's a very simple millivolt setup. My problem is the wiring in this (rental) house is a mess and none of the normal fuses in the box cut power to the furnace. It appears to be hardwired to the house main, which I don't want to deal with.

It's only a 24 V system so I'm not too worried about getting shocked, but I've read it can hurt. If I wear gloves is it safe to do this work with the wires energized? What order to remove and install them? I'm guessing remove W then R. On new thermostat wall plate install R then W? The wall plate is separate from the control unit so I don't think I need to worry about frying the control unit since it's made to pull off with the power still turned on. Is there much chance of hurting the furnace transformer if I do the wires in the correct order?

 

[Stopsignhank]

I installed a honeywell thermostat here at work, very similar to yours. I plugged and unplugged it in at least a dozen times, probably closer to 2 dozen. I did it bare handed and had no problems. Pretty sure I even touched the wires, but not real sure. You will be fine. I will caution you though if you touch the wires while they are live they will arc and cause the beautiful blue light that causes your heart to beat fast. However it was a case of no harm no foul for me.

Of course my case is different than yours because our transformers are different, but just use common sense and you will be fine. If you wear gloves make sure they are tight fitting. That way you can get a better grip on the wire and be able to move the wire around better (that is the safety guy in me talking).

One thing is to put the wire in the terminal, tighten the screw and then pull on the wire. You want to make sure it is seated right.

 

[HeXen]

User rubber handled needle nose pliers to to handle the wires perhaps? At least I assume rubber handles won't conduct.

 

[etrigan420]

I will caution you though if you touch the wires while they are live they will arc and cause the beautiful blue light that causes your heart to beat fast.

Wait, what?

I have nothing to contribute to the thread...other than to state that there's no way in hell I'd change out a live wired *anything*.

Let us know how it goes!

 

[boomerang]

If you know where the transformer is located, why not disconnect one wire at the transformer prior to doing the swap? What's going to occur is that in fishing the wires out of the backing plate they are going to touch. You'll try to not have that happen but you know Murphy's Law, right? It will be the same situation when feeding them through the new backing plate.

Disconnect one wire at the transformer or cocoon the first wire you remove in electrical tape so if they touch the circuit can't be completed.

The order in which you remove and connect them doesn't matter.

Edit: Getting shocked is not pleasant. But remember kids, it's not the voltage that's going to harm you it's the amperage. Is that relevant in this situation, no, but why create sparks when you can very easily not create sparks? That's why I gave the advice I gave.

 

[master_shake_]

lol the thermostat is just a switch that's it.

all it does is connect them together when you need heat/ac and disconeect them when you don't.

not smoke and mirrors folks.

if you have ac you should have 3 wires iirc

oh and some AA batteries.

 

[HeXen]

How electrical shocks happen
http://lee65.hubpages.com/hub/How-do-you-get-electrical-shocks

(a) When two wires are at two different voltages and they come into contact with each other, a current will pass through them.

If they are not in contact, but your body connects the two wires by touching both of them at the same time, then an electric current will pass through your body and you will get the shock.

(b) If you accidentally touch the energized LIVE wire or other, live parts of an energized electrical appliance, while another part of your body is also in contact with the NEUTRAL wire, a current will pass through your body. Then you will get an electric shock.

(c) If your body is in contact with the LIVE wire, while another part is touching a grounded object, you will also get an electric shock.


(Notes: A grounded object is:

1. Anything that in good contact with earth mass, like water pipes. Most authorities require that water pipes be grounded. However, even if some pipes are not grounded, the pipes that are laid partly or wholely underground (i.e. the mater mains coming to the house) are actually in good contact with earth mass.

2. Or anything that is purposely connected to the earth mass, like the street lamp posts in front of the house. Do not be fooled by the innocent-looking lamp posts, the compound lighting posts or the short bollard lighting posts at the park or by the street in front of the house. Many grown adults have actually been electrocuted and died just by leaning to the metal posts.

3. Or anything that is electrically connected to the house earth wires, like the metal conduit, the metal casing of the washing machines, etc.)

 

[Sukhoi]

If you know where the transformer is located, why not disconnect one wire at the transformer prior to doing the swap? What's going to occur is that in fishing the wires out of the backing plate they are going to touch. You'll try to not have that happen but you know Murphy's Law, right? It will be the same situation when feeding them through the new backing plate.

Disconnect one wire at the transformer or cocoon the first wire you remove in electrical tape so if they touch the circuit can't be completed.

The order in which you remove and connect them doesn't matter.

Edit: Getting shocked is not pleasant. But remember kids, it's not the voltage that's going to harm you it's the amperage. Is that relevant in this situation, no, but why create sparks when you can very easily not create sparks? That's why I gave the advice I gave.

No clue where the transformer is. Wires come out of the crawlspace, so I assume it's down there or behind an odd painted-over switchplate cover near the furnace.

I had already been planning on taping the wires as I remove them, so I'm just going to go ahead and do it. They are quite separated on the old thermostat, and removal should be easy to do without shorting anything.

 

[Stopsignhank]

This is old but still applies today.

How many mail list subscribers does it take to change a light bulb?
Answer: 1,331

1 to change the light bulb and to post to the mail list that the light bulb has been changed

14 to share similar experiences of changing light bulbs and how the light bulb could have been changed differently

7 to caution about the dangers of changing light bulbs

27 to point out spelling/grammar errors in posts about changing light bulbs

53 to flame the spell checkers

41 to correct spelling in the spelling/grammar flames

156 to write to the list administrator complaining about the light bulb discussion and its inappropriateness to this mail list

109 to post that this list is not about light bulbs and to please take this email exchange to another list

203 to demand that cross posting to other lists about changing light bulbs be stopped

111 to defend the posting to this list saying that we all use light bulbs and therefore the posts *are* relevant to this mail list

3 to post about links they found from the URLs that are relevant to this list which makes light bulbs relevant to this list

306 to debate which method of changing light bulbs is superior, where to buy the best light bulbs, what brand of light bulbs work best for this technique, and what brands are faulty

27 to post URLs where one can see examples of different light bulbs

14 to post that the URLs were posted incorrectly, and to post corrected URLs

33 to concatenate all posts to date, then quote them including all headers and footers, and then add "Me Too."

12 to post to the list that they are unsubscribing because they cannot handle the light bulb controversey

19 to quote the "Me Too's" to say, "Me Three."

4 to suggest that posters request the light bulb FAQ

48 to propose new change.lite.bulb newsgroup

47 to say there is already an alt.light.bulb newsgroup

143 to ask if anyone ever did change the lightbulb

 

[olds]

I've never, ever turned the power off to swap out a thermostat. But I haven't done one for about 6 years. I doubt anything has changed though, probably still all low voltage.

 

[imagoon]

lol the thermostat is just a switch that's it.

all it does is connect them together when you need heat/ac and disconeect them when you don't.

not smoke and mirrors folks.

if you have ac you should have 3 wires iirc

oh and some AA batteries.

It is more like 3 - 24 with 5-7 being the most common for anything in that last 40 years.

AC unit would be typically common - fan - AC - heat
It also could be: common - fan 1 - fan 2 - AC stage 1 - AC stage 2 - heat stage 1 - heat stage 2 - 24Vac
Or even more convoluted if heat pumps and aux heat and cooling with stages and emergency heat get added in to the mix.

 

[Sukhoi]

Yay, did it hot with no issues. I'm still alive, and more importantly the furnace seems to work.

 

[Stopsignhank]

Yay, did it hot with no issues. I'm still alive, and more importantly the furnace seems to work.

Cool. Glad to hear it and thanks for the update.

 

[richardycc]

Yay, did it hot with no issues. I'm still alive, and more importantly the furnace seems to work.

I've done it without turning off the furnace/central AC in the past twice, once in my house, once for my in-law. My AC transformer blew within a yr or so afterward. Some thing happened to my in-law unit too, don't know the exact details, but she got talked into getting a new AC and furnace...Coincident? not sure, but I will make sure I turn them off next time I change the thermostat.

 

[Sukhoi]

I've done it without turning off the furnace/central AC in the past twice, once in my house, once for my in-law. My AC transformer blew within a yr or so afterward. Some thing happened to my in-law unit too, don't know the exact details, but she got talked into getting a new AC and furnace...Coincident? not sure, but I will make sure I turn them off next time I change the thermostat.

I think mine should be fine. The way this new thermostat is you pull a connector apart when you take the main unit off the mounting plate to replace the batteries. I never touched the wires to anything during the whole operation, so I was dealing with an open circuit the whole time. I'm sure it's designed to replace the batteries with the furnace powered, as about 1% of homeowners would turn off the power to do that even though the manual says to.

 

[Scarpozzi]

The real key here is to make sure you wire it up correctly, keep the wires from touching each other, and don't have the batteries in the thermostat when you wire it. Other than that, it'll be fine. Thermostat wire is like 16-18 gauge wire usually....it's not exactly a conductor of more than a few milliamps.

 

[HeXen]

Batteries? Mine doesn't have batteries.

 

[Sukhoi]

Batteries? Mine doesn't have batteries.

Well aren't you special?

 

[stormkroe]

Master shake is right, it's only a low voltage switch.
R, G, W, Y, and C are just about the most you will see these days, most 2 stage units that would require w2 and y2 have been replaced with auto staging units. They automatically go into 2nd stage if the new target temp isn't reached withing a set time.
Close action
RG = fan only
RW = heat
RY = A/C
C = common (used to power smart stats battery free)
You will only get shocked if you stick your tongue on it, in which case you more than likely deserve it.

 

[HeXen]

Well aren't you special?

Apparently so going by this thread. I stand on a mountain overlooking the peasants purchasing batteries for their thermostat.

 

[imagoon]

Master shake is right, it's only a low voltage switch.
R, G, W, Y, and C are just about the most you will see these days, most 2 stage units that would require w2 and y2 have been replaced with auto staging units. They automatically go into 2nd stage if the new target temp isn't reached withing a set time.
Close action
RG = fan only
RW = heat
RY = A/C
C = common (used to power smart stats battery free)
You will only get shocked if you stick your tongue on it, in which case you more than likely deserve it.

None of the HE ones recently [that I have had to work with] are "auto staging." Even the one just put in 2 weeks ago still uses a controller. Most that I have dealt with use the 5 wire cable as a "Remote control panel" that connects to a 12-24 wire + control box attached on the furnace itself. Stages are controlled via indoor / outdoor delta and "temp delta requested" type decision on the on the furnace controller.

On the modern HE 95% or 97% efficient furnaces there are to many controls to hit that number with a basic 5 or 7 wire 24V stat. Something quite a few people are learning in the Google / Nest world where tstats like the Nest simply can't control furnaces with variable burners / variable blowers / humidity management among others.

-edit-

FYI I know that some of the 70-80% efficiency units allow you to bridge the W1+W2 and then set a switch to for "x amount of minutes on -> enable stage 2" for the people that can't or cant be bothered to install a proper multistage tstat. These are slowly being phased out because they tend to lower the efficiency ratings and burn the unit longer with a burst of heat at the end of the run which makes the furance feel "drafty" until the second stage kicks in and finally warms the house. Most people with these units complain of wide temp swings in the winter / summer. This is less "auto" and more of a "last ditch effort" mode.

 

[unrlmth]

Before we had a thermostat, I turned the heat on and off by connecting wires and never got shocked.

 

[Zorba]

I used to always change thermostats hot, but once when moving out of a rental I had my wife swap the thermostats (we had installed a programmable touchscreen and they had a $10 POS). I am not sure which wires she touched together, but she somehow fried the transformer. Now I always turn off the power to the furnace before I swap one.

24Vdc isn't going to hurt you though, especially when it is powered by a dinky transformer. My FIL was an electrical contractor for 30 years, he always works on 120Vac and breaker boxes live, he will shut off 220V, though. I've worked with 120V live my whole life, too, not a big deal, just a little zap every now and then. You just want to make sure you aren't grabbing the end of the wire or could fall into the breaker panel, since you muscles will tense making it to were you couldn't let go.

Disclaimer: Of course you should always turn off the breaker and double check that wires are dead before you do any work, don't be dumb.

Commercial aircraft power always worried me, as it was 120V but 400hz. I heard that the 400hz would tense your muscles so much they'd hurt for days and it could be very hard to let go/move away.

 

[stormkroe]

None of the HE ones recently [that I have had to work with] are "auto staging." Even the one just put in 2 weeks ago still uses a controller. Most that I have dealt with use the 5 wire cable as a "Remote control panel" that connects to a 12-24 wire + control box attached on the furnace itself. Stages are controlled via indoor / outdoor delta and "temp delta requested" type decision on the on the furnace controller.

On the modern HE 95% or 97% efficient furnaces there are to many controls to hit that number with a basic 5 or 7 wire 24V stat. Something quite a few people are learning in the Google / Nest world where tstats like the Nest simply can't control furnaces with variable burners / variable blowers / humidity management among others.

-edit-

FYI I know that some of the 70-80% efficiency units allow you to bridge the W1+W2 and then set a switch to for "x amount of minutes on -> enable stage 2" for the people that can't or cant be bothered to install a proper multistage tstat. These are slowly being phased out because they tend to lower the efficiency ratings and burn the unit longer with a burst of heat at the end of the run which makes the furance feel "drafty" until the second stage kicks in and finally warms the house. Most people with these units complain of wide temp swings in the winter / summer. This is less "auto" and more of a "last ditch effort" mode.

Perhaps the difference is canada vs the us for models, I haven't lived in 'Murica for 10 years, but over the last 3 years I've hooked up 100+ high efficiency furnaces (carrier, train, coleman, napolean, lennox, etc), not a single one has been traditional 2 stage. They don't even have w2 and y2 terminals to bridge. The efficiency is unaffected as they are all certified 96+ with this operation. I agree that the second stage cfm makes the furnace feel drafty, but that's a characteristic of forced air heat in general and you get that whether you set it to stage 2 or it sets itself there automatically.
Also, might just be my area but I don't think you can even buy a mid efficiency here anymore.

 

[imagoon]

Perhaps the difference is canada vs the us for models, I haven't lived in 'Murica for 10 years, but over the last 3 years I've hooked up 100+ high efficiency furnaces (carrier, train, coleman, napolean, lennox, etc), not a single one has been traditional 2 stage. They don't even have w2 and y2 terminals to bridge. The efficiency is unaffected as they are all certified 96+ with this operation. I agree that the second stage cfm makes the furnace feel drafty, but that's a characteristic of forced air heat in general and you get that whether you set it to stage 2 or it sets itself there automatically.
Also, might just be my area but I don't think you can even buy a mid efficiency here anymore.

Fair point. The ones I deal with allow the unit to start in 2 stage if conditions are right and provide connections to do that. Example would be say "night mode" set to 62F then "morning mode" set to 72F would kick on stage 2 from the start because of the 10 degree F delta. (16.5C to 22C there about) Another case is when outside is below "x" degrees changes the stage timing as does the delta(temp) / time.

This is generally done with a "controller" bolted to the side of the unit with the all the wire connections for stage control, humidity etc Then the 5 wire that is in most houses gets hooked up to the controller and a control panel/display/temp sensor is placed where the old Tstat is.

This makes the furnace basically "uncontrollable" via any of the cheaper systems but allows installs of new HE furnaces in existing homes with out trying to pull a new 24 wire group up.