Mick - I checked the voltage across the two wires this morning and I’m chagrined to say that it’s 28 v, so should be sufficient, correct? I’d assumed the voltage was low because I had tried to connect a Honeywell digital thermostat last year and the two wires didn’t provide the voltage that device needed to activate the display. My other frustration was that neither of the two Nest tech experts I spoke to suggested that I check the voltage. Thanks for your quick reply.
- Mike
Making a Nest thermostat work with a two wire system
Re: Making a Nest thermostat work with a two wire system
ProfMikeT wrote:Mick - I checked the voltage across the two wires this morning and I’m chagrined to say that it’s 28 v, so should be sufficient, correct? I’d assumed the voltage was low because I had tried to connect a Honeywell digital thermostat last year and the two wires didn’t provide the voltage that device needed to activate the display. My other frustration was that neither of the two Nest tech experts I spoke to suggested that I check the voltage. Thanks for your quick reply.
- Mike
My issue was the tiny current draw of the Nest leeching power from the furnace was enough to set the primary on the burner to think the thermostat was switching the furnace on. This resulted in a noise coming from the furnace like it was about to start, but never did. This sound pulsed due in some way to the design of the Nest charging circuit. So be on the look out for that.
Outside of that if the wires cant provide enough for the nest it will tell you. with my setup it very occasionally tries to tell me that it cannot get power from the heating wires, but works fine
Mick...
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phil
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Re: Making a Nest thermostat work with a two wire system
I'm not too sure about the nest brand. I added a digital programmable thermostat and it has batteries but can run without them installed so I think it picks up some current from the furnace. My furnace is so old it really doesn't have any digital circuity so I think it just runs on AC. I doubt it has a rectifier.
I guess that makes sense because if the furnace supplies AC then the peripheral device could likely just use a coil transformer to transform some of the AC and it would indeed have some sort of rectifier to make it to DC for the electronics in the thermostat.
if the Nest is taking power from the furnac's AC circuit I would think you'd also see a voltage drop across that and you might be able to measure that and using ohms law maybe figure out the amount of draw ( in amps) to see if you think it is significant?
anyway I just noticed you quoted a voltage but didn't specify weather you were measuring AC power or DC power and thought hey I wonder if maybe you are reading the meter readings without noticing the significance of it being AC or DC.
you can't really transform DC power in the way that you can with AC as the traditional transformers only work on AC.. that said it is possible with electronics to turn AC into a pulsed DC which will then resemble AC and you can transform the pulsed current to the voltage you want.. the boards might just be doing that but if there is a wire coil type transformer involved they don't work on straight DC.. I don't know if that solved anything or just added confusion.
I thought you might try an old analogue multimeter (with a needle) rather than an electronic meter as sometimes you can get erroneous readings. That happen sometimes because you are measuring stray voltages and things.. I think you might be able to add a resistor to one of the probes or maybe just try an analogue meter as they might not get "confused" for example giving you a reading of a voltage where there is miniscule actual potential.. You might measure power where there is actually very miniscule amperage and it could get introduced into a circuit even by interference between wires that run near to each other for example.
if you are measuring DC powere you miught also sense "dirty power" or in other words a lot of ripple in your AC power. your digital meter might show that as a DC current which you were expecting but a higher AC component that expected.
a scope would be able to display this and it can be caused by a bad ca[pacitor and these days with junky chinese components you can still have bad capacitors.
also a 9 volt meter can introduce 9 volts and in some cases probing circuits can cause damage to the circuitry if it isn't expecting that high of voltage.
if you try to measure some points with a DMM sometimes you can also be trying to take a sample from a sensitive circuit and affect the circuit ecause the meter is loading the circuit you are testing. the workaround might be to use a vacuum tube volt meter. this type of meter doesn't load the circuit that you are trying to test in the same way that a digital meter does. someone a bit smarter with electronics than me might explain why adding a 22K resistor to the probes of your digital meter helps with this sort of measurement issue. I think its basically just a way to filter out a voltage that appears because it has enough voltage get the DMM to measure it but which isn't of significant enough potential ( almost 0 amps) to be actually meaningful in troubleshooting.
basically I've seen it in reality where I forgot to try both AC and DC and getting weird readings because I was expecting it to be other than what it was. sometimes it can throw ya. so my simple suggestion is to take meter readings with both AC and DC and you may only be at all concerned with one or the other of course.
Phil
I guess that makes sense because if the furnace supplies AC then the peripheral device could likely just use a coil transformer to transform some of the AC and it would indeed have some sort of rectifier to make it to DC for the electronics in the thermostat.
if the Nest is taking power from the furnac's AC circuit I would think you'd also see a voltage drop across that and you might be able to measure that and using ohms law maybe figure out the amount of draw ( in amps) to see if you think it is significant?
anyway I just noticed you quoted a voltage but didn't specify weather you were measuring AC power or DC power and thought hey I wonder if maybe you are reading the meter readings without noticing the significance of it being AC or DC.
you can't really transform DC power in the way that you can with AC as the traditional transformers only work on AC.. that said it is possible with electronics to turn AC into a pulsed DC which will then resemble AC and you can transform the pulsed current to the voltage you want.. the boards might just be doing that but if there is a wire coil type transformer involved they don't work on straight DC.. I don't know if that solved anything or just added confusion.
I thought you might try an old analogue multimeter (with a needle) rather than an electronic meter as sometimes you can get erroneous readings. That happen sometimes because you are measuring stray voltages and things.. I think you might be able to add a resistor to one of the probes or maybe just try an analogue meter as they might not get "confused" for example giving you a reading of a voltage where there is miniscule actual potential.. You might measure power where there is actually very miniscule amperage and it could get introduced into a circuit even by interference between wires that run near to each other for example.
if you are measuring DC powere you miught also sense "dirty power" or in other words a lot of ripple in your AC power. your digital meter might show that as a DC current which you were expecting but a higher AC component that expected.
a scope would be able to display this and it can be caused by a bad ca[pacitor and these days with junky chinese components you can still have bad capacitors.
also a 9 volt meter can introduce 9 volts and in some cases probing circuits can cause damage to the circuitry if it isn't expecting that high of voltage.
if you try to measure some points with a DMM sometimes you can also be trying to take a sample from a sensitive circuit and affect the circuit ecause the meter is loading the circuit you are testing. the workaround might be to use a vacuum tube volt meter. this type of meter doesn't load the circuit that you are trying to test in the same way that a digital meter does. someone a bit smarter with electronics than me might explain why adding a 22K resistor to the probes of your digital meter helps with this sort of measurement issue. I think its basically just a way to filter out a voltage that appears because it has enough voltage get the DMM to measure it but which isn't of significant enough potential ( almost 0 amps) to be actually meaningful in troubleshooting.
basically I've seen it in reality where I forgot to try both AC and DC and getting weird readings because I was expecting it to be other than what it was. sometimes it can throw ya. so my simple suggestion is to take meter readings with both AC and DC and you may only be at all concerned with one or the other of course.
Phil
Re: Making a Nest thermostat work with a two wire system
phil wrote:I'm not too sure about the nest brand.
The nest is a bit fussy and fairly specific in it's needs - get it wrong by going too far away from what the install instructions say and it's a $200 mistake - I know, been there. These are essentially computers that leach a power recharge from the device they are switching - as suchthey dont like to be plugged into wrong voltages etc.
Mick...
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phil
- Has many leather bound books
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- Joined: Tue Aug 18, 2015 6:11 pm
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Re: Making a Nest thermostat work with a two wire system
right. the instructions should specify how to connect it and maybe its best to jump to the last resort and break out the instructions at this point 
mine does similar I think my furnace has an AC transformer similar to a doorbell transformer. it used to just run to the mercury switch in the thermostat. for some reason I remember a third wire being used and can't remember why exactly, maybe it needs a constant hot for the charging. Yes I think it supplies some AC and the thermostat gets its low voltage by using a little transformer to as you put well to leech power away The electronics of the little computer do not need much power to operate and with mine if the batteries are ok I can pull it off the wall and adjust the time zones and stuff. so it will run off the batteries but I haven't
changed them in so long Im pretty sure its just operating on the power it is "stealing"
I think its just using two power wires from the transformer to provide the "leeching power" constantly and then opening and closing continuity to the other wire to trigger the heater to come on or stop. Maybe the nest one is a little more complex.
I imagine internally it is basically a computerized timer and that would be triggering a transistor to open or close this loop but you can bet if you look at the board and try to reverse engineer it it is complex enough that it isn't easy to do unless you are a bench technician or at least have a wiring diagram for the thing.
sometimes if you call a tech support line and dont; get good advice the trick is to wait and call back and get a different tech to help. The techs might have different attitudes. If you call and sound confused and you are trying a lot of goofy things they may think you dont; have the skillset to continue so maybe let them lead and just give them the info they need to avoid frustrating their train of thought.
I used to do remote support for some specialized and complex machinery so one thing I'd get good at was listening for little signs from the customer. Id usually ask them if they have a meter and if they are comfy doing some continuity checks and voltage checks and if I found I had to tell them where to put he meter switches and things like that then I'd just end it because I didn't wan the responsibility of leading a customer into a power box to troubleshoot if they didnt' have the skills to be aware of the high voltage parts also present. If they seemed to have a good grasp of things I'd be more apt to work with them and we would always make some notes on every call so sometimes Id put a little warnign in there in case notes so if someone else picked up the case when they called in at a different timethey could continue. If you want to avoid a site visit then it might help to get whoever you can to talk to them at a higher level like maybe a local electrician.
a lot of electricians don't; know much about electronics so that varies too.. If they just do house wiring their knowledge might not be great on electronics but many have some of both.
I think if one could use a scope you'd be able to probe the wires and get a better picture of what is happening. using a cheapo meter in an incorrect way could end up blowing some component and it's worth being careful. likely it has some LEDs that can be used to tell a bit more.. or maybe a point on the board labeled TP ( test point)
sometimes if I am stuck Ill just try to measure power at each point and write it down.. then later that could be good info to be able to provide, but you should be careful just shorting one pin to another with a misplacement of a probe might blow the little electronic board and then it might not be obvious you are now working on two problems , a dead board plus whatever the original problem is and that would add a lot of complexity.
if there is a cable between it might be ok to unplug that completely from both ends to check continuity through the pins in the cable itself without involving the electronics. look for things like bent pins or maybe a wire coming loose from the pins. wiggle the wire about as you check it in case it is an intemittent bad contact.
tech support may ave a policy that they don't lead the customers into the furnace wiring as higher voltages may be present with covers off and its about liability, but they might at least be more comfy with an electrician who may be able to follow their lead through some tests with at least knowledge of what not to go touching.
Micks advice to be careful is probably worth listening to and you cant; really blame the tech support people if they decide they might be leading someone untrained into where there is 110 V present as they don't want to be the cause of an accident. for a little unit worth a couple hundred the cost of a site visit might easily be higher than the cost of the unit itself.
mine does similar I think my furnace has an AC transformer similar to a doorbell transformer. it used to just run to the mercury switch in the thermostat. for some reason I remember a third wire being used and can't remember why exactly, maybe it needs a constant hot for the charging. Yes I think it supplies some AC and the thermostat gets its low voltage by using a little transformer to as you put well to leech power away
The electronics of the little computer do not need much power to operate and with mine if the batteries are ok I can pull it off the wall and adjust the time zones and stuff. so it will run off the batteries but I haven'tchanged them in so long Im pretty sure its just operating on the power it is "stealing"
I think its just using two power wires from the transformer to provide the "leeching power" constantly and then opening and closing continuity to the other wire to trigger the heater to come on or stop. Maybe the nest one is a little more complex.
I imagine internally it is basically a computerized timer and that would be triggering a transistor to open or close this loop but you can bet if you look at the board and try to reverse engineer it it is complex enough that it isn't easy to do unless you are a bench technician or at least have a wiring diagram for the thing.
sometimes if you call a tech support line and dont; get good advice the trick is to wait and call back and get a different tech to help. The techs might have different attitudes. If you call and sound confused and you are trying a lot of goofy things they may think you dont; have the skillset to continue so maybe let them lead and just give them the info they need to avoid frustrating their train of thought.
I used to do remote support for some specialized and complex machinery so one thing I'd get good at was listening for little signs from the customer. Id usually ask them if they have a meter and if they are comfy doing some continuity checks and voltage checks and if I found I had to tell them where to put he meter switches and things like that then I'd just end it because I didn't wan the responsibility of leading a customer into a power box to troubleshoot if they didnt' have the skills to be aware of the high voltage parts also present. If they seemed to have a good grasp of things I'd be more apt to work with them and we would always make some notes on every call so sometimes Id put a little warnign in there in case notes so if someone else picked up the case when they called in at a different timethey could continue. If you want to avoid a site visit then it might help to get whoever you can to talk to them at a higher level like maybe a local electrician.
a lot of electricians don't; know much about electronics so that varies too.. If they just do house wiring their knowledge might not be great on electronics but many have some of both.
I think if one could use a scope you'd be able to probe the wires and get a better picture of what is happening. using a cheapo meter in an incorrect way could end up blowing some component and it's worth being careful. likely it has some LEDs that can be used to tell a bit more.. or maybe a point on the board labeled TP ( test point)
sometimes if I am stuck Ill just try to measure power at each point and write it down.. then later that could be good info to be able to provide, but you should be careful just shorting one pin to another with a misplacement of a probe might blow the little electronic board and then it might not be obvious you are now working on two problems , a dead board plus whatever the original problem is and that would add a lot of complexity.
if there is a cable between it might be ok to unplug that completely from both ends to check continuity through the pins in the cable itself without involving the electronics. look for things like bent pins or maybe a wire coming loose from the pins. wiggle the wire about as you check it in case it is an intemittent bad contact.
tech support may ave a policy that they don't lead the customers into the furnace wiring as higher voltages may be present with covers off and its about liability, but they might at least be more comfy with an electrician who may be able to follow their lead through some tests with at least knowledge of what not to go touching.
Micks advice to be careful is probably worth listening to and you cant; really blame the tech support people if they decide they might be leading someone untrained into where there is 110 V present as they don't want to be the cause of an accident. for a little unit worth a couple hundred the cost of a site visit might easily be higher than the cost of the unit itself.