I reconciling an apparent contradiction with thermostats and 24V

[fourthindiana]

TL;DR Summary

There was 28 volts from R to Y. Why was there zero volts from C to Y?

The attached photograph contains a diagram that represents my understanding of how thermostats are wired internally. I recently did six experiments with the low voltage wires in the air handler of my heat pump. Here are the results:

Experiment 1# With the heat pump running in cool mode, R to Y was zero volts.

Experiment 2# With the heat pump running in cool mode, C to Y had 27 volts.

Experiment 3# With the heat pump running in cool mode, R to C had 27 volts.

Experiment 4# After the thermostat was satisfied, R to Y had 28 volts.

Experiment 5# After the thermostat was satisfied, C to Y had zero volts.

Experiment 6# After the thermostat was satisfied, R to C had 28 volts.

The contradiction I see here is between experiments #4 and #5. It might be the case that I see a contradiction because I am relying on a false premise of some sort. My understanding is that when the thermostat is satisfied, the switches are like the switches in the diagram in the attached photograph, with all the switches open. I know that when one reads voltage on each side of a wire across an open switch, one gets a voltage reading, not zero volts. I also know that when one reads voltage on each side of a wire across a closed switch, one gets zero volts because there is no potential difference between the two points. In experiment #4, the switch from R to Y was open. Therefore, I understand why there were 28 volts from R to Y. However, on experiment #5, C is receiving 24 volts from the transformer, and Y is not receiving any volts from the R side of the transformer. Therefore, on experiment #5, I would have expected there to be 24+ volts from C to Y since C is receiving 24 volts from transformer, and C is not connected to Y. On experiment #5, why was there zero voltage from C to Y?

___________________________________________________________________________-
P.S. I think I should mention that I don't know for a fact that my thermostat is wired up like I depicted my thermostat in the diagram in the attached photograph. That might be a false premise that I am relying on. The diagram in the attached photograph is just my understanding of how my thermostat is wired up.

 

[PeterDonis]

The diagram in the attached photograph is just my understanding of how my thermostat is wired up.

This diagram looks incomplete to me. A thermostat has to have a temperature sensor somewhere, or at least something that opens and closes electrical contacts based on temperature. I don't see any such thing in your diagram. I also see a bunch of points just hanging in the air, which doesn't make sense.

 

[fourthindiana]

This diagram looks incomplete to me. A thermostat has to have a temperature sensor somewhere, or at least something that opens and closes electrical contacts based on temperature. I don't see any such thing in your diagram. I also see a bunch of points just hanging in the air, which doesn't make sense.

I am assuming that the "bunch of points just hanging in the air" refers to the Y, W, O, and G terminals on the thermostat. I did not depict the wires going from the Y, W, O, and G terminals to the air handler and the outdoor unit in the interests of brevity and simplicity. Please just imagine that there are wires going from the Y, W, O, and G terminals to the air handler and to the outdoor unit. The imaginary wires going from the Y, W, O, and G terminals to the air handler and to the outdoor unit are irrelevant to the question of this thread.
Please just imagine that this thermostat has a thermistor for a temperature sensor. You can answer the question that is at the heart of this thread without these extraneous details.You can still answer the question of this thread.

 

[PeterDonis]

I am assuming that the "bunch of points just hanging in the air" refers to the Y, W, O, and G terminals on the thermostat.

Yes.

I did not depict the wires going from the Y, W, O, and G terminals to the air handler and the outdoor unit in the interests of brevity and simplicity.

How can you expect anyone to correctly analyze the behavior of a system if you leave out information?

Please just imagine that there are wires going from the Y, W, O, and G terminals to the air handler and to the outdoor unit. The imaginary wires going from the Y, W, O, and G terminals to the air handler and to the outdoor unit are irrelevant to the question of this thread.

Connections to the air handler and outdoor unit might be irrelevant, yes. But there's something you're leaving out that is clearly not irrelevant: where is the temperature sensor and what is it hooked up to?

Please just imagine that this thermostat has a thermistor for a temperature sensor.

Connected to what? A thermistor doesn't work by magic. It works by having electrical connections to other parts of the system. Where are they?

You can still answer the question of this thread.

No, I can't, not with the information you have provided so far. And since you obviously can't answer it with the information given (since you posted it), I don't see how you can possibly believe you know what information is needed to answer the question, and that no other information is required. Wouldn't it be a lot easier to just assume that, if other people, who you are asking for help, are asking for more information, they might have some legitimate reason for doing so? Something like, oh, let's say, needing more information to answer the question?

 

[fourthindiana]

How can you expect anyone to correctly analyze the behavior of a system if you leave out information?

I addressed that in post #3, IMO.

Connections to the air handler and outdoor unit might be irrelevant, yes. But there's something you're leaving out that is clearly not irrelevant: where is the temperature sensor and what is it hooked up to?

The temperature sensor is a thermistor in the thermostat. I don't know what it is hooked up to. I would hazard a guess that the temperature sensor is hooked up to Y. WOuldn't that make the most sense? Do you know what the temperature sensor is hooked up to on most residential thermostats? Why don't we just assume that the temperature sensor is hooked up to whatever a temperature sensor in a residential thermistat is normally hooked up to?

I completed a 1-year long HVAC program at a trade school. I saw my instructor make dozens of electrical diagrams during my time at the trade school. I never have seen my instructor or anyone else anywhere depict the thermistor in a thermostat on any type of wiring diagram. No, not once.

No, I can't, not with the information you have provided so far. And since you obviously can't answer it with the information given (since you posted it), I don't see how you can possibly believe you know what information is needed to answer the question, and that no other information is required. Wouldn't it be a lot easier to just assume that, if other people, who you are asking for help, are asking for more information, they might have some legitimate reason for doing so? Something like, oh, let's say, needing more information to answer the question?

WHy don't you just assume that the thermistor is hooked up to whatever a thermistor is normally hooked up to in a residential thermistat? You can do it.

 

[PeterDonis]

I addressed that in post #3, IMO.

Well, if you claim the question can be answered based on the information given in your OP, then why don't you just answer it?

The temperature sensor is a thermistor in the thermostat. I don't know what it is hooked up to.

Well, then that seems to me to be a vital piece of information you are missing, since where the thermistor is hooked up to is going to determine what electrical connections are open or closed when the thermostat is satisfied or not satisfied.

I would hazard a guess that the temperature sensor is hooked up to Y.

It might make sense for one side of it, yes. What about the other side?

I completed a 1-year long HVAC program at a trade school. I saw my instructor make dozens of electrical diagrams during my time at the trade school. I never have seen my instructor or anyone else anywhere depict the thermistor in a thermostat on any type of wiring diagram. No, not once.

Ok, fine. Then why can't you answer the question yourself?

WHy don't you just assume that the thermistor is hooked up to whatever a thermistor is normally hooked up to in a residential thermistat?

I don't know what a thermistor is normally hooked up to in a residential thermostat. I've never had occasion to try to figure out how one works since I've never had one malfunction and reverse engineering one has never been high enough on my list of things to do.

You can do it.

Sorry, I can't, not with the information given. I might be able to figure out where the thermistor might be hooked up if I took enough time to consider it, but I don't have the time right now. But you've got the thermostat in front of you; can't you look at it and see where the thermistor is hooked up? Why should I have to try to figure it out if you've already got the information in front of you?

 

[fourthindiana]

Well, if you claim the question can be answered based on the information given in your OP, then why don't you just answer it?

I am no prima dona. I don't know the answer.

Well, then that seems to me to be a vital piece of information you are missing, since where the thermistor is hooked up to is going to determine what electrical connections are open or closed when the thermostat is satisfied or not satisfied.

I will hazard a guess as to how the thermistor is hooked up.

It might make sense for one side of it, yes. What about the other side?

The other side would be hooked up to R.

Ok, fine. Then why can't you answer the question yourself?

I am far from a master at this stuff.

Sorry, I can't, not with the information given. I might be able to figure out where the thermistor might be hooked up if I took enough time to consider it, but I don't have the time right now. But you've got the thermostat in front of you; can't you look at it and see where the thermistor is hooked up? Why should I have to try to figure it out if you've already got the information in front of you?

I am attaching a photograph to this post which has a diagram that shows how I would guess that the thermistor is wired up to the transformer.

The diagram in the above photograph is my supposition of how the temperature sensors in a thermostat are wired up to the thermostat. The thermistors in the diagram are the temperature sensors in the thermostat. The thermistors in the diagram act as temperature switches.

 

[PeterDonis]

I completed a 1-year long HVAC program at a trade school.

Did the program cover what the typical meanings of the letters labeling thermostat circuit diagrams are? The R, Y, C, etc?

 

[PeterDonis]

I am no prima dona. I don't know the answer.

Then I would strongly suggest that you not try to tell someone else what information they need to answer the question.

 

[PeterDonis]

The diagram in the above photograph is my supposition of how the temperature sensors in a thermostat are wired up to the thermostat.

Ok, so it looks like the "switch" between R and Y would actually be a thermistor (which would make sense in the light of your #1 and #4). Is that consistent with what you see visually inside the thermostat? (A photo of the inside of the thermostat would be very helpful.)

Why do you think there would also be thermistors between R and W and R and O? (And why not between R and G?) Also, why are you labeling the line from C to the transformer as "neutral" and the one from R to the transformer as "24 volts"? Please note that I'm not saying you're wrong, I'm just trying to understand your reasoning, since you have the thermostat and transformer and air handler right there and I don't.

 

[fourthindiana]

Did the program cover what the typical meanings of the letters labeling thermostat circuit diagrams are? The R, Y, C, etc?

Yes.

The R wire divvies out 24 volts to Y, O, G, and W when called upon by the thermostat.

The Y wire controls the compressor.

The O wire controls the reversing valve.

THe G wire controls the indoor blower motor in the air handler.

Usually a heat pump does not have a W wire, but the heat pump I worked on at my job the other day did have a W wire that controlled the heat strips.

 

[fourthindiana]

Ok, so it looks like the "switch" between R and Y would actually be a thermistor (which would make sense in the light of your #1 and #4). Is that consistent with what you see visually inside the thermostat? (A photo of the inside of the thermostat would be very helpful.)

I don't have the thermostat in front of me to look. Besides, I am not really sure what the thermistor would look like exactly.

Why do you think there would also be thermistors between R and W and R and O?

Because W is the heat strips, which are controlled by temperature. Now that I think about it, there would not be a thermistor between R and O. O would be controlled by a person pressing buttons on the thermostat.

(And why not between R and G?)

G would be wired to come on whenever the heat strips are on and whenever the compressor is on. There would not be a need for a temperature sensor on G.

Also, why are you labeling the line from C to the transformer as "neutral" and the one from R to the transformer as "24 volts"? Please note that I'm not saying you're wrong, I'm just trying to understand your reasoning, since you have the thermostat and transformer and air handler right there and I don't.

I started communication with a co-worker of mine who is an HVAC technician AFTER I made the OP of this thread. When I created this thread, I did not know that the C wire coming out of a transformer is neutral. Now that I know that the C wire coming out of a transformer is neutral, I don't think that there is a contradiction any more.

I learned today that the low voltage side of all transformers on residential heat pumps have one hot leg and one neutral leg. Everyone labels the R wire coming out of the transformer as the hot leg. The hot leg of the low voltage side of the transformer is 24 volts. The neutral leg coming out of the transformer is always labeled as C.

 

[PeterDonis]

The R wire divvies out 24 volts to Y, O, G, and W when called upon by the thermostat.

The Y wire controls the compressor.

The O wire controls the reversing valve.

THe G wire controls the indoor blower motor in the air handler.

Usually a heat pump does not have a W wire, but the heat pump I worked on at my job the other day did have a W wire that controlled the heat strips.

Ok, good, this is all helpful information. Have you checked the voltages between R and O, G, or W under the different conditions?

When I created this thread, I did not know that the C wire coming out of a transformer is neutral. Now that I know that the C wire coming out of a transformer is neutral, I don't think that there is a contradiction any more.

I learned today that the low voltage side of all transformers on residential heat pumps have one hot leg and one neutral leg. Everyone labels the R wire coming out of the transformer as the hot leg. The hot leg of the low voltage side of the transformer is 24 volts. The neutral leg coming out of the transformer is always labeled as C.

OK, so does this mean the OP question has been answered?

 

[fourthindiana]

Ok, good, this is all helpful information. Have you checked the voltages between R and O, G, or W under the different conditions?

No.

OK, so does this mean the OP question has been answered?

Yes. The (false) contradiction was that with the thermostat (stat) satisfied, there was 28 volts between R and Y, but there was zero volts between C and Y. When I first created the thread yesterday, I thought that with the stat satisfied, the voltage between R and Y should be the same as the voltage between C and Y. But that was all based on my false premise that R and C are both hot legs of power. Now I know that R is a hot leg, but C is neutral. Since C is neutral, it makes sense that with the stat satisfied, there is zero voltage between C and Y, even though there is 28 volts between R and Y.

 

[sophiecentaur]

I never have seen my instructor or anyone else anywhere depict the thermistor in a thermostat on any type of wiring diagram. No, not once.

The 'thermostat' in basic domestic equipment is usually an on/off switch which is operated by a (mechanical) bimetal strip. There is no use for a thermistor unless the control is proportional and that simple diagram doesn't describe the system that way. Perhaps the OP has been railroaded into considering the presence of a 'thermistor'. If the thermostat has a part / model number then we could find out what it actually does and the functions of the contacts.
It has to be said that the circuit details in typical service manuals for typical equipment are always sketchy and they don't tend to follow the standard (ISO) practice. This doesn't help at all except when a 'service engineer' was 'taught by numbers', rather than on an Electronics Theory course. Two cultures exist.

 

[fourthindiana]

The 'thermostat' in basic domestic equipment is usually an on/off switch which is operated by a (mechanical) bimetal strip. There is no use for a thermistor unless the control is proportional and that simple diagram doesn't describe the system that way.

Do most residential digital thermostats have an on/off switch that is operated by a mechanical bimetal strip?

Perhaps the OP has been railroaded into considering the presence of a 'thermistor'. If the thermostat has a part / model number then we could find out what it actually does and the functions of the contacts.

My thermostat is a digital Carrier brand thermostat. I cannot give you anything more specific than that at this time.

It has to be said that the circuit details in typical service manuals for typical equipment are always sketchy and they don't tend to follow the standard (ISO) practice. This doesn't help at all except when a 'service engineer' was 'taught by numbers', rather than on an Electronics Theory course. Two cultures exist.

I don't know what you mean here. Do the circuit details in typical service manuals for typical residential thermostats accurately reflect the electronics inside the thermostat or not?

 

[sophiecentaur]

Do most residential digital thermostats have an on/off switch that is operated by a mechanical bimetal strip?

No but they have simple switched contacts at the output (which was my real point - no proportional output). Any 'proportional control' for advanced domestic heating is based on duty cycles of an on and off cycle. My comment still holds in the context of the OP.

My thermostat is a digital Carrier brand thermostat. I cannot give you anything more specific than that at this time.

Doesn't it have some identifying mark on it? How could a spare be ordered without some information. If there are actual screw terminals on it, you can find which pins are connected and not-connected as you operate the control around the existing room temperature.

I don't know what you mean here. Do the circuit details in typical service manuals for typical residential thermostats accurately reflect the electronics inside the thermostat or not?

Of course not; it doesn't need to. The diagram will normally have some indication of one or two switch contacts inside, perhaps, a rectangle drawn with a dashed line to show the relevant function. This is just the same as the idea behind the functional diagram for a system with subsystems shown as boxes with numbered pins with abbreviated names.

But that is a diversion. You have referred to "thermistors" (i.e. Inputs) early on but the connections seem to be Outputs - which makes sense. Imo, you should be measuring all your volts between C and the terminals. That will tell you what is supposed to be switched on or not. When you measured 24V between R and Y, for instance, that just means that one was powered and the other was not.

Summary: There was 28 volts from R to Y. Why was there zero volts from C to Y?

Experiment 4# After the thermostat was satisfied, R to Y had 28 volts.

Experiment 5# After the thermostat was satisfied, C to Y had zero volts.

That means that Y is either connected to R or to C (or possibly just disconnected from R). No confusion if you understand what is 'really' going on.

What you wrote later shows that you are beginning to get a hold of the situation but you can do better if you present your data more suitably.
I suggest you re-write your results with actual names as well as contact letters and leave out the non-relevant voltages. That will tell you the story of what is on and what is off, according to the conditions.
If the 'Thermostat' box has no inputs then it is the only source of information is a single Temperature. If one of the lettered contacts is fed from another sensor then you can draw up a 'truth table' to show the results from different combinations of the two inputs (Temperature high enough and the value of the other input).

 

[Averagesupernova]

I did a quick Google and in all the diagrams that show up I fail to see a thermistor. Trust the OP when he tells you drawing in the thermistor is irrelevant. Thousands of HVAC techs work on these daily and understand the switching action and yet have no idea what makes the switch contacts move other than the obvious answer of temperature.
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https://www.google.com/search?q=the...KHY4lDyAQ_AUoAXoECA8QAQ&biw=360&bih=512&dpr=3

 

[fourthindiana]

Doesn't it have some identifying mark on it? How could a spare be ordered without some information. If there are actual screw terminals on it, you can find which pins are connected and not-connected as you operate the control around the existing room temperature.

On the circuit board inside the thermostat, it says TC-NHP01 1311N049453.

I think it is the TC-NHP01-A thermostat.

https://www.carrier.com/residential/en/us/products/thermostats/non-wifi-thermostats/tc-nhp01-a/

 

[sophiecentaur]

Thanks for that. The documents make it clear what's supposed to happen. It just remains to confirm or disprove that it's doing the right thing.
This link
https://resource.carrierenterprise....arrier_tb-nhp01-a_article_1463560348148_en_ii
gives you an installation manual that I found. It's fairly full of useful stuff. It's much simpler than I feared, at first.

 

[fourthindiana]

Thanks for that. The documents make it clear what's supposed to happen. It just remains to confirm or disprove that it's doing the right thing.
This link
https://resource.carrierenterprise....arrier_tb-nhp01-a_article_1463560348148_en_ii
gives you an installation manual that I found. It's fairly full of useful stuff. It's much simpler than I feared, at first.

The manual at that link does not say what type of temperature sensors that the thermostat uses.

 

[sophiecentaur]

The manual at that link does not say what type of temperature sensors that the thermostat uses.

Why would that matter - unless you are planning to open up the thermostat unit? Not an easy task; a step up in difficulty. It’s the functionality that counts. Why would you expect sensors plural in just one box?
Is there still a conflict with results in the light of my suggested different approach?

 

[fourthindiana]

Why would that matter - unless you are planning to open up the thermostat unit? Not an easy task; a step up in difficulty. It’s the functionality that counts. Why would you expect sensors plural in just one box?
Is there still a conflict with results in the light of my suggested different approach?

The manual at your link does not say what the functionality of the switches is in the thermostat.

 

[Asymptotic]

Summary: There was 28 volts from R to Y. Why was there zero volts from C to Y?

However, on experiment #5, C is receiving 24 volts from the transformer, and Y is not receiving any volts from the R side of the transformer. Therefore, on experiment #5, I would have expected there to be 24+ volts from C to Y since C is receiving 24 volts from transformer, and C is not connected to Y. On experiment #5, why was there zero voltage from C to Y?

'C' is common. 'Y' is cooling output.
In experiment #5 (thermostat satisfied), neither heating or cooling was commanded on.
No voltage would be expected between 'Y' cooling output and common because the cooling output relay within the thermostat would not be turned on.

One side of the 24V transformer secondary coil is connected to 'C' (common), and the other to 'R' (24V 'hot').
Supply voltage (nominally 24V) should always exist between 'C' and 'R'.

24V will be present at the 'W' (heating output), 'Y' (cooling output), and 'G' (fan output) whenever the thermostat's internal logic decides those functions ought to be on.

Table 1 from the manual shows which outputs ought to be on, depending on how it is configured.

 

[sophiecentaur]

@fourthindiana does that table help you? Your OP mentions inconsistency with what you expect but is there an actual fault?

 

[fourthindiana]

@fourthindiana does that table help you? Your OP mentions inconsistency with what you expect but is there an actual fault?

The table does not help me because I already knew that information. I reconciled the apparent contradiction with thermostats and 24 volts late Sunday afternoon. I mentioned that I reconciled the apparent contradiction on post #12, so I knew the answer to the OP before you ever posted on this thread.

My purpose on my posts to you (you specifically) on this thread has been to learn about the internal switches in my thermostat, not the purpose of the OP.

 

[sophiecentaur]

My purpose on my posts to you (you specifically) on this thread has been to learn about the internal switches in my thermostat, not the purpose of the OP.

Ahah - so the thread has shifted. Your question is still a bit too vague for a good answer. I think that the details of what's inside that box are absolutely anyone's guess. There will likely be a processor in there that follows some complicated algorithm that tries to second guess what will happen to the temperature as the surroundings change in relation to the heater and fan etc. actions. I don't think there would be a simple on/off feature, with a binary decision about the measured temperature being above or below the desired temperature.

I have a couple of room thermostat units that control the radiator valves in two rooms. They use proportional control (the radiator valves are proportional) and it takes a couple of days for the system to settle down. Once it does, the temperature around the thermostat sticks to what you order within a fraction of a degree - there is none of the wild swings that you used to get with simple mechanical control.

If you really want to get into climate control systems then you need to start at the beginning, rather than to try to reverse engineer a specific unit that you have. That would be the equivalent of learning how computers work by looking at a circuit diagram of your laptop - no chance.

If you want to find out about logic circuits then start off with basic reading and then progress to simple construction projects.

Do a search for domestic climate control system strategies and avoid the manufacturers' adverts. You would need to be very lucky to find a PF member who happens to know about such specialist stuff - for a start, there haven't been any responses yet. But you never know; not everyone looks at PF every day.