Questions about wiring Nest for two-wire boiler system

mfvreeland

I recently installed a Nest thermostat to replace a Honeywell dial controller on my old fashioned oil-powered boiler heat setup. Although the old system only required two wires (R and W), a third wire was already run by the previous installer, and I took advantage of this to run common wire for the Nest. At first, it didn't work: I ran into the same issue described in an older thread here:

https://forum.heatinghelp.com/discussion/190784/nest-loses-power-when-call-for-heat

I was able to solve my issue by following the examples in that thread, but I don't fully understand why what I did worked, and as a curious DIYer, I would like to learn more.

At the suggestion of @EdTheHeaterMan, I am starting a new discussion about my questions.

My setup has two zones controlled by valves and no zone controller — the thermostats control the valves directly. To simplify the discussion, I will just focus on the zone where I replaced the thermostat with a Nest. Before I installed the Nest, it had been wired by whoever installed it previously like this:

Transformer R ——— Valve ——— Thermostat R
Transformer C ——— Thermostat W

Adding the common wire, it looked like this:

Transformer R ——— Valve ——— Thermostat R
Transformer C ——— Thermostat W
Transformer C ——— Thermostat C

As was the case in the other thread, the thermostat received power this way, but it would fail when a call for heat was made, reporting no power before resetting itself. This would keep happening.

Following the tips in the other thread, I was able to fix the issue by rewiring as such:

Transformer R ——— Thermostat R
Transformer C ——— Valve ——— Thermostat W
Transformer C ——— Thermostat C

To get to this state without messing with any of the valve wires, I swapped BOTH R and C at the transformer and R and W on the thermostat. C to C was unchanged.

This works, but I would like to better understand one thing:

In the initial setup, it appears to me that there is always a complete circuit available from Transformer R through the valve control and exiting from Thermostat C to Transformer C. Given this, my expectation would be that it would be calling for heat constantly while configured this way, not losing power when the threshold is met. What's really happening here?

mfvreeland

Apologies for replying to a two-year-old topic, but I stumbled upon this thread when experiencing the same problem with a Nest thermostat on a similar old-fashioned setup. Changing the wiring as suggested worked for me even without adding the relays, so first of all: Thank you. I am still curious about one thing, though:

In the original setup, with R from the transformer going through a valve before landing at R on the thermostat, returning via W direct to C on the transformer, how is it that installing the common line direct from C on the thermostat to C on the transformer doesn't just constantly complete the call for heat circuit? I observed the exact same behavior as the OP: The thermostat received power, but as soon as it went to call for heat, it would lose power — and this was fixed by changing both the R and C wires at the transformer and the R and W wires on the thermostat, essentially mirroring what EdTheHeaterMan suggested in his diagram, minus the relays. I do understand why the second configuration is correct and works properly, but I don't understand what was stopping the heat in the original configuration.

What am I missing?

Jamie Hall

When the switch in the thermostat called for heat, it shorts Rh to W. Then there is no difference in voltage between Rh, W, and the way yours was wired, C, so… no power available.

Jamie Hall

Remember that a thermostat is really only a switch. Sometimes a complicated one, but… only an on/off switch. When the thermostat is off, the switch is open.

Now one side of the switch is labelled on the thermostat as "R" and the other side as "W". So, in your first illustration, you had a circuit from the transformer R (hot) side, through the valve to the thermostat R. Which, when the thermostat was not calling was a kind of dead end.

Except, in a Nest and other powered thermostats, it isn't a dead end. The internal electronics in the thermostat are powered by having 24 volts AC between R and C. In your first wiring, you have that: transformer R through the valve (which has some resistance, but much less than the thermostat electronics) to the thermostat, through the electronics and back to the transformer through C. The resistance of the valve is low enough so most of the voltage is left over to power the thermostat electronics.

Now. The thermostat calls for heat. What happens is that inside the thermostat R is now connected directly to W, and in your first illustration that goes directly to C. The thermostat gets no voltage — the elctronics are bypassed completely — the drop is now all in the valve — and it dies.

In the second illustration, when the thermostat is off W isn't connected to anything and the thermostat is powered by R and C. When the thermostat turns on, W is now connected directly to R, as before, but the voltage drop in the valve is between W and C, and R and C keep their constant voltage. Some of the power (most of it) goes through W and the valve, but some of it still goes through the electronics, through R and C. The valve and the electronics are connected in parallel. And all is well.

mfvreeland

Thank you, Jamie, for that in-depth explanation. I think you've completely cleared up why the first configuration would cause the electronics in the thermostat to lose power once R and W were connected in a call for heat. That makes sense.

Before that happens, though, is there not a complete circuit which includes the valve already? What prevents the valve from opening *before* the thermostat calls for heat? I thought that essentially all a call for heat did ordinarily (i.e., with regular thermostats) was complete this circuit with the valve. Once you add the common wire to this configuration, why isn't that circuit immediately (and constantly) complete? It must be to run the electronics, but how does it pass through the valve without activating the motor within?

Jamie Hall

One certainly might wonder just that! The answer is in the difference in the resistance of the two. The thermostat electronics have much higher resistance than the valve motor, so when they are connected in series like that, almost all of the voltage drop is through the electronics in the thermostat, and very little through the motor. And nowhere near enough voltage to activate the motor is available.

In_New_England

• In circuit A (old config) the thermostat power was effectively wired parallel to the switch (thermostat relay) which shorted itself out when the thermostat called for heat. A kind of electric harakiri if you will.

• In circuit B (the working config) the thermostat power is wired across the relay which means that when the thermostat calls for heat (switch closes) it still gets power since it is now a load in parallel to the relay.

mfvreeland

@Jamie Hall Ah ha! That was the piece I was missing. Thank you for helping me understand this. I can rest easy now -- until the next mind bender.

109A_5

Be careful with these circuits. To me it looks like the 24 VAC goes directly into the T-T, so that may be bad. Also if the zone valve end switch closes it would short out the transformer's secondary.

This would be better.

EdTheHeaterMan

OK this takes a basic understanding of how basic electricity works.  There is something called an electrical circuit.  You need three basic things and you can add some others to make the circuit but you need at least these three.  Source , Load, & Return path 

I like to use a flashlight to illustrate the basic circuit.   In a flashlight there is a source, usually a D Cell battery.  The Load is the light bulb, and the Return path is the wire that connects the light bulb back to the source.  You can add a switch to make it easy to turn the flashlight off and on. That switch can be added to the return path or between the source and the load. Anywhere that will open up the path so the light bulb can turn off. 

Fast forward in the class on electricity to the basic thermostat that has only a set of contacts. There is nothing inside it but a temperature sensitive spring that opens and closes a set of contacts.  It does not use any power to operate.  You know this because there is no C wire terminal.  

With a digital thermostat, you have both a set of contacts and a load that uses electricity.  The smart thermostat that has a couple computer chips and microprocessor, display screen and other things that use more of the electricity has both a load and a set of contacts. That is a load on the source which in this case is the 24 VAC side of the transformer. That transformer is there to control the gas valve, circulator relay, compressor relay, or whatever else needs to happen to make heat or air conditioning happen in your home.  

So the thermostat still needs to have the switch contacts that open and close, but it is also called on to power up the load side of the thermostat. All those chips, and microprocessors and screen display.  When you connect the transformer to R on the thermostat, That becomes the R side of the transformer.  If you then connect the other side of the transformer to something else that needs to be on the R side of the transformer, like a gas valve, then you have a problem.  The other items need the return path and the thermostat C needs to go to the return path.  But only one side can be the source and only the other side can be the return path.  So you end up with a conflict, and that conflict can, at times, let the factory installed smoke out of the transformer or other parts on some very expensive controls.  

By making a R on the transformer for one of the terminals and making a C on the transformer’s other terminal, then you can avoid the mistake of having conflicting source and return paths . 

In your original diagram

Transformer R ——— Valve ——— Thermostat R
Transformer C ——— Thermostat W

Adding the common wire, it looked like this:

Transformer R ——— Valve ——— Thermostat R
Transformer C ——— Thermostat W
Transformer C ——— Thermostat C

looks like this:

Now I have closed the W to R contacts as if there is a call for heat. You can plainly see in the diagram that all three thermostat wires R W and C become connected. In order to have the Load part of the thermostat working, you need R and C on the thermostat to be separate and powered with a return path, when calling for heat and when satisfied. Your first diagram fails that test. It actually makes a dead short from R to C for the on board battery in the thermostat. That will cause the thermostat to reset and start over.

If you want to read more about this problem look at this comment on how you can wire a

https://forum.heatinghelp.com/discussion/comment/1785090#Comment_1785090

Taco zone valve at least 3 different ways that work just fine. However, when you combine all three wiring styles on the same system, you end up with a Dead Short that can really mess up your day. You may need to read it 3 or more times to get the idea.

mfvreeland

@In_New_England and @EdTheHeaterMan , your explanations make this even clearer. Thank you very much for taking the time to explain it with visual aids. This is quite a helpful community!

109A_5

There is a complete circuit, since there is continuity through the motor in the zone valve. The resistance of the zone valve motor is much lower than the equivalent resistance of the NEST system logic so most of the circuits load Voltage is dropped across the NEST (during no call for heat), which is how it charges its battery on a two wire system.

Since the NEST system logic circuitry and its charging functionality is kind of a parasitic relationship there has been issues with various heating equipment. So best practice is to provide a 'C' wire or a dedicated power source. Also, for example, some Taco zone valves intentionally opens the circuit by design so the wax motor does not overheat, see illustration below. This circuit opening was also an issue for the NEST too, possibly no more, with newer units.

BTW there was a wiring error in that other thread, I posted the way it should be.

HeatingHelp.com

@mfvreeland, we've merged your comments from the older thread into your new discussion here.

@EdTheHeaterMan, thanks for the heads up!