Nest Thermostat + C Connector with Oil burner and AC Unit

[ozzium]

Hello,

I had 5 different experts come to the house to figure things out but they couldn't, in fact I suspect the first one burned out my Fan Control Center unit (a transformer with a relay) that I had to replace myself, After everything was done to no avail I decided to check voltage on the transformer and verified its dead, I don't have a control board on my HVAC system instead I have an Oil Burner with two TT terminals giving out 27Volts through a +Red and a -White, and an AC unit outside giving a Red and White wires, going into a Fan Control Center unit that is a Relay/Transformer Combo , this is my setup now

Which finally worked for me, well for few days anyway, my issue is that now I'm getting random Wi-Fi disconnects, weird temperature readings (90 F inside) when it's actually 66 F, I took the thermostat off the wall checked its not overheating or the wall is hot.. replaced the batteries and put it back. Still didn't solve the issue. then I received an email from Google Nest saying that [Action needed] Nest Thermostat installation issue and that I'm using a secondary transformer into a wall outlet, which I'm not. and that they recommend me using a a Google Nest Power Connector or a C connector, which I am using as you can see in the diagram. On a different forum someone suggested a single pole relay on the TT terminals for the Oil Burner (pictured Honeywell model) but that's just for protecting the Oil burner itself over time. I'm at wit's end with this whole deal since I wanted to upgrade from a basic Emerson thermostat to this new nest. 

Thanks in advance

[ozzium · Accepted solution]

[Patrick_Caezza]

What is the make and model number of your system?

Do you have a picture of the original thermostat's wiring that you can post?

How many wires go from your system to the thermostat location?

[ozzium]

Hi Patrick, thank you for a quick reply I have the original old setup discussed in a different thread here

It should have all the original and new setup pictures, Since then as you can see in the newer replies I have cancelled the original thermostat wires(the old wires were bad upstairs coming off the wall some were hanging by the isolating plastic so the blue and I think yellow and green were bogus not connected to anything downstairs, also replaced the fan control center with a new one, bought a new thermostat wire roll and installed the thermostat closer to the furnace in the hallway downstairs with the new wires and google nest c connector.

[Patrick_Caezza]

It looks like Pjmax in the other forum has hit the nail on its heat. 

You have a dual fuel system and only the Nest 3rd Gen Learning Thermostat is compatible with it. 

The TT connection is normally just a switch connection (like a light switch). If you short those two terminals together, the oil furnace will turn on. So what you need is a relay to control the oil furnace.

Does the A/C unit also have heat? If it does, you need to wire it differently than if it is A/C only.

[ozzium]

for cooling I have an A/C unit, and for heating I have an oil heating unit. My system is a dual transformer setup, 1st one is the blower speed splice block which has a transformer providing the TT terminals on the Honeywell Oil Burner, and I have a Fan Control Center that is a Transformer/Relay Combo that is giving 4 wires to the thermostat. 

The Nest I have (Google Nest Thermostat) which is neither 3rd or E has worked for a little while (about 10 days) without problems I could control it via App or Alexa or Google home or directly. now it's reading 90F inside when its around 66F, losing wi-fi, and displaying different error codes.

Note: I'm thinking a software bug (the nest OS or firmware in the Thermostat unit) . I shut off the mains to the oil burner, to the AC, and to the Fan control (separate switches) , waited a couple of minutes then powered everything on again and so far it's been running good for the last 9 hours. 

No my A/C doesn't have heat.

[dhx227]

What does the block in the middle of your diagram represent?   Is that just a wiring block?   Or is it the terminals of a controller from your oil burner system?

What I am confused about are these statements:

"I have an Oil Burner with two TT terminals giving out 27Volts through a +Red and a -White"

"Fan Control Center unit that is a Relay/Transformer Combo"

What is confusing is where the ~24vac is actually coming from.   In reading specs from the Honeywell R7284, it is expecting to be connected to a thermostat with only R & W.   In this type of connection, it assumes the thermostat is merely acting as a relay and calling for heat merely by connecting R&W together.   The actual voltage coming out of R might be unknown and can't easily be measured without the common reference.

Then there's the "Fan Control Center" that you say also has a transformer.

One thing that will never work is to take one half the output from one transformer and connect it to the other half of another transformer.    I suspect something like that may be going on in this wiring setup.

What you probably need to do is isolate the Honeywell R7284 power from the Fan Control Center power by using a relay to control one or the other.

[ozzium]

Hi dhx227, 

The block in the middle is the Fan Control Center unit labeled screws and looks like this one

Amazon.com: White Rodgers Fan Control Center : Tools & Home Improvement

as for your other thing: "I have an Oil Burner with two TT terminals giving out 27Volts through a +Red and a -White"  is this > Honeywell R7284B1024 Electronic Oil Primary with 15 seconds lock out timing - Hvac Controls - Amazon...

The first two screws say T T, which are TT terminals, first on the left is white and the one next to it is Red and is a lead reading around 27volts when shorted with the white wire. and is getting power from the blower speed splice block under the furnace. 

My setup in a picture looks like this (without wiring):

Transformer is the block you were referring to and is the board on the Fan Control Center 

My setup currently is pictured in the first post on this thread. and has worked for 10 days before problems arose (wifi disconnects, incorrect thermostat sensor reading (90F inside when its actually around 66F), and various error codes.

without isolating the oil burner this has worked with a basic Emerson thermostat (4 wires) in the past, but if i really need to isolate the oil burner, how would I connect everything together? having A/C , Oil Heating, and a smart thermostat ?

[dhx227]

What's possibly happening is that the Nest is experiencing very subtle power instability issues due to be connected with different power transformers.   That could make things like the WiFi operate unpredictably and give uncalibrated readings from the sensors.

The main issue here is that most smart thermostats (the Nest included) are completely solid state.   Older thermostats used small physical relays inside to control the switched heating/cooling circuit and were themselves isolated from the HVAC system.   Also, the method that the Power Connector supplies power to the Nest over the two R & W/Y wires that it also is switching, is also very sensitive.  By having two transformers connected, you are introducing some very small power disturbance due to it being nearly impossible for the transformers to be in perfect phase with each other.   You have power going to the Nest from one transformer (the Fan Control Center that's connected to the Power Connector), while it is also directly connected and interacting with the power from the Honeywell R7284.   If these two power sources are even just slightly out of phase with each other, that can cause a disturbance in the power to the Nest and for it then act erratically.

I think you have a couple of options here:

A) You could try removing the isolation jumper in the Power Connector.  This is usually meant for when there is no Common available from the HVAC system and you need to instead supply power from an external transformer.   In your scenario, you could think of the Fan Control Center as this external transformer and it needs to be isolated from the switched circuitry (controlling the Honeywell).   The jumper is located under a dust cover between the two sets of wires come out of the Power Connector.   However, this may not isolate the circuitry enough.

B) Introduce a relay that isolates the Honeywell R7284 from the Fan Control Center.   The relay would then "switch" the Honeywell R7284 on/off as controlled by the Nest.  You would need a simple 24vac SPST NO relay.   The Nest R/W wires would connect to the relay power and the switched side of the relay would connect to the TT terminals on the R7284.

C) Switch to a 3rd-gen Learning Nest that already has internal isolation circuitry between the Rc and Rh power inputs.   Note, you likely would also need to upgrade your wiring to provide a C-wire and avoid the Power Connector (wiring in two Power Connectors - one for each system - would make it extra complicated).

Hope that helps.

[dhx227]

A little deeper research on this, and thinking about the Fan Control Center as an external power supply (which it pretty much is) and what you really need to do is isolate the Honeywell and then operate the Nest and A/C from the Fan Control Center.

So, based on this diagram for connecting the Power Connector in the external transformer scenario:

What you might want to try is to first re-wire the setup, but with the A/C unit left out.   Use the above diagram with the Fan Control Center as the transformer and the Honeywell as system on the right.  Be sure to have removed the jumper as mentioned above.    Then wire in the A/C unit with the yellow wire going straight from the Nest to the A/C unit's Y wire.   I know this is not how the Power Connector instructs you to wire it in a heating and cooling application, but I think it should still work and it would properly isolate the one piece that has it's own power source.

[ozzium]

Thank you for all the effort dhx227 I think I'm getting a good understanding of this thought process and it's making a lot of sense to me now considering the Fan Control Center as an external transformer, that's being said I don't know why it was installed and connected to a basic emerson thermostat in the first place, but that's aside I have a 3rd gen Nest supposed to arrive in the next few days that I purchased used off ebay, confirmed its tested and the wifi works, I know how to replace the battery if it's dead. and also another friend is willing to sell me in like a new condition an ecobee 4th gen. I will lay down the setup you provided in photoshop and follow it, will let you know how it goes. thank you again have a great weekend.

[dhx227]

Wait, are you saying the Fan Control Center is not being used to control an external system fan?   All that module provides is an external 24vac transformer and a relay packaged together so you can operate a 120/240vac fan (or whatever else you really want to).   I was assuming it was controlling a fan since you do have the (G)reen wire hooked up to it.

[ozzium]

I am not sure now that you mention it, because the green wire coming from the FCC transformer is going to the Nest thermostat, and It actually is controlling the fan ( I tried to set it to 15 minutes, turn it off, use auto, or 30 minutes) and the Nest is kicking it off so the transformer is connected to my blower fan 

here's a picture of the Blower splice board under the furnace 

[dhx227]

Well, the G terminal on the fan control center is by default connected to the relay incorporated into it (you should see another black wire going from the G terminal around to the back of the fan control center, you'll also see a similar wire from the C terminal going around back).   You would have to look at what is connected to the high-voltage wires on the back side.  Since you say it does control the fan, it sounds like those are connected to the fan blower motor.

No need to change anything, other than just to understand that is why you have a fan control center.   I would still suggest proceeding to wire it up like that is an external transformer.

[ozzium]

Now that I received the Google Nest 3rd Gen learning thermostat and the backplate has more wiring options, Rh and Rc for example, do you think I still need the C connector adapter? and how will I connect this instead of the newer nest model that I have? I will attach a picture of what I have  without connections.

[dhx227]

Yes, you need a C wire.   The only reason you really need the Power Connector is if you don't have enough wires going to your Thermostat to connect the C wire.  How many wires do you have available at the thermostat?

[ozzium]

Right now 4 are connected and 1 is tucked in,  

Red, White, Yellow, Green, and the Blue is not used for anything. 

[dhx227]

That will depend on whether you want to use an additional relay to control the Honeywell R7284 or configure the Power Connector to use the Fan Control Center as an external transformer.   You definitely should isolate the R7284 one way or another.

[ozzium]

Yes I'm planning to do this the right way

1- isolating the Honeywell R7284 with a relay (should I follow pjmax's wiring?) 

What I don't understand is why the R from the Honeywell going to the R terminal on the FCC transformer? since they both have power (24V) ? and why is the AC Red wire going to the C terminal on the FCC transformer? isn't that a common wire? and if so, shouldn't I be using that to feed the thermostat blue wire to the C slot? I'm just trying to wrap my head around things because what I'm thinking is  (Honeywell R to Relay to Rh, and W to W on FCC to W1 on the Nest) then A/C 's White to Y on FCC and Y to the nest, A/C's Red to R FCC and Rc to the Nest) then C to C, G to G , would that make sense? My apologies I'm confused , I can fix computer (hardware and software) which I do as a side job, but for some reason this is more complicated to me.

[dhx227]

Yes, that drawing is correct.

The thing I think you are not following is that the Honeywell has it's own transformer in it.   It supplies power from that transformer out one of the TT terminals and expects the "thermostat" to switch on and complete the circuit with the other TT terminal.  That's how it then knows to activate.   You can't mix the power output from one side of that transformer with some other transformer.   AC current is a lot different that DC current in this way.   If this were DC, you can take some DC+ voltage and add it to some other DC+ voltage and you'll still have some DC+ voltage.    In AC current, the current is constantly flowing back and forth between the two wires of the transformer at 60hz.   If you connect one side of that to some other transformer, you are going to mix the two oscillating currents together in some fashion.   There are generally a couple of scenarios depending on which outputs of the transformers get combined and how they are combined (eg, in parallel or in series):  1) the combination will cancel itself out and you'll have no voltage and depending on current rating some hot wires; 2) they get combined in a series and you'll get twice the voltage; 3) they get combined in parallel and you'll have twice the current.  Also another factor, especially when combined in parallel, is that no two transformers are wound exactly perfectly, and all wire has an attentuation.  Since the AC is oscillating at ~60hz, what can happen is that you get a slight offset in phase from each transformer that then can cause a "noise" problem.   Net-net: is that you don't want to combine transformers.

Which, brings up another potential issue I had overlooked - you have the two wires going to the "A/C condenser".   Are these really going just to the condenser relay, or are they going to another controller that also has it's own power supply?   The easy way to know is whether you can just short the two (red and white) wires together and does the A/C then kick in?   If so, then it is supplying power down one of those wires and expecting a "switch" to merely connect it to the other wire to call for cooling.

[dhx227]

Also note, that based on the wiring in that drawing, if you were to additionally connect your spare blue wire that goes to the thermostat to the (C) terminal on the FCC and then connect the other end of that to the (C) terminal on the Nest, you can eliminate the Power Connector.

[dhx227]

Sorry, I just realized there is an error in that drawing:  Terminal 1 from the 90-380 should be connected to (C) on the FCC.

Also, you can use either a 90-380 relay like you have in the drawing and which includes both a normally open switch (terminals 2 & 4) switch and a normally closed switch (terminals 5 & 6) ; OR you can use a 90-360 which just has the normally open switch (terminals 2 & 4).   They probably cost about the same though.

[ozzium]

Thank you dhx, does this look right?

[dhx227]

Close.   The red wire to the A/C unit should get connected to (C).    Again, this is assuming those wires go to a compressor relay or soleniod and not a control board.   The easiest way to test is to temporarily combine them together (just touch the red and white together).   If the A/C Unit starts up, then they go to a control board that is also supplying power down one of the wires and the unit is just expecting a switch.   If the A/C unit does not start up, then you can temporary connect one to (C) and one to (R).  That should then start up the A/C unit and you can proceed with your above (corrected) wiring plan.

Note: the most likely reason those wires are red and white is just because that is the most common coloring for a two-wire bundle.    Also, whenever I encounter wires that are colored a different color than is typical for their use, I put either some colored tape on them indicating their function, or even better, a label identifying it's purpose (eg, (C) or (Y)).    This is also why you can't always just assume what some wire does based solely on it's color.

[dhx227]

Additionally, if the A/C unit does start up when you connect it's two wires together, then you are just going to need another 90-360 relay and configure it just like the one for the Honeywell.   Except, terminal (3) will connect to (Y) instead of (W), and then (2) & (4) will connect to your A/C unit.

[ozzium]

"Close.   The red wire to the A/C unit should get connected to (C). "

That would mean both R's from the Honeywell oil burner and the R from the A/C will merge on the FCC C terminal before going to the thermostat?

The reason I'm asking is because I have a dual transformer system, I know the HoneyWell has its internal power (reads 27V between T1 and T2 without any wires out , T2 being the lead (hot) red wire) , and  FCC terminal board also giving out 27v ( I think outta the R terminal) ?

What about the Rh and RC on the thermostat? what's going to them? I know the thermostat can jump those automatically if it needed to. and I read on other forums that the nest prefers power coming from the AC side instead of the Burner side (this issue is addressed here https://youtu.be/-c1zlfz1IeY ) 

[dhx227]

The Honeywell should only be connected to the 90-360/380 relay.   The (1) terminal from the relay is what should be connected to (C) on the FCC.   You might consider using a piece of blue wire for that if you have that available as that would then match common color coding (and perhaps help you understand that it's common).   The red and white wires from the Honeywell are appropriately colored as red is signifying a hot power leg and white is signifying a switched (call for heat) leg.

As for the Rh/Rc on the thermostat - yes, the thermostat will jumper those automatically so it should not matter which one you plug it into since we are isolating it down to the one transformer from the FCC.

However, if the A/C unit is using a control board like I discussed above, then you alternatively have the option of using the Power Connector to isolate it.   Unfortunately, you do not have enough wires running to the Nest to use the built-in isolation features of the Nest - you would need one more wire to provide the power leg from the A/C unit all the way to the Nest.   But, please confirm whether the A/C unit is just a solenoid/relay OR if it has an independent control board before we go any farther down that path.

[dhx227]

I also just realized that you must have a voltmeter since you are reporting voltages.   Please report back whether you read any a/c voltage across the red and white wires coming from the A/C unit.

[ozzium]

Hi dhx, I finally got to test the A/C red and white wires with 3 different multimeters, and they have no power, I think when they first installed the A/C they installed the Fan control center as a transformer and relay to power on/off the A/C and the green wire for the fan.

[dhx227]

Okay, that is good.  Then you should just proceed with wiring as discussed above and use the one extra 90-360/380 relay for controlling the Honeywell R7284.

[ozzium · Accepted solution]

[ozzium]

correct?

[dhx227 · Accepted solution]

Yes, that is correct.

Note: you do not need the jumper from Rc to Rh on the Nest.  Just plug your R wire into either Rc or Rh.  The Nest has an internal jumper that it will sense is needed.

[ozzium]

Just reporting back!
Installed using the last Diagram without the jumper between Rc & Rh ,  everything seems to be working as it should, Thanks again dhx277 for all your help and support, have a great weekend.

[Ryan_G]

Hey folks,

Thanks for visiting the Google Nest Community. 

Since this thread hasn't had activity in a while, we're going to close it to keep content fresh.

If you have additional questions, feel free to submit another post and provide as many details as possible so that others can lend a hand. 

Hope this helps!

Kind regards,

Ryan