[jstricks]

Hi everyone. I am trying to connect the Common wire to my Nest thermostat, so it charges the battery. Fortunately, the therm wire has a 5th wire running from therm to the air handler. It's currently disconnected at the therm. What is confusing me is that this 5th blue wire is connected to the R terminal on the air handler.

There is a second wire also connected to the air handler's R terminal, which travels a bit and then connects to the Rh line from thermostat and the R from the boiler at a wire nut.

Outline of the Thermostat Wiring

G (therm) connects to G (air handler)
Rh (therm) connects to R (air handler) and R (boiler). These three Rs connect in the 'junction' photo I've linked to below.
C (therm) was connected to the R (air handler); I disconnected the side at the thermostat; other side is still at R on air handler
W (therm) <--> W (boiler)
Y (therm) <--> Condensation pump's Red. Condensation pump's White goes offscreen to A/C compressor Red. Compressor's white connects to air handler's Common terminal. From what I understand, this is done so that if condensate tank overfills, 24vac is removed from the A/C compressor.

(I'll post a link to photos of the wiring in a minute.)

I've been cautioned that this situation is not optimal because the boiler R and the air handler's R are connected to each other and go to a single Rh at the thermostat. That's worked well because there isn't a Common.

So the main question is how to determine whether I can disconnect the Blue 5th wire that runs to the thermostat from the air handler's R terminal, and move it to the C terminal. Then attach the other side to the C terminal on the Nest. Or would this compromise the systems because the boiler and air handler would now both be connected at the C and R.

Thanks so much for the help.

 

[jstricks]

Here is a Photobucket link with images of:
Air handler connections and notes
Junction (air handler, boiler, thermostat lines) with and without notes

s482.photobucket.com/user/jstricks/library

 

[beenthere]

Doesn't the Nest have an RH and RC terminal.

 

[jstricks]

It does, but I don't have separate lines from the boiler and air handler running to it. The house was bought with the two systems' reds connected, and then a single red line run to the thermostat connected to Rh. That's been the setup for years with no problems. Just trying to confirm that connecting the Common line won't upset the balance.

 

[yuri]

Normally a boiler comes with it's own transformer and controls and so does the air handler.

Then you run R from the boiler to Rh and R from the air handler to Rc and remove the jumper between them. Both of them have a C as each has it's own transformer. W/O being there we cannnot see what kind of homemade wiring the last person did. Sounds wrong to me.

No you cannot assume damage won't occur. I would recommend you get a experienced heating tech to trace the wiring and see what you have to be safe.

 

[jstricks]

Thanks Yuri. I have traced the wires, maybe these images and mores help.

Here is a Photobucket link with images of:
- Air handler connections and notes
- Junction (air handler, boiler, thermostat lines) with and without notes

http://s482.photobucket.com/user/jstricks/library

 

[jstricks]

Sorry, meant 'notes' not 'mores.'

 

[jstricks]

Over and over again, I read about examples where the boiler and air handler share a connection at Rh. Nest even directly addresses it here: https://nest.com/support/article/Ho...rmostat-had-a-short-wire-connecting-Rh-and-Rc

How could that be the case unless the boiler R and air handler R were connected together?

 

[beenthere]

No, it says nothing that they share the RH. It says they each go to their own respective terminals, like on most thermostats.

Boiler-RH
Cooling-RC

 

[oldbikes]

What it looks like to me is that someone has hooked the 2 transformers together and knew enough to get them in the same phase. This works however if a transformer is replaced or any wires are changed then you have the penitential to create 48 volts which will cause damage to components attached to the circuit.

The best way to deal with this is to use more conventional wiring methods or make sure that 24 volts is all that is present.

Phasing is simply accomplished by trying different wire combinations from the transformers with a meter until the output is 24 volts. What is happening is that the 2 transformers are sharing the same wire which works until something goes wrong and then it is a bit more complicated to repair.

The short answer to your question is if you take a meter and the potential between the r terminals at the thermostat and the c wire hooked to the transformer in the air handler reads 24 volts it should be fine, however it is risky to do this.

 

[beenthere]

The NEST thermostat is a bit touchy. Doing that could work for a week. And then stop working with no advance symptoms, and not work a gain.

 

[yuri]

It is ALWAYS best to have it wired properly. Saves you a LOT of $$ when troubleshooting it later with a Pro. If I run into some homemade hacker wiring it takes me 2-3X longer at their expense to figure out what they did wrong and then we have to rewire it anyway. I would get some 18/6 LVT wire from HDepot on a roll and wire the boiler and air handler properly.

 

[supers05]

Will it work? As you described it, and you haven't missed anything, then yes it will work. As it's been mentioned, the 2 transformers are in parallel. I'm on the side of Yuri and Been, and would pull a new cable.

Make sure you don't forget that you have 2 transformers hooked up like this when things don't work. Any pro, or us would need to know that for troubleshooting reasons. You're also more likely to blow both transformers if something shorts instead of just one.

Cheers!