I've managed to add a C wire to my thermostat. I then decided to check the voltage between the R and C wires, and find that it read as 27.8 volts, not the 24 volts it was supposed to be. Could this cause problems? I do have a Lux thermostat which is having trouble with it's ambient temperature reading, but I'm not sure if it's because of the voltage or something else.
Is voltage too high in C wire going to thermostat
hvacthermostatthermostat-c-wire
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This has been idle a while, but my guess is you need to connect the Nest to the R terminal of thermostat 1 on the controller instead of R1.
It looks like your control system was designed and wired for a heatpump but you no longer have one (there are only 4 wires on the lower left block which I think is the output to the furnace/AC compressor). If so the O and B wires are unneeded and can be removed from the control panel.
So what I would try:
- Zone 1 R to Nest Rh (24VAC Hot)
- Zone 1 C to Nest C (24VAC Common)
- Zone 1 W to Nest W1 (Heat)
- Zone 1 Y to Nest Y1 (Cool)
- Zone 1 G to Nest G (Fan)
Only thing I don't understand is how the fan gets turned on by the second thermostat in Cool mode. (In cool usually the thermostat has to tie both G and Y to R (24VAC Hot), in heat only W has to be tied to R (+24VAC Hot)... Since it works it must be something the controller does automatically.
By connecting the Nest to the C terminal it gets access to 24VAC without trying to power it self from "leakage current".
It you want to test, jumper Zone 1 R to Y for a minute and see what happens. Does the compressor come on outside? Does the interior blower fan?
Double check this against the manuals..
clarification as to how it is supposed to work.
There are so many variations within and between countries that it is hard to answer without knowing more about your system
Where I live, a very common system is Honeywell S-Plan and variations of this
L = hot/live
CH = Central Heating to radiators (pumped hot water).
HW = Hot Water to faucets/taps for sinks/basins/baths etc.
In S-Plan, electric power only reaches the furnace (AKA boiler) after passing through a series of devices:
- power from the supply is connected to a timer/programmer
- during set times, the timer/programmer allows power to the room thermostat
- when temperature is low, the room thermostat allows power to the zone valve
- when the zone valve is fully open, it allows power to the pump and furnace/boiler
Some furnaces/boilers have an independent power supply and control the pump so that they can overrun to circulate water to remove residual heat from the furnace/boiler.
None of the domestic systems I've had in my homes have ever needed to start the furnace/boiler unless the thermostat is calling for heat. I changed my room thermostat to a model with an indicator light that comes on when it is calling for heat.
The system described above is a high-voltage system, I believe many US systems use a completely different low-voltage control arrangement.
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Best Answer
It's a bit high but whether it will be a problem depends on your thermostat and how it tolerates the high voltage. Also consider that the furnaces controller board is also receiving this voltage.
Before you start anything else, check to see if your thermostat can be calibrated and complete this step first.
To rule out the higher voltage as the cause of the faulty temperature reading, find another transformer that gives you close to 24VAC and connect it to R and C - basically use that power source instead of the transformer in the furnace. If the temperature reading is correct now then you have verified the cause. If other transformers are also giving higher than expected voltages, check your input line voltage - if that is high, so will the transformer output.
If line voltage is OK and other transformers are OK then you can replace the transformer in your furnace; it's a pretty straight forward replacement usually. Just remember to turn off the power to your furnace first!