There's no C-wire from my boiler to the old Honeywell Thermostat. I need to replace this with a wi-fi enabled thermostat -- so as to see the temperature when away. The boiler is merely switched on and off with the old Honeywell.
Almost all wi-fi enabled thermostats need the Common to provide 24VAC. It's probably rectified and used to power the transmitter/receiver. I wonder whether it just might be easier to use an SMPS chip and little transformer to run off the "AA" cells, rather than snaking the "C" and "Rc" wires. Probably replace the alkaline "AA" cells with higher mAh units.
Almost all wi-fi enabled thermostats need the Common to provide 24VAC. It's probably rectified and used to power the transmitter/receiver. I wonder whether it just might be easier to use an SMPS chip and little transformer to run off the "AA" cells, rather than snaking the "C" and "Rc" wires. Probably replace the alkaline "AA" cells with higher mAh units.
Now I am confused. There may be 24 volts AC across the existing thermostat.
The thermostats I use have back up batteries inside so while the furnace is turned on the batteries really power things. When off it runs on the 24 VAC at low enough current it doesn't trip the furnace relay.
If the latest whiz bang gizmo really needs a non stop current I would consider a wall wart. But I suspect it does not or power outages would scramble its' brains.
The thermostats I use have back up batteries inside so while the furnace is turned on the batteries really power things. When off it runs on the 24 VAC at low enough current it doesn't trip the furnace relay.
If the latest whiz bang gizmo really needs a non stop current I would consider a wall wart. But I suspect it does not or power outages would scramble its' brains.
I put in a couple zwave thermostats recently, same issue, need 5 wires including the c wire.
I had 5 wire cables running from my heaters, but the C wire wasn't connected. I connected the C wire at the heater end, and then all was good for me.
I've seen little converter gizmos that combine and then separate the signal, so it lets you connect a thermostat even if you only have 4 wires running between heater and thermostat. 4 or 5 wires is for a combined AC/heater system, single speed.
I would think you would go through batteries pretty quickly with a wifi thermostat.
From what I understand, there is 24VAC coming from the heater, but for some reason older thermostats only used one leg of the 24VAC. The newer wifi/zwave thermostats need both legs. So not sure how you get around this easily.
I had 5 wire cables running from my heaters, but the C wire wasn't connected. I connected the C wire at the heater end, and then all was good for me.
I've seen little converter gizmos that combine and then separate the signal, so it lets you connect a thermostat even if you only have 4 wires running between heater and thermostat. 4 or 5 wires is for a combined AC/heater system, single speed.
I would think you would go through batteries pretty quickly with a wifi thermostat.
From what I understand, there is 24VAC coming from the heater, but for some reason older thermostats only used one leg of the 24VAC. The newer wifi/zwave thermostats need both legs. So not sure how you get around this easily.
They used to be "just switches". Same as a light switch on the wall. US light switches break the Black, do not need the White in the switch box. (OK, when you buy only Black/White pair, one of the White is re-identified as a Black, which shows it is not the "neutral".)
See attached.
This works fine with no "smarts" in the thermostat. It even works with a "wee bit of smarts": a bit of resistance wire uses the solenoid current to warm the thermo-coil and shut-off a little early so the last bit of heat does not over-shoot much.
When the burner is OFF, a *small* amount of 24V current may be drawn by the thermostat, which was often used to trickle a battery to hold "smart" settings through the burner-ON cycles. If this vampire current is not small enough, the fire solenoid chatters or turns-on.
Some estimations of typical solenoid action suggests a half-Watt is possible, though maybe not for "all" solenoids. And the typical user or even installer will have no clue what the problem is.
Wi-Fi may need this much or more? Or less? I am not hip to this.
When I re-did my HVAC, I had 2-wire heat-only cable. Pretty ratty stuff. And I could see that 5 or 6 wires were now common, especially since I was adding AC. No two ways about it: I drilled up through the floor and along molding into the wall behind the thermostat location. Nicer (still cheap) wire, 7-core.
Yes, in principal a 24VAC wall-wart at the tstat across R and C should power the brains and waves. I'm less sure what happens if one or the other 24VAC source fails. (My furnace 24VAC is wired to 'fail' if the condensate pump gets full. This can happen summer or winter because the burner is condensing. Beats a puddle in the cellar; worse if it was mounted above the ceiling like southerners do.)
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The Nest thermostat does not need a C wire. The Ecobee comes with a device that installs at the furnace and allows for an installation without a C wire.
Let's have a look here and see what the C wire is all about:
Thermostat Wiring Explained
So, if you look at the 4th blue image - you will see that C is a common AC phase in a two-stage heating system. It's not ground, it's not earth referenced - it's common to both transformers in a two-stage system...
I would just suck it up and fish a new wire...
This is on of many reasons I prefer PoE (Power over Ethernet) to Wifi.
What does this have to do with it? We are talking about a thermostat. As I said there are options where you do not even need the C wire.
Support | Nest
In the interim, I'll snake a pair of wires.
It's got me reading some stuff from Linear Tech on "energy harvesting".
Right. The Nest attempts to steal power in idle periods without triggering furnace operations. If this works, no C is needed. If it does not work as hoped, your gas relay chatters, your zones go nuts, it works when cold but when COLD the long run-times let the Nest battery go flat, etc.
I'd think if you are running a wire, it is about the same labor to run a full 7-wire cable. Less confusion for the next guy. (Which may be you some cold morning.)
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