Anybody hooked up an external power supply for a Wi-Fi thermostat?

[MMM]

Hey all,

I'm shopping for a thermostat for my newly rebuilt house, and would like to get a Wi-fi enabled one to allow monitoring during travel. The thing is, these things draw the larger power required for Wi-Fi circuit from a 24V AC power source (sometimes provided by existing furnace wiring and called a "c" wire).

My new setup doesn't have that, since it's a radiant heat system. You just connect two wires to trigger the pump, which starts pumping water, automatically triggering the tankless water heater. So far I'm just connecting it manually and it is working well.

Anyway, Nest and others try to hide this technical detail and just say "if you don't have a C wire, you can't use the Nest". It's not true - really you can just buy a 24V AC power supply and hook those wires up to the right terminals. But this practice is a bit rare, so it's not well documented on the web.

So my quick questions are:
- has anyone here done this for the Nest or another Wi-Fi thermostat?
- if so, which 24V supply transformer did you use?
- how much current is required to run the thermometer? I found an old 500mA one in my parts bin which implies 12 watts - is this enough juice?
- any recommendations for any particular Wi-Fi model? (being an Enginerd, I like big displays with lots of stats and information - the more the better).

Anything I learn here I will share with others through the main blog when I do an update on the radiant heat article.

many thanks!

[swick]

Hi MMM,

I tossed your post over to my husband as he is more familiar with "Science and Magic" (Electricity) then I am. While not something he has done, he found a good resource for how to add a C wire. Hope it helps :)

http://tombuildsstuff.blogspot.ca/2012/10/how-to-add-c-wire-to-thermostat.html

[woodnut]

Hi MMM,

I just had a new hi-efficiency 2-stage furnace installed and have been looking into redoing my T-stat & H-stat setup.  I've been looking at and researching a Nest, but haven't pulled the trigger yet, because it's pretty anti-mustachian in my situation (I have a perfectly working T-stat).  Anyway with my old setup, I did have a separate 24VAC supply for the electronic H-stat & humidifier.  I just used a 20VA doorbell style transformer.  I just checked my new furnace and it has a 50VA transformer.  It originally came with a 35VA but the installer put a bigger one in because of a fancy ass air cleaner I had installed with it.  I would think your 12VA transformer is adequate for a WiFi Tstat.  With that said I've found that the doorbell style laminated transformers are pretty bad at vampire power.  My doorbell transformer draws 3W with no current draw on the secondary side.  My furnace transformer isn't that great either.  A toroidal style transformer is much more efficient.  I found some from DigiKey that would work well for a 24VAC furnace, etc, but they are not cheap.

As far as hooking up your radiant heater, I'm not sure you can hook it up directly to a thermostat without the "C" connection from the heater.  You may need an additional relay in between.  Since the water heater is expecting a "dry" contact.  I think you need the thermostat W connection to power the relay and hook up the two wires from the water heater to the relay contacts.  The other thing to note is that the Nest does not have relay contacts.  It is all solid state.  The other thought is that I'm sure your water heater has a "C" connection but probably not easily accessible.   Let me know if this doesn't make sense.  I can draw up some diagrams.

[guitar_stitch]

You can run an external supply for the T-Stat.  It shouldn't require amazing amounts of current.  2000 mAh should be more than enough.

As for recommendations, I just went with the lowest tier Honeywell that had WiFi.  I have the app on my mobile phone and use it to keep an eye on things.

I installed it when I had a room mate and noticed a severe power bill increase.  The Honeywell system will email you if the temperature falls outside of your predefined range.  ;)