WiFi thermostat C wire

cameronS793

Hey all first post here, with the price of oil at an all time high, I’m looking to add a smart thermostat to my oil fired boiler with radiators. The only issue I’m seeing is most require a C wire to power the new thermostat, which my system doesn’t have. I currently have the classic 2 thermostat wires only. It is a very short and easy run for me to add a C wire, but I have no idea where I would hook it up in my boiler Hydrostat 3250 plus. My whole house is on a single zone so I can’t imagine it being difficult, maybe I’m over thinking it. Any help on where I would have to hook this up is greatly appreciated. The nest thermostat is a 24V thermostat if that information is required. Pictured is my current thermostat and my hydrostat 3250 plus located on my oil boiler. Thanks!

HVACNUT

Sorry. No can do. You need to get a 40 va transformer and an isolation relay. There's no Common available in the 3250 Plus.
On the up side, you've got yourself a cool hockey puck.

STEVEusaPA

You’re wasting your money. It won’t help.

Big Ed_4

Best to run up a new thermostat wire . You would need three 18 gauge wire . I would run up four .

What other controls are around the boiler ?

HomerJSmith

See post: https://forum.heatinghelp.com/discussion/189927/c-wire-for-thermostat-from-lochinvar#latest

HVACNUT

Apologies @HomerJSmith. I misread your post. I'm curious what the rating of the boiler transformer is, and why there's a zone valve at all if it's only one zone.

HomerJSmith

That's a good question, but is it possible you are referring to the other post? The point of this discussion is the connection of a wifi thermostat on a single zone. I would use an aux transformer and relay. Then there wouldn't be any conflict with the boiler control board. The reason is one would need a thermostat charging means.

HVACNUT

Yes @HomerJSmith, wrong post. I'm 2 for 2 now. Maybe I should see a doctor. 

STEVEusaPA

HVACNUT said:

Yes @HomerJSmith, wrong post. I'm 2 for 2 now. Maybe I should see a doctor. 

Dude, get some sleep & make sure you're wearing your personal CO monitor at all times.

murf1179

Having the same issue with a Nest thermostat. Would you hire an electrician or an HVAC person if your system does not have a transformer for the C wire?

Jamie Hall

Goring to be a really good HVAC tech., familiar with your particular boiler and control setup and with the Nest. Good luck.

HomerJSmith

Use this relay Fan Center: https://www.ebay.com/p/1050552145 or https://www.supplyhouse.com/Packard-FC90113-Fan-Control-Center-120-VAC-Primary-24-VAC-Secondary-SPDT-Relay

You will have to mount it on a 4X4 metal box and bring 110Vac power into it thru a plug-in cord with ground and ground the metal box, connecting the black transformer wire to the smooth side of the cord and the white transformer wire to the ribbed side of the cord. Wire up the Nest per drawing and plug the 110Vac cord into an outlet receptical.


Wire it this way to the Nest:

melvinmelvin

Here’s a video on how to do it. 

https://youtu.be/OoGyLWjFaTw

SteamBoiler

@cameronS793

Please see my schematic and parts below for Nest learning thermostat wired to my Weil McLain steam boiler. As @HomerJSmith says it is imperative to use an independent power source and an isolation relay, and you need to run a C wire. His box combines the 2 functions, mine separates the transformer from the relay. I have my setup running reliably for the bulk of last winter and this current one.

You need to wire up the hand drawn portion and instead of the relay switch pins 3 and 4 going to G and Y of my boiler, it would go to TT of your boiler. When Nest calls for heat, W is energized, the normally open switch on the relay closes, and shorts TT of your boiler.

120V to 24V AC 40VA transformer, I recommend it for the 1.67A current output. Nest by itself draws 200mA and the isolation relay could potentially draw another 200mA and you want a transformer with plenty of margin, not the 500mA output ones:
https://smile.amazon.com/gp/product/B083R621RV

Emerson 90-380 24V fan relay: https://smile.amazon.com/gp/product/B000LDCPQS

SteamBoiler

@cameronS793

As an aside I am not sure if you will achieve any savings with a Nest if that is your primary motivation.

In my case, I did save about 20% usage for this November with the Nest compared to last November with an old 2 wire Honeywell Chronotherm. I believe it was because the Honeywell's calls for heat were too short in duration and left the basement warm but the home cold. The Nest's typical heat call is 20 minutes long, long enough for my radiators to get saturated with steam (the earliest rad gets steam 10-12 minutes into the heat call). With the Nest, you will be able to see the heat call durations (I look at them every day as a proxy for system health) and can potentially fine tune setbacks (periods of lower temperature).

I have disabled all Nest smart learning features. I have my home set to 68F at 7am, 67F 8am-3pm, 68F 3pm-10pm, 64F 10pm-3am, and recover to 68F at 1F per hour from 3am to 7am. This slow recovery helps the boiler not cut out on pressure. I also like to set the Nest to 55F or so when traveling and bring it back up slowly on the way back. This is flexibility that even a typical 5-2 thermostat doesn't have.

If you want additional tuning, Ecobee may be a better choice. Ecobee lets you control the dead band (undershoot below setpoint at which thermostat will call for heat) and the minimum heat call duration (these are hardwired into the Nest, it calls for heat about 0.5F-1F below setpoint and stops so home coasts to maybe 0.5F above setpoint). If you want, you can sacrifice steady comfort for some undershoot and overshoot and potentially save energy.