C wire with two Nest thermostats

HeatingHelp

This discussion was created from comments split from: C wire & Aquastat L8148E with 3 zones.

mrgoodbar

https://forum.heatinghelp.com/discussion/comment/1714010#Comment_1714010

Apologies for necromancing this post. I have a similar scenario. I have a Honeywell Aquastat L8148E with two Honeywell V8043 zone valves. I do not have an external transformer. I'd like to know how I can provide a C wire for two Nest thermostats. I've searched the forum, but alas have not been able to find this solution discussed anywhere. Would be incredibly appreciative of some assistance!

HeatingHelp.com

@mrgoodbar We added your issue here as a new discussion (with a link to the related thread). We didn't want it to get lost at the bottom of an old thread. Thanks and welcome to HeatingHelp!

EdTheHeaterMan

The L8148E has 4 additional numbers after the letter E. This will determine if you have a standard 24 VAC with15 to 20 VA rating or if you have the 24VAC with a 40 VA rating. Also a photo of the inside wiring will also indicate if you have a 40 VA transformer. If you have the larger capacity transformer, it can handle 2 zone valves, and you do not need a dedicated transformer for the zone valves.

These two photos show the difference between the L8148 with a standard transformer and two thermostat terminals marked T T (black and white photo) and the L8148 with the larger transformer with four low voltage terminals marked T, TV, W and Z. (color photo). See the difference in the size of the transformer? (Green arrow). If you have the control on the right, then you may not need a separate transformer for the zone valves.

In order to find the C terminal for your Smart Thermostat, you may need to change the wiring on the thermostat and zone valves. With a 2 wire thermostat (labeled R and W) it does not matter if you place the R on T or TV on your control in which case the W from the thermostat will be placed on a Yellow of the zone valves. When using a Smart thermostat it is important to put the R from the thermostat on the TV of the L8184 ONLY and the W from the thermostat one of the Yellow wires of the zone valve, that will make sure that you can use the W on the L8184 as the C for the thermostat and the zone valve other Yellow wire.

It is important to wire both zones identical or you may connect a dead short by accident. I recommend this device to be placed on the TV terminal of the L8148 control then connect all the TV wires to the other end of the fuse.

I do not actually sell these, it was presented as part of another post as a device that does not let the factory installed smoke out of the control transformer

Assuming you have the L8148E 1265 or similar control, here is the diagram for using the internal transformer of the L8148E 1265 to operate 2 zone valves and 2 smart thermostats. That is the maximum load you can place on this transformer. That is because this control transformer is powering 2 zone valves, one vent damper motor, one ignition control, one gas valve, one (burner and circulator) K1 relay coil, and now two smart thermostats.

TV on the L8148 is connected to the R of both thermostats and also gets connected to one of the Red wires of each of the zone valves.

T on the L8148 is connected to the other Red wire of both zone valves

W on the L8148 gets connected a Common to both zone valve motors and both thermostat by connecting it to C on both thermostats and the other Yellow wire of both zone valves. Leave the factory installed jumper on the L8148 connected to Z

mrgoodbar

Simply incredible, @EdTheHeaterMan. I'm awed by both your knowledge and your willingness to share it with laymen like myself.

Thank you!

Adding images of my set-up to answer your questions re: the L8146E, which is indeed the 1265 variant, and a bit more context in case any of this extra info alters the recommended approach.

Context

1. The system is already set up with the two zone valves and works perfectly fine with "dumb" thermostats.
2. I've run new 18/5 thermostat wire from the thermostat locations down to the boiler so I will be rewiring everything.
3. I purchased two nest power adapters last year, prior to having run the new 18/5 wires, but hit a roadblock and let the project sit idle until now.

Additional Question:

How crucial is The Smoke Keeper? Is this something I can install later on, when it arrives? I ask because I'm looking to get the system functional by EOD since temperatures here in the Northeast are dropping fast.

Aquastat

Zone Valves

EdTheHeaterMan

The Fuse is not 100% important until you make a mistake and burn the transformer on the control. Then you will say Ooops. I wish I went to the auto parts store and picked up a package of 3 amp automotive fuses and a couple of insulate spade connectors.

EdTheHeaterMan

Even though the zone valves are working just fine with the current wiring design, please please please please look carefully to see that you follow the wiring diagram exactly, and that both zones are identical.

If that L8148 fails, You will have no heat tonight! and no heat until you get another L8148.

By adding a C wire to the thermostat, you make a T T connection into a R W connection because the W R C configuration will not operate the thermostat. Only the R W C configuration will operate the thermostat.

If you follow my drift.

mrgoodbar

@EdTheHeaterMan: Thank you so very much for taking the time to walk me through this wiring exercise. I followed your advice and everything is working just like it's meat to. Was even able to install the "Smoke Keeper".

Thank you!