Honeywell Aquastat L8148E and 3 Nest Thermostats

Yoyo0214

Looking for some help. I've seen other that have posted regarding the C wire on the L8148E. Now here is my issue. I have 2 zones for Central Air and 5 Zones for Heating (Baseboard Heating). I've attached pics to show what I'm working with here. Now my issue is the 3 nests where I have connected have a red, white and green wire. The green wire is disconnected and I would like to repurpose it as a C wire, is that possible? I noticed a brown wire that is connected to the "Z" where it what recommended in this forum to connect the "C" wire.

The second photo shows where the "G" wire is connected connected to several "G" Wires (3rd) Pic that leads to all the nests. However on the other end leading to the L8148E, the "G" Wire is not connected to anything on the red and white as seen in Pic 1. Could I connect the "G" wire under the "Z" connection and make it a "C"?

Or do i use a separate transformer like in the last pic to add all the nests to the "G" wire and how would i go about doing that?

Or scrap the entire idea and pay a pro to change all the "G" wires to "C" wires and add a bigger transformer for $1000 quote he gave me?

Thanks in advance!

Jamie Hall

Not sure I followed all that, but…

First, what you need to do is get constant power to the Nest. That is a wire — usually red but it needn't be red — connected to Rh or Rc, and a wire, usually green but it needn't be — connected to C at the Nest. The other end is to be connected to a transformer supplying enough power at 24 volts AC, red or whatever to the hot side and green or whatever to the ground or low side. The wire from W on the Nest — usually white but it needn't be — goes to the controlled device control terminal, and then a wire goes from the other side of the control back to the transformer.

Any way you can do that will work. Now you can also use an independent transformer — and may have to, as the Nests draw a fair amount of power. One terminal of that gets connected to Rh/Rc and the other to C. The control circuit is independent.

Or you can power the Nests from the AC system — hot to Rc and low to C.

Lots of ways to go.

Yoyo0214

Thanks for the reply. Sorry if I wasn't clear in my previous post. I have 3 nest thermostats that are wired with R and W as in the pic below. However all the nests refuse to work using the Google Home app due to not having the C wire. I've tried every combination within the app with no luck. There is a green wire (Not seen in the pic, but wrapped around the red and white in the wall) on all three nest connections. I would like to repurpose the "G" to a "C" wire for each. If i go the transformer route does it connect to nest?

As for the AC system…I have 2 zones for Central AC. Those have separate wiring and I was able to connect the heating (baseboard) along with the AC (Central) using the "C" wire from the AC. The other 3 Zones done have a "C" wire. Only red, white and green.

Jamie Hall

Wire colour doesn't matter — electricity is colourblind. Connect that green wire or whatever to C on the Nest and the low side of the transformer powering the system. If it has enough power…

HVACNUT

I would add up the draw to see if the 40 va transformer can handle 5 zone valves, damper motor, gas valve, plus 3 Power robbers. You'd be better off going with something like this.

You don't get an Rc and Rh with those? T

EdTheHeaterMan

First of all, The L8148E is a gas boiler control. It is equipped with a 6 pin plug for the automatic vent damper and has a 24 VAC at terminals B1 and B2 to operate the gas valve circuit. What are the last 4 numbers of the L8148E control? There are some L8148 controls that have a 24 volt 50 VA transformer in order to wire up 3 zone valves without using an additional transformer. The maximum number of zone valves that can be connected to that L8148E is 4 Honeywell zone valves. You have 5 zone valves now so I wonder if you are using that small 16/24 VAC transformer for at least one of the valves. this photo illustrates the two different transformer sizes that come in these Aquastat relay controls

There is more going on here than you may understand.

The bottom line for any Nest thermostat is that you need to connect the C wire of the thermostat to the C on the transformer that is operating the zone valve yellow wires. That may not be the transformer on the L8148 control.

If you are currently using the L8148 transformer to power any of the zone valves, then you may already be maxed out on that transformer and you will need a complete wire redesign.

What thermostats are you using for the other two zones that are also connected to the Air Conditioning? There is a better way to connect those Nest Thermostats that will free up some of the power on the L8148E control.

I will be happy to provide step by step instructions for you with diagrams if you provide me with this info

• Picture of the L8148E control with the cover off so I can see the wires and what the transformer looks like.
• Are you using a separate transformer to power any of the zone valves?
• Model number of the Air Handlers that the other two thermostats are connected to.
• Do you have at least three wires for the 3 heat only thermostats?

With this information I can send you a wiring diagram I made for another person that has a similar situation.