Nest Thermostat 12v to 5v Circuit Conversion

Thread Starter


Joined Dec 10, 2020
Hi everyone,

I recently installed a boiler in our basement. The boiler is plugged into the wall (i.e., it uses AC power) and can be controlled by a thermostat. The thermostat connectors on the boiler create a very simple circuit: when the circuit is closed, the boiler turns on and starts heating water. When the circuit is open, the boiler is off. Using a multimeter, I have determined that the circuit runs at very near to 5 volts.

I would like to install a Nest thermostat to use with the boiler. Most Nest thermostats use the power delivered by the heating unit to charge the thermostat's battery. The catch is that the Nest thermostat needs at least 24 volts to charge; when I hook it up to the two wires coming from the boiler, the Nest's battery slowly dies and the unit powers off. I have purchased a wall-mounted power converter that delivers 300mA at 24 volts. When I hook the converter up to the Nest Thermostat, the unit powers on, charges, and runs great. There are also two wires coming out of the Nest thermostat that are supposed to go to the boiler: when the thermostat senses that the room is too cold, it completes a 24 volt-circuit that runs through those two wires (i.e., it sends 24v of power through two wires coming out of the Nest thermostat).

Is it possible to make my boiler and thermostat work together? To simplify things even further: I have a 24-volt circuit and a 5-volt circuit. Every time the 24-volt circuit turns on, I need the 5-volt circuit to turn on, and every time the 24-volt circuit turns off, I need the 5-volt circuit to turn off. Is there a converter or switch of some kind that will accomplish this for me? If you need more information, please feel free to ask and I will get the additional information for you.

I really appreciate it.



Joined Jan 23, 2018
There is a simple and inexpensive way to achieve the interface between the two systems. The one question is about the 24 volts that the Nest thermostat sends out: Is it AC or DC?
What you need for the interface is a 24 volt relay with a single pole normally open contact. That is the minimum contact requirement, more contacts are OK if that is all you can get. The relay coil will connect to the Nest 24 volt wires and the contacts will connect to the boiler's 5 volt control wires. When the Nest requests heat the relay will operate and the boiler will switch on. That is as simple and reliable as it can get.


Joined Dec 31, 2017
Is the 5 volt supply on the boiler AC or DC.
The normal output from a thermostat is an open relay contact. Should be able to directly connect the Nest to the boiler.
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Joined Jun 22, 2012
Post the model of the boiler and we can look up the service manual, most modern Combis are 24V and Mains thermostat operating.


Joined Jan 23, 2018
Since the boiler will be controlled by the closing relay contacts it does not matter if the boiler control is AC or DC, does it?? And since the Nest drives the relay I did say that the TS needs to verify AC or DC for that, and select the correct relay.


Joined Dec 31, 2017
Since the boiler will be controlled by the closing relay contacts it does not matter if the boiler control is AC or DC, does it??
It shouldn't and I don't think there's a need for a relay. I don't think the Nest sends 24 vac when activated it's just a closed contact.

Art Mezins

Joined May 26, 2019
The original basic house "thermostat" had a single pair of 19 gauge solid wire between the thermostat and furnace. The thermostat consisted of a coiled bimetallic strip that rotated a mercury switch used to open/close a 24 VAC control circuit in response to its temperature setting (by adjusting the coil's tension). Closing the circuit energizes a circuit that causes the furnace/boiler/etc. to "activate heat" (and typically used a 24VAC relay). It really is that simple. Everything else is fodder. See:

I'm sure there are as many different "smart thermostats" as there are manufacturers, due to patents. There have been lots of different modifications to this basic operating principle to make alternative home thermostats, especially those for heating AND cooling. I suspect the Nest self-charging relies on the fact that the 24VAC either doesn't have to be fully shorted to make the heat work, or (more likely) that the heat is OFF (i.e. which is when it can scavenge electricity to charge) more than it's ON, and its charge rate is scaled for a standard 24VAC control transformer source. Check YouTube -- someone may have tried this already.