How do I replace a line voltage thermostat with a 24-volt one?

Its not as easy as it sounds. It can be done but there is no prepackaged kit that I know of because how to do it depends on your system. Some questions that have to be answered before you start a conversion: Are you using a heating only baseboard/fancoil/radiant/etc? Is this controlling a hot/cold water pipe change over system, or is it a 4 pipe system? Are you controlling two completely different pieces of equipment during cool/heat modes, are both those devices active the whole year or is one off? What about separate fan controller (see bottom)? Is it 120volts or 240/208 volts? How many AMPs? You must know these answers before you begin. Since I can't go over what to do in all situations I will start with the basics.

There are two fundamental kinds of line voltage thermostats, switches (the most common) and rheostats (or volt/amp variable controllers).


A switch type basically will open or close an electrical contact to provide or disconnect power to the system. This is akin to a standard light switch in your house, turn it on, you get electrical flow – hence lights, off no light. The difference is that instead of flipping the switch whenever you want to, a thermostat flips the switch based on what the temperature is or it DIVERTS power between two possible paths.

A heater only line stat (thermostat) will only turn on when the temp gets to cold (use this for baseboard, radiant heaters, etc) otherwise its off ( a true light switch). But if you have a changeover system you need one that sends (diverts) power between LESS than SETPOINT or MORE than SETPOINT terminals (NC, NO). Often in this case a subbase is used to switch the control type, so heating equipment can react to the normally closed (NC) terminal and cooling to the normally open (NO) and you can have a true OFF. This allows a line stat which diverts power between NC and NO terminals have an off, but you should pair the subbase with the stat per manufacturer’s instructions. A standard heat only line stat will not send electricity down the NO terminal so it can never tell a device to go on when it is hot (MORE than SETPOINT) and can't be used with a heat/cool subbase or a changeover system. (Confused?, more below to help you, read on!)

A thermostat is akin to a dimmer switch on your lights. It literally changes the volts/amps delivered to the device based on switch position. So more amps might mean more heat, turning the knob changes output temperature, not overall room control. These are very rare and frankly not very safe for home use, as a broken rheostat can start a fire in your heating system. Some radiant floor heaters are like this. If you have one of these do NOT attempt a 24volt conversion, it is too complicated for you as evidenced by the fact that you searched for how to do it. In fact I would recommend hiring a HVAC engineer to design such a conversion, do not allow an HVAC contractor to figure it out him/herself.

I will speak no more about this type and nothing below is designed to help you with rheostat controls.
With the switch types there are two types of conversion, a voltage converter/transformer or a voltage relay. I highly, highly, recommend a relay. A relay is a switch that is controlled by a signal. In this case you want the 24volt switch to turn on/off the 120/240 system.

BUT wait, do you have a 24volt power supply???? Huh? In other words, where is the power to send a signal on the 24volt system coming from? The relay won't necessarily give you 24volt power, it merely reacts to 24volt line changes. Some relays have a built in transformer, this is used to provide 24volt power so you can use an unpowered thermostat. I recommend these. An EXAMPLE is Honeywell Solid State Relay RT850 (don't just go buy this without understanding your system). This provides 24volt power regardless if the system is on or off. I see a lot of people recommending a 24volt transformer only. I say this is bad.

Here is the reason, a transformer takes all the power of the system, turns it to 24volts, sends it through the thermostat, which acts like a switch, and then converts it back to 120/240 volts. Here are the many problems: One the amps going through the 24volt system are way too high for most thermostats to handle, two if the transformer stops working well it can damage your thermostat or worse (like start a fire), three the ability to switch on/off is heavily dependent on the nature of the transformer itself and how it handles electricity, which is too complicated to go into and four, it gets hot/very power wasteful.
If you go the way I don't recommend in theory all you need is the relay and a thermostat with two wire control. Good luck, don't say I didn't warn you.

If you go my way with the relay w/ built in transformer you have to see if you have a subbase and what it does. Depending on what kind of equipment you have to control will determine if you need to leave in the subbase or take it out. With the heat/cool/off system I will usually take it out, there are some odd cases where its best to leave it in.

But you might need TWO (2) relays if you control two totally different pieces of equipment (one to heat, the other to cool and they are both active all year) or have a 4 pipe system (a quick check -but not sufficient enough to replace an investigation – to see if you need two relays, you will have a whole bunch of wires coming/going to the thermostat). In this case (needing two relays) you need to realize that you will have 6, 24 volt terminals (three from each relay): the two 'C's (common) get wired together, the heating system relay W is a true W (white - heat) but the cooling system relay W is really a Y (yellow - cool) and the two 'R's (reds) can be wired together so you end up with a 4 wire system (W,Y,R,C) and you need a 4 wire thermostat (easy to find).

For a seasonal changeover system (think apartment building where during one half the year you can only heat vs other half you can only cool) you should only need one relay as all you’re doing is turning on the system or turning it off. This config will work for a changeover system with or without a subbase previously used.
{A changeover with no subbase means that during the winter when the Temp setpoint is satisfied it actually TRIES to cool, but the cooling system is disconnected somewhere else so it really turns off. During the summer the opposite occurs. In a two pipe changeover system this doesn't work because there is no off, during the summer after setpoint is reached it would shift over and try to heat, but turning on heat is the same as turning on cold (it opens a valve and turns on a fan, whether hot or cold water flows through the valve is inconsequential to the switch) so it would end up just staying on all the time, a total waste. The subbase allows for OFF, in essence that's all it does. Hopefully this clarifies what I said above. }

Then you just need a two or three wire thermostat.

If you have a fan, particularly a variable speed fan in the loop (like the person asking the question seems to) it gets tricky. You will know this because your thermostat gives you separate fan controls (like fan high/low/off/auto). You need to realize what you are trying to do. You need one controller that can turn off and on the heating/cooling system at the right time and another that can turn on or off the fan at the right time, AND for variable speed fans you need the relay to somehow signal the right amount of power to send to the fan. PLUS the fan controller needs to talk to the heating/cooling controller because you want it to go on and off with the heat/cool system OR have a FAN ON option. At a minimum to do this you need two relays and a thermostat with a separate fan control capability (usually a G or green terminal) - a higher end thermostat can do this.

Explaining how to get the variable fan speed is too complicated for here. But understand the relay is merely a switch it can't regulate how MUCH power the fan gets, you need something that can change the amount of power to the fan. Honestly I would just leave the fan power regulator as a separate line voltage switch and allow the thermostat to turn on the fan and I would choose if I want high or low fan power. Trying to do this with 24volt controls requires an HVAC controls engineer. So in other words, get the 'two relays with G terminal thermostat' and on the relay controlling the fan, on the relay output (red 120/240 volt wire) have a fan high/low power controller there and place the switch somewhere accessible.

Closing – always check the kinds of wires you have, the volts/amps etc. Know what they all do. Practice safe electrical procedures. When in doubt or if you are not comfortable performing this work, higher a licensed electrician.