What is the best way to remote-control 4+ 240v wall-heaters, each with their own line-voltage thermostats, and their own 240v circuits?

Question

We live in a warm climate and rarely heat our tiny 2-floor townhouse fully, but when we do, we cannot use the existing electric heaters efficiently at all.

We will be installing a floor heating system (heat-pump water heater plumbing under new floors) in the next few years, but meanwhile, the costs of electricity pile up, and the waste of a scarce public utility in my area bothers me a lot. So we want to install a single smart thermostat like Nest to manage these evil inefficient toasters and/or their line-voltage units, until then. The goal is to minimize electricity use on wall heaters and maximize efficient heating, while monitoring/controlling all wall heaters with a smart unit that comes with smart sensors. I just need help with that.

• We have 4 old school, independent Cadet RM202 in-wall heaters, 240 volts;
• 2 of them in bedrooms in 2nd floor, 2 of them heating the shared space (living room+kitchen) in 1st floor;
• each individually wired (THHN red, black, ground) from separate 20 amp breakers on the main panel;
• each controlled by old school TH110 line-voltage thermostat units with their own local temperature sensors, inefficiently just 3 feet over each heater;
• no C-wire or any other lower voltage wiring, thermostats powered directly by each of the 240 volt lines.

(No other existing controllers, relays, etc. And no other HVAC in the house. For a bunch of "tmi" reasons, mini-splits and the like are not option.)

I have no problem with adding new wiring, or individual relays, or setting up my own new zoning relay, as well as opening walls and drywall repair, etc. But I also want this to be no more complicated than it needs to be (coding a DIY zoning-relay, while possible, feels a little overkill for example).

I did my research, as well as having checked a few similar (but not exactly same) questions here. But instead of throwing a plan, I'd really prefer to hear direct suggestions for the best and/or the most efficient way to control these 4 heaters and/or their thermostats with a smart unit.

(I'd prefer to have a Google unit, since I'm already all-in on the "hey google" stuff. But open to different alternatives with Ecobee, etc.)

Thanks!

Answer 1

Based on you statement

We live in a warm climate and rarely heat our tiny 2-floor townhouse fully, but when we do, we cannot use the existing electric heaters efficiently at all.

and since you are remodeling, I would suggest switching to Mini-SPLIT AC for heating. It is most energy efficient, install them only in the rooms you want to use. The installation is not to complex ( no air ducts)

Answer 2

I don't know if this is the "best" option but I can lay out what I'd do. I did something similar to what I'm about to describe and apparently it's been working well for the homeowner where I set this up. Oh, perhaps I need a disclaimer, I'm no HVAC professional or licensed electrician but I did study electrical engineering and did some independent study into how HVAC systems work. My guarantee is that if following my advice breaks something then you may keep all the pieces.

It appears that you want to control heating in three zones in the dwelling, I'll call them master bedroom, spare bedroom, and living space. Most thermostats only allow for control of two zones but I'd "cheat" and use the fan circuit as a control on a third zone.

Presumably for occupant comfort and convenience you'd want the smart thermostat in the master bedroom. For maximal control on temperature over time in the spare bedroom I'd set that as the second zone. The living space heaters would live on the fan circuit. Find an appropriate 24V transformer to power the smart thermostat and put it on a "safe" circuit, a circuit where it's not likely to trip because someone plugged in something they should not have. Run the 24V line to the master bedroom in a convenient place, then run a pair of low voltage wires to contactors on each heating element. I'm going to be a bit nonspecific on how to wire the thermostat since that can vary on the model.

The thermostat will sense the temperature in the room it is in for the first zone, the spare bedroom will have a remote temperature sensor and wire that as instructed for the second zone. The living space will be a bit different.

I'd have an "old school" bimetallic strip thermostat in the low voltage fan control line between the smart thermostat and the contactors for the living space. The idea is that you can control the bedroom temperatures remotely using the smart thermostat app, with the living space having just on/off control by using the fan circuit to control it with the app.

In the setup of the thermostat you can have the fan be only on or off, or you can set the fan circuit to turn on when either zone calls for heat. This is the classic "on/auto" fan setting seen on thermostats, where you can force the fan on with the switch while allowing it to cycle with the furnace. Whether this option proves useful or not is up to interpretation. Maybe have the living space on "auto" at night so when either bedroom needs heat the living space gets a bit of heat too so that it's not absolutely freezing in the morning. Then have the fan circuit come on with a schedule or manually to restore heating function in the living space and having the old school thermostat hold the temperature from there.

Adjust and rearrange the idea as desired, perhaps with the spare bedroom on the fan circuit since that is not used often.

Answer 3

I'm adding another answer to offer an alternative after some of the comments lead me to more research. Apparently there's enough of a market for line voltage smart thermostats that there's a number of options on the market to choose from. I did a search on "line voltage smart thermostat" in a popular web search engine and found a few articles like this one: https://thesmartcave.com/best-line-voltage-smart-thermostat/

I have no affiliation with the linked site or any of the products they reviewed, I offer it only as an example of what can be found. Apparently many of these thermostats offer the ability to be ganged up in software so they can be controlled as a single thermostat. Some are compatible with Google, Amazon, Apple, and/or other home automation systems. This should avoid any need to run new wires.

Maybe with a fifth "master" unit that is not connected to any heater there's a fix for the poor placement of the current thermostats. It is likely possible to put the "master" unit in a central location for where the temperature is detected and then the "slave" units turn on and off as needed to maintain a comfortable temperature at the "master" unit.

I've discovered that the best solutions can be found if only the proper nomenclature is known for what is being sought. It's a bit of a catch-22. Unless I'm missing an important detail your heating units are using what is called "line voltage", and you seek a "smart thermostat" to enable programmability and remote control, therefore "line voltage smart thermostat" appears to be a good place to start searching.

For completeness I'll add that other common thermostat control voltages are a nominal 24VAC or "low voltage", and there's what is called "millivolt" which doesn't seem to have any nominal voltage other than it being much lower than "low voltage". Maybe I'm mistaken but that's my understanding so far. Low voltage is common on most central HVAC units. Millivolt thermostats are common for fossil fuel heating units like fireplaces. Obviously "line voltage" control is common for electric resistance heating.

Answer 4

All the heaters are already wired to a central location: the circuit breaker panel. So that's the obvious place to put remotely-operable line voltage controls. In other words: a set of relays, also called contactors.

Contactors with 24 V AC coil are abundantly available. You'd need to select a model rated for 240 volt resistive loads at least 2000 W (8.3 amps). These are cheap; plenty of models retail for just US$10-15. Install them in vacant space in the breaker panel (if there is any) or in a metal wiring box beside the panel.

Interface the contactors to your hobby microcontroller of choice with a relay board.

As an alternative to the conventional contactor, a "solid state relay" is a nice choice. These have the advantage that they can wire directly to a 3 V or 5 V logic pin on your microcontroller without need of the intermediate relay board.

From there.. yes, "fiddle with an arduino overnight" is what's left. You can choose whether to set the room thermostats absurdly high so that the heater will always run when the arduino turns on the power to that zone, or set the room thermostat to a reasonable temperature and use the arduino only to block the heater from running by cutting its power (this seems to be what you're angling for to "minimize electricity use on wall heaters and maximize efficient heating").

Add BLE or wired temperature sensors to round out the solution if you're so inclined.