Smart Thermostat Compatibility Guide

Smart thermostat compatibility is a hot topic these days, and that’s not just because these units are used to switch on the furnace! Not all smart thermostats work with all HVAC systems.

Some homeowners have found that they can’t get their shiny new wireless gadgets to interface with the equipment that they currently have in place.

Fortunately, a few simple tricks will ensure that you don’t have to worry at all about whether or not a thermostat will work with what you have in place. With the right unit and a few tools, you can bypass all of these compatibility concerns and get back to enjoying your heating and cooling system like normal.

Thermostat Wiring

Some smart thermostats like the Honeywell RTH9580WF need an additional power cord to keep the wireless and programmable functions going at all times. Wiring won’t be an issue if you’re replacing a smart thermostat with another smart thermostat, since you probably already have a C-Wire in place.

You might have to improvise in other cases, though. If you’re looking at an open hole in the wall with wires sticking out of it, then you can probably tell right away if you have a C-Wire. Extra wires with blue insulation are normally common, which means they can be used as a C-Wire for thermostats that require extra juice.

Should you find one that’s hooked up to a terminal labeled Common or 24V Neutral, it means you already have one in place. All modern thermostat wires provide 24 volts of power, which means you can also use unused proprietary wires as a C-Wire if you can find them.

Now, there’s also the chance you have a much beefier 120 volt LV system, which you can spot easily because you’d have multiple thermostats controller individual heaters. These are usually present in small apartments or duplex homes.

If you have this kind of a system, then the wires inside the wall will be quite thick. Instead of running around trying to replace these wires, you’ll want to opt for a smart thermostat that can handle a line voltage system.

Make sure that any smart thermostat you get handles 120 volts out of the box, which will prevent compatibility problems altogether. You’ll probably need relays and transformers in this case, which means you may wish to call in an electrical professional for help.

HVAC System Types

You’ll possibly fry out your new smart thermostat or even cause an electrical fire if you hook it to the wrong type of HVAC system. Most electricians would never recommend installing a smart thermostat to control a multivolt gas furnace or an electric baseboard system.

Specialty systems like the Mr. Slim from Mitsubishi won’t be compatible with any type of smart thermostat on the market, so you’re better off just using the included equipment if you have one of these.

On the other hand, you’ll be able to work with heat pumps, conventional forced air systems and even those based on a boiler if you have one. If you don’t know what kind of system you have, then these tips should help you identify it:

  • If you notice vents throughout your house that blow air out the top, then you mostly like have a compatible conventional forced air system.
  • If you have an outside unit that handles both heating and cooling and live in a place like Florida where the weather doesn’t change often, then you mostly likely have a compatible heat pump system. These work with or without auxiliary heat sources, and should be fine with most smart thermostats as long as they don’t have extra stages.
  • If you have radiant heat pipes that warm up your floors, then you have some sort of boiler system. This should work with most smart thermostats as well.
  • Have you noticed any big electrical metal units on your floor? These are electric baseboard heaters and they usually don’t work with smart thermostats. They might even fry them out!
  • Do you have a long narrow white box near the ceiling that forces out cool air? This is Mitsubishi’s Mr. Slim unit or a clone of it at least, and it doesn’t work with smart thermostats.

You’ll want to use the regular included controls on window air conditioners as well, since smart thermostats usually can’t control these systems properly either.

What are HVAC System Stages?

Two-stage cooling refers to air conditioners that have compressors with high and low settings. There are also two-stage furnaces that have two different settings for the furnace. Compatible smart thermostats mention that they can control these systems, though none of the regular wireless consumer grade designs can handle HVAC systems that have more stages than two.

Once you’ve popped your thermostat off the wall, take a look at the wires that are headed into the terminals. Single systems have only one wire for heating. Heat pumps using a single stage only have a single wire that leads into the Y1 terminal for both heating and cooling.

You’ll notice extra wires headed into terminals marked either W1 and W2 or Y1 and Y2 if you have a multistage system. Owners of triple systems will have W1, W2 and W3 terminals on the back of their thermostat. Unfortunately, smart thermostat technology doesn’t really work with these kinds of systems in most cases so you’re better off with using an advanced programmable thermostat.

For what it’s worth, engineers refer to any thermostat with an onboard computer as smart so you might not be loosing all that much with a downgrade!

In some cases, HVAC technicians in the past might have creatively wired the back of your thermostat. If this is the case, then you’ll want to simply count the wires going into the terminals.

Whenever you find that there are tons of extra wires going into a unit, you can usually safely assume that it has some sort of odd multistage heating or cooling system. This will cause compatibility issues for most brands of smart thermostats.

Systems that are Usually Incompatible with Smart Thermostats

As you know, electric baseboard heaters and specialty air conditioners won’t work with smart thermostats. While heat pumps will, there are unfortunately some designs that won’t work either so you’ll need to pay close attention to the equipment that you have if you live in an area where heat pumps are common. Multiple stage heat pumps are particularly problematic and usually don’t work with smart thermostat technology.

Honeywell RET97B5D1002 and Lux WIN100 thermostats don’t normally work with heat pumps at all. Some people have recommended taking a photograph of the wiring behind your existing thermostat then emailing it to the manufacturer, but this isn’t necessary if you have a regular heat pump. Just check the specifications sheet for compatibility and keep in mind that these two models usually aren’t compatible at all.

More complicated systems don’t work under normal circumstances either. Three-speed fans are a problem, since smart thermostats usually don’t know how to control them. They’ll just got stuck at whatever speed the thermostat designates as correct. Usually, this is always the fastest speed that it can move at.

Humidifiers get ignored by smart thermostats, which can cause some very unusual problems depending on how your system is configured. They might stay on all the time or even get fried out, so you’ll probably not want to try interfacing a smart thermostat with any HVAC system that has one.

If you really feel the need, then you could disable or remove the humidifier entirely. Most homeowners don’t really have a need for them, and you could always get an inexpensive floor model.

Some people have hooked up a separate thermostat control to manipulate just their humidifier while leaving their AC unit and furnace under the control of a smart thermostat. This really isn’t recommended, since it defeats the purpose of having a smart thermostat to begin with.

While they’re extremely rare, there are some furnaces and AC units designed to work with proprietary computer interfaces to present a sort of fake smart thermostat interface to the user. They were popular before the Nest Learning Thermostat became took most of their market share, but unfortunately these kinds of devices can’t interface with modern smart thermostats.

There are some very unusual pieces of technology out there, like wireless smart space heaters. Even though these are technically networked pieces of equipment, the protocols that they use aren’t compatible with smart thermostats either.

One Last Piece of Advice

Always make sure to read the full specifications of a smart thermostat before you buy it. Many of them have different requirements. While it might seem boring, taking the time to do it now can save a good deal of time in the future.

Nest Learning Thermostats might not require a C-Wire but Honeywell’s solutions often do. Very few devices can work with the powerful line voltage HVAC systems installed in vacation homes.

By making sure to read all of the specifications before you buy, you won’t end up with something that won’t do you any good.

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