Ever since smart thermostats hit the scene, homeowners have clung to them like ticks to a buck’s hindquarters. They’re excellent little machines, and they really can help you stay comfortable while saving energy.

But eventually, you’ll need a new air conditioner. Can you keep your NEST or Ecobee thermostat when you get one?

The answer is yes… except for those times when the answer is no. Allow us to explain.

If you’re replacing a standard AC with another standard AC, you can keep your smart thermostat.

Standard, single-speed air conditioners pair well with most off-the-shelf smart thermostats like nest and Ecobee. After all, controlling a standard air conditioning system is what smart thermostats are made to do.

So, when you replace a standard AC with another standard AC, you’re replacing like with like. Sure, the new system will be more efficient than your last one, and it may even be a different HVAC brand. But at its core, it’s the same type of air conditioner.

It’s kind of like when you buy a new 13″ laptop and sell your old 13″ laptop on eBay. Since the new laptop is the same size as your old one, you can keep the same carrying case.

Will your new AC be paired with a variable speed blower?

Here’s where things get tricky for your NEST or Ecobee thermostat. Any time you upgrade to a variable speed system (which is a very smart decision, by the way), keeping an existing smart thermostat may weaken one of the biggest advantages of the new equipment: dehumidification.

You see, one of the biggest benefits of variable speed fans is the ability to run at a low speed for a long time and dehumidify your air – even after the system has met your home’s cooling needs. Here in Atlanta, extra dehumidification goes a long way toward keeping us comfortable indoors!

This feature poses a problem for your NEST or Ecobee device. Unlike the thermostat that comes with your new variable speed furnace or air handler, NEST and Ecobee thermostats can’t turn the system on to dehumidify your air. In other words, you forfeit your ability to effectively dehumidify by keeping the NEST or Ecobee.

These thermostats can lower the blower speed to dehumidify your air, but only after the system is already running. If you’ve already met the temperature you’ve set on the thermostat, they won’t turn the system on to dehumidify.

At PV Heating, Cooling & Plumbing, we mostly install Carrier equipment. The thermostat that comes with your new Carrier Infinity variable speed furnace can read temperature and humidity and turn the system on when relative humidity (not just temperature!) spikes beyond your comfort threshold. Similar systems from other manufacturers also come with integrated thermostats that allow the system to perform at its fullest potential.

The bottom line? Sure, you can keep your NEST or Ecobee with a variable-speed furnace or air handler. But it will never be as effective as a thermostat that’s made to work with your new system.

Can you keep your smart thermostat when you get a communicating HVAC system?

No, you can’t. Fully communicating systems only work properly with the manufacturer’s approved thermostat. Otherwise, they don’t, you know… communicate.

Let’s use Trane as an example. There are several Trane furnaces to choose from, but two really popular ones are the XV80 and the XC80. Both have variable speed blowers, but only the XC80 is fully communicating.

Say you get the XV80 (the non-communicating one). You can still use your NEST or Ecobee thermostat, but as we explored a moment ago, you lose the ability to turn the system on for dehumidification. It’ll work, but it won’t do one of the things you presumably bought it for (dehumidification).

Now let’s say you get the fully communicating XC80. This one has a variable speed blower and the system is designed to control temperature and humidity in a very precise fashion by communicating with a Trane ComfortLink™ 1050 or 850 thermostat. You simply cannot forgo Trane’s thermostat and keep your NEST. If you do, the system will not work at all. It’s made to communicate only with Trane’s thermostat.

So, the XC80 (communicating) will not work with your nest or Ecobee. The XV80 (non-communicating) will, but you’ll still lose the ability to begin dehumidification when the system is off (see the previous section).

The verdict: Don’t keep your NEST or Ecobee if you want a communicating HVAC system. It just won’t work.

The thermostat that comes with variable speed systems is still really smart.

That’s the thing to remember. When you “give up” your NEST or Ecobee in exchange for the thermostat that comes with your new variable-speed HVAC system, you’re still going to have a smart thermostat.

With the ComfortLink™ 1050 or 850 thermostats, you get pretty much all of the features your existing smart thermostat has. Plus, you know it’s going to work perfectly with your variable-speed equipment.

In the end, you’ll be even more comfortable. And you’ll probably save money thanks to the variable speed blower’s built-in efficiencies.

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