New Honeywell Round Thermostats Suck: Do Not Buy

I ordered one to use as a mechanical, low-voltage thermostat for my electric heating after I converted it to low-voltage control. It came up on my Amazon product search, and had what seemed to be a decent rating of 4.2 out of five stars. That, plus the appearance of familiarity with the product (I have lived in homes with older versions of them before) prompted me to choose it.

Big mistake! Turns out it’s not a mechanical thermostat at all; it’s an “intelligent” (I use the term very loosely here) electronic one, complete with a printed-circuit board hidden inside. It’s just disguised to look like a traditional mechanical thermostat.

If Honeywell had properly engineered such a design, there would be no problem. But they did not. Look at the actual written reviews for it on Amazon’s product page and you will see something unsettling: a large number of one-star reviews, with those bad reviews rated as being the most helpful.

For further amusement, go to the HVAC-TALK site, feed “Honeywell CT87” into the search box, and you will be rewarded with some archived discussions of this model by HVAC professionals. The general consensus is that they are junk and should not be installed.

The main problem is apparently how the firmware emulates a traditional mechanical thermostat’s anticipator (a tiny electric heater inside a thermostat that “anticipates” the tendency for heating systems to overshoot past the set temperature). It assumes a fixed and unrealistically rapid rate of temperature increase when the heat comes on. It works OK if it’s barely cold enough to need to run the heat, but as the outside temperature drops, it gets less and less accurate; you have to set the thermostat ever higher to get the same inside temperature. This has apparently even caused frozen and burst pipes for some homeowners!

To make a shitty product even worse:

  • They have an internal, undocumented lithium battery that will die within a decade, degrading performance further.
  • They have an undocumented power-stealing design that is incompatible with some systems.
  • The case design, in contrast to the traditional Round case, has poor air circulation which makes for poor sensitivity.
  • The temperature-sensing thermistor is mounted directly on the circuit board, making its sensitivity to air temperature worse yet.
  • Their circuitry is unreliable and prone to failing entirely within a year or two.

Honeywell Round thermostats didn’t used to suck; in fact, they used to be the most popular thermostat out there, and would last decades. That was when they were mechanical. But that design used mercury switches and ended up getting banned*. Instead of choosing to go with a mechanical magnetic snap design, Honeywell chose the electronic route, and badly botched it.

* For good reason. Mercury is toxic, so old mercury thermostats should be recycled. Instead, they generally end up in the trash, causing toxic waste problems.

What’s infuriating is that this has been a problem for most of a decade, and Honeywell is still selling these defective-by-design pieces of junk, apparently because I am not the only one suckered by their retro appearance into believing they are simple, mechanical, and reliable.

Caveat emptor!

Converting a Line Voltage Thermostat to Low Voltage

Holes in wall, with wires coming out of them, during upgrade to low-voltage thermostat.

During installation.

Completed installation, showing new thermostat, cover plate, existing heater.

After installation.

Thermostats come in two basic kinds: line voltage, for control of electric heaters, and low voltage, for everything else.

I have electric heat, so my condo naturally came with line voltage thermostats. That has allowed me, over the past heating season, to be reminded that line voltage thermostats generally suck, for two basic reasons:

  • There is not as much a market for them, so their makers basically don’t care very much about quality and accuracy. If you have electric heating, you probably have line voltage thermostat, and you are stuck with the limited selection of generally lousy options. Sucks to be you.
  • Electric heaters draw a lot of current, and that tends to make line voltage thermostats heat up after they close. A thermostat that heats up will tend to open and cut the heat off before the room has reached the set point. How much, depends on the wattage of the heater and how much heat loss there is (i.e. how cold a day it is).

You might think that a line voltage thermostat rated at 22A could easily switch 14.6A of current (which is what the 3500W of heating in my main living space draws) without appreciably heating up, but revisit Point No. 1 above. Neither the one that my unit came with, nor the one I replaced it with, could properly do that.

It was so bad I had to crank the thermostat all the way up to its 90°F setting just to prevent the heat from cutting off prematurely on frosty cold mornings. Once I left fairly early and that day I ended up wasting energy and money heating my home summer-hot for a few hours because I forgot I had turned the heat all the way up. So the status quo was not just annoying, but wasteful as well.

Naïvely replacing a line voltage thermostat with a low voltage one will lead to failure at best and catastrophe (i.e. a fire) at worst. Thankfully, there are combination transformer/relay devices like this which enable one to use a low-voltage thermostat to control electric heating.

I had done that at a previous home where this issue plagued me, but there I had the benefit of a crawlspace with a large electrical box to bolt the thermostat relay onto. Not so here. Initially, I gave up, because the electrical box the existing thermostat was mounted on was too small for me to bolt the relay onto, and I didn’t want to replace it with a double-gang box, because that would make for a large, ugly double-gang cover plate.

Then I realized that the thermostat and one of the heaters shared the same inter-joist wall space, meaning it was possible to remove the 12/2 cable running from the box to the heater, replace it with 12/3, mount the relay on a spare knockout on the heater enclosure, and use the extra wire to feed the switched current back to the box (where I could then connect it to the other heater).

Of course, with existing construction, nothing is ever as simple as it seems. In this case, there was a staple on the old cable which at first prevented me from removing it, resulting in my lying on my back, sticking my arm most of the way up into the inter-joist space, and probing around blind with a pry bar until I could locate and remove the pesky staple. Then there was the unexpected structural member inside the wall. I could only assume it was load-bearing, so as much as it would have simplified my life to cut a notch out of it, that was an absolute no-no, and the resulting extra bit of fiddliness caused by the unexpectedly tight clearances easily added another hour to the project.

No matter; it’s done now.

P.S. Yes, I used a simple, mechanical thermostat, not a fancy computerized one with lots of setbacks. Why? Two main reasons: 1) I don’t work a regular schedule, so don’t really have any sort of regular thermostat program that makes sense, and 2) I prefer to keep it easy, simple, and un-computerized; complicated systems tend to be unreliable and unpredictable systems.