Blackcap Blog

Published at 13:48 on 21 October 2020

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!