Honeywell RTH7500: Long Term Verdict

Surprisingly, the most annoying feature of this thermostat turned out to be lack of programmable hysteresis, a.k.a. deadband.

This particular one, installed in this particular location, allows runtimes as short as three bleeping minutes - whereas the time required to get into more or less efficient operating mode is no less than about five minutes for furnaces and about fifteen minutes for an A/C or a heatpump.

Likewise, there is no minimum run time protection (sometimes called short cycling protection).

In other words, this thermostat will make your unit work much less efficiently than it is designed to, and the only way to prevent that is to put the thermostat in a place where the temperature changes very, very slowly - like a closet or a thermally insulated box. Which is not quite that practical.

So, thumbs down, no go.

Likewise, if I ever have to select a thermostat again, I will pay particular attention to presence of programmable hysteresis and minimum run time.

The Competition: Honeywell Prestige HD

The best things to do with competition are to know them and to respect them.

Here's what our competition is, sort of:



Go read the Honeywell Touchscreen Thermostat and Portable Remote Review and keep in mind that there is a subtle distinction between Prestige HD and Prestige SD (about a hundred dollars or so).

Then go do some shopping (zoning system not included) and hopefully come back - if not today, then in a few weeks or months when the GUI (and other unannounced bells and whistles) is back to its former glory and beyond, for the price of Prestige with comparable feature set is steep indeed.

By the way, DZ is going to look nothing like Prestige :)

UPDATE: This is what it looks like. This is what it looks like :)

Favorite quote from the Homeowner Brochure (emphasis mine):

You can even name each of your zones on the Portable Comfort Control by having your contractor customize it with more than 60 different naming options - upstairs, master bedroom, office - even wine cellar and home theater!

(Image Credit: Gizmodo.com)

Informal Review: Honeywell RTH7500

Following up the RiteTemp GMPG808C review...

This thermostat is actually simpler to describe. It has no character. You don't notice it.

Actually, one might take it as a compliment - this is exactly what you want from an appliance - all you need of it is to perform its function and not struggle for getting a piece of your attention all day.

But it'll sure make you work to get to that point and will punish you if you try to upset the status quo.

First of all: don't lose the manual.

The rest of installation is simple - just follow the installation manual. It would be a good idea to read all of it beforehand - there's a couple of quirks you need to be aware of, in particular, configuring the thermostat to work with your exact HVAC system configuration.

Configuration takes a while - again, it's better to have the unit on the desk before you, and not in th wall cradle, when you do it. Can't do it without the manual - options are numeric. Did I mention - don't lose the manual?

That's basically it - unless you are really off with setpoints and schedules, you won't have to touch it again.

A few observations:

• It is nice to be able to move period boundaries. Different people, different schedules.
• You may want to think twice if you really want to use the "recovery" option (the one that makes sure that if you set the period starting at 9PM having setpoint of 78°F, you will have the room at 78°F at 9PM, instead of HVAC unit starting to struggle to cool it down at 9PM) - this option will throw a wrench into your energy savings if you are on a time of day usage plan. The worse is your HVAC (or, should I say, the more properly it is sized), the less useful this option is.
  
• Have to imprint it in your mind that all you need to do to change the temperature temporarily is to press up/down arrows a few times. There is no need to press "hold" - this makes the thermostat hold the temperature forever. Kind of counterintuitive in the context of other interaction patterns - closure is expected, but missing.
• I absolutely hate the fact that the only way to see the screen in the dark is to actually press the button, and, it does something just as you press the button.Watch what you're doing. The only way not to screw things up is to learn what button is idempotent when pressed twice, and find it in the dark (oh, by the way, it is not the temperature up/down arrows... they invoke a "temporary temperature change" context, which you have to explicitly get out of or let it time out - of course, not before the light goes off). Or just forget it and switch on the lights - to me, this option eventually won the argument. In other words, if you're specifically looking for a backlit LCD thermostat, have a chuckle and move on.
• Summarizing the above, one could say that the user interface design violates the principle of least astonishment and thus may not be well suited to people who still have their VCRs blinking at 12:00.
  
• There is no apparent way to adjust hysteresis, so if you are big on energy savings, you'd better consider thermostat that can do that - GMPG808C is not the worst other choice.

That's basically all. I can make an educated guess (but it is a guess nevertheless, please let me know if it is correct) that other thermostats of the same product line are probably identical to this except for different scheduling options:

• RTH7500 supports 7-day programming (for $100);
• RTH7400 supports 5-1-1- day programming (for $80);
• RTH6300 supports 5-2 day programming (for $60);
• RTH3100 doesn't support any programming at all (for $50).

Bottomline: brand name may be overrated and overpriced.

The only other consideration is - it's been less than a year since both Honeywell and Rite Temp thermostats have been installed, so let's see which one lives longer, and by how much.

UPDATE: No Go.

Short Take: Hysteresis in RTH7500

Frequently asked question with a very vague answer - what is the hysteresis (a.k.a. dead band) of my thermostat?

For Honeywell RTH7500, the measured (therefore not exact) typical value is about 1°C (about 1.8°F) - for the sensor located in the room behind the wall where the thermostat is.

Worst observed value is 2.5°C (about 6.3°F) - for the sensor located on the first floor (the thermostat is at the second) - this is not actually a property of the thermostat, but rather an indication of how badly the airflow was designed.

Another question - can hysteresis be controlled in RTH7500? The answer is: probably not.

If it can, there's no way to do it that is described in the documentation that is sold with the thermostat (yes, I found the manual, feel free to ask me what those numbers mean).

Short Take: Where's My RTH7500 Manual?

Exactly where you put it, dummy.

Correction: I am the dummy.

So here I am, trying to figure out the way to configure the hysteresis and, at the same time, save myself a long walk to the shelf at the second floor where I *think* I put the plastic bag with the manual and spare parts. Also, it's middle of the night, it's dark, the light switch is out of the way, everyone in the house is asleep and I don't want to step on the cat strategically placed somewhere down my path.

So I'm thinking: it's 2007, let's google it up.

Yeah, right.

I guess my patience is thinned by previous encounters with HVAC machinery and service, unlike one of this gentleman.

Mind you, all configuration settings in this thermostat are not described by words. They are described by numbers. There's no way in hell I can remember numbers I have to use once in a few months, if not years. And the paper manual is nowhere to be found. And what other companies usually put up (a printable PDF document) is nowhere to be found, either.

Nice.

Another score for Rite Temp.

And don't lose your manual, otherwise you end up with a quite expensive brick on your wall.

Update: I've found the manual...

Thermostat Wars

The predominant kind of thermostat war is the one between those who understand the concept of the thermostat and those who don't. It is difficult to believe, but there are still people who don't get it. They would crank the thermostat all the way down when they feel hot and all the way up when they feel cold. I kid you not, several years ago there was a guy who came to the Residential HVAC forum with a question, "how to educate the significant other not to do that". After few days and several dozen messages, the community agreed that the only way to deal with his particular problem was to install a fake thermostat. This case is beyond repair, let's concentrate on the other.

Out of remaining, two kinds can be distilled: those that require marriage counseling and cause HR violations , and those that don't. The latter is the kind I would like to cover in this article.

The final division is the wars between the occupants of different rooms, and, those apparently happening just in the mind of a single person waging the war.

The first kind is actually pretty easy - this is what the zoning systems are for. I mean, the real ones that do allow you to set different temperature for different zones.

As for the last kind... Imagine a situation when there's a person who claims they are absolutely comfortable with a certain temperature - let's say, for the sake of argument, it's 78°F. And indeed, when put into a scientifically controlled environment, they feel just fine. But in real life...

They fiddle with the thermostat all the time. Sometimes they push it up. Sometimes they push it down. Sometimes, they claim the thermostat is not calibrated (it is). When cornered, they are claiming the temperature is not right, and demand a precision thermometer. And you know what?

Most of the time they are right.

The cause for it is trivial from a control systems engineer standpoint, but is carefully hidden in a plain view for the rest of the world.

Almost all thermostats, except the very advanced ones, are based on a very simple concept of a hysteresis. Temperature goes X degrees above the setpoint, cooling on. Temperature goes X degrees below the setpoint, cooling off. Simple?

Not quite.

First of all, not all thermostats (as a matter of fact, very few) will let you change X. According to some claims (google it up if you want), X = 2.5 (I suppose Fahrenheit) for mechanical thermostats, somewhat less for digital. 5°F spread is a lot.

Side note: Rite Temp GPMG8085C (and possibly other Rite Temp thermostats) will let you control hysteresis. Doesn't mean I endorse it, though - it's got its quirks. Doesn't mean I condemn it, either - it's doing a good job comparable to Honeywell RTH7500 for $20 (20%) less.

Then, the situation gets very interesting as temperatures get milder. The closer is the ambient temperature to the setpoint temperature, the more suffering there is (that'll be mornings, afternoons, and the whole mid-seasons). The reason?

Deviations of ambient temperature are sufficient to cause severe discomfort, but are not sufficient to kick the temperature at the thermostat beyond the dead band and thus to force the HVAC unit to actually do something.

Things can become really bad if the house is big enough, and the part of it where the thermostat is is insulated better than other parts - all in all, "mild" is a relative term. A typical case would be a tri-level house somewhere in Arizona.

Which is where and how it all started. Is there an answer? Yes, and it's been already implemented several years ago, but I'm not telling. It would suffice to say that if you want to find out, by this time you know where to look for it.

Imitation is the most sincere form of flattery

Here's a screenshot of a debug console for DZ, published on February 22 2002:




(also, here, here, here, here, and here, and the whole discussion around it here)


Here's a picture of a Honeywell VisionPro Touchscreen Thermostat, released several years later:



Hmm...