Thermostat

[Frank Cox]

I had to replace a thermostat today and discovered that a mechanical thermostat that doesn't require a battery still exists.

I thought they had gone extinct when mercury switches were outlawed.

Honeywell CT87N

If you need a thermostat that never needs a battery, there it is.

It looks like the old round Honeywell thermostat that used to be on everyone's wall in the 60's.

[Frank Angel]

Another reason to wish we were back in the 60s.

[Randy Stankey]

Galinstan is an alloy of gallium, indium and tin with a bit of flux thrown in for good measure, which is liquid at room temperature. It is almost as good as mercury when used in "liquid-in-glass" type thermostats. It does have a tendency to stick to surrounding materials if the surface oxidizes so the glass capsule also has to be filled with a non-reactive gas such as argon.

The only other drawback to Gallinstan is that its melting point is -19º C where mercury melts at -39º C but if your house gets THAT cold, you've probably got more important things to worry about than the melting point of metal alloys.

[Frank Cox]

The thermostat is still electronic since it has a little circuit board in it and you set dip switches for the type of heating you have. Since it has a mechanical dial for setting the desired temperature there's no need for any kind of battery backup to store the setting.

[Randy Stankey]

Does it use the 12v source from the furnace for power? When you said that it doesn't need batteries, Galinstan is the only thing besides mercury that I know would work in place of a thermocouple and wouldn't need a source of electricity to work.

Even non-battery-powered thermostats might have a circuit board inside them, nowadays. It's just easier to manufacture that way but even mechanical thermostats might need to have switches inside in order to configure them for the type of furnace they are used for. (e.g. Whether the system has both, heating and cooling, or just heat.)

[Frank Cox]

I didn't see anything that looks like a mercury switch (or close relative), though that might be hidden behind the plastic casing. It does have a "place level here" line on it but I don't know if it really needs to be level or if it's just be for aesthetics. I levelled it anyway.

Thinking about it now, I don't know what, if anything, powers it. It has has three connectors, RWY, but my installation just uses RW. The idea is close circuit=open valve, so if it was powering itself from the incoming power wouldn't it would be opening the valve too? Obviously that's not happening. Maybe it doesn't need power at all, but it does have two little dip switches on a circuit board inside that apparently do something.

Now I'm wondering how it works.

[Randy Stankey]

That's what I was wondering.

If it doesn't need batteries or electricity from the furnace, how DOES it work?

[Mike Blakesley]

Must be magic. I've got a thermostat in the theater that has a digital readout on it and it's been working for 15+ years and I don't even know if it has a battery in it.

[Harold Hallikainen]

Did older thermostats have a mercury switch? I always thought they had a coil of bimetal that would flex as the temperature changed, causing it to hit a metal contact completing the circuit, or opening the contact. With gas furnaces, the main gas valve was powered by a thermocouple on the pilot light. If the pilot light went out, the main gas valve would not open (preventing your house from exploding).

Today's stuff is a lot more complicated! Our furnace has a big board with a microcontroller that does a bunch of stuff. It's something like: Turn on exhaust fan. Verify exhaust airflow. Turn on gas to pilot light. Make sparks with the igniter. Verify that the flame started. Wait a bit for the air to hear up. Turn on the main blower. If internal temperature gets too high, shut everything down.

We recently had to replace that board because it was intermittent. There is a flashing "idiot light" that is supposed to give you an error code. But removing the furnace cover to see the board and its light removes power from the board. If you hold the interlock switch to power the board up again, the error is gone and the furnace might run fine for a couple weeks. The techs just replaced the board, and our furnace now works fine (and I have an interesting board to play with).

On thermostats, my brother had a fancy thermostat that he could not figure out how to use. He bought a simple thermostat with a temperature dial and a heat-off-cool switch. There are something like a half dozen wires going to it. The "manual" does not tell you what the wires or terminals do, but, instead have rather convoluted instructions of what color wire goes to which terminal unless it's Tuesday, then they go somewhere else. We had a wire that I disconnected from the old thermostat that was none of the colors mentioned in the manual. So, I had to call customer service. Their answer was "leave that one unconnected." Well, it works, so I guess they are right. But more information in the "manual" (one sheet of paper) would have helped.

A bimetal thermostate has just two wires for heating only or three for heating/cooling. Not too complicated.

Harold

[Leo Enticknap]

We went the exact opposite route to Frank, and installed a Honeywell T9 smart thermostat with several satellite combined temperature/motion sensors (so it knows what rooms are occupied and what not) around the house. It was a significant up front investment, but we think it was worth it. That having been said, it's difficult to know if it's paid for itself (or will do so during its service life) in efficiency savings, but the algorithms it uses are certainly clever. It has three modes: heat, cool, and heat or cool as necessary to keep the ambient temperature between a lower and upper limit. We can program it to prioritize certain parts of the house during certain parts of the day or week. For example, between 10p and 6a seven nights a week, it is programmed to keep the temperature between 70 and 77 in our bedroom and our son's, but to ignore the rest of the house (with the result that the kitties all want to be in one of those two rooms overnight, too). During weekdays between 7a and 5p, it's programmed to keep the main downstairs living area between 68 and 80, and ignore everywhere else. On weekdays between 5-10p, it's set to try to keep the temperature between 72 and 77 in all the rooms in which it detects motion; and so on and so forth. It only has two blunt instruments at its disposal, because all it can do is to fire hot or cool air into all the rooms at once. But even so, I've noticed that it has become more accurately focused on when it actually does that.

It's obviously gathering data as to our daily routine, because during the year or so after I installed the system, I noticed that it would anticipate ahead of time what was likely to happen, and start the furnace or backyard cooler proactively, a little before it knew that the living room, office, bathroom, etc. would be in use. I guess the only worry is that that of privacy: we're allowing Honeywell to gather "big data" about or routine and movements, which presumably they then monetize. The main base station thermostat is wifi-equipped and needs an Internet connection, so it is clearly phoning home on a regular basis.

The main base station takes its power from the 12v feed from the furnace's motherboard, and the satellite sensors each use a pair of AAAs. Rechargeables work in the satellites, with each charge lasting around 8-12 months, so the cost in batteries is very much livable with.

[Frank Cox]

20250120_175703.jpg
Here is a mercury switch thermostat that controls the heat in my living room. It's about thirty years old. Works fine and no power of any kind is required to make it operate.

I also solved the mystery of the new Honeywell thermostat power source.

A close reading of the manual reveals a sentence that says "This thermostat contains a Lithium battery which may contain Perchlorate material. Perchlorate Material—special handling may apply, See www.dtsc.ca.gov/hazardouswaste/perchlorate". So I guess there's a tiny permanent battery hidden somewhere in there. Here I thought I had a permanent thermostat but I guess it's a thermostat that will last until it's little battery quits.

Oh well. Still better than having to keep changing thermostat batteries.

[Harold Hallikainen]

Our "smart" thermostat consists of turning the switch to off when we go to bed, then turning it to heat when we get up. In the fall or spring, we use a swamp cooler with a fairly complicated thermostat since it has to control water pumps as well as a blower. In the summer, when the swamp cooler does not work due to the monsoon humidity, we set the one thermostate on cool with the temperature at 80F during the day and 70F at night.

There are some interesting Internet connected thermostats that electric utilities can use for load shedding, dropping off some people's air conditioning for a bit during peak loads. Utility load management has been interesting over the years. A while back people could get a discount on electricity if they put a radio linked box on their air conditioner so it could be remotely shut down. A lot of people bypassed the box and kept the discount. I think that now utilities are giving away smart thermostats so you can control your heating/AC from across the world. And they can too.

Harold

[Randy Stankey]

Yeah, the traditional mercury in glass thermostats were just a coil of bimetalic ribbon with a three-pin mercury capsule switch balanced on the end.

When we were kids, our parents used to yell at us for turning up the thermostat in the winter. What my parents didn't know was that the round thermostat on the wall was just a little bit loose and you could tilt it, just slightly, on its mount so that the switch would tip on, making the thermostat run until the temperature gets up to 80 deg. instead of the 72 deg. my parents set it at. Mom might walk into a room and say, "Gee, it's hot in here!" then tell me to go turn down the thermostat. At which point, I'd just twist the whole thing back to its original position. How could anybody tell that I was turning the whole thermostat instead of just the dial? They'd have to be burning your hands with their eyes. The thing is ROUND, for cripes sake!

The secret is to make sure you don't do it too often. If something always seems to happen only when you are around, people are bound to start asking questions.
Still, I don't think anybody knew. If they did, they didn't say anything. Or, maybe my brother was doing it, too, so he couldn't really rat me out for something he was doing himself.
Or, maybe everybody knew but nobody really cared.

Oh, yeah! Years ago, I got a call from the electric company making me an offer of a discount if I let them install one of those load shedding devices.

I told them:
"You're telling me that you want to install a device that can control mechanical and electrical systems, inside my home, that can turn things on or turn things off, at any time, with a few seconds to a few minutes warning, at best?

The person was like, "Yeah, I guess..."
I said, "No freakin' way!"

That was the end of THAT!

[Tony Bandiera Jr]

Does it use the 12v source from the furnace for power? When you said that it doesn't need batteries, Galinstan is the only thing besides mercury that I know would work in place of a thermocouple and wouldn't need a source of electricity to work.

Even non-battery-powered thermostats might have a circuit board inside them, nowadays. It's just easier to manufacture that way but even mechanical thermostats might need to have switches inside in order to configure them for the type of furnace they are used for. (e.g. Whether the system has both, heating and cooling, or just heat.)

All HVAC equipment (with the exception of baseboard electric heaters) uses 24 v AC for the controls.

A two wire thermostat is a simple switch which turns on the gas valve coil, oil burner relay or heating elements relays, depending on if the furnace is gas, oil or electric, respectively. The newer electronic versions of a two-wire stat, if they don't need a battery, just draws the power it needs using the valve coil or relay's coil resistance in series. (like the old christmas lights in principle.) Otherwise a third wire is used to bring 24v AC directly to the stat, or the stat uses batteries.

The additional wires in other thermostats are for the A/C condenser (outdoor unit), first or second stage A/C compressors (in larger units) and emergency heat strips (heat pump systems) and finally, manual indoor blower controls.

Wire colors are supposed to follow a standard, but rarely do. If you are replacing an old stat. take note of what color wire is on which lettered terminal ( R,G,W, Y1,Y2, G etc.) and put them back the same way on the new stat.

The real fun begins on commercial systems with BMS (Building Management System) controls. I did a few jobs for Trane on medical buildings and their BacNet controls, it got ridiculously complex. The system operated in either heating or cooling mode (depending on the season) with very large and dangerous blowers supplying the conditioned air to multiple rooms/spaces, which had thermostats and VAV's (Variable Airflow Valves) controlled by the thermostats. Here in Idaho, the cooling was accomplished by chilled water as normal during the summer, but in the winter the cold outside air was mixed with the hot water coils to regulate the indoor air temps. All of which were ran by proprietary PLC master and slave controllers.

Another common trait of HVAC equipment in general is extremely sloppy wiring, controls and otherwise. Open the cover on a typical furnace and it looks like a wire factory puked inside.

[Harold Hallikainen]

When i started college in 1969, the dorms had a clever system. Above the door in each dorm room was a heating/cooling vent. Behind that was a heat exchanger. The heat exchanger had a bypass valve. A thermostat in the room controlled the bypass valve and a blower. If it was hot outside, cold water was piped around. If it was cold outside, hot water was sent. The thermostat would let you tap inro the hot or cold as needed. Very sime system!