I had my old heat/ac system replaced a year or so ago with a new Rheem heat/ac system. I noticed the other night that it was cold in the house. I checked the thermostat and it said "replace the batteries", which I did and all was well again. I thought the Honeywell thermostat ran on ac from the furnace. Apparently mine doesn't but runs on the batteries only. Is this the way it is supposed to be or did the techs who installed the system not wire it right? I could run wire with more connections if there wasn't enough wires. The old round Honeywell apparently ran on ac because for 30 years it didn't have batteries.
On Saturday 23 February 2013 08:19 snipped-for-privacy@fake.sig wrote in alt.home.repair:
Some do, out of design laziness. They find it easier to require batteries and drive a relay ensuring complete isolation between mains and control interface.
Most digital thermostats will run on batteries or 24 volts AC with the batteries being used to maintain memory for the settings. Your old round Honeywell thermostat had a coiled bi-metallic spring attached to a mercury switch to control your HVAC system. It was mechanical requiring no extra power to function since it was a simple switch. The cable for the older mechanical heat/cool/fan thermostats had only 4 or 5 wires and when your new digital thermostat was installed there may have been only enough wires to provide basic control signals to the furnace and no spare wire to provide a common ground for 24 volt power from the air handler/furnace. ^_^
You should call the installer, and ask him. We'd only be guessing. Many thermostats are line powered, but yours appears to be the exception. Who knows, maybe it was wired wrong.
Not sure about most running on 24V or batteries though. There are certainly both kinds though, but agree with the analysis. If it was a system with an old mechanical thermostat, it's very possible that there was not an extra wire available to supply the 24V. In which case, using a battery only thermostat is probably what almost every installer would do.
There should be an installation manual for the thermostat, available online if he doesn't have one, which answers the question of if the thermostat supports being powered by 24V. If it does, then the question is if there is an extra wire available, if he wants to run new cable, if it's worth it, etc. There are a lot of thermostats out there that are battery only. Another thing that may be of interest is if the thermostat has a failsafe mode where a mechanical switch closes at around
45 or so to prevent freezing in case the batteries go dead.
That old round stat had a glass bulb filled with mercury. When it expanded and contracted with changes in temperature, it would make or break the contacts. Simple, reliable, repeatable, albeit in a fairly big off and on range for the setting.
with electronics, you can program different temperatures for different days and different times of day and for heating and cooling. They also hold it steady at a fairly narrow range.
Regardless of what actually power the workings, you need power constantly to maintain the memory. Enter a battery. Most will last at least a year, some much more. Make it a habit to replace the battery once a year at the same time you do the smoke detectors.
Of the digital thermostats I install I always pickup the dual powered units at the supply house. They're not that expensive and I will usually go with the non-programmable unless a customer requests one. ^_^
If your thermostat cable contains 5 wires and you have a single stage heat and cool thermostat there are enough wires to get 24 volt power to the thermostat unless the installer is lazy. ^_^
C -- Red 24 volt hot W -- White Heat Y -- Yellow Cool G -- Green Fan
B -- Blue 24 volt common/ground often connected to the air handler cabinet.
On Saturday 23 February 2013 13:19 Ed Pawlowski wrote in alt.home.repair:
There are a couple of other options:
E2PROM which is common on many microcontrollers (eg Atmel AVRs, my personal favourite 8 bit) and you don't need much even on a 7 day stat with 4 periods/day.
Supercapacitors which at low voltages are very small for something with a farad odd of capacity which would keep a bit of SRAM going for a long time in power outages.
Though you really need the supercap either way to keep the clock running - but programming in E2 is cute in that it relly will *not* go away by accident and makes doing a device reset much easier.
I think the real reason is that testing/approval for anything with a mains (120 or 240V) PSU is considered "hard" (and is probably harder than it was say 30 years ago). That's why so many things these days have wallwarts - leave the PSU isolated and the design to mains specialists.
A modular PSU is not so much of an option for a tiny form factor device.
Ironically, you could virtually run these things with a reactive dropper and a rectifier as the demands are so low - but then your circuits are no longer isolated from the mains, which is not necessarily a problem (eg the pot on a dimmer switch is wired into the mains) but it gives designers heebie jeebies and does demand more testing.
BTW - did the US really have mercury stats? I've never seen a non electronic one that wasn't a bi-metallic mechanism in all of 40 years in the UK and that includes a fair few Honeywells.
The old round honeywell didn't "run" on anything - it was a simple mercury switch on a bimetal spring. My electronic stat runs on the 24 volt thermostat supply transformer but has a battery to back-up the memory settings in case of power failure. It will still operate with a dead battery but I believe if the power were to go out the clock would lose it's setting and the thermostat would revert to a preset fall-back setting.
What contacts are used on your "non mercury" bimetallic Honeywells??? Every one I've ever seen had a "wet contact" bulb - which had mercury in it. The bulb was about an inch long and 1/4 to
5/16 diameter with 2 wires coming out - 3 on the odd one.
Yes, those were very common back in the 60s, 70s. Don't know when they stopped making them. I also remember back in the 60s they had AC wall switches that were the silent type that used mercury.
The big issue with thermostats is not so much the memory loss if the batteries die. It's that in areas subject to freezing you could freeze the house due to a bad battery, if it's unoccupied for a period. Some thermostats do have some kind of failsafe 45F or so mechanical that kicks in, but not all of them.
If you go to a "Nest" thermostat, a new round one but electronic, it has a self charging battery using power from the 24 volt transformer on the furnace. While no larger than the old Honeywell round, it is chock full of goodies such as Wi-Fi and proximity sensors to detect when the house is occupied.
Thanks for the help guys. I called the installer and when I can get an answer back I will know what to do. It looks easy enough to run an extra wire or two for getting the 24v ac to the unit if that was the problem. Probably the installer was lazy and didn't want to do it right. Either way I think I want ac to the thermostat in case I'm away from the house and the batteries die and the house freeze up.
On Saturday 23 February 2013 15:41 snipped-for-privacy@snyder.on.ca wrote in alt.home.repair:
Dry contacts - some sort of alloy that was common in switches - possibly silver/cadmium - there were a few variants...
The bimetal mechanism had a "snap" action engineered into it to cause a clean make and break. These stats tended to have quite a large hysteresis - probably due in part to the snap action. So it was quite common to have a small heater (nothing more than a 5W ish resistor under the bimetal strip to cancel some of the hysteresis. It was wired between the stat output and neutral to cause a bit of local heating when the stat was on "call for heat". This was optional and required a neutral present at the stat, which some installations did not have.
Looking at Wonkypedia, I do see a lot of historic info on thermostats with mercury switches. Curious - never seen one here - and we never used to be shy of mercury until the last decade or so...
I have one in the house we're selling. It's a great thermostat but on the pricey side. I'll probably buy another when we sell the place (in a couple of weeks, hopefully) and change both in this house.
Hi, You're talking about nalog vs. digital 'stat. You can make the new one run on 24V AC using a power adapter. Ours is a wireless one so no wires between 'stat and system. I use Lithium batteries which last ~3 years.
If your 'stat is not on battery, when there is a prolonged power outage, programmed settings could be lost.
Hi, Yes, our wireless one has RAS(return air sensor) mounted on the return air duct. If whatever reason 'stat fails to communicate to the system and the temp drops near freezing automatically temp setting falls back like that to prevent freeezing.
Hi, You must be real OT like me. This is digital era. SS relays, AI logic ASIC what you are talking about is past history. Fuzzy logic, AI are taking over things like that.
It's possible it just needs one more wire, a common for the 24VAC, that's my guess.
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Thanks for the help guys. I called the installer and when I can get an answer back I will know what to do. It looks easy enough to run an extra wire or two for getting the 24v ac to the unit if that was the problem. Probably the installer was lazy and didn't want to do it right. Either way I think I want ac to the thermostat in case I'm away from the house and the batteries die and the house freeze up.
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