Digital Thermostat Quit Working

Last night it began getting cold in my house. I went to the thermostat and found out the Digital readout was blank. I pushed the UP button and the furnace did not turn on. I popped it off the wall and found two AAA batteries. Of course this is 3am and there are no stores open closer than a 25 mile drive (I live in a rural area).

Then I remembred that my small LED flashlite has AAA batteries, so I used those. The thermostat had alkaline batts, but the ones in the flashlite were carbon-zinc, but they worked fine and I had heat again.

That thermostat was installed in here about 4 years ago, when I had some furnace work done. I never knew it even had batteries, thinking it was powered from the transformer on the furnace. It's NOT a programmable t-stat, so aside from the digital readout, it's nothing more than a plain thermostat.

I never knew those thermostats would completely fail to start the furnace when the batteries went dead. Apparently so.

Now that I know it has batteries, I will replace them yearly, but I'm actually thinking about replacing that t-stat with a standard analog one. The last thing I need is for the pipes to freeze because the batteries failed, when I was not at home. (Even new batteries can be bad). There is no advantage to having a digital readout for me. I hardly ever change the temperture anyhow.

My question is how does this sort of thermostat switch the furnace on and off? Is it a semiconductor type of switch or what?

Yea, I did buy some new alkaline batteries today and put those in this thermostat.

(I still think the best Thermostats made were those old round Honeywell ones with the mercury switch. They lasted decades and never needed any repairs. So much for so called "improved technology").

Reply to
oldschool
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I had at least one fancy stat that ran off Line with battery backup. Most I've seen have a two way mechanical relay of some sort except the one that ran on line I think. The digital ones have better accuracy with less temp swing, and some have adjustable swings. I don't like the many that limit cycles. By the way, I've had a few electronic types go bad. The mechanicals are fairly reliable.

Greg

Reply to
gregz

Mercury is prohibited because of environmental considerations.

Coould you precise the exact failure ?

Are the batteries in the right position ?

Did you check them with a meter ?

Reply to
Look165

Yep, but there are lots of them still around. I know I have several of them in my shop, if I can remember where I put them.

I said it works now, since I replaced the batteries. No need to check anything more. I am just wondering how this thing is switched.

By the way, I got a surprise. I found a manual for it on the Honeywell site, and it IS a programmable 'stat. After having it for 4 years, I found this out.... Not that it matters, I dont live on a schedule and I dont like it cool when I'm sleeping. I just keep it at 68 deg all the time. Once and awhile I may raise it a few degrees when I take a bath, othrsise it stays the same.

When that guy installed it, all he told me was to use the up / down arrows to set the desired temp, and press the HOLD button to keep it at that temp. He never told me it was programmable or gave me a manual. But now that I have the .PDF manual, I know what it does, and I did set the correct time on it, which I never knew how to do, nor did I really care, since I got plenty of clocks.

I still might replace it with an analog 'stat. Knowing this could cause a loss of heat, just because of batteries, makes me uncomfortable. Normally I am home in the winter, but like when I visited distant relatives and stayed overnight for Christmas, I could have ended up with frozen pipes if the battery failed. I dont like that idea....

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>> Last night it began getting cold in my house. I went to the thermostat 
>> and found out the Digital readout was blank. I pushed the UP button and 
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Reply to
oldschool

The thermostat on my wall, a moden programmable type, also has batteries and will not work without them. I don't understand why it was not made to be powered by the furnace. It has a mechanical relay inside, a reed relay I think. I can hear it when it switches on and off. The 3 AAA batteries will power it for about 3 years. Eric

Reply to
etpm

As it applies to two-wire heat-only stats:

a) Imagine a programmable thermostat that wiped-and-reset at each power failure. b) Furnace/boiler power can vary from micropile DC up to 24V AC. c) Imagine an expensive device that could be spiked very easily from many sorts of causes.

Hence the need for batteries. All the on-board relay does is switch the AC to the unit. Furnace/Boiler power is entirely isolated from the thermostat power.

Not to mention the additional complications with 5 and 7 - wire heat-pump systems.

Peter Wieck Melrose Park, PA

Reply to
pfjw

e:

ailure.

sorts of causes.

C to the unit. Furnace/Boiler power is entirely isolated from the thermosta t power.

systems.

Small correction/clarification: The *micropile* (thermocouple driven by th e pilot light) fires a *millivolt* gas valve. Usually, such devices operate at/around about 0.700 VDC or so. And the typical thermocouple puts out abo ut 0.750 DC - or *just enough*.

Reply to
pfjw

If designed correctly the thermostat should hold all its memories during a power giich or fail without depending on the batteries. It should have some short back up battery or 'super capacitor' to keep the time for a few hours or more.

I would want it powered by the furanace for the problems stated above. Dead batteries and none in the house late at night or snowed in.

Reply to
Ralph Mowery

If a digital thermostat only had to run a furnace, life would be easy. However, it's also used for heat pumps, staged HVAC, solar, radiant (pump with pipes in the floors and walls), exhaust fans, etc. Note the maze of wires and functions. However, I think the real reason they run on battery is that batteries are cheaper and smaller than a regulated, protected, and approved power supply.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I'm guessing it could be any of those. mechanical relay or SS relay.

When my battery wore out years ago I wired in the Mercury relay that was right next to the new electronic one. (previous home owner had not removed it.) Like you it was at night and I just wanted heat. I've never un-wired the Mercury switch. To me it seems like the perfect technology for controlling a furnace.

George H.

Reply to
ggherold

If the thermostat only turned the furnace on and off I could see battery power because there would be only two wires. But my thermostst will and does control much more and there are several wires connected to it. Not only that, my house is only 8 years old so the furnace is a modern one. A memory backup battery I can understand but my thermostat uses the barreries to power the complete thermostat, even powering the mechanical relay. And anyway, either the memory is non-volatile or there is a super capacitor on board because the memory is not lost when the batteries are replaced. Eric

Reply to
etpm

When I changed the batteries for a second time yesterday, the digital numbers were still there after I removed the batteries, but by the time I opened the batt package they faded away.

I just have 2 wires. I had a new furnace installed this last Novemnber. It still only has 2 wires. The guy never even touched the thermostat.

Reply to
oldschool

My two AAA batteries lasted 4 years and 3 months. I checked my paperwork for when that 'stat was installed. Like I said, I never knew it even had batteries or I would have changed them long ago.

Reply to
oldschool

Since tou mentioned thermocouples, I always wondered how they produce a voltage. I've probably installed/replaced at least 10 of them, but never understood how they work to create a voltage.....

Reply to
oldschool

OK, I'll bite.

a) A micropile/millevolt system will not make enough power to operate an el ectronic programmable thermostat. Here in this region of very much older ho uses, many of which had central heat first installed in the 1930s, many of which are gravity-types (both air and radiators), such furnaces are probabl y 10% of the operating base. There are several on my block alone. b) So, let's exclude that base and go with standard systems (2 - wire for n ow). Voltage can be anywhere from 12VAC to 24VAC. Ours is a 24V system (WM Ultra). Our neighbor is at 12V. So, at the very least the stat would have t o convert from AC to DC, be smart enough to understand both 12 & 24V, keep a super-cap or rechargeable battery charged and so forth. Adding to the co st. d) And, of course, we are still left with potential spiking issues - and it is not exactly as if one might unplug a thermostat in the case of nasty we ather, storms, or potential power-outages.

These are $40 items unless one wishes to get a "Nest" or some similar devic e. With all these refinements, probably the cost would double.

As it happens, all this applies even more-so to heat-pump stats as large mo tors (and associated spiking) are involved, as well as many more variations in voltage and functionality assigned to those 5-7 wire stats - some of wh ich are even proprietary by brand. Those, as it happens, are often mains-po wered via the system, due to that complexity.

Writing for myself, our 'annualized battery program' includes smoke/heat/ga s detectors, the thermostat, the flashlights (car and house) wireless mice and similar devices. Removed-still-good batteries go into things like my ra dios that take D or C batteries, remote-controls and other non-critical ite ms until death. As we use only name-brand batteries with good warranties, a nd within their use-by dates, we have no worries about explosive outcomes, if any. We keep spares in the house for all critical applications (we once went 9 days without power after Sandy, so this is no small thing.

Does it cost us a few bucks per year? Do we contribute some nasty chemicals to the waste stream - Yes and probably. But there is peace-of-mind.

Peter Wieck Melrose Park, PA

Reply to
pfjw

And unless they can get power from the control line to the furnace, it may not be convenient to have a power supply. There are suggestions on where to place the thermostat, and there may not be a convenient outlet just below.

Michael

Reply to
Michael Black

They convert a small amount of the heat differential (hot end to cold end) into DC electricity. It is, mechanically, a bi-metallic strip inside the pr obe that converts the heat. They are quite elegant little devices, but, sad ly, do not make much voltage (0.750V) at tiny, tiny currents. Just enough t o trip a gas-valve. Certainly not enough to drive electronics on a thermost at.

Reply to
pfjw

My programmable thermostat has an auxiliary thermoswitch that will turn on the furnace if the temperature goes below around 50F or so, even if the battery fails or the thermostat itself fails. It's a Robertshaw, about 20 or so years old. The thermoswitch is clearly visible on the baseplate assembly. The thermostat itself is removable from the baseplate so you don't have to stand at it when setting the program.

I'd expect most decent battery powered thermostats to have a similiar feature. Otherwise I could see lawsuits against the manufacterers or the heating contractors when the battery fails and the pipes freeze and cause extensive water damage.

Reply to
Jerry Peters

Have the same here.

24 volt transformer on the AC -furnace and the units either pass the 24 volt current or not to switch the relay on the furnace _ Ac depending on whether it's set on heat or cool. Relay on the AC - Furnace has a evaporator fan sequencer to keep the fan running for some minutes after heat or cool and prevent the AC compressor from restarting so soon as to damage the compressor.
Reply to
Wayne Chirnside

I'm gonna check mine and see if it has the extra thermoswitch. If not I should wire one in. It is a great idea. A friend of mine had his house freeze while he was gone due to some sort of malfunction and a pipe burst inside the house. Water ran for over a week before the problem was discovered. The leak was quite small and still damaged a lot of stuff. Eric

Reply to
etpm

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