Digital Heat Thermostat: If Batteries Run-Down, What happens ?


Hello:
Have a gas, forced hot water heating system. Was about to order a Honeywell thermostat over the web, but had the following concern which just dawned on me.
Am interested in their digital model RTH 5100 which is non-programmable.
It says on their ad sheet that it maintains the set-point if there is a power outage. O.K., that's great.
But even if there is house power--
These units require a battery or two.
What happens if the battery runs down while you are away ? I know, use new, fresh, batteries, etc. But-
Will the furnace shut off if battery runs down ?
Or, is the battery only for the display, and the circuitry still works ? If so, how, as there's no current when the circuit is calling for no heat thru the 2 wires.
e.g., with no thermostat battery voltage, does the curcuitry default to a circuit closed condition, which would keep the furnace active, or... ?
A bit worried about this.
Thanks, Bob
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Replace the batteries once a year. I've had programmable state for about 20 years and never had a problem from battery failure.
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Edwin Pawlowski wrote:

Some of the digital models have a failsafe mechanical backup that will kick in at a fixed low temp like 45. You'd have to pull up the specs to see if this one has it.
For digital thermostats, I like the Honeywell LCD touchscreen. Not sure of the model, 8600?. Two key features I like are adaptive recovery, where you just set it for the time you want the new temp to be reached, eg 70 at 7AM. Based on experience, the thermostat gets the furnace going at whatever time prior to 7am that it figures out it needs to. It also has vacation hold mode, where you can set a temp to be held for X days before it resumes it's regular schedule.
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replying to trader4, Frank wrote: The only problem is that is that at 45 for a long period of time, the condensate will build up in the pressure switch tubes and the furnace will quit working.
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On Thursday, August 11, 2016 at 9:14:05 PM UTC-4, Frank wrote:

Says who, Mr. Responder to ten year old threads? I've had my furnace set to 45F while on vacation in the middle of winter, with no problems. Sounds like you have a crap furnace.
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Robert11 wrote:

    As Edwin stated, if you replace the batteries occasionally you will never have a problem. But I believe the settings (day, time, and temp) are lost if the battery gets too low. There is however a default setting I believe the thermostat will revert to, and that is somewhere around 70 degrees. So even if you lost your settings, it would not heat or cool the home excessively. Again, I believe that is what most would do and that is what mine do.
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Snip
That is what my Honeywell did, too,
aem sends...
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Robert11 wrote:

I don't know about all models, but many if not all just use the battery as backup to remember your settings. If the battery dies and if the power goes out, you will need to rest your preferences.
--
Joseph Meehan

Dia \'s Muire duit
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I added a regular thermostat set at 50 degrees, just in case the electronic one flakes out.
Cheap insurance if you ask me:)
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snipped-for-privacy@aol.com wrote:

Provided that the electronic one does not fail to say ... 110 degrees :-) Many years ago a buddy came home after a weekend away .. to a temperature of 90 degrees or so (I don't recall the exact temperature). I use the trusty old manual thermostats. I have a screwnail stop point at about 64 degrees ... and I turn it down a 1/2 " when I go out and at night, or a 1/4 " if I'm working around the house ... and to about a 1/8" of the stop point for sitting around. And I have a wall thermometer beside it for the reference. Those electronic thermostats are just not flexible enough for me ... or maybe it's the going to get my frikken glasses every time I need to access anything electronic these days :-).
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I am not sure of your concern but I went and read the manual to this thermostat and it is entirely digital as it has no manual switch. It does not say what it defaults to in case of no power battery or otherwise. My guess would be off but that is just my guess. In any case if you do not think you can remember to put new batteries in it or may be away from it for extended periods of time this may not be the thermostat for you. You may be better of with one like I have. I have a Robertshaw 9520. It is a digital thermostat that requires no batteries. I has a manual switch though for Heat, Cool and Off. With mine If the switch is in the Heat position and the power fails it comes back on with the heat set at 60, if the switch is in the Cool position it comes back on with the air set at 80, if the switch is in the off position it simply stays off.
Joe

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