Heat Thermostat Question, Please


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Old 12-31-06, 07:32 AM
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Heat Thermostat Question, Please

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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Old 12-31-06, 10:51 AM
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That is a very good question. I know from experience. If you only have 2 wires at the thermostat, you have no wire for the common terminal. That seems to be the most common configuration. When the battery goes dead, the heat goes off. The low battery warning does come on ahead of time but when it is not seen or is ignored, heat goes off.

Ken
 
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Old 12-31-06, 10:54 AM
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The batteries only power the clock and programming. The T-stat draws very little current. For peace of mind, change the batteries once a year, when you change the smoke detector batteries, and don't worry about it.
 
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Old 12-31-06, 11:09 AM
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I don't want to be argumentative but without either a common wire connected or batteries, the thermostat will NOT energize the heat. If you think about it (which I did not until my house got cold) the heating relay in the thermostat needs power to close. If the batteries are dead, as soon as the relay closes using the 24 volts from the system, the voltage goes to 0 and the relay will drop out. I wish they allowed rechargable batteries that would be charged by the 24vac during system off time. The old thermostats used to be like that but I believe it was cost prohibitive to keep building thermostats like that.

Ken
 
 

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