Thermostat Questions

Hello:
Have just started looking for a new heating thermostat for my gas, forced hot water system. Probably a Honeywell.
Been many and many a year since I've looked at these.
Have no schematics avail. for any of them, hence my ignorance, in part.
I guess most of them are now with a backlighted digital display, and either programmable or non-programmable.
They all seem to require 3 wires, power from 24 V, BUT also a few batteries, surprisingly.
Questions:
a. Why the batteies ? Can't everything be derived from the 24 V easily ? If it's for memory backup, don't they use a memory that doesn't require constant battery power ?
The implication is that if your batteries run down, your heqting system stops. Not good. What actually do you think the batteries do ?
b. My system now is quite old. Only 2 wires going to a simple SPST mercury swith in the thermostat.
Will these new thermostats work on 2 wires ? I don't see how they can ? If so, how ?
Thanks, Bob
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Robert11 wrote:

The batteries provide two functions: Keeping the thermostat clock running during power outages. Providing power to the t'stat internals during two wire operation when it closes its heat contacts.
Many t'stats (programmable ones) have clocks to keep track of time and day of week to implement temp. setback points.

Many will do so just fine. They 'rob' a few milliamps through the furnace gas solenoid (not sufficient to close it) to charge the t'stat batteries and operate its uP. When the t'stat requests heat, it closes a contact across these two wires. Since this removes the 24V input supply, the batteries carry the internals over during this period.

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Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
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This is a multi-part message in MIME format. --------------040702080308030100060005 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8bit
I have had two different digital thermostats (both from Hunter) over the years. Each used either AA or AAA batteries. Both failed after a year or two of operation. From the front panel, there was no overt indication of battery failure but the units could not pull in their heat pump relay. The first time I spent a day checking all the wiring to the heat pump and was ready to place an expensive service call. This last time, I was sure the batteries were the problem and they were.
If you buy one be sure to mark the unit with the install date and change the batteries every year.
Paul Hovnanian P.E. wrote:

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Joe Leikhim K4SAT
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Not all thermostat locations have both ends of the 24V transformer brought out to power the things. In fact, it seems few do. Until programmable ones existed, there was little reason. One of my programmable ones doesn't even have the option to use 24V power. (I tried, since it has an unused spare wire and I have access to both ends, I wanted to use 24V power and the batteries as backup only)
You need batteries to maintain the clock, as well as to power the thing when calling for heat.

The programmable thermostat I first had had a clever setup to work off of 2 wires. It robbed a few mA when the heat relay was open, and when the relay was closed it had a winding of a transformer in series with the t'stat wires to get power then as well. Unfortunately it was sensitive to contact bounce in a limit switch (in series with t'stat) which a capacitor and MOV paralleling the t'state took care of. It failed after many years.
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No answers to your questions, just a note about what happened with mine. I replaced a mechanical thermostat with an elecronic one. I wired it according to the instructions. Most everything worked fine. Except that the AC wouldn't shut off once it started. There seemed to be some backfeed putting about 9 volts to ground on the neutral for the furnace transformer secondary. Things worked fine with the mechanical thermostat but not with the electronic gizmo. Grounding the neutral of the transformer secondary fixed the problem.
Dean
(The EEs are probably cringing since it's really a grounded conductor instead of a neutral).
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http://toad.net/~jsmeenen/wiring.html
http://toad.net/~jsmeenen/thermostat.html
http://yourhome.honeywell.com/yourhome/Applications/Wizard/Wizard.aspx
Robert11 wrote:

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