Troubleshooting a digital thermostat

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How do you verify that a battery-powered digital thermostat is working like it should? From what I've read, it seems that it is supposed to electrically bridge the connection between R wire and the W wire, when it turns on heating. I detached a thermostat from the HVAC system and tested the connection between the two terminals using a multimeter *when the room temperature was well below the heating set-point*; the contacts on the thermostat corresponding to the R & W wires didn't seem to allow current to flow through. Given that it's an electronic device and may not respond to the small DC voltage from the multimeter the same way it may to 24 VAC, I can't tell whether the thermostat is good or bad.
Any ideas?
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Put it back on the wall. Run the temp way up. Go see if there is a 24 volt signal at W.
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Like making a test circuit that provides the necessary 24 volts?
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I think he's right. Most T-stats have "power stealing" circuitry, meaning they depend on the 24VAC from the HVAC system.
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Hook it back up. With the room temp above the heat setpoint you should have 24VAC between the wires that turn on the furnace. Set the thermostat above the room temp and the voltage should drop to close to zero, indicating the circuit is closed.
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In article

In the UK the switching volts can be mains. So these universal programmable stats seem to have some form of electrically operated switch which is isolated from the actual device - rather in the same way as a relay. But because they only take current from the battery when they alter status are more suited to this sort of app.
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On Sat, 30 Jan 2010 03:06:51 -0800, Mikepier wrote:

That is correct thermostats electronic or mechanical always depend on the power source in the HVAC system they control for power and are never self powered.
The battery is just there for power interruptions to keep the thermostat from resetting to a different temperature nothing more.
To test it you will have to provide the 24v source for it to operate from otherwise nothing will happen.
Gnack
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Gnack, you are mistaken:: a lot of thermostats ARE battery powered and work independantly of the voltage they are controlling. Some can even control millivolt systems for wall/floor furnaces. True, some are power stealing, and some must have a common wire connected, and use batteries to keep the program if the power is interrupted, but don't say ALL are that way, because it isn't so. .
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In the UK that's simply not true. There are many different makes that operate entirely on battery power. And can be used to control a mains *or* low voltage circuit. I'd be very surprised if the same models weren't on sale in the US. Something like these:-
http://www.tlc-direct.co.uk/Main_Index/Heating_Index/Therm_3/index.html
which are a very good way of upgrading an older system. Better modern ones tend not to have a room stat but a simple temperature sensor with the house temp being set via the boiler electronics.
I know US systems can be different from UK ones - but the same principles must apply. And much needed in these days of high energy prices.
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Most thermostats have a relay. One has to know the circuit to troubleshoot it. Some of the best thermostats have power stealing circuitry and will opperate without batteries, but most of the cheap ones just have a small relay, and contacts could have some resistance over time.
greg
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Battery operated thermostats don't have a conventional relay - ie one which takes power all the time it's made. That would run down the battery in short order. They use some form of 'motorised' switch, which only takes power when it changes state. On mine you can hear it operate.
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I always assumed a relay, and yes they are loud. On a soild state unit I could also hear a click, but I think that was from current flow. If units use a switch, I hope they have a safety overheat breaker.
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snipped-for-privacy@zekfrivolous.com (GregS) wrote in wrote:

the relay would be in the AC unit itself.The TS just switches the low voltage to the relay.
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Certainly in the UK it's a form of mechanical switch. It can carry mains or low voltage AC or DC - not really possible with a solid state device. My guess is it may be piezo operated. But that's only a guess. Mine is rather louder than a relay.
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In article

Thing is in the UK it was pretty common to have mains to a mechanical thermostat. Which also supplied an accelerator coil round the bi-metallic strip to improve accuracy. So aftermarket types can all handle this. On my last system here everything electrical was mains - gas valve, pump, three way valve, room stat and water stat. A very simple system which worked well for years. I've replaced the boiler with a condensing type which has electronics so has to have a low volt supply for that. But uses sensors instead of thermostats for house and water temperature which is now set via the programmer, rather than locally.
Aftermarket thermostats will most often be fitted to older systems here - so have to cope with either AC mains, or low volt AC or DC. Which I'd guess rules out a solid state switch.
But even a solid state switch will take some current when made - does it not? So not ideal for battery operation.
The programmable thermostat I retro-fitted to the old system - allowed you to set different temperatures for parts of the day and days of the week - used four AAs for everything - no power from the line - and they lasted about 2 years. They are a very worthwhile addition to an older system.
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On Tue, 02 Feb 2010 09:30:13 +0000 (GMT), "Dave Plowman (News)"

Sounds a bit like my Danfoss TP75. It uses an Omron latching relay type G6CK-2117P which works off a 20mS 3V pulse:- http://www.omron.com/ecb/products/pry/121/g6c_2.html
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Interesting - but nothing like those on my board. I no longer have it so can't take pics. The description isn't that clear - does it take (near) zero current when made or just a reduced amount? For long battery life it would need to be zero. Nor did the noise it made when the 'switch' operated sound anything like a relay.
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On Tue, 02 Feb 2010 14:54:02 +0000 (GMT), "Dave Plowman (News)"

No current at all. Imagine a balanced metal arm pivoted in centre and a coil at each end with some remanent magnetism. A current pulse through either coil is sufficient to overcome the small static magnetic pull and trip the bistable to the other condition. The Danfoss WP75H uses a much larger Gruner 703H to switch up to 25A for an immersion heater.
This article has a short explanation and some driving circuits:- <http://www.electronicsweekly.com/Articles/2008/11/21/44977/circuits-drive-single-coil-latching-relays.htm
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<http://www.electronicsweekly.com/Articles/2008/11/21/44977/circuits-drive-single-coil-latching-relays.htm
Excellent - thanks for that. I can think of a few uses I could put them to.
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snipped-for-privacy@zekfrivolous.com (GregS) wrote in

I believe the batteries power the clock. there's probably a diode to allow the 24VAC to power the clock until the mains drop off,and then the batteries take over.But the batteries don't power the relay that controls the system.
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