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.
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.
Can you hear it operate? Mine has a sort of electrically operated switch rather than solid state or relay type to reduce battery consumption and are clearly audible when they make or break. It also only operates when in its housing so to test you'd need to bring out wires from that to connect to a DVM.
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Dave Plowman dave@davenoise.co.uk London SW
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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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If it's a two wire circuit then the contacts should show a short when the thermostat is set to the 'Heat' mode (if applicable) and the thermostat is set for at least 2 degrees above room temp.
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.
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:-
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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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Dave Plowman dave@davenoise.co.uk London SW
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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.
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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Dave Plowman dave@davenoise.co.uk London SW
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snipped-for-privacy@zekfrivolous.com (GregS) wrote in news:hk6n4m$qui$ snipped-for-privacy@usenet01.srv.cis.pitt.edu:
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.
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.
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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Dave Plowman dave@davenoise.co.uk London SW
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There are solid state relays that will supply fairly high current at
240vac with a 24vdc switching voltage. They are very common on automated machinery, I'm not sure if they would be used in a thermostat, the ones we use are pretty spendy. You can get ones that are latched, and will only need a signal to change state. Some of the cheaper digital thermostats that I have installed in rental properties never make a sound. I've never really had to back engineer one though, if it fails and I don't have voltage coming through, then I toss it and get another. Of coarse, if you have a higher end thermostat then that may not be the thriftiest way to go.
When I test...
-I test to make sure I have a 24vdc supply at the thermostat, by checking for potential across the wires incoming wires with the thermostat off. This doesn't work all of the time, if there is a broken wire or a burnt out relay or fuse, or a bad ground.
-Then check the voltage drop again after the thermostat is supposedly engaged
-If that's good, check the voltage of the thermostat leads at the furnace or boiler or whatever. This can be dangerous if you don't know what you are doing. Usually there is a point on the outside of the furnace where you can tie in to test the wires.
The rest depends on the end piece of equipment that the thermostat runs. On many furnaces now, the controller will flash a code if there is a detectable failure, kind of like the computer in a modern car. So that's something to consider as well.
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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Dave Plowman dave@davenoise.co.uk London SW
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