Troubleshooting a digital thermostat
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From: DGF on 29 Jan 2010 20:01 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?
From: Stormin Mormon on 29 Jan 2010 21:02 Put it back on the wall. Run the temp way up. Go see if there is a 24 volt signal at W. -- Christopher A. Young Learn more about Jesus www.lds.org .. "DGF" <noname(a)noplace.nospam> wrote in message news:hk00ce$f9b$1(a)speranza.aioe.org... 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?
From: PeterD on 29 Jan 2010 21:16 On Fri, 29 Jan 2010 20:01:01 -0500, DGF <noname(a)noplace.nospam> wrote: >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? Like making a test circuit that provides the necessary 24 volts?
From: Mikepier on 30 Jan 2010 06:06 On Jan 29, 9:16 pm, PeterD <pet...(a)hipson.net> wrote: > On Fri, 29 Jan 2010 20:01:01 -0500, DGF <non...(a)noplace.nospam> wrote: > >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? > > Like making a test circuit that provides the necessary 24 volts? I think he's right. Most T-stats have "power stealing" circuitry, meaning they depend on the 24VAC from the HVAC system.
From: trader4 on 30 Jan 2010 06:34
On Jan 30, 6:06 am, Mikepier <mikep...(a)optonline.net> wrote: > On Jan 29, 9:16 pm, PeterD <pet...(a)hipson.net> wrote: > > > On Fri, 29 Jan 2010 20:01:01 -0500, DGF <non...(a)noplace.nospam> wrote: > > >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? > > > Like making a test circuit that provides the necessary 24 volts? > > I think he's right. Most T-stats have "power stealing" circuitry, > meaning they depend on the 24VAC from the HVAC system. 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. |