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From: GregS on 1 Feb 2010 09:09
In article <ecf06c6d-6550-4716-9829-9249e92ed919(a)21g2000yqj.googlegroups.com>, Mikepier <mikepier(a)optonline.net> wrote:
>On Jan 29, 9:16=A0pm, 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.

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

From: Dave Plowman (News) on 1 Feb 2010 09:24
In article <hk6n4m$qui$1(a)usenet01.srv.cis.pitt.edu>,
GregS <zekfrivo(a)zekfrivolous.com> wrote:
> 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.

--
*A dog's not just for Christmas, it's alright on a Friday night too*

Dave Plowman dave(a)davenoise.co.uk London SW
To e-mail, change noise into sound.
From: Jim Yanik on 1 Feb 2010 09:33
zekfrivo(a)zekfrivolous.com (GregS) wrote in
news:hk6n4m$qui$1(a)usenet01.srv.cis.pitt.edu:

> In article
> <ecf06c6d-6550-4716-9829-9249e92ed919(a)21g2000yqj.googlegroups.com>,
> Mikepier <mikepier(a)optonline.net> wrote:
>>On Jan 29, 9:16=A0pm, 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.
>
> 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
>
>

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.

--
Jim Yanik
jyanik
at
localnet
dot com
From: GregS on 1 Feb 2010 09:40
In article <50e2ca86fcdave(a)davenoise.co.uk>, "Dave Plowman (News)" <dave(a)davenoise.co.uk> wrote:
>In article <hk6n4m$qui$1(a)usenet01.srv.cis.pitt.edu>,
> GregS <zekfrivo(a)zekfrivolous.com> wrote:
>> 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.
>

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.

greg
From: Jim Yanik on 1 Feb 2010 09:41
zekfrivo(a)zekfrivolous.com (GregS) wrote in
news:hk6ovs$rb0$1(a)usenet01.srv.cis.pitt.edu:

> In article <50e2ca86fcdave(a)davenoise.co.uk>, "Dave Plowman (News)"
> <dave(a)davenoise.co.uk> wrote:
>>In article <hk6n4m$qui$1(a)usenet01.srv.cis.pitt.edu>,
>> GregS <zekfrivo(a)zekfrivolous.com> wrote:
>>> 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.
>>
>
> 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.
>
> greg
>

the relay would be in the AC unit itself.The TS just switches the low
voltage to the relay.

--
Jim Yanik
jyanik
at
localnet
dot com
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