Honeywell thermostat

LSMFT

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Oct 29, 2010, 11:16:47 PM10/29/10

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This thermostat I have has a mechanical relay. But.... When closed is a
closed circuit or dead short, fine that works for me. BUT when it's open
it still has 20 ohms and will not shut off a circulator. Why in hell do
they make it have 20 ohms when it's open?????? It should be infinity.

--
LSMFT

Simple job, assist the assistant of the physicist.

Michael A. Terrell

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Oct 29, 2010, 11:35:54 PM10/29/10

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LSMFT wrote:
>
> This thermostat I have has a mechanical relay. But.... When closed is a
> closed circuit or dead short, fine that works for me. BUT when it's open
> it still has 20 ohms and will not shut off a circulator. Why in hell do
> they make it have 20 ohms when it's open?????? It should be infinity.

Not if you want the anticipator to work.

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

Dave Platt

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Oct 30, 2010, 1:34:45 AM10/30/10

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>This thermostat I have has a mechanical relay. But.... When closed is a
>closed circuit or dead short, fine that works for me. BUT when it's open
>it still has 20 ohms and will not shut off a circulator. Why in hell do
>they make it have 20 ohms when it's open?????? It should be infinity.

I suspect that might be the resistance of the "anticipator"... a small
built-in heater, which is normally energized when the furnace is
running. The heat it generates, helps cancel out the effect of
thermal lag (i.e. the room air heats up faster than the bulky
thermostat does) and improves the ability of the thermostat to maintain
good temperature control.

--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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Art Todesco

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Oct 30, 2010, 9:01:57 AM10/30/10

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On 10/30/2010 1:34 AM, Dave Platt wrote:
>> This thermostat I have has a mechanical relay. But.... When closed is a
>> closed circuit or dead short, fine that works for me. BUT when it's open
>> it still has 20 ohms and will not shut off a circulator. Why in hell do
>> they make it have 20 ohms when it's open?????? It should be infinity.
>
> I suspect that might be the resistance of the "anticipator"... a small
> built-in heater, which is normally energized when the furnace is
> running. The heat it generates, helps cancel out the effect of
> thermal lag (i.e. the room air heats up faster than the bulky
> thermostat does) and improves the ability of the thermostat to maintain
> good temperature control.
>

All of the heat anticipators I've seen, have been in series with the
heat contact, not in parallel. The anticipators for cooling are
generally across the cool contact. That's the way I've seen it for
mechanical thermostats. But, most of the good electronic
thermostats have the anticipator function built into the software.
So if the thermostat is electronic with a relay driving the output,
I'd suspect something is wrong. Some electronic thermostats have
triacs in the output, which can have leakages. I've had one that
actually slightly hold open the gas valve. Honeywell confirmed that
they had a problem with some gas valves playing nice with some of
the triac models and sent me a new one, with a relay output.

LSMFT

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Oct 30, 2010, 12:47:28 PM10/30/10

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I put voltage across the relay contacts that were supposed to be open
and fried the electronics that was holding it partially closed.
Everything else still works and the relay still clicks. Now I can use
the contacts like a switch like I wanted in the first place.

Jeffrey Angus

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Oct 30, 2010, 12:53:18 PM10/30/10

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On 10/30/2010 11:47 AM, LSMFT wrote:
> I put voltage across the relay contacts that were supposed to be open
> and fried the electronics that was holding it partially closed.
> Everything else still works and the relay still clicks. Now I can use
> the contacts like a switch like I wanted in the first place.

Well...that's one way to fix it.

Jeff

Michael A. Terrell

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Oct 30, 2010, 2:47:20 PM10/30/10

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The Bill Turner way. :(

Art Todesco

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Oct 30, 2010, 3:10:42 PM10/30/10

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There was probably a contact protection network across the relay
contact that got partially destroyed. There are lots of possible
protection networks including varistors, diodes, RC or RLC, etc.
You say it measured 20 ohms. Was that done with a standard
ohmmeter? And if so, if you reverse the meter's polarity, what
does it read. Well, I know you can't do that because it was smoked.
Of course, you are not supposed to put raw power across the
contact, even if it is open.

Michael A. Terrell

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Oct 30, 2010, 3:19:19 PM10/30/10

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That's ok. The contacts will probably weld themselves together
before long, and he'll have to replace it.

LSMFT

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Oct 30, 2010, 5:31:50 PM10/30/10

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It would not shut off the circulator so current still flowed. It could
have been 200 ohms instead of 20. It was useless that way.