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Simmyster

@comcast.net

Common Wire to T-Stat Adds Delay

I have a newer construction home in S. Florida. Tech came out for unrelated issue and found that a common wire was not connected to my battery powered Honeywell Pro2000 thermostat. He mentioned that although not required for use (not a high-end t-stat with wifi that requires extra power), its very important to have this connected anyway for the following reason: it adds a couple minute delay to click on compressor after a power outages, when power is restored rapidly. Without at least a couple minute delay, the compressor could cycle on too soon and could cause extensive damage due to built-up pressure. He mentioned this delay was different that the "compressor protection" feature thats included on the t-stat.

We simulated a power outage test at the breaker first without common wire attached (battery power only). We set the t-stat very low and turned off the air handler at the breaker, and then waited 20 seconds. After 20 seconds, we turned on the air handler at breaker and the compressor clicked on immediately! Very bad.

We did the test again with the common wire attached. This time, there was a 5 minute delay before the compressor clicked on.

I have not been able to find anything about this phenomenon on this board or any other HVAC boards. Can any of the expects here confirm why/how common wire provided the delay? For my own curiosity.


shdesigns
Powered By Infinite Improbabilty Drive
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join:2000-12-01
Stone Mountain, GA
Reviews:
·EarthLink
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It would have to be a feature of the thermostat. From Honeywell the manual:

Compressor Protection (Setup Function 15): Forces the compressor to wait a few minutes
before restarting, to prevent damage. During the wait time, the message Cool On or Heat On
(heat pumps only) will flash on the display.
--
Scott Henion

Embedded Systems Consultant,
SHDesigns home - DIY Welder


Tex
Premium
join:2012-10-20
kudos:2

1 recommendation

reply to Simmyster
I've never heard of a thermostat having an extra time delay feature because a common wire is being used. As far as I know, compressor protection is a function of the thermostat whether or not there is a common wire attached. It should not matter if the thermostat uses batteries as its primary source of power or whether it uses 24V from the transformer as its primary source of power.

aroberti
Sarcastic? Me? Never
Premium
join:2000-12-21
Norwalk, CT
But with a battery powered t-state and no C hookup, how would the t-stat know power was lost/restored?


Tex
Premium
join:2012-10-20
kudos:2
Because of the electronics built into the thermostat. It senses when there is a call for cooling.


StillLearn
Premium
join:2002-03-21
Streamwood, IL
Reviews:
·AT&T Midwest
reply to aroberti
said by aroberti:

But with a battery powered t-state and no C hookup, how would the t-stat know power was lost/restored?

Without the C wire, I don't know how the thermostat could distinguish between there being no power vs nothing (cooling, heating, fan) is calling for action.

On the other hand, if the compressor was not running in last 5 minutes, what is the problem with the compressor being turned on immediately after power is restored? I can understand it would be better to wait a several seconds to let other loads get past their surge, but still, what is the big deal.

I would run a C wire in case you want to replace the thermostat with an internet WiFi thermostat some day.


Simmyster

@comcast.net
reply to aroberti
"But with a battery powered t-state and no C hookup, how would the t-stat know power was lost/restored?"

I think we are getting to something here.

Again, the thinking is that "compressor protection" feature only activated during normal cycles. How would it activate after a power outage?


leibold
Premium,MVM
join:2002-07-09
Sunnyvale, CA
kudos:10
Reviews:
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said by Simmyster :

Again, the thinking is that "compressor protection" feature only activated during normal cycles.

The "cycle" is completely irrelevant. If you want you can change the settings during the time of power loss and the compressor protection will still work.

The compressor protection feature is a simple timer that gets started whenever there is a transition from 0V to 24V AC on the C and R/Rc terminals (which is the reason why connecting the C wire matters). Until the timer expires the thermostat will not call for cooling (or heating in heat pump mode).
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Simmyster

@comcast.net
reply to StillLearn
"On the other hand, if the compressor was not running in last 5 minutes, what is the problem with the compressor being turned on immediately after power is restored?"

Isn't the risk if the compressor was running, then power shuts off/returns quickly?


pende_tim
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join:2004-01-04
Andover, NJ
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Reviews:
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reply to Tex
Sounds like this 'stat may have 2 timers in it:

One timer powered from the C wire that senses power loss/restore

and

a Second timer that provides short cycling protection during normal operation to prevent the compressor from restarting if the compressor just stopped running and someone changes the temperature immediately after that.

The second timer probably gets reset if there is a power loss and no battery in the thermostat.

A model number would be helpful to investigate.
--
The difference between genius and stupidity is that genius has its limits.


Simmyster

@204.16.76.x
said by pende_tim:

Sounds like this 'stat may have 2 timers in it:

One timer powered from the C wire that senses power loss/restore

and

a Second timer that provides short cycling protection during normal operation to prevent the compressor from restarting if the compressor just stopped running and someone changes the temperature immediately after that.

The second timer probably gets reset if there is a power loss and no battery in the thermostat.

A model number would be helpful to investigate.

Model number is TH2110D1009. I like your theory about the two distinct timers. Problem is, the installation instructions and all previous web searches seems to indicate that only the second timer you mentioned is a feature on my model. I can't find anything about t-stat adding delay when C wire senses power loss.


Msradell
P.E.
Premium
join:2008-12-25
Louisville, KY
said by Simmyster :

said by pende_tim:

Sounds like this 'stat may have 2 timers in it:

One timer powered from the C wire that senses power loss/restore

and

a Second timer that provides short cycling protection during normal operation to prevent the compressor from restarting if the compressor just stopped running and someone changes the temperature immediately after that.

The second timer probably gets reset if there is a power loss and no battery in the thermostat.

A model number would be helpful to investigate.

Model number is TH2110D1009. I like your theory about the two distinct timers. Problem is, the installation instructions and all previous web searches seems to indicate that only the second timer you mentioned is a feature on my model. I can't find anything about t-stat adding delay when C wire senses power loss.

Maybe it's an undocumented feature. It obviously exists because you tested it to see what happens.