# Thread: Carrier thermidistat staging logic

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Hmmmh. I just can't figure this out. Perhaps someone can tell me the answer to this mystery. I have a Carrier 2 speed condenser (38TDB) and 2 stage variable speed gas furnace 58cva and a Carrier thermidistat. How does the thermidistat determine what stage should operate? I looked at all of the Carrier literature and it indicates that no higher stage will operate until the preceeding stage has operated for at least fifteen minutes. But if the setpoint on the stat is only one degree from the room temperature, how long would the lower stage operate until switching to a higher stage (in both heating and cooling modes)? There is absolutely no way to know from the stat whether high or low is in operation unless you look at the circuit board on the furnace or condenser! It would be somewhat nice to be able to determine if high or low stage is operating for economic reasons. I have looked at every schematic, diagram, pamphlet, operating guide and technical manual--nothing indicates what the stat logic is in regards to staging. help someone?

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I think that it operates for 15 minutes in the low stage and then the higher stages will turn on, unless there is 5 degree difference.

There should be a triangle below set point display. It will flash when the staging or cycle timers are active.

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Originally posted by dhanna
I think that it operates for 15 minutes in the low stage and then the higher stages will turn on, unless there is 5 degree difference.

There should be a triangle below set point display. It will flash when the staging or cycle timers are active.

thanks for your reply. I was wondering if you knew more, If I may query you further.

Is time the ONLY factor in determining staging logic or does "PROGRESS TOWARDS THE SET POINT" count too?

For instance if the stat is "holding its own" with the lower stage, say within 1 degree of the set point--would it still kick to the higher stage after fifteen minutes no matter what?
thanks again.

4. You are asking a rather complicated question, wish it was simple. The Thermidistat does use a very impressive logic; let me see if I can explain without getting too carried away.

The Algorithm is time and temp, for initiating a call and bringing on stages.

To bring on a stage it waits 15 minutes, then asses the rate (time and temp) of how the demand is being satisfied and determines if 2nd stage is needed.

For example: 70o room temp

70 set point
69.8
69.6
69.4
69.2
69.0 heat on 1st stage 15 minutes, raises to 69.8 = 80% satisfied.

Algorithm sees that the demand was 80% satisfied in 15 minutes; logic says it does not need to bring on 2nd stage. (Gain was sufficient)

Let’s look at another example: 70o room temp

70 set point
69.8
69.6
69.4
69.2
69.0 heat on 1st stage 15 minutes, raises to 69.3 = 30% satisfied.

Algorithm sees that the demand was 30% satisfied in 15 minutes, 2nd stage is now energized. (Gain was insufficient)

Stat looks for an 80% gain in 15 minutes to help judge 1st or 2nd stage, this is defeated if setpoint and room temperature have a differance of more than 3 degree's.

Make sense? Now that is for the thermostats, furnace board logic is different when determining 1st and 2nd stage…but that is another thread.

Good luck

[Edited by hvac r us 2 on 10-11-2004 at 04:37 PM]

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Great explanation, but does that that mean that the thermidistat is a "two stage" thermostat. How does the humidity control compare to the infinity in terms of accuracy %?

6. The Thermidistat can be set up for single stage, two-stage or multi stage…it’s all in the set up.

As far as the Infinity, it has better control for humidity and a broader range.

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I can't thank you enough. Nobody seemed to be able to answer this question except you.

Another couple of questions if you don't mind.

What is the difference in system operation with the thermidistat in the "cool to dehumdify" and plain "dehu" mode and when does the "superdehumdification" occur?

the literature is confusing on this issue-- The way I understand it is--

dehumidify lowers the blower speed to 86% of normal for the stage that you are operating when there is a cooling demand and a dehumdification demand--thereby giving you a colder evaporator coil and giving the air more "time" moving across the coil--thereby removing more moisture. IF there is no cooling demand then the system doesn't operate no matter what the dehu setpoint is in the "plain dehu" mode.

In the cool to dehumidify mode the system will operate even if there is no cooling demand. If there is a cooling demand then the system will drop the blower speed to 65% of the speed for the stage you are in for the first 10 mins of the cycle. (super-dehumidify?) If there is no cooling demand then the system runs in a similar fashion overcooling if necessary to meet the dehu demand--10 mins on, 10 mins off. Overcooling will occur up to 2-3 degrees to meet the dehu demand.
With the dehu function off--the system just operates normally irrespective of the humidity--temperature setpoints only determine system operation.

8. Ken...Navy Nuke? Just curious...knew a fellow with same name in Navy...stationed in Charleston and Kings Bay with him...he would've asked the same inquistive questions...those Nukies have lots of questions.

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That would be a negative, sir. Not navy but my old man was air force. Now he is a civilian SES and works in the pentagon. I have always been fascinated with how things work and I guess I get that trait from my upbringing/environment. He was most responsible for the development of JDAM (Joint Direct Attack Munition)..essentially a dumb bomb with a smart guidance system with near pinpoint accuracy. It has been very successful in Afghanistan and the current bull going on in Iraq. Look it up in a search engine if you are curious.

I am a registered nurse of eleven years bachelors degree and married two years live in the Big Easy (N.O., LA.)

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Ken, hopefully you've gotten your question answered by now but just in case..

Your description of the dehumidification options are basically correct. However, in the cool-to-dehumidify (super-dehumidify) mode, the equipment configuration dictates how much the fan speed is reduced. If your air handling is done by a furnace, you do not get a second dropdown of air speed in super-dehumidify operation. The fan speed is reduced the same amount for any dehumidification. If you live in a warm climate and have a typical split A/C system your inside air handler (there is no furnace) moves the air. This dedicated air handler provides the two air speed reductions you described.

I too was confused by the info in their manuals. I happen to have two houses - one with a furnace (WI) and one with an air handler (FL). My WI system had some wierd problems and Carrier "factory" got involved. Their tech explained the above. This is probably why their manuals are unclear on this issue - since it's equipment dependent.

And, jrbenny, I'm not a nukie either - just a curious fellow.

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Thanks for your reply.

I upgraded my thermidistat to the Infinity Control. I have noted much more seamless operation and more precise humidity control. I don't know if it was worth the extra money but I got a little tired of the annoying "clicks" when the system adjusted the fan speed with the thermidistat. No clicks with the Infinity Control--great unit. I actually installed it myself. ABCD connections--unbelievable simplicity, intuitive programming. The thing even gives you static pressure readings!

I highly recommend the Infinity System.

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I'm very glad you said that. We're building a new home and I am pleased with the ComfortZone II zoning and controls in my current home. But, the Infinity looks really nice so we'll almost certainly go with that for the new one. Still waiting on pricing though so we'll see ...

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The infinity system has a zone control unit with dampers also..very similar to the Comfort Zone II. I would be wary of replacing it if what you have works.

I have information which may be helpful to you about pricing (wholesale prices). I cannot discuss it here. My wife worked for United Technologies that owns Carrier--we were able to get pricing info. This was helpful information with bids that we received from different AC and Heating Contractors.

Please email me if you are interested

kenneth.d.little@att.net

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