Nest- Furnace Short Cycling with A/C RC connected

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Old 04-12-17, 05:35 PM
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Nest- Furnace Short Cycling with A/C RC connected

Hello I just installed nest few weeks ago and I'm having some strange issues with furnace short cycling only when the outside A/C unit is connected to the nest and power is active, the A/C works fine though. If I just have the furnace connected, remove the AC Y1 and RC to the nest it runs beautifully no issues. The current configuration is furnace W1 and RH A/C is Y1, RC ,and Common I added the common wire recently to see if that would help things out but no luck, I've been in contact with nest support (with no luck referring me to Pro installer) and even received new thermostat back plate to see if that was an issue. The old thermostat was a lux programmable with on off /Fan / Heat/Cool, no issues using that thermostat. The furnace is propane thermo Pride and the AC unit is split system ICP. Both systems utilize there own 24v transformer on own Breakers. I'm at my wits end trying to figure why its not working out, Scouring the internet for days coming up empty handed. This seems this should be a cut and dry minimalist setup, any help would be greatly appreciated.
 
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Old 04-12-17, 06:46 PM
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Welcome to the forums.

You have a split system A/C and a propane furnace.
Do they share the same duct work and blower ?
If yes.... why two transformers ?

You are missing a G (blower) connection in your A/C wiring. How are you handling that ?

Some furnaces and A/C units are sensitive to power robbing thermostats. Basically the thermostat is like partially shorted allowing unwanted current to flow.

One way to eliminate this problem is to add a relay for the heating system.

Once we understand what you have... we WILL give you a working solution.
 
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Old 04-12-17, 06:55 PM
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this is a rental house and the AC unit (PAM330K00A1) has its own 10' flex duct work from outside that ties into existing duct work so it has its own blower as it was later added on to home , the old thermostat didn't utilize the G wire the unit is like an all in one once the Y1 and RC connect the unit starts and runs on its own. Thanks for the quick response
 
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Old 04-12-17, 07:04 PM
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You could try basic things like reversing the Y1 and Rc connections and see if that helps.

You said the furnace short cycles with Rc connected. Does it turn on and off by itself when heat is not selected ?
 
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Old 04-12-17, 07:20 PM
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Hmm, wasn't sure if reversing the Y with RC would cause the nest to mess up or not. I can have everything connected and breakers on AC and Furnace and doesn't try to turn anything on or cycle on until the thermostat tries to call for heat, that's when the furnace starts short cycling, the nest shows VOC 35.20 VIN 35.20 and LIN 200ma (if that helps) if I turn the breaker off for the AC unit I have to remove the RC from the nest otherwise it errors and says no power to RC ( i can leave the common and Y1 (RC removed)with AC breaker off and furnace runs fine
 
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Old 04-12-17, 07:32 PM
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If you try reversing the Rc and Y .... don't use the C connection.

The problem here is you have two transformer working against each other. That's why you're seeing 35v.

If reversing Rc and Y doesn't help. Try reversing Rh and W instead.
 
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Old 04-12-17, 08:01 PM
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ok so I tried reversing the Y1 and RC removed the common switched it back to then tried switching RH and W1, still short cycles, also i noticed when i turn the breaker of for the furnace of the nest still shows power on RH and RC, but when the ac breaker is off and furnace breaker off shows no power to RC and RH flip the furnace breaker on with ac breaker off shows only power to RH(not sure if this has any meaning *shrug*. nest readings AC breaker on furnace breaker off with no common VOC 41.25 vin 39.12 lin 40ma (just the ac breaker on) Readings for furnace only ac breaker off VOC 38.56 VIN 37.35 LIN 40ma
 
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Old 04-12-17, 11:09 PM
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I'm thinking on this one.

The voltages are too high even when only one transformer is running. It's almost like there are three transformers involved.

Do you have a voltmeter ?
We may need to do some testing.
 
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Old 04-13-17, 06:59 AM
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I may have something I can use I'll look around. I work until 7PM ill let you know then Thank You.
 
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Old 04-13-17, 05:28 PM
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I found a Amp clamp test meter with test leads, i measure 27.5 V between W and R on furnace control board, not sure what other things to test for, currently the AC circuit breaker is off
 
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Old 04-13-17, 05:31 PM
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27.5vac is a good measurement on the heating system.

Check from Y to Rc on the A/C system.
Also check from Rc to C.... should be the same as Y to Rc.
 
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Old 04-13-17, 05:59 PM
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29.3V Y to RC and RC and RH to their common reflect exact same , on side note I'm not sure what happen now my nest is showing no power to RH and 0.01V I moved the RH to RC took out the AC Y! and RC wires and it reads back to normal I think something in the back plate on the RH terminal went screwy, not sure what happened just taking some measurements shouldn't have cause any issues.
 
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Old 04-13-17, 11:16 PM
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Note that Nest is unusual with Voc and Vin displayed as peak voltages, not RMS. Need to multiply by .707 to get real world working values. So 41.25 displayed becomes your measured 29.3 vac.
 
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Old 04-14-17, 12:00 AM
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Thanks for that information.... makes sense.
 
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Old 04-14-17, 08:14 PM
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I got off phone with Nest Senior support, they are willing to mail me another baseplate, but would like me to contact a NEST pro installer they think its related to the "Power sharing" and possibly willing to reimburse me up to an hour of their troubleshooting time. I let him know that I tried a common wire and still short cycles, it must be the way they prioritize the Y1/RC to share power from even if trying to run furnace possibly. I tried to think of ways of adding a second thermostat before the NEST to select between Heat/Cool, but the way the weather changes her in central NY still would kinda be a pain. This is turning into a bigger headache than expected with a $250 thermostat. I'm still open to some suggestions while I wait for a new backplate. I might try giving a ring to A NEST installer in area to see if this is something that can be fixed fairly reasonably.
 
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Old 04-14-17, 09:48 PM
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The nest should ignore differences between Rh and Rc. Most stats would need a physical jumper to join them together. The nest does it electronically.

I'm thinking out loud and this is what I'd be testing for.

I guess what I'd want to know is if both transformer secondaries had a connection to ground. Some systems ground one side of the secondary and some don't.

I would check from both heating wires to actual ground.
I would expect to see 24vac on the Rh wire and 0v on the W wire to ground.

Same for the A/C system.
I'd check from Rc to ground and expect to see 24vac.
Then I'd check from Y to ground and expect to see 0v.
 
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Old 04-14-17, 11:10 PM
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What do you see when the furnace short cycles. Is the Nest still showing orange/calling for heat? Or does the Nest shut down? Is there an LED at the furnace blinking a status? It would be interesting to know the voltage when this happens.

I think sending a pro out in this case is more to find out what really is there. Since the a/c does not have G connection it probably is simple as they jumpered it to Y somewhere but it may hooked to some type of controller that is confusing things. How does the outdoor unit connect to ductwork? Is there a damper and/or other controls in the middle?

For the old thermostat - did you review its switches and settings? Sometimes that can reveal details of what's going on.

When you say that you added a common - did the other end go to the furnace or a/c?

Ideally you would map out all of the wires and post what connects where.
 
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Old 04-15-17, 04:23 PM
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If i recall i checked the VOC settings during a short cycle and the readings were the same or close to the same if it wasn't running. the nest shows no errors and appears to be functioning normal. the sounds it makes is the normal initial click then the first blower turns(for the exhaust fan) on clicks again shuts down tries to start again clicks and repeats.(never gets to ignition stage ) Ill try to get a diagram up. As for the A/C the wires all go to control board separate no jumpers believe the (Fan) wire is just used if you want just the fan flow on without cooling. I added the (C) common wire later to the AC unit Brown common wire matched up with control board. The old thermostat only had switches for gas/elect,etc and swing times. I have 24vac on the Rh wire and 15v on the W wire to ground. haven't got to to the AC portion yet requires removing top cover screws like 8-10 of them I'll get to that later. As for the duct work it comes from the ac unit via 10 inch flex duct with insulation then connects with a metal flange into existing metal ducks, don't believe theirs any damper or flaps, probably loosing heat this way to.
 
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Old 04-16-17, 12:13 AM
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the sounds it makes is the normal initial click then the first blower turns(for the exhaust fan) on clicks again shuts down tries to start again clicks and repeats.(never gets to ignition stage
That is the power stealing of the nest doing that. You need to use a relay to control the furnace.

Easiest way is to connect the stat using C to the A/C system.
So you'll have Rc, Y1 and C connected from the A/C system.

Now using a 24vac N.O. relay. Connect the coil to W1 and C.
Connect the heater to the N.O. contacts on the relay.
You can put the relay at the heater.
 
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Old 04-16-17, 09:39 AM
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The Nest is not supposed to power steal if there is a C wire connected. The original Nest design was for it to use Y1 and C for power. If no C it steals between Y1 and Rc first. If no Y1 it steals between W1 and Rh.

15 volts on W doesn't sound right so may be related to the issue. Possibly a bad base that is power stealing when it shouldn't - try measuring with and without the thermostat connected. PJ's relay solution should work isolating the furnace.
 
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Old 04-16-17, 09:48 PM
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Using a relay sounds like it may work out, I'm a little confused how to wire it from furnace to relay to thermostat, and whats a fairly good relay to use. I understand some of it the W1 from furnace to coil and Common to seccond coil but not sure on the rest
 
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Old 04-16-17, 11:15 PM
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A relay is very easy to add. There are two wires between the stat and the relay and two wires from the relay to the furnace. So this relay can go anywhere in the circuit. You will be using the A/C to supply relay power for heat.


I would put the relay at the furnace.
Connect your two furnace wires at the nest to C and W1. Since you will have two C wires at the nest you may need to connect them behind the stat and bring out a single wire to go to C.

Follow the wiring in the diagram to wire the relay. You are working with AC so there is no polarity. No polarity on the relay.

Name:  nest relay.jpg
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As far as relays. My diagram shows a double throw relay. You only need any 24vac normally open relay. Just some picks. There are many.

Industrial-Grade-6AZU2-Enclosed-Relay/
Emerson-90-290Q-Enclosed-Fan-Relay
 
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Old 04-16-17, 11:45 PM
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this is a rental house
Are you the renter or owner?
 
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Old 04-22-17, 06:46 PM
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Hello, sorry for the delay and I thank you I finally got the relay and replacement nest backplate in. got it all wired up and every thing seems to be in order now furnace kicks on and runs fine now, I really appreciate the help and time looking into this and maybe this post will help someone else out in the future.
 
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Old 04-22-17, 08:25 PM
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Thank for letting me know how you made out. You really stuck with it.
 
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Old 08-02-17, 11:29 AM
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Hello.
I was having the same problem and after a lot of research I got a Venstar at amazon. It was easy to hook up and I have not had any problems with short cycling since then. I purchased it off amazon .
https://www.amazon.com/Venstar-ACC04...ywords=venstar
 
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Old 08-02-17, 10:33 PM
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Welcome to the forums.

The venstar you linked to added the C wire. The problem in this case was two different systems with two different transformers.

Thanks for the additional info.
 
 

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