Multiple RC840T's on a single thermostat

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Old 09-09-19, 11:47 AM
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Multiple RC840T's on a single thermostat

I have 3 wall heaters that I'm trying to control using a single Honeywell RTH2300 thermostat. I have 1 RC8402-240 and 2 RC840T-120. If I wire just one (any one) of the heaters up, it works fine. But it doesn't work if I connect all 3. I believe this because all 3 of the RC840T's are trying to drive 24V to the thermostat. I believe it's possible to have just one RC840T provide the power to the thermostat and still signal the other 2 units. In fact there's another thread on this forum that discusses that. Unfortunately the link that was provided in that thread is no longer active. That thread is https://www.doityourself.com/forum/e...ighlight=rc840

and in there the very last comment says that what I want to do is possible. I can replace 2 of the 840T's with 840's if necessary.

I'm thinking to connect the R wire of the thermostat to just one of units; connect the W wire on the thermostat to all 3, and then connect the 3 C wires on the 3 RC840T units together but not wire them to the thermostat.

I'd very much appreciate any guidance or pointers to wiring diagrams that show me what to do. Thanks!
 
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Old 09-09-19, 12:13 PM
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Welcome to the forums.

The problem is anytime you connect transformers together you run into problems. You are proposing connecting the three C connections together. That may or may not work.

I would use a double pole relay. There are two sets of contacts. Based on the diagram below you would connect the coil to the primary unit. You would connect terminal 1 and 3 to the second transformer unit on the W and R terminals. You would connect 4 and 6 to the third transformer unit. This way you could keep what you have and they would be isolated from each other.

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Double Pole relay
 
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Old 09-09-19, 01:21 PM
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Thanks Pete. That's very helpful. Please clarify the part where you say "you would connect the coil to the primary unit". I assume I connect the R and W wires of the primary to the thermostat; do I use the same wires to drive the coil?
 
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Old 09-09-19, 01:24 PM
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You would connect the coil to W and C on the first unit.
 
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Old 09-11-19, 04:35 PM
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Thanks Pete. I bought and wired up the relay you specified but I'm still having a problem.

I confirmed that the wiring to the 2 remote units is correct (when I connect one remote unit to contacts 1 and 2 that unit turns on, and when I connect the other remote unit to contacts 4 and 5 the other unit turns on. I then connected them correctly to 1 and 3, and to 4 and 6 so that they are off by default).

The problem is with relay activation. I connected the coil on the external double pole relay to the C and W wires of the RC840T. When the RC840T is turned on by the thermostat (red light comes on on the RC840T), a loud buzzing comes from the relay (I think) and none of the units turn on. If I disconnect either the W or C wire from the relay, the buzzing stops and the directly connected unit turns on. I've used a voltmeter to confirm that there is 24VAC between them W and C when the unit is turned on and one of the wires is unplugged from the relay. Is it possible that the current draw for the 2 relays is more than the RC840T can provide and so neither the internal relay nor the external double-pole relay is turning on? Is there a way to address this?
 
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Old 09-11-19, 04:43 PM
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Check the AC voltage to the external relay when it's active.

Since you're using a battery operated stat...... the main 840 should be able to run the aux relay.
 
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Old 09-11-19, 06:35 PM
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It drops to 11VAC when it's connected to the relay. It jumps back up to 24VAC if I disconnect either wire from the relay.
 
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Old 09-11-19, 10:21 PM
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Wow..... they've really reduced the size of the transformer in those units. That relay draws minimal power.
I'm not sure what to offer. I have connected relays to them in the past with no problem.
 
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Old 09-12-19, 06:39 AM
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Hi, the output rating on the RC 840 is listed at 40ma. what is the rating on the relay?
Your sure the coil voltage is 24VAC.
Geo
 
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Old 09-12-19, 09:46 AM
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Here are the specs on the coil for the relay. If this requires more power than the 840T can provide, is there an alternative part I could buy instead? Or is there a way to use an external 24V transformer and give up on using the 24V from the 840T?

Specifications

Coil Voltage24 Vac, 50/60 HzTerminals 1-2-3PowerTerminals 4-5-6PowerCoil Data17.5 D.C. Res., 334 mA, 9.5 Nominal VA, 22 Inrush VACoils, Operate85% of nominal coil voltage; 110% maximum safe operateCoils, Duty CycleContinuousApprovalsU.L. file number E12139C.S.A. file number LR13360Coils, Operate85% of nominal coil voltage; 110% maximum safe operateCoils, Duty CycleContinuousAgencyU.L. file number E12139C.S.A. file number LR13360
 
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Old 09-12-19, 09:51 AM
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Sorry -the formatting got screwed up. Hopefully it's still legible but trying again
Coil Voltage 24 Vac, 50/60 Hz
Coil Data 17.5 D.C. Res., 334 mA, 9.5 Nominal VA, 22 Inrush VA
 
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Old 09-12-19, 02:23 PM
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I have it working. I decided to buy an external 24V transformer and use that instead. In case anyone else has the same issue, I followed the 2-wire wiring instructions from this page:
https://www.warmlyyours.com/en-US/pu...ON-GUIDE-A.pdf

Thank you both for all of your help.
 
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Old 09-12-19, 02:49 PM
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Good job. Name:  thumb.jpg
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Size:  1.6 KB That relay I had you use must be slightly higher draw than the straight 840 relay.
 
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