Trouble with wiring a thermostat

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Old 06-05-18, 11:48 AM
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Trouble with wiring a thermostat

hi everyone..

i am am in a bit of trouble and need help figuring this problem out.

i have a Mitsubishi split system that gives out heat and cool and a separate radiant heating sysytem. I bought a t5 Honeywell thermostat and trying to wire it. What I would like to accomplish is that the primary heating sysytem is the radiant and the secondary is the split system heater. What I have wired so far: R to R heat and W to W heat. That is first pair of 24v coming form the on demand water boiler. Now.. the split unit. Red to Rc, white to W2, Fan to green, y to compressor and c to common. the issue to the sysytem turns cool and radiant fine, but does not turn 2 stage heat from the split system. Any advice how I can make this work is very appreciated. Thank you in an advance. Gene.
 
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Old 06-05-18, 12:38 PM
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I believe your issue is that when the thermostat activates W2 it connects it to R instead of Rc. The safe solution is to use a relay to isolate the boiler's R and W wires. Those wires would go the NO contacts of the relay. The relay's 24vac coil would be fed from thermostat's W and C. R and Rc would be bridged at 'stat and fed from the split.
 
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Old 06-05-18, 02:22 PM
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This sounds like a great solution. Thank you. Will it be too much to ask how would I wire a relay like that and what relay do i need to accomplish this. Thanks.
 
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Old 06-05-18, 04:48 PM
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I thought Astuff's explanation was pretty clear. A relay has two main components: the coil, and the contacts. What Astuff said to do is to connect the R & W wires that come from the boiler to the NO (Normally Open) contacts of the relay. The relay's coil wires would connect to the W & C terminals on the thermostat. You would also tie the R & Rc terminals together at the thermostat.
 

Last edited by Bob14525; 06-05-18 at 06:05 PM. Reason: Made mistake
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Old 06-05-18, 06:00 PM
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Relay's coil terminals need to connect to the thermostat's W and C. With C also going back to the split system.

Relay such a https://www.amazon.com/Emerson-90-29.../dp/B00BTLN78Q or https://www.amazon.com/Industrial-Gr.../dp/B004ZROX4S

Someone with better artistic skills than me could put together a drawing.
 
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Old 06-05-18, 11:05 PM
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Bob.... the boiler is fine. It's the split system heat that is the problem.

The 90-290, or the 6AZU2 or even 90-293 will work. We could fine tune the diagram based on which exact one you get.

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Old 06-08-18, 03:40 PM
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Hi Guys,
Thank you so much to you all for helping me on this issue. I did order the 6AZU2 relay and have it on hand. I am very new to this and Iíve been told that it is very easy to burn a board on the Mitsubishi unit. I am taking every step to make sure that it doesnít happen. I drew up the wiring diagram how it is wired now ( which I know is wrong). Please guide me how to wire the relay in so I donít mess thing up for myself. Thanks again. You guys rock!!!

My diagram
 
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Old 06-08-18, 10:19 PM
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This should help....

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Old 06-08-18, 11:05 PM
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PJ - I think it is the boiler that needs the isolated R and W.
 
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Old 06-08-18, 11:27 PM
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system turns cool and radiant fine, but does not turn 2 stage heat from the split system.
Maybe I'm reading it wrong. It looks the boiler heat is fine but not the split system heat.
 
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Old 06-09-18, 09:40 AM
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PJ - The issue is two transformers - C is from the Rc transformer so C to Rh is 0 volts. The Nest will connect W2 to Rh on a heat call which will still make the C to W2 voltage 0 and not trigger the relay.
 
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Old 06-11-18, 08:12 PM
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Hi Guys, Wired according to PJís diagram and everything turned on, but the heating stage 2 on the split sysytem didnít fire up. Everything works just like it did before installing the relay. Is there anything else that I can try? Thanks.
 
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Old 06-11-18, 09:24 PM
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It is because the C is from the air handler but the R is from the boiler.

Move the boiler wires to relay #2 and #4.
Install a jumper wire between R and Rc.
Connect thermostat W1 and C to the coil on the isolating relay ( 1 and 3).

Connect thermostat W2 to air handler W.
 
 

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