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[HVAC] Adding a C Wire to an Ancient Air Handler

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Ok so I got a Honeywell wireless thermostat but I only have five wires - (G, Y, RH, RC, W) with no C.
I found a guide that allowed me to put the G wire to the C wire and then put a jumper cable from the Y wire to the G. »diy.stackexchange.com/qu ··· ermostat

Thermostat powered on. Heating works fine but my AC is not working. I have to manually press "Fan On" to have my AC Unit outside to spin, even thought it is not the "Fan" in my attic. This wiring is messed up I concluded:

So after rewatching this Honeywell video I realized I have to change the G wire to a C wire at the Furnace as well.

In the picture attached is my Ancient Split System Air Evaporator, however I have no idea how to change the G wire to a C wire in this really old Air Handler. I've bought a 5 wire to 6 wire mod ( Venstar »www.hvacproblemsolver.co ··· stat.asp , I don't even know how I can apply it my case (see attached pic).

Must I resort to a new transformer or power adapter for the C? for example: »www.amazon.com/LockState ··· 6&sr=1-1

Is this a safe option? Does it need to be grounded? Any suggestions are welcome.
TheSMJ
join:2009-08-19
Farmington, MI

4 edits

TheSMJ

Member

It's kinda hard to guess just by looking at the pics you provided. If you have an electrical diagram of your current setup, a photo of that would help a lot.

Be careful with swapping out the G wire - you may lose heat and/or AC if you try switching it with something else. Not all HVAC systems trigger the fan automatically. At the very least, you'd lose the ability to run the fan on its own without calling for heat or AC. Y is used to call for AC, so I can't imagine how disconnecting it will allow AC to continue functioning at all.

What colors are the wires that run to your old thermostat, and what are they labeled as?

The C (AKA "Common") wire should be connected to the transformer on the right, directly above that switch above the terminal where the red wire is connected (which I'm willing to bet is either Rh or Rc). The same terminal where there is already a grey and white wire attached.

That block to the left of the transformer also looks like another, older (my guess is the original) transformer for the heating system. Can you check the label on it and confirm this?

EDIT: Also, keep in mind that Rh and Rc are usually the same wire and jumped together either on the thermostat itself or at the furnace terminal, as they're both 24V power. You *may* get away with disconnecting Rh and using that as your new C. If you do that, you'll have to remove one of the two transformers.

Judging by the looks of your current setup, I'd hazard to guess that the AC system was added later on after the furnace. That may be why separate Rh and Rc wires were ran, but they both may not be necessary.

nunya
LXI 483
MVM
join:2000-12-23
O Fallon, MO

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Pictures of the wiring of the thermostat please!
Also note: there are countless threads in this forum regarding the exact same subject. Have you reviewed those yet?

Tex
Dave's not here
Premium Member
join:2012-10-20

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As far as I can tell, you have no heat. From what I can see, your wiring goes like this:

Cloth Cable
Red - R (24 VAC)
Green - G (Fan)
White - Y (Outside Unit)

Vinyl Cable
Red - (24V from cloth cable White wire to outside unit)
White - 24V Common (To outside unit)

I've never used one of those 5 to 6 wire things or a power adapter thing, so I can't help you there.

Is it too difficult to pull another thermostat cable? Where's your heat?

Also, your duct work looks terrible.
dougone611
join:2007-01-03
Tappan, NY

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Thank you guys for looking at this post so there are more pics of my thermostat and a diagram I made.

I concluded from THESMJ that I need to add a C wire in one of the Spots of the "Air Handler". Do I have to add a transformer or do I simply just add a wire like in Spot C and twist?

Sorry if this is really noobish, but I have looked on the forums regarding the c wire and their system seems to be different from mine.

Thanks again.

mackey
Premium Member
join:2007-08-20

mackey

Premium Member

Undo the blue/green wires in "spot c" and add the blue wire to the red/white ones just below it (so you have 3 wires under that wirenut) and move the green wire to "spot A1" (so you have 3 wires under that screw; you may need to add a short jumper wire and use a wirenut instead). At the thermostat, remove the jumper between G and Y; it's not needed. Also trim those wires down so there's not all that bare copper showing.

/M
dougone611
join:2007-01-03
Tappan, NY

dougone611

Member

hey mackey, ok but one problem i see is that there is no wire connected to the g slot of the thermostat... does it no longer matter with your modification?

mackey
Premium Member
join:2007-08-20

mackey

Premium Member

That's fine. It's tied to the Y (white wire in this case) at the air handler.

/M

nunya
LXI 483
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join:2000-12-23
O Fallon, MO

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You don't have enough wires. Something isn't adding up. You have 3 at the furnace, and 5 at the t-stat. Where do those extra 2 wires at the t-stat come from (or go)? It's not the furnace.

Do you have a different heat source somewhere?

Tex
Dave's not here
Premium Member
join:2012-10-20

Tex

Premium Member

said by nunya:

You don't have enough wires. Something isn't adding up. You have 3 at the furnace, and 5 at the t-stat. Where do those extra 2 wires at the t-stat come from (or go)? It's not the furnace.

Do you have a different heat source somewhere?

My guess is he does. That's why there's a separate Rh wire. Rh and W go to his heat source from a separate transformer.

OP - I wouldn't recommend running the heat with mackey See Profile's wiring configuration if your heating system depends on the air handler blower. On a call for heat in that wiring configuration, with no wire going to the "G" terminal (blower relay), your blower will not operate. I'd highly recommend pulling another thermostat cable with the appropriate number of wires.
dougone611
join:2007-01-03
Tappan, NY

dougone611

Member

As in the diagram, I have 2 from the furnace (boiler) ( Rh, W) the third wire seems dead (was taped up), 3 from the Air handler - aka the attic/split system (Rc, G, Y)
So all I need is C so. The Air handler is not for heating its only for the AC. The furnace is a boiler downstairs. No blower is used in the heating, but it is used in cooling.

So it still doesn't make sense how Mackey's wiring configuration is going to work when there is nothing going to the G spot of the thermostat...

mackey
Premium Member
join:2007-08-20

1 edit

mackey

Premium Member

said by dougone611:

So it still doesn't make sense how Mackey's wiring configuration is going to work when there is nothing going to the G spot of the thermostat...

What makes you think it's needed at the thermostat? There are no wires going to K, Y2, or W2 either.

When the t-stat calls for cooling it powers on "G" and "Y" and sometimes "Y2."
When you slide the "Fan" switch to "on" it powers on "G" only.

In the above pictures you can see the "G" wire from the t-stat goes to the blue wire from the air handler. By wiring the blue wire at the air handler into the wire from Y you are making it so it still turns the fan on whenever it calls for cooling without needing to use the G terminal.

/M

nunya
LXI 483
MVM
join:2000-12-23
O Fallon, MO

1 recommendation

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How hard is it to do it right and just replace the cable with one that has enough wires to do the job?
dougone611
join:2007-01-03
Tappan, NY

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Thanks Mr. Mackey. This makes sense. I will try this and I hope it doesn't destroy my air handler for some odd reason. Fingers crossed.

BillRoland
Premium Member
join:2001-01-21
Ocala, FL

BillRoland

Premium Member

said by dougone611:

I will try this and I hope it doesn't destroy my air handler for some odd reason. Fingers crossed.

That's a vote of confidence right there!

Jack_in_VA
Premium Member
join:2007-11-26
North, VA

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Why not just replace the multi wire cable with one that has the required number of conductors you need? That will solve the problem.