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Old 08-23-2013, 08:03 PM   #1
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Central a/c wiring question


Hi all,

I've gutted the whole house and had a hvac specialist come in and run ductwork for a central air conditioning system (buying the actual equipment next year after I finish the house).

I'm keeping my gas boiler steam heat system, and am curious about how to wire in the thermostat. The a/c is a single zone system.

Only 1 thermostat to control both heat and A/C systems

How do I connect the thermostat?

I think I need the following, but would like confirmation if possible.
- 4 lead low voltage wire from air handler in attic to outdoor condenser
- 4 lead low voltage wire from attic air handler to thermostat
- 4 lead low voltage wire from boiler in basement to thermostat

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Old 08-24-2013, 10:45 AM   #2
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Central a/c wiring question


for futures run 6 bell from stat to air handler and 6 to bolier from stat only a pair required out to condenser from air handler..use spare for boiler.having 2 Transformers you must have an isolating relay for the boiler into the star for R to W

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Old 08-24-2013, 11:27 AM   #3
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Central a/c wiring question


Thanks for the reply.

Im completely new to this type of wiring(only done lights and receptacles before). Any chance you could elaborate a little?
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Old 08-24-2013, 11:52 AM   #4
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Central a/c wiring question


It is NOT advised to run both off one thermostat as extra relays or problems could/may occur. I would mount the AC tstat near the heating one or on any wall in the center of the house where sunlight is not hitting it or near any heat source. Depending on the brand of AC and air handler determines the wiring and you can get that from their install manuals. I would run a 18/4 LVT wire to the tstat from the air handler, and a 18/2 LVT to the outside unit from the air handler. The super expensive high end ones need three. The install manual has wiring info but R-R Y-Y G-G C-C is the normal hookup and the outdoor unit goes Y-Y and the other wire to C on the airhandler. I had a 88 RX7, went like he*ll, not turbo but still blew away some Firechickens and Camaros. got too expensive with speeding tickets and with the snow I get was not any good in the Winter. Had lots of fun with it for awhile.
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Old 08-25-2013, 03:23 PM   #5
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Central a/c wiring question


mount a 24v relay with Normally Open contacts and that is your isolating relay,as mentioned can't have 2 transformers and one stat..so that relay becomes the R-W on the coil connection and the boiler pair of control wires thru the N.O.contacts.only need Y and C out to the condenser
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Old 08-26-2013, 12:35 PM   #6
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Central a/c wiring question


Awesome, thanks guys. Just a few follow up questions:

1. Does the 24v relay make it so I can use just one T-stat or do I still need 2 T-stats in addition to the relay?

2. Where to I wire the relay in (basement?), sorry, have not worked with this before at all and only familiar with wiring in relays in cars haha

3. Regarding the comment about running 6/18 cable for higher end equipment, how high end is this? I'll be saving up all winter for my equipment, and wanted a 2 stage condensor unit to help control humidity, would this mean that I need the 6/18, or should I be sticking with the 4/18?

4. Is there any chance someone wouldn't mind throwing together a quick wiring diagram to help explain it in a little bit more detail for me? It would be a huge help.

Thanks again for all the feedback so far.
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Old 08-26-2013, 01:02 PM   #7
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Central a/c wiring question


IMO and experience it is not a good idea to add extra relays as eventually they will stick closed or open and XMAS morning you will be trying to solve a unnecessary problem. all depends on the brand and model of AC you buy as too how many wires it needs. the higher end ones have monitoring computer boards and need extra wires.
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Old 08-26-2013, 02:02 PM   #8
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I run 10 strand wire to everything I install. I always have plenty of wires for high end equipment (even though some high end is communicating and only needs a couple of wires) It sucks to go to a house where only 2 wires were run to an outdoor unit and there are no spares available or customer wants to upgrade to a heatpump.

So if the walls are open and you can run wires, run plenty....especially if its a path that cannot be accessed after the sheetrock and siding is up.
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Old 08-26-2013, 02:13 PM   #9
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that is a good idea but it depends on the cost of the extra wires, could get expensive after awhile.
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Old 08-26-2013, 02:30 PM   #10
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Quote:
Originally Posted by yuri View Post
that is a good idea but it depends on the cost of the extra wires, could get expensive after awhile.
Why do people save meaningless amounts of $$$ when the benefits always outweigh the savings????

It costs about $2 each to put balancing dampers on a duct system so a typical resi duct system takes maybe 12 dampers....so for $24 there is no proper control?

At another site some guy has a high end airhandler that has many more features he can take control of, which he isn't because of a limiting cheap ass thermostat.....but for $200 more, needless to say about the extra $$ he paid for variable speed features....and he's not getting the value out of it..
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Old 08-26-2013, 03:19 PM   #11
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Central a/c wiring question


I'm not afraid of running some extra wires, so I'll do the 10/18 as well.

But back to the single thermostat issue, I've been doing a lot of searching and it seems like a lot of people successfully use a single for a HVAC system like mine (separate heating and cooling equipment).

Are there any other opinions on it, how often do relay's fail? I'm ok with keeping an extra one handy if that'll solve the problem.
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Old 08-26-2013, 05:14 PM   #12
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Central a/c wiring question


You can use one thermostat but it has to have a feature where R and Rh can be separated on the thermostat subbase. Some have a wire jumper and others don't ( don't ask exactly which ones do as I rarely do what you are trying). I like to keep systems simple for later troubleshooting. Your R would be the R from the airhandler and Rh would be the hot from the boiler and W to the boiler if it has simple 2 wire wiring. Or you can use an isolating relay and regular tstat. we are volunteers here and not going to do major wiring schemes just giving simple effective solutions.
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Old 08-26-2013, 05:16 PM   #13
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As long as what ever thermostat you are gonna use. Has RH and RC terminals, no real problem using 1 thermostat for your 2 systems.

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