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bajwae · Jan 9, 2008, 07:00 AM

Hi all - longtime reader but this is my first question asked on the site.

I am in a newly constructed house, upgrading the cheapo digital thermostats to programmable ones. Have two thermostats, one upstairs and one downstairs, a gas furnace, and central AC outside.

The current wiring is:

RH (red)
RC (red)
W (white)
Y (yellow)
G (green)
And there is a blue wire that is not connected to anything.

I wanted to upgrade to something like this model: 7-Day Programmable Digital Thermostat - T1800
Installation manual: http://www.smarthome.com/manuals/30411a_install.pdf

My question is, the new thermostat calls for hooking up R and C wires. How do my existing RH and RC wires translate to that? What about the blue wire, is that C? Any way to figure this out?

I'm pretty capable with DIY work but have never attempted any type of HVAC work. Should I be hiring someone for this?

Much appreciated!

-Ed

hvac1000 · Jan 9, 2008, 03:24 PM

And there is a blue wire that is not connected to anything.
DO not connect the blue leave it alone just as it is.
RH/RC is basically the same as R and C.
You will need to find out if your system uses more than one transformer. I cannot tell from here.
Follow all the directions that come with a new thermostat and I recommend Honeywell or White Rodgers only for a trouble free install.

Frdbrkl · Jan 9, 2008, 03:43 PM

Other than the confusion between the RC/RH thing, hvac1000 is correct-use Honeywell or White Rogers, and you won't have these issues.

hvac1000 · Jan 9, 2008, 04:49 PM

Frdbrkl you are correct. This flu is getting the best of me. LOL

I cannot understand why people keep buying junk thermostats.

KISS · Jan 9, 2008, 05:47 PM

With Rh and Rc both cnnnected to Red, there isn't two transformers.

Rc is connected to Rh because the old stat can accommodate 2 transformers.

Check to see if Blue is connected at the furnace side. It probably isn't. You can then connect this to common at the furnace. Red and Blue will then have 24 VAC between them at all times.

C is a relatively new comer to thermostats because of electronic thermostats and the constant need for power. T-stat manufacturers have responded to DIY'ers with thermostats that require batteries for power or are power-stealing.

Common is just the side of the transformer not connected to Rc/Rh or R.

bajwae · Jan 9, 2008, 06:57 PM

Thanks guys, that is very helpful.

hvac1000: I had been looking at that thermostat because it can accommodate an Insteon control unit for home automation. Will definitely check out Honeywell and White Rodgers.

KeepItSimpleStupid: Actually there are two separate RC and RH wires that connect to different terminals on the old thermostat. I'll check on connecting the blue wire to Common.

KISS · Jan 9, 2008, 07:26 PM

Terminal Rc is connected to terminal Rh on the old thermostat and are connected to a single red wire, correct? That single Red is equlivilent to terminal R.

bajwae · Jan 9, 2008, 07:41 PM

Oops - yeah, that is correct, didn't understand what you meant. Thanks.

bajwae · Jan 10, 2008, 05:29 AM

Could anybody recommend a good thermostat that can be easily integrated with a home automation system? Much appreciated.

Frdbrkl · Jan 13, 2008, 08:05 AM

Originally Posted by hvac1000

I cannot understand why people keep buying junk thermostats.

Because >sigh< somehow Lux managed to win some sort of Consumer R*****ts award for best value for the dollar or somesuch... now I can't seem to impress on people that read this magazine (which is great for the most part) that they are in error regarding this issue.

Lux may have won the "best bang for the buck" award, but they didn't even *place* In my opinion when it comes to ease of installation or clarity of instructions.