Expert: Master Bob - 1/23/2007

Question
Hi,

I want to replace the existing manual thermostat in our apartment with a programmable unit.

We have a 'fan-coil' electrical system as far as I know, and it is a heating and A/C system.

Right now, we have a Honeywell T6575D1009.
Upon removing this unit, I found 8 separate wires going into it. The new thermostat only takes upto 4. And of course, the wires don't match up.

Here's what the current Honeywell wiring looks like:
(column format is terminal number, colour, usage)

1  Black   L1(Hot)
2  White   L2(N)
3  Orange  Heat
4  Yellow  Cool
5  -none-  Damper (not connected)
6  Violet  Fan Common
7  Red     Fan Low
8  Blue    Fan Medium
9  Brown   Fan High
10 -none-  Energy Management System (not connected)
11 -none-  Energy Management System (not connected)

The new unit accepts G, R, Y, W wires. According to the wiring diagram: G is for Fan Relay, Y is for Cooling Relay, W is for Heating Relay, and R is not labelled (but connected).

Help is greatly appreciated!

Thanks,
Imran

Answer
Hi Imran,

  Based on your description.....your original thermostat (the Honeywell model # T6575D1009) and the programmable one (model unknown) are NOT compatible. The existing Honeywell thermostat operates on LINE voltage (120 volts 60 hz)...your progammable unit (model unknown) appears to be a LOW voltage model designed to operate the fan/heat/cool cycles by relays. The old unit you have....and the new model are simply not compatible....you will need to exchange it for one that is compatible with your current system and wiring.
 
  The new unit can not be installed on your system...it is not designed for line voltage control.
 
 I attempted to cross-reference your existing thermostat with Honeywell for a programmable replacement - they did not show a replacement for your model (you can review this at:

http://customer.honeywell.com/Techlit/pdf/63-0000s/63-9610.pdf

 This does not mean that you won't be able to find a suitable replacement - just that Honeywell apparently does not make a programmable one to replace the one you have. In any event - the new one you have will not work with the existing set-up as is....you will need to exchange it.

  To help you get a correct thermostat match-up...I HIGHLY recommend you contact a local HVAC contractor in your area - they may have (or be able to recommend) a replacement model that is compatible and will fit your system. Your local HVAC company should have a wide selection of thermostats where they deal strictly with heating and cooling systems.

 Hope this helps Imran....if you need more details - please follow-up. If you should find me "maxed out"...please use the e-mail below.

Best Wishes,

Bob Osgood
Master Electrician / Owner

RKO ELECTRIC


www.rkoelectric.com

info@rkoelectric.com