I have a conventional furnace running on a LUX500 4 wire thermostat. I know that the thermostat has a 24v AC supply to it and the digital display is b/u with battery. I am changing to a Lennox G61V hi -eff furnace with 2 stage heating and variable speed DC motor. I was told that I could continue to use my existing t-stat but it would not give me the benefit of a true hi -eff 2 stage heat. Will I have to run new wires if I replace my t-stat or is there one that will give my real 2-stage heat with 4 wires?

Check the terminal numbers on the lux500. They should be Y R G and W. I could not find a manual. I don't know if the model is power stealing, but in all likyness it doesn't have a common wire (C) which would make it get power rom the furnace. So, you may actually need two more wires: C and (usually W2)

This thermostat runs with 3 wires. Buy Honeywell VisionPro IAQ Programmable Universal Thermostat with Total Home Comfort Control | Honeywell YTH9421C1002 (http://www.prothermostats.com/product.php?p=honeywell_yth9421c1002&product=110092)

Warning: The above is considered a professional thermostat. Help direct from Honeywell will be non-existant.

By placing an interface box at the furnace.

There may be other communicating thermostats that can work.

There are products such as add-a-wire which are difficult to use/install.

Check the terminal numbers on the lux500. They should be Y R G and W. I could not find a manual. I don't know if the model is power stealing, but in all likyness it doesn't have a common wire (C) which would make it get power rom the furnace. So, you may actually need two more wires: C and (usually W2)

This thermostat runs with 3 wires. Buy Honeywell VisionPro IAQ Programmable Universal Thermostat with Total Home Comfort Control | Honeywell YTH9421C1002 (http://www.prothermostats.com/product.php?p=honeywell_yth9421c1002&product=110092)

Warning: The above is considered a professional thermostat. Help direct from Honeywell will be non-existant.

by placing an interface box at the furnace.

There may be other communicating thermostats that can work.

There are products such as add-a-wire which are difficult to use/install.

Thanks for the detailed reply - the Lux 500 has RC RH W Y and G. RC and RH are jumpered out. The 24v supply is taken directly off the power supply inside the furnace through a transformer. I also have 3 more wires which are strung along with the 4 wire sheathed cable but are not connected at either end ( i.e. at the furnace / transformer or the t-stat. ) In your opinion will I still have to run extra wires?

The new stat will need W2 for the second stage.

C is generally an optional connection. If C is not used then the thermostat is powered entirely by batteries or through power stealing.

Between C and (Rc/Rh jumped) you have 24 VAC all the time.

You have the wires. You can move C and W2 from the furnace to the extra cable and label the ends of the wires.

OR use this

Venstar Inc. - Add-A-Wire ACC0410 (http://venstar.com/Thermostats/ACC0410/)

Or on some lennox units there are provisions to allow two stage operstion without a two stage thermostat. Read your directions to find out if your unit is one of them that this can be done to.
OR

ADD - A - WIRE PART # P474-0410 - eBay (item 160373247349 end time Nov-26-09 11:55:53 PST) (http://cgi.ebay.com/ADD-A-WIRE-PART-P474-0410_W0QQitemZ160373247349QQcmdZViewItemQQptZLH_De faultDomain_0?hash=item2556fd8d75)

OR this company makes a real nice setup

FAST-STAT (http://www.fast-stat.net/)

Both of these items have been tested and used at the university in the HVAC lab and they work well. I personally hate to run new thermostat wires from the inside unit to the thermostat or to the outside unit from the insyde unit for a replacement heat pump install.