Hi all,

I have scoured the internet and have yet to find a definitive answer to my question I'm about to ask. I have a Trane heatpump (XE800, model TWB718A100B0) that was made in 11/1988. It's got backup electric heat and that's it. Works fine since I moved into my condo and I have recently replaced the old Baystat 239 thermostat with a Honeywell 7600D. Everything works fine, but I'm noticing that every once and a while, it's kicking on the AC while the heat is "on" (the hi/lo pressure lines coming off the inside handler are ice cold). I'm guessing this is the defrost cycle but I've never known it to come on as often as it does now. I've seen this mentioned in a few posts, but my X2 and W on the Baystat were jumpered together (see attached). Well, it's more like the X2 was extended to W as well. Might this have been a way to prevent the defrost cycle from coming on? Not sure why this was done or if I should do the same on my 7600D (ie, jumper AUX and E together even though there is a wire in each now). Any assistance is greatly appreciated. :)

http://img3.imageshack.us/img3/7813/dsc00817hc.th.jpg (http://img3.imageshack.us/i/dsc00817hc.jpg/)

Actually, I think I may have figure it out on my own. There is an actual wiring diagram for a Baystat located here on page 4: http://customer.honeywell.com/techlit/pdf/69-0000s/69-0392.pdf

I jumpered E and AUX on my 7600D and it did what I think my old thermostat did (but really didn't pay attention to it); when it runs a defrost cycle (which seems to be frequently), it kicks on the Aux heat (electric) so that the air blowing through isn't cold-'er' due to the defrost running (cool air in, hot air out). As soon as the cycle is done, the Aux goes off and heat pump blows warm air again (I verified by standing by the handler as it did all of this). Might be a good tip for anyone who doesn't want their heatpump running all the time because of defrost cycles. :)