How hard is it to change an old-style thermostat for a new-fangled digital/programmable one?  Is it just a matter of unhooking the wires from the old one and replacing them on the new one?

Depends.....

A:  Why?  I scrap more of the new crappy ones than the old doorknob Honewells, by far!

B.  If the new one requires a 4th wire for common and you ain't got it, then wire is required.

C:  Be sure to turn off power to furnace and AC unit while you do the deed.

D:  Reconsider what you are about to do!!!!

MickeyMouse is an old grouch!   [:D]

Get a way cool Enerzone system.
(it'll only set 'ya back a few thousand and a bunch of cat-5 cable) [;)]

[url]http://www.i2automation.com/aprilaire.htm[/url]

[img]http://www.i2automation.com/pdf/aprilaire/ct_monitor.jpg[/img]

Is your answer dependability related only. Don't they help with fuel savings? I know almost nothing about them but I like mine I can set it to turn off and on when needed and not just have the thing run all day at the set temp. I am not questioning your[bow] wisdom[bow] just questioning.

WOW
[:)]  

Mickey's probably correct...  I was "sold" on the energy savings benefit when we installed our system.

(spent about 4k just on thermostats, sensors and a controller)

The payback time will probably exceed my lifetime. [;)]


BUT...  It IS cool to press a button and see the outside temperature on the screen !
(helps to decide the daily apparel. [:D])

our digital fangled dealie seems to save us some ching every year-just set the temps and it will havve the house all warm for ya when you get home.  Of course you will want to recycle the old one with mercury in it.  Ours you had to set to whether you had gas or electric heat too.  The instructions look tough but just sit down and read em.

For whatever it's worth, some background info:
1) I'm renting.  I likely can stick the landlord for the cost of the unit, if I do the install myself.  The old thermostat is broken right now so it has to be replaced anyway.
2) The AC is not working very efficiently (i.e. doesn't cool the damn house and runs all the time, but by the morning it's cooled down).  We haven't even gotten to our hottest months yet, and I already can't cool the house?  My electric bill is going to be through the goddamn roof.
3) What I'm considering is one of the $100 boxes from Home Depot [url=http://www.homedepot.com/prel80/HDUS/EN_US/diy_main/pg_diy.jsp?CNTTYPE=NAVIGATION&CNTKEY=market%2fpg_zip_code.jsp&BV_SessionID=@@@@1158012204.1052317261@@@@&BV_EngineID=ccddadciffeefdlcgelceffdfgidgnk.0&search_text=thermostat&DRC=4]here[/url]

I have a cheaper coil cleaner-garden hose.

OK, I see the fins, you're talking about the ones all around the outside of the unit right?  They dont really look particularly dirty to me.  Some light dust & debris, sure, but that's about it.

Maybe tonight I'll open up the unit inside and see how things look in there.  Otherwise I guess I'll just call a repair man and have him tell me that I need the ducts cleaned for $500 or something.

I run commercial HVAC for a living. Clean the coil on the unit and change the air filter. Unless you have major fungus problems, the inside of the ductwork should be fine. As to the thermostat, if you buy a simple unit that has a timer to run the system, you shouldn't need to rewire anything.

Yes, RBAD, I am an old grouch!!  Been doing the HVAC thing for 25 years or so and have grown weary of the hype and BS that is rampant.  I have an electronic background (braodcast engineer etc.) BEFORE the HVAC gig so am well aware of what goes on in the little box.


You guys asked for the LONG answer so here goes.


Remember, K.I.S.S. as in keep it simple, stupid.  A bimetal t-stat is a POS IMHO but they ARE simple.  Still better than electronic!   A mercury bulb one, Honeywell "door knob" T-87 model is also pretty simple.  Millions are in use, most for untold decades.  RARELY do anything but what they are intended to do.  Anybody can adjust and understand them.

Electronic ones are a bit complex to program.  They lose their mind every now and again due to power fluctuations and other demons. Need a battery to "remember".   After the novelty wears off, many owners tire of screwing with them.  I then get to sell them a NEW HW unit like they had before.  Real smart.  Spend two hundred bucks to go full circle.  Electronics DO have the capability of maintaining a tiny bit narrower temperature range and doing a better job of anticipation.  SOMETIME.

The cost to heat or cool your space is based mostly on the "envelope" which is doors, insulation, glass, sun exposure, HOME SIZE, fuel used, fuel cost rate, your lifestyle, weather.  Very minor is HVAC unit efficency, temperature maintained, setback/setup.

Setback / setup saves during that period of discomfort, no question.  It costs MORE money to return to comfort.  If gone for a week and you set heat to 50 degrees, you save.  When you "Jack" the thermostat over just a few hours, most savings are lost on the "reheat".  In the case of cooling, it may be impossible to cool the space back down at all.  If you are stuck with a stinking %#@$$$$$$$&*^ heatpump, a setback t-stat can easily RAISE your bill.  Sometimes a LOT.   You save during setback when eff. is 200 - 400%.  You reheat on strip heat at TWICE up to 4 times the cost!!  To a lesser extent, same applies to cooling.  AC works best at night and early morning when outside is coolest.  That is when you want to cool the space and "store cool" by reducing stored heat in the structure and contents.  You want to maintain that accomplishement, not let it be lost so you can cool it back down when outside temps are high and unit works the hardest.  Steps on dollars to pick up quarters!!

Most "fancy" T-stats save nothing, but because the owners WANT them to perform "magic" they THINK $$ is being saved.



COIL CLEANING:

VERY important.  Often inored.  A strong stream from a hose and nozzle works best, aimed STRAIGHT in at 90 degree angle to coil.  ALL coil cleaners eat away the fins of your unit.  Makes 'em pretty but very destructive if not REALLY well rinsed, at which time they only do SOME damage.  Wash coil twice a year from the INSIDE like I do.  Opposite the airflow.  All that crap is sucked in at 40 MPH or so then cooked at 250 degrees for months.  It DOES NOT want to let go!!  In general, if you see ANY dirt the thing is filthy.  (What hurts you most is the crap between the fins.)  I can "lay hands" on the unit tubing and tell you if it is dirty and many other things.

Please note that LONG run times are best.  They use LEAST energy if unit is matched to load.

Poor AC performance can be dirty filters, dirty condenser coil, low charge (common), unit poorly matched to load (VERY common), inadequate indoor airflow (almost always) ignorant resident that has closed "some registers" to [save] money (false economy), poor service pratices allowing dirt or moisture into system, plugged refrigerant filter / driers, "low pumper" compressor, among other things.

There is no "magic bullet" to save energy!

Short of buying a unit that isn't 20 years old, right?

Twenty years?  The change is so small as to make any savings a fraction of the replacement cost.  If the unit is in good condition otherwise, it is impossible to justify.  You can be sure the life of the new unit will be far less than the one being replaced.