Thinking about replacing 20-year old Carrier "midrange" gas furnace with new two-stage high-efficiency model. Big investment. What's the payback? In general, what sort of difference does this make in fuel consumption/heating bill?
What size and efficiency is your current furnace?
There are lots of "ifs" to your question.
What is your location?
It is important that your furnace is sized correctly.
Ductwork system should be thoroughly inspected including leak test. Any hot/cold spots in home should be looked at with suggestions for improvement.Adequate return s essential for good airflow and improved comfort.
If you are going two stage with var speed blower motor, it is important to have correct thermostat to reach best operational capabilities. If you don't already have a good whole house air filter cabinet, now is the time to add one.
You average fuel savings will be in the 15-20% range depending on which furnace you choose and the condition of your current furnace. Your savings will be a few percentage points higher if your old furnace has a standing pilot light.
Tigerdunes, this is a 125,000 BTU carrier system in a 1930's masonry house in the Washington DC suburbs with ductwork to match; I can't put a number on the efficiency, but it's not two-stage or "high-efficiency." It has electronic ignition. If mike home is right and the general savings is in the 15%-20% range, then it will take a very long time for a new unit to pay for itself. In the heating season, my gas bill is from $100 to $220 higher than in the summer (we have gas hw heater and stove). Doesn't seem right to spend $7,000-$10,000 to save, at best, $50 a month half of the year. (The estimates include new a/c coil, humidifier, thermostat, condensate pump, some sheet metal work, etc.)
You have to look at it from another perspective. How long do you plan to live in this house? Will it be more than 5 years? Then there is a high probability you will need to replace the furnace before you sell this house. Every year you wait to replace it will be one less year you will get to enjoy your new furnace.
There is also a savings if you are replacing the AC. Is it also 20 years old?
Your furnace was rated at 78% efficiency when it was brand new assuming it was properly installed. How large is your house and what is the condition of the insulation and windows? A 125,000 BTU furnace is very big. I would bet it is grossly over sized. This degrades the overall efficiency of the furnace.
Mike, exterior compressor unit is 12 years old and contractor said it will function OK with new interior unit and coil. Existing furnace is probably too large; it sometimes overheats and activates its high limit switch, and has always done so. Improvements to return ducts a few years ago greatly diminished that problem, but it hasn't gone away completely.
A furnace which has been running hot for 20 years could have a cracked heat exchanger. My advice would be to do some research on contractors and furnace options. If you are looking for a better price than plan to do it in spring or fall when contractors are not swamped with work and manufacture's rebates are available.
What size home?
Payback might be better if you are oversized more than you need.
And there s always the intangible about improved comfort with a var speed blower.