We have an old Westinghouse Forced Air Furnace (model FGUB137EG). It's been working fine, with the occasional pilot lighting issues since we moved in 6 years ago. We've been thinking about adding central air conditioning with a heat pump and part of the upgrade would be replacing the furnace, with a new variable blower furnace.

I am trying my best to figure out IF we would save on energy - and heating cost - with the new setup in winters. We are in NYS and the furnace definitely gets its use. Anyway, according to the furnace plate, it's "INPUT BTU/H" is 137,500 and the "BONNET CAP BTU/H" is 110,000. Does that make it this a 110,000 BTU furnace with 80% efficiency?

We would be switching to a 96-98% efficient 100,000 BTU furnace with variable or 2-speed blower, along with a 4-ton 24-SEER heat pump. The cost of the everything is coming to around $13K (including labor), in case you can comment on that as well.

  • If you see a 10 or 15% improvement in efficiency, how many years will it take to recoup 13k? A simple analysis... But don’t forget the extra use of the new one as you will then have energy use for heating and cooling...
    – Solar Mike
    Aug 22, 2020 at 7:01
  • @SolarMike, the $13K includes the heat pump / central AC, which we currently don't have.
    – 0pt1m1z3
    Aug 22, 2020 at 12:49
  • So how does that change what I told you?
    – Solar Mike
    Aug 22, 2020 at 12:52

2 Answers 2


What is “old” a 20 year old system may have the same specs or close.

The serial # can help find the date but if it has an open combustion chamber your efficiency will improve.

As far as pay back that really never happens I say this because by the time even a 10% increase in efficiency you save $10, $20 per month by the time it pays back its time to replace it again ,,, how could that be? you don’t need heat half of the year watch the sales scams that throw in 12 months of savings.

I tell my friends the way to look at it if you have a 30-40 or 50 year old system is it may be getting close to the end of its life and proactive replacement makes sense. And is usually less expensive (most all furnace failures happen when it is cold that makes sense) having the system installed off peak heat or cold is when the contractors are hungry for business and if you shop around you get a better price. But prices go up in the peak heat and cold times because they can get it.

You would like to add AC /heat pump. To your system so if it is a truly old system that you would gain efficiency that makes sense but trying to justify based on payback in reality is a shell game , what you gain in comfort is the main thing because adding AC your annual cost of heating/cooling usually goes up but the home is more comfortable throughout.

If you do get a ac/heat pump that makes sense to me. a high end thermostat that can use the outside temp and select the cheapest heating method may increase your savings on heating when it’s not as cold outside. Again it takes a long time to pay for bells and whistles like that but it is also greener in the long run.

  • Thanks! Yes, the major motivator is the comfort, and part of the cost is adding registers in the basement which currently has none. Yes, we are getting a system that monitors outside temp and according to the company switches to the gas furnace when outside temps go below 35 degree (and that happens every year for us in NY!). What I am very interested in knowing (and I will ask the sales person), is if we are able to switch between the furnace or the heat pump for heating at a whim, granted temps are above 35. The reason being is variable electricity and gas costs.
    – 0pt1m1z3
    Aug 23, 2020 at 13:07
  • Most thermostats that have that feature can toggle the primary heat source.
    – Ed Beal
    Aug 23, 2020 at 15:36

Yes, you have calculated the efficiency correctly. Note that since 1980, all furnaces in the USA have been required to have an efficiency of 80% or better.

The new unit has an efficiency in line with what is now common in the market, 96-98% efficiency.

So the question of whether or not this is a good idea financially cannot be answered with the information provided. You need to know the cost of the fuel or electricity for the old unit and how much it used. Then you need to calculate the cost of fuel/electricity for the new unit and estimate how much it will be used.

Generally the company trying to sell you the new system should be able to make some estimates for you and help you understand your payback period. If they can't or won't, then keep shopping.

  • I guess I need them to separate the quote between the furnace and the evaporator coil & heat pump. The rub maybe that installating both together likely has lower labor costs VS installing the furnace now and the AC / heat pump later.
    – 0pt1m1z3
    Aug 22, 2020 at 12:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.