A new Furnace is actually a wise "Investment".
With the rate that natural gas and other fuels are rising these days, what could be a more sure investment than a higher efficiency furnace? After all, if a penny saved is a penny earned, then a new high efficient furnace is a money making machine.
A.F.U.E. (Annual Fuel Utilization Efficiency ) roughly refers to the percentage of heat that actually makes it into the home as opposed to what is wasted out the chimney. With a 20 year old furnace that is rated at 60 A.F.U.E. that would mean that 40% of the gas burned was wasted, or on a $200 monthly gas bill $80 of it simply went up the chimney. Where as, a 95% A.F.U.E. furnace in this case would have saved you at least $70 just that one month. Now multiply that times 6 months and you can see how a new furnace could pay for it's self in just a few short years, the rest from there is profit.
What are the differences in furnaces' efficiency?
The main options in furnace efficiency are, basically, 80%, 2 stage 80%, +90%, 2 stage +90% and a modulating +90%.
80% Furnace- This is the minimum efficiency allowed in furnaces today and it vents into a properly sized, lined, masonry chimney or a class B metal chimney. The heat exchanger consists of s-type tubes that restrict the hot gases and allow more air to pass across them to better transfer the heat into the home before it is exhausted out the chimney.
2 Stage 80% Furnace- is essentially the same as the 80% except that it has a 2 stage gas valve. The furnace basically fires at 60% capacity for the first 12 minutes and if the thermostat is not satisfied in that time it steps up to 100% until the demand for heat is satisfied. Although the actual efficiency percentage hasn't increased, the significance of a 2 stage furnace is this; When we size a furnace in Michigan we use 0 degrees as the outdoor design condition, so that furnace has to be big enough to heat that house to 70 degrees when it is 0 degrees outside. Obviously, most of our heating months are far above 0 degrees, meaning that our furnaces are over sized the majority of the season. Enter the 2-Stage furnace, if you take a 100,000 BTU furnace @ 80% A.F.U.E. it is wasting 20,000 B.T.U.s per hour when it is firing, but if you make that furnace just 60,000 B.T.U.s it is only wasting 12,000 B.T.U.s per hour, saving 40%. This is the furnace I recommend for those that do not plan to be in a house long enough to see a return on an investment in a 2 stage 90%, or for rental properties where cost efficiency is a more determining factor.
+90% Furnace- These furnaces do two things that increase their efficency over the 80% furnace. First, after the hot gases of combustion pass through the heat exchanger, as they do in the 80% furnace, they are then drawn through a secondary heat exchanger to further remove heat before it is exhausted. In fact, so much heat is removed that these furnaces no longer exhaust enough heat to vent in to a vertical chimney and are instead vented out in PVC pipe. The typical exhaust temperature on these furnaces range from 90 - 110 degrees as opposed to an 80% furnace that has exhaust temperatures in excess of 300 degrees. The second thing these furnaces do, is that by cooling the hot gas in the heat exchanger they actually condense water vapor that was produced by the combustion process (Hydrogen in the gas and oxygen in the air equals H2O). As this water vapor is condensed, B.T.U.s are gained via "Latent" heat, this is why these furnaces are sometimes referred to as "condensing" furnaces.
2 stage +90% Furnaces- This furnace combines the best of the 2 Stage Furnace with the +90% furnace and truly gives you the best "bang for your buck" when fighting high fuel costs. It is actually usually a very dependable furnace and it is the furnace I would highly recommend to anyone that is going to live in the home for at least 5 years.
Modulating +90% Furnace- These furnaces go beyond 2 stage and actually modulate the amount of B.T.U.s and C.F.M. delivered from your furnace to meet the changing heat loss due to varying outdoor temperature conditions. Although there are some applications for this furnace, if your ducting was not originally set up with a modulating damper system this furnace will never do what it is “sold” to do. As a service technician of 25 years, I do not recommend these types of furnaces in replacement situations. Aside from the costly repairs, it has been my experience that they often cause air flow and balancing problems in a ducting system that was not designed for modulating static pressure.
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