Other than the obvious 90, 91.2,92,93,or 95 to 97 % efficient furnaces what are the differences in layman terms?
First and foremost the percentage efficiency on a furnace or boiler refers to the amount of heat gained vs the amount of gas applied. simply put for every dollar you put in you get 90 to 97 cents of heat out.
The most important factors are combustion efficiency and heat exchange efficiency.
Combustion efficiency is effected by the amount of free air and fuel mixture. Heat exchange by the air over the heat exchanger.
Each furnace is rated for the amount of temp rise the manufacture has required during the design process in order to achieve it's steady state efficiency rating.
It only stands to reason that the efficiency requires a minimum amount of air flow. This is verified by a good contractor by performing a temperature rise check after he has installed the furnace and while doing a clean and check it is one of the final steps to assure proper operation.
The Home owner's responsibility is to insure that the filter is changed periodically according to the type of filter.
One inch filters should be changed every month while the furnace is in operation and the same applys for A/C operation. 4 inch filters every 4 to 6 months and pleated media filters should be chnged ones per 6 months to once every year. The Aprilaire media filter is one of the best media filters and it is suggested once a year unless your operating the fan continous and have heavy traffic in the house. The whole point is to maintain the best air flow through the furnace to maintain a high level of heat exchange thus higher efficiency.
90 % plus furnaces are made to create and handle condensation in the secondary heat exchanger. Temps lower than the range indicated by the manufacture will creat condensation in the primary heat exchanger and could cause furnace damage. This is caused by too cool of return temps back to the inlet of the furnace or too much air flow.
80% furnaces are designed for 80 % efficiency and no higher with out condesate being created. It is also required that they have a liner in the masonary chimney do to the temp of the flue gas being so close to condensing level.See 80 % furnace article for further information.
The higher end 93% to 95% and up furnaces made today have a variable speed blower available as do some 80%. Variable speed blowers are more than just a simple change and have revolutionized the forced air furnace and other ventilation equipment such as exhaust fans.
The low wattage DC drive fans have cut the operation cost of furnaces and air conditioners by more than 50%. In Mr Heat and Cool's humble opinion, Any one who doesn't go the extra step in providing a variable speed blower is wasting time and energy.
The varable speed blower has also proven to be very trouble free due to the diminished wasted heat energy the bearings will last much longer. Trane was the first to come out with these blowers and I have seen no failures do to natural causes. Usually water is involved with electronic damage or perhaps dirt due to lack of filter and blower maintenance causing unbalanced blower wobble which is a secondary failure.
In closing, I have seen the invent of 80 and 80plus(which were a huge failure) and then the 90% plus. The kinks have been worked out of mostly everyones brands and now we see the new blowers adding even more class and advantages for the home owner and small business owners. Prices are driven on warranty and the installation.
The home owner and building owner has a decision to make based on the contractor. Will they be there to back their installation? Will they do a post installation check out to assure the furnace is running the way it was intended? Is the filter sized according to the installation guide? Is the duct work sized appropriately ( even the old furnace may have been installed on inadequate ducts, robbing the system of efficiency.
The efficiency of a 90 plus furnace is more than just a number. The efficiency is driven more by the installation and set up than the rating. The contractor you chosse could cost you much more than the final invoice following the install.