Efficiency is the key property of all furnaces. It shows how much of the energy from the fuel the furnace is able to turn into heat and deliver to your home. This will further determine how much fuel of any type you will need to have the desired temperature in your home over a heating season.

The Annual Fuel Utilization Efficiency rating is the standard measure for furnace efficiency. The AFUE rating is a must for all manufacturers who need to show it with all furnaces they offer. AFUE shows efficiency of a furnaces calculated on an annual level, as some systems can operate differently in different conditions.

How do you assess AFUE (Annual Fuel Utilization Efficiency)?

If you have a furnace with declared AFUE of 90% it is expected to deliver 90 per cent of actual heat capacity of all fuel put in it over a year as heat, while the reminder 10% will perish. For example, if you put oil which has 100 000 British thermal Units (BTUs) of heat as its potential, your 90% AFUE furnace will deliver 90,000 BTUs of heat from it. Note that there are no perfect furnaces with 100% AFUE. However, some cutting-edge furnace designs have AFUE ratings of up to 98.5%.

residental_furnaceHow much heat you will get also depends on the state of your ductwork. Ducts may have leaks you don’t notice which release heat where it’s not intended to be. That is why you need to include duct check in your annual system check-ups.

You might have moved into a home with an old furnace for which you can’t find out the AFUE rating. In this case you can use continuous pilot-light to test the furnace. This kind of furnace requires you to relight the pilot whenever it goes off manually.  If you find out your system is one of these, you need to know that it has low efficiency of between 65% and 70% AFUE and you will only benefit from its immediate replacement.

Somewhat better models feature a fan that controls airflow. These systems usually came with electronic ignition. Furnaces of that generation have AFUE of 80% to 83%. You should consider its replacement in the following years.

Highly-efficient furnaces with AFUE of 90% and more have two heat exchangers installed and are sealed to eliminate heat or fuel leaks. The best among such furnaces can have efficiency of 98.5%. These furnaces are almost new and you can rely on them for many years.

Some of the old systems can be retrofitted giving them an increased AFUE efficiency rating. Oil furnaces, for example, can be improved by adding vent dampers and boilers can be improved by installing modulating aquastats and time-delay relays. You should carefully calculate whether these measures, albeit being inexpensive, can still save you enough money to be more justifiable than replacing the entire furnace or boiler. Retrofitting will never result in an AFUE as we can get by installing a whole new furnace.

Minimum AFUE requirement for home heating furnaces are currently set at 78% for weatherized oil furnaces and 80% for weatherized gas furnaces, 78% for electric systems, 84% for oil water boilers, 82% for gas water boilers and 82% for oil-fired steam boilers. These requirements are valid from 2015 which saw AFUE requirement increase for most systems from the previous 2013 levels.

However, majority of the systems already have efficiency of above 85%. Highly efficient systems with AFUE of 95% and more are somewhat more expensive, but that pays itself over a long run.

Highly efficient units usually have labels which mark them as such. Those include EcoLogic and Energy Star (the same you can spot on any other kind of device). A high efficiency energy system is also very safe for operation.

You should always look for a high-efficiency model, as every percent of AFUE will pay for itself. Fuel costs will always increase, slower or faster and the main way to prevent the negative impact of it on your budget is to increase energy efficiency wherever and whenever you can. In the Northwest, heating has the greatest stake in every home’s energy consumption and this is why you should prioritize the investment in a new high-efficiency furnace and do it as soon as possible. Otherwise you will have yet another season of high oil, gas or electricity expenses. Sealed high-efficiency systems are more reliable as well.

All systems will tend to lose its efficiency measured as AFUE over time. To minimize that you need to properly maintain your system. Annual checkups, air and fuel filters replacements whenever needed, timely fixes of all breakages and not neglecting your system when it’s not in use (i.e. during summer) are ways to maintaining high efficiency of your heating system for years.  Although all these things cost certain amount of money, you will save in a long run through a better fuel economy of your system. If you have an oil furnace or boiler, you will need to replace the nozzle, clean the tank and put additives in your oil tank whenever needed, too. Other measures can include choosing quality fuels (if you use heating oil or any other heat source delivered to your home by truck). Be aware that furnace manufacturers always measure their furnaces and boilers for AFUE under the best available conditions which include the best fuel. To benefit from it you would use oil or propane price comparison services, to find out which suppliers in your area deliver high quality fuels at reasonable prices.