Do You Need to be Concerned if Your Furnace Exhaust Pipe is Hot?
Older style furnaces had their exhaust run up through the home and then up through the roof. Because the chimney is out of reach, most people have no idea how hot it can become. Whether the furnace is a standard (a conventional furnace 50-70%), mid-efficiency (70-80%) or a high-efficiency furnace, the provision for combustion air and flue gas vent systems is very important. Mid-efficiency furnaces have an AFUE (Annual Fuel Utilization Efficiency) of 80%, and high-efficiency furnaces have an AFUE of 90-98%. In the case of a mid-efficiency furnace with an AFUE of 80%, that means 80% of the energy of the fuel becomes heat for the house and the other 20% escapes through the chimney. So even though the furnace is much more efficient than old furnaces there is still a small amount (~20%) of the energy is sent into the exhaust is to keep the flue gas temperature from condensing, which could otherwise cause the flue pipe to corrode.
Unlike the old furnaces that used metal pipes, the new flue pipes for high efficiency furnaces are made of plastic (PVC). They are plastic due to the lower temperatures left in the flue gases. It is a specially rated plastic however, which is engineered to stand up to the caustic condensation and byproducts of a high efficient furnace. With high efficiency furnaces becoming more and more popular, their exhaust systems are now very visible on the side or the back of houses with their obvious white pipes. A higher efficiency furnace with 90% to 98% efficiency, is significantly more efficient than an 80% efficiency furnace and should produce slightly cooler exhaust. These highly efficient furnaces have a heat exchanger and a condensing chamber that help maintain a considerably low temperature on the pipes. The only thing you may have to additionally invest in is a drain line to drain the acidic water that is a result from the condensation from the exhaust.
In colder climates you may see icicles hanging from the plastic exhaust pipe. This is the high humidity exhaust which is simply freezing as it is getting exhausted. The majority of the moisture drains back to the furnace where it is directed into a drain. (This is why these PVC pipes are sloped ¼” per foot back towards the furnace, to assist in the draining process). These ice formations can be alleviated and /or at least minimized if it becomes a concern. Sometimes, as these pipes are now easily within reach, many curious people touch them and find them to be quite warm, which can cause them concern.
If you feel the exterior of the plastic pipe while the fans are running it will be warm to the touch. This small bit of lost heat is not something that you should be worried about. You are actually saving money which you would have otherwise spent on the total gas bill by having that high efficiency furnace.
A few reasons for an elevated temperature could be a possible blockage in the chimney, dirty filters, low blower speed, some form of restriction in the duct. If the pipe is awfully hot to touch, please get in touch with our office and one of our service technicians can give you their opinion. Our technicians are both experienced and reliable, and can have a look at the furnace, inspect it, diagnose, and address any issues that they may find.
Purchasing a newer furnace with a model that is more efficient is a big decision, and there are lots of important factors to consider. We at Weather Tech Heating and Cooling replace and install these units every week. We would be happy to discuss your options, do an inspection and make our recommendations.