Educational Information about 92% Furnaces

Due to client interest in how these high efficiency condensing furnaces operate we will attempt to clear up some mis information that circulates by word of mouth as well as on Internet Chat Forums.

First let us say we have been installing furnaces since 1989 with next to NO repair problems. This includes the 92% versions. We ran a new house HVAC business for 10 years installing not only the furnace, air conditioner or heat pump but all the ductwork, wiring, in other words a complete turn key installation. We did this on between 150 - 200 new homes a year for this 10 year period.

To give you all some insight to why some companies try to discourage you from installing 92% furnaces please read on.


Installing these furnaces MUST be done correctly or they WILL NOT work right. Companies who hire inexperienced installers and service people already know they will have problems with the installation.

If your basement ceiling is finished with sheet rock it may be difficult to run the P.V.C. flue pipe or pipes to the outside wall of the building. These pipes USUALLY have to run horizontally AND have to have a 1/4" pitch back towards the furnace or air handler. If you don't have this pitch you will get at the LEAST erratic operation of the burners on the furnace or cycling of the air handler.

The size of the flue pipes ( diameter ) runs between 2" and 3". The size of the furnace and the length of the runs of pipe as well as the number of elbows will dictate which diameter of pipe is needed. Here again, if the installer AND the estimator doesn't understand this, you can count on problems! In other words if 3" pipe will be needed SOMEONE has to be sure there is enough clearance to allow for installation AND the 1/4" pitch.


The P.V.C. flue pipes must not be closer to an opening door or window than 1 foot.

Most manufacturers recommend they not be closer than 1 foot from grade level outdoors. Inspectors will read this in the instructions, put their tape measure on the pipes to the ground and if it is 11" will flunk the installation. This is because they know nothing about how the system operates and are going strictly by the instructions. Here is what is going on here. Let's say it snows and the pipes get blocked. Inside the single stage heating furnace there is a pressure switch, if it is a 2 stage heating furnace there is 2 pressure switches, either way the pressure switch will " sense " there is an obstruction and after 3 tries of lighting the furnace will go into lockout. After 1 hour the furnace will again try to relight. If the snow hasn't yet melted or otherwise been removed, it will again go into lockout mode. This is strictly for safety! Ice build up during certain humidity and outdoor temperatures can also cause this problem. The answer is simply to go out and remove the obstruction.

Other requirements are to keep the flue terminations away from high wind areas, in alcove areas of the home and a few other things. These aren't requirements but rather recommendations.


Another thing that we hear from clients getting estimates from other companies on 2 stage heating furnaces is that "We don't have to change your existing thermostat or change the thermostat wire from the thermostat back to the furnace." This is not totally true. The 2 stage furnaces have a low fire and high fire gas valve which should have a thermostat with low and high fire capability along with 6 wires going back to the furnace. The terminals are designated W1 (low fire) and W2 (high fire). If the thermostat and wire isn't changed, the furnace terminals must have a jumper wire installed between W1 and W2. The furnace circuit board will start out on low fire and after 10 minutes it will go to high fire. This will be alright in a newer home since newer homes have good insulation and will not run more than 10 minutes in most cases. Old, leaky homes with poor insulation and single pane windows will have longer run times and will be on high fire more than you are expecting. Homes in colder climates will be even worse in this regard. We feel however, you should try to pull a new thermostat wire if at all possible! This isn't always feasible however. One answer to this problem is to install a wireless thermostat (see our thermostat page by clicking the t-stat button on the Category page). If the price sounds too good, it probably is. Ask your questions, since if you are talking to a salesman or an otherwise inexperienced installer they may HONESTLY not understand, or the company is trying to get away with something? We have been beaten out of jobs by companies playing this game.


This is where it gets time consuming or even impossible! Try to remove your existing thermostat and tie a new wire on to your existing wire. One person pulls the wire from the furnace area while another person feeds the new wire by the thermostat. More often than not however, the wire will be stapled to a stud behind the sheet rock or the wire is jammed somewhere and can't be pulled. When this happens you will have to re locate the thermostat. If it can't be relocated you may NOT be able to use a 2 stage furnace effectively, depending on the age of your home. A furnace change out is expensive and needs to have a lot of thought by you and the installing company. EFFECTIVELY is the key word. Beware!! You may decide to use a jumper on low and high fire if the thermostat is a big factor. Then again a wireless thermostat corrects many problems also!


When installing a 92% furnace in place of an old furnace, we are removing many B.T.U.'s from the existing double wall vent pipe which runs through the home to the roof. Usually the only thing left producing heat is the water heater, which may not produce enough heat to properly vent. An experienced installing company should test the drafting of the water heater when finished with the installation, if it has affected the draft of the water heater you may need a liner installed in your existing vent to DECREASE it's size to get the water heater drafting right again. This seldom happens, however some City Inspectors will require it be done. This liner is stainless steel and is expensive and very labor intensive!


Due to the nature of the products we sell directly to homeowners we must have you read and understand our terms of use page. If for any reason you can't understand or agree to these terms we suggest you call a reputable Heating and Air Conditioning Contractor in your area for assistance. Your safety and the manufacturers warranty also requires it. Please click this link.

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