Venting of a replacement appliance

Home Inspector with About The House

Replacement Appliance Venting


In today’s world, a home inspector is required to know a lot of information in a lot of different areas. I cringe whenever I read a HI report that states something like “further review is required” because it seems this has become the normal cop-out statement. However, there are times when that is exactly what is needed. Let me explain.


New Jersey state law prohibits a home inspector from ‘Performing or offer to perform engineering or architectural services unless appropriately qualified…’


When it’s time to replace a furnace, a homeowner has options. Two of the most popular types of furnaces are what’s referred to as 80 or 90 percent efficient. A 90 % furnace is required to have special, direct venting systems that usually are constructed of plastic pipe and terminate thru the sidewall or the roof of a house. We will deal with those types of units later. For today, let’s talk about 80% or ‘fan assisted’ furnaces.


The first thing is let’s clear up a very large misconception;

The exhaust of a fan assisted appliance is not under pressure. The appliance is considered Category I, which is an atmospheric venting appliance.


When replacing the older ‘natural draft’ furnace with a ‘fan assisted’ one, the installer needs to calculate the exhaust and the size of the flue. Let’s say the old furnace is installed in the basement and it’s connected to a masonry chimney. There are 2 types of masonry chimneys; interior & exterior.

An interior chimney is one that runs up, thru the interior of the house while an exterior chimney has 3 sides exposed to the weather.

Here in New Jersey, the common rule of thumb is if the new FA (fan assisted) furnace is connected to an exterior chimney, relining of the chimney will be required. If the FA furnace is connected to an interior chimney, relining MAY be required.


What happens is the FA furnace is more energy efficient; the flue gas temperature is much lower than the natural draft type. The cooler flue gas condensates quicker and will not reach the top of the chimney (particularly when 3 sides of a chimney are exposed to the weather). The flue gas turn to condensate, drips down the masonry flue liner and cause the liner to deteriorate. In quick order, the liner will crumble and fail. In order to prevent this, the masonry chimney needs to be relined. The relining needs to be sized according to the appliance(s) connected to it.


While a home inspector should know general requirements about appliance venting, it will take some engineering to determine sizing and types of exhaust venting.


So the next time your HI report states something to the effect that a “a fan assisted furnace is venting into a masonry chimney, a qualified HVAC contractor needs to review the venting” of a replacement furnace, your home inspector is really looking out for the occupants of the house.

Seriously, incorrect installation of a fuel fired appliance could have deadly consequences.


Here is what a 'fan assisted' furnace looks like.



Comments (4)

Mike (Inspector Mike) Parks
Inspector Mike - Circleville, OH
Inspector Mike


Can you show me a furnace that is allowed to be vented into a chimney?

I have never seen one that was allowed to do so.

Jan 24, 2010 04:39 AM
Darren Miller
About The House - Succasunna, NJ


Here in NJ, lots of old furnaces along with Water Heaters vent into chimneys. The problem occuring now is the chimney flue is too big for modern appliances.

There are tables in the Fuel Gas Code [G2428.3(4)] that are specific for a masonry chimney.

Another problem we see here is an 'orpahaned'; the old Cat I furnace was replaced with a Cat IV but they left the water heater connected to the masonry chimney.

For something REALLY cool,  and click on HVAC/R, go down to Gas vent and watch the demo.


Jan 26, 2010 03:48 AM
Mike (Inspector Mike) Parks
Inspector Mike - Circleville, OH
Inspector Mike


I should have said that I have not seen a manf. spec. that allows this.

Can you point me to one?

Jan 26, 2010 08:00 AM
Darren Miller
About The House - Succasunna, NJ


Here's what Lennox has to say on their FAQ area on their Web site:

"Furnace technology has advanced significantly in recent years, raising concerns over chimney use. As a result of changing technology, many existing masonry chimneys aren't able to meet the specific demands of new furnaces.

There are several reasons for this furnace-chimney incompatibility. First, the size of the chimney can be an issue. Modern, higher-efficiency furnaces transfer more heat into your home and less heat up the chimney than older, less-efficient units. While this means more efficiency for your energy dollar, it also means that the existing chimney might be too large for the new furnace. The result could be improper ventilation of flue products, which can cause condensation problems inside the chimney.

Other considerations include chimney height and location, proper lining and condition of the chimney. Building codes must also be kept in mind to ensure proper draft in the chimney for adequate ventilation."


All they are saying is what I've been saying- sometimes it can work, but many times it won't. Calculations are needed to determine if the new furnace will work with the existing chimney.

Jan 26, 2010 11:17 PM