Mutiple filters

I noted that the system needs to be evaluated due to the multiple filters.
I believe this can restrict the airflow. I also wasnt able to remove the paper filter due to the fins being jammed.

Secondly there where some disconnected wires in the cabinet and It appeared they were not being used any more.

Lastly, the exterior chimney had excessive damage and flaking and it appeared they should of installed a metal liner. The furnace was manufactured in 98. The ducts appeared to be newer.

When or does code state that a liner is required?



Well, since no one else is answering your questions, I’ll give it a try:

I’m not a furnace expert, but that fat filter looks like a combination of parts from different systems. The thin filter looks almost ok. I like to see the filter slide all the way into the plenum with a door to shut it in tight.

The chimney needs to be repointed. This is more likely to do with the weather outside than the furnace exhaust. It looks like the chimney is lined with tile, if it is properly maintained it should be able to handle the exhaust from the furnace.

It looks like there are two sets of thermostat wires, both with brown insulation. One set with two wires is not being used, one set with at least 3 wires is being used.

Hope this helps,


Pretty good guess, Ed.

The larger filter is/was a “static filter”–they were never worth a damn–
and the small filter should have a door to eliminate draw from so near the furnace.

The chimney should have a screened cap.

The chimney would need a liner if the furnace is a high-efficiency.

The large filter is April air/Spaceguard and is all that is required. The extra filter is not needed. The large pleated filter is able to go six months to a year without changing. The extra t-stat wires are going to the condensing unit or maybe a humidifier.

Hey Jae,
I appreciate all your helpful posts.
Help me with my train of thought here:
I assume a high efficiency furnace would produce less exhaust heat than an old inefficient model. Some of these new furnaces vent horizontally w/2" PVC. What am I missing?

I think Jae meant “unless the furnace is a high efficiency.” However, if he didn’t, then I have the same question as you, Ed.

The return air plenum should not be using combustion air and if it’s a natural draft furnace could be sending products of combustion into the supply air by creating a negative draft. That plenum should definitely be sealed. Right?

High-efficiency furnaces today use up to 97% of the heat capacity of the fuel. Older furnaces were about 45% - 65% efficient and the flues were very hot with the exhaust fumes. By draining all the heat from the gases to heat the house, there is very little left in the inert gases that are exhaused outside. Thus PVC is more efficient in removing these gases than is metal.

Since the flue gases are not warm, they would not create a heat-induced updraft in a large flue, so the smaller PVC, usually vented outside, would have to have a liner that would make the chimney small enough to draw the gases out. The best way, however, is to vent immediately from the furnace to the outside through the wall.

“The return air plenum should not be using combustion air and if it’s a natural draft furnace could be sending products of combustion into the supply air by creating a negative draft. That plenum should definitely be sealed. Right?”


Thanks Jae and James,
Big flue + small cold exhaust = backdraft. That makes sense.
That’s what I was missing.

I’m not finding myself agreeing with that.

First, if one is using the furnace, that probably means that it is cold outside. And I think that even the 3% of the heat that is not used is hotter than the cold air outside. And since heat rises, I think that 3% would naturally draft up the chimney, unless, possibly, there was a hurricane outside; that would definitely interfere with the draft of the chimney.

Second, all the high-efficiency furnaces I’ve inspected here definitely have warmer air coming out the plastic flue.

Third, considering how expensive it is to line a chimney, I think it would be more reasonable to simply run a piece of plastic up the chimney for a high-efficiency furnace.

Fourth, all this thinking has made me thirsty. And you know what that means, boys and girls? That’s right. Margarita break. :cool:

That is why the manufacturers require a liner in most cases.

…of course, if margaritas were induced into the flue I would suppose
the flue would be much warmer…

I don’t think one needs to heat the air in the chimney. Warm air will rise, so any amount of warm air will find a way to rise. Air that is only 5° warmer than all the air around it will try to rise above that air around it. And a chimney provides a natural way for that warm air to rise, especially if it is directed into the chimney to begin with.

I dont think the reason for the liner is the draft, I believe the high efficency furnaces produce more acid byproducts which break down the mortar in the chimney. This is why you will sometimes find sand particles in the furnace.


I thought that was the purpose for the liner with old chimneys and wood. And I thought that the high-efficiency furnaces were better at removing those byproducts. After all, those byproducts come from inefficiency. If something were 100% efficient, what would we get? Less of everything, right?

And acid will eat away at anything, including plastic. So if the high efficiency units produced more acid, plastic would not be my first choice for flues on those high-efficiency units. Metals, such as aluminum, even glass, would be much better.

The filter arrangement is not good installation practice, the use of the additional standard filter prior to the Aprilaire filter is done to “pre filter” the dirt to keep from changing the more expensive filter as often. The air flow restriction of the Aprilaire when new is very high, when dirty the system is close to failure from low air flow, by adding the “pre” filter were compounding the restriction and reducing the air flow even more and not adding any measurable life to the Aprilaire filter which is up to 97% efficient. This was mot likely a sales gimmick playing on the home owner request or just an idiot HVAC guy.

In situations where the high performance filter here the Aprilaire (one on the left) was not present and you only had the standard filter in the slot of the return air drop no need to “cap” over the filter, code normally will not dictate this and the risk of recalculating combustion air in to the house air is almost non existent, and “using up” the make up air or combustion air by drawing through the area around the filter is also almost non existent and most codes will not address this. Caps over the filter are higher end sheet metal work by better HVAC companies that charge more, while it looks better lack of a cap is not wrong under most older codes, although this is changing as more codes include provisions from the energy conservation act requiring tighter ducts.

The furnace you have pictured here by the look of the circuit board is a 80% efficient furnace, and when installed on masonry chimney with out a metal flue liner will cause mortar break down but may not have been required under code and installation guidelines depending on what year it was installed.

Prior to the 80’s we had furnaces in the efficiency range of 60% and lower, the amount of heat sent up the chimney was sufficient to warm the brick and provide proper venting.

In the early 80’s the 78% , 80% & 82% efficient furnaces were introduced. The amount of heat removed from combustion was greater resulting in cooler flue gases, but in the beginning no one knew this would be an issue. But after about 5 yrs home owners were having chimney failure, heat exchanger failure, carbon monoxide and carbon dioxide was leaking into the home through the mortar of the chimney as it broke down. Result many law suits.

The reason then and today that the chimneys break down is that the gases cool as they rise up the chimney, they cool and condense on the inner chimney with or with out a clay liner, the acidic level of the gas breaks down the mortor and rusts the heat exchanger. So liners became recommend under certain conditions. The liner protected the chimney, and the smaller surface area of the liner would heat properly to allow complete venting to the roof.

In the late 80’s into the 90”s the manufactures standardized on the 80% efficient level and recommend a flue liner only on chimneys on out side walls with out a hot water heater, again after several years more law suits.

Today liners are recommend by most manufactures for all installations of 80% furnaces on masonry chimneys

80% furnaces have a power venter but this does not push the flue gases up the chimney, this is basically still done by convection of hot air rising, the venter only pulls the gases through the heat exchanger.

Furnaces in the 90% range do not use liners nor do they vent by convection of hot air rising, they use only PVC pipe and the flue gases are pushed out side by the power venter motor. They can not be vented by convection because the average temp of the flue gas is around 90 degrees, far too cool for any type of chimney or metal vent. The PVC can be vented vertical or horizontal with each manufacture having a limit on the distance the PVC can be ran and if it is a one or two pipe system.

Appreciate the post, John.

NACHI RR&R (rocks, rolls, and rules).

Great info. thanks John.