HVAC experts: I have a fairly efficient gas furnace for my house. It is a Goodman But it is somewhat like a fireplace. It draws its combustion air from within the house so when it is running it sucks warm air up the chimney. I am aware that the most efficient systems draw the combustion air from the outside. My first question is do the systems that draw air from the outside compensate for the temperature of that air by changing the air/fuel ratio as do modern fuel injection systems in cars? Can I gain in efficiently with my present system by simply sealing off the unit and drawing the combustion air from the outside? I am really concerned about heating my home next winter. Thanks for your help.

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Welcome to the forums.

I don't recommend any modifications directly to the furnace but a fresh air in line brought near the furnace is ok. If your furnace has the plastic pipe exhaust it should have a fitting for intake air.

You can post a model number of the furnace. It's on a tag inside on the side wall near the burner.

I have installed outside air lines into restricted utility rooms. I use a damper inline to close the line off when not in use to reduce the amount of incoming cold air.

Your not really changing furnace efficiency.... just not drawing unwanted outside air into the house.

 

One of the benefits of having an fresh air intake is that the furnace will not be drawing air into the house from ever crack or gap in the house thus providing less potential cold spots.

The furnace doesn't care what the incoming air temp is, it just runs until the thermostat says enough.

Sooo, it's not really an efficiency improvement, that comes from the combustion/heating efficiency of the furnace to something more like a bit more comfort.

An 80% efficient furnace will always be an 80% efficient furnace.

 

I understand what I am trying to accomplish, increase overall efficiency by limiting outside air infiltration. Perhaps I did not explain myself very well. I know much more about modern fuel injection systems in cars then gas furnaces. In a car the air fuel mixture is constantly being adjusted based on the information gathered from the oxygen sensor in the exhaust stream. In my 80somthing % furnace, the air fuel mixture is only adjusted when the unit is serviced. This is acceptable because the furnace in my cellar is not subject wide temperature swings. I guess what I should have asked is if the high efficiency furnaces have reached the sophistication of automobiles where the exhaust is constantly monitored to adjust the air/fuel mixture. My concern is that if I run an outside air intake that the varied temperature could cause such inefficiency in combustion that the modification would be counter productive. By the way my current unit uses the old style metal pipe not PVC. It is a Goodman GMP050-3 I believe it is around 55,000 BTU. Thank you very much for your informed opinions.

 

The outside air is not directly connected to the burner. Even on a high efficiency furnace. It discharges in the area so that air mixing occurs. You would not want to connect cold air directly into the burner chamber.

The burner has no adjustments based on air temperature.

 

Your gas furnace most likely has a failed heat exchanger - gmp was from when goodman was very poor, they had tons of problems with the heat exchangers cracking as well as popping the rings.

Furnaces do not have oxygen sensors, etc. The changes due to air density, etc are too minimal to have noticeable impact on efficiency.
By design, they run with far more air than needed to ensure very clean, complete combustion even if the btu input is 10% or so above rating.
The exhaust of a properly running furnace has 6 to 10% oxygen and less than 100ppm co -> 100 being max limit, 30ish or less being normal.
The difference in heat output isn't worth the extra complexity and risk of high emissions.
Cars on the other hand run within closer tolerances - are optimized for raw efficiency, maximizing force from exploding fuel.
The exhaust off of an engine has co in the thousands of ppm, and the catalytic converter cleans it up.

Some high end furnaces do use a variable speed inducer/exhaust fan -> the inducer only runs at speed necessary, and tries to minimize excess air to enhance efficiency. These help compensate for length of exhaust/intake pipes and lower firing rates ->These furnaces are either 2-stage (most 2-stage have 2-speed inducer) or modulating, and at the minimum firing rate still run more excess air than when on maximum because of the venturi effect and the burner/heat exchanger openings being sized for maximum firing rate.

2-stage = a low heat and a high heat, low is normally 2/3rd high
modulating = variable heat output, range 35 to 100% on most.


Overall, most of the savings of a high efficiency furnace actually come from having a secondary heat exchanger to extract more heat out of the exhaust and also recover the latent heat by condensing moisture out - not combustion air from outside or better fuel to air mix.

 

I thank everyone for their responses. User 10 this was exactly the information I was looking for and I especially thank you for your detailed information.