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My house is heated by a condensing natural gas furnace that is rated as 92% efficient. It has the option to draw combustion air directly from outside. I know the main advantage of this is avoiding negative pressure within the house to reduce drafts. However, I thought of a few questions:

  1. Outside air is colder. Does this decrease the maximum temperature of the flames?
  2. As the outside air is colder, it is also denser. Does the furnace's performance increase due to increased oxygen content?
  3. The furnace's exhaust temperature exiting outside is about 43 degrees Celsius, so there is still some heat in the exhaust that is not going towards heating the house. What could be done to utilize more of that heat?
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the outside air is colder, but the flame temperature is still much much hotter than that so the difference cold inlet air makes on the outlet temperature of the furnace will be small.

The same commentary applies to the density argument: yes, but the effect is small.

The fact that the outlet air is at 43 C represents the 8% efficiency loss of your furnace. This can be captured with a device called a heat pump, in which case the discharge air will be at or below ambient. This is the most efficient kind of furnace you can buy but a heat pump-topped gas furnace will cost about twice as much as one without the heat pump.

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    $\begingroup$ If your situation and local regulations permit it, it is also common to run both the intake and the exhaust using a coaxial duct (small exhaust pipe inside a bigger intake pipe). This has the benefit of serving as a heat exchanger that recovers whatever heat is left in the exhaust and use it to preheat the intake air. It is also safer than two separate pipes, because any leak in the exhaust cannot result in combustion gases (CO) poisoning anyone. They will just be sucked in by the furnace again and trip its protection sensors. $\endgroup$ – TooTea Dec 26 '20 at 20:47
  • $\begingroup$ @TooTea you have to beware of condensation and resulting corrosion in the exhaust pipe if you do this. $\endgroup$ – Tiger Guy Dec 31 '20 at 21:55
  • $\begingroup$ A heat pump that takes the heat out of the flue gas? Who makes these? $\endgroup$ – Tiger Guy Dec 31 '20 at 22:00
  • $\begingroup$ everyone. Easy to do, you put the heat capture coils in the flue plenum. critical issue is managing condensate on the coils from a corrosion standpoint. straightforward HVAC engineering. $\endgroup$ – niels nielsen Dec 31 '20 at 22:27
  • $\begingroup$ @TigerGuy Sure, but that is always a concern with a condensing furnace. The exhaust is always at the dewpoint, because that's where the condensation inside the furnace stops. Given the low temperatures, the typical solution is to simply use plastic coaxial piping or mixed plastic/aluminium. Just a random example (no endorsement): coxgeelen.com/en-gb/1/71/concentric_ppalu.aspx $\endgroup$ – TooTea Jan 1 at 6:44
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Outside air is colder. Does this decrease the maximum temperature of the flames?

  1. Your furnace will always need air for combustion. This combustion air must ultimately come from the outside. Combustion air can either come directly from outside (say it's 30F outside) or it can leak into your house, go through the heater and be warmed up from 30F to 70F, and then go into the furnace.

In the first case, your flame temperature is a little cooler than the second case, but all the heat is used for your house.

In the second case, the flame temperature is a little higher than the first case, but some of the heat is diverted into heating the combustion air.

Ultimately, it works out the same.

  1. Yes, denser air would increase your furnace's maximum output but it is seldom that your furnace would be running at maximum output. What you always care about is your fuel bill which is affected by total BTUs required and efficiency. Denser air would not increase your furnace's efficiency.

  2. As others have already pointed out, a coaxial heat recuperator can increase your furnace's efficiency and is also safer. My two furnaces (for a two story house) do have coaxial heat recuperators.

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  • $\begingroup$ Thank you, Phil Sweet. I have changed my answer to respond to your comment. I hope you like it. $\endgroup$ – Philip Ngai Dec 28 '20 at 19:59

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