Efficiency vs Emissions

[iron]

In researching a new ZC fireplace, I have been seeing efficiency ratings ranging from 66% - 76% and emissions ranging from ~1.0 - 4.4g/hr. While I have not tracked the numbers and performed a side by side comparison, I have certainly observed that some units with low efficiency also have low emissions, whereas some units have high efficiency and high emissions. I'm unsure how this would work. Is it basically saying that it a unit could burn clean, but not extract as much BTU from the wood (maybe b/c of firebox shape, size, insulation, etc)?

 

[NorMi]

Efficiency is mainly down to overall heat exchanger performance at a given flow rate, where emissions are more complex, mostly what firebox temperature and excess o2 you are using at your test burn rates, and how good the o2 is distributed, etc. You can't compare emission g/hr directly, because the BTU/hr changes what that number means. You need g/mmBTU (emissions per unit of heat output) to compare apples to apples on that one.

 

[Highbeam]

You have discovered what many people get confused by. Emissions and efficiency are not related. The best designs have good marks for both but many sacrifice efficiency for low emissions in this emissions driven epa environment.

Same thing happened with automotive engines.

All 2020 approved stoves are sufficiently clean, choose based on efficiency if you would like. There’s still quite a spread on efficiency.