Not much heat could be escaping from that type of a unit , interesting it must have a built in heat exchanger.

My Direct Vent Gas fireplace is 85 or 88% efficient , the Vent system is crude to say the least its Made by Heat n Glo

Usually direct vent means it exhaust vents into the living space, as in there's no flue. They sometimes say its 100% efficient because there's no loss out the flue. Of course you are also getting 100% of the carbon monoxide.
All furnaces with blowers have a heat exchanger to keep the carbon monoxide out. Even the run of the mill 78-80% efficient ones built in the last 25 years.

Your thinking of a No Vent fireplace

With My Fireplace No air leaves my house or enters it from the Fireplace , its sealed.


here is a Direct Vent Fireplace

https://www.youtube.com/watch?v=QobD_22S_vc

I'm re-visiting my options for replacing a gas furnace, and see that Yanmar is offering both CHP units, and pure gas-powered heat pumps with exhaust heat recovery. https://www.yanmarenergysystems.com/vrf/ Has anyone here tried an off-the-shelf system?

If you find some prices, I'd love to know them. I've read about natural gas heat pumps being available in Asian markets for years, but haven't seen any available here in the USA.

Thanks,

Charlie

Before I ask for prices, I want to have a size in mind, and my current furnace has no indication of BTU output by the numbers. Is there a decent formula for estimating that from the bills on the cold months? On the very worst days, it runs half the time.

No brand/model number? If you can find that, you can research the model to determine btu.

The most discouraging thing for me was that it looked like the smallest model was 8 tons. That might be about right for max output for my whole house, but we currently have 2 units and we close off over half the square footage unless we have a house full of company. I'd wonder if it could ramp down to less than a quarter of capacity efficiently when we don't need much in our mild climate down south (MS, USA).
Charlie

Model number? How many burners? Is it a natural gas furnace? Is the exhaust vented using metal or plastic pipe?