In the cold months,the exhaust fan in the one shop building would pull to much cold air into the room and it was hard to get anything to dry decent. I recently rigged up this make up air heater to take the edge off the fresh air that is drawn in.The water to air HX is mounted in an opening in the wall opposite the exhaust fan.When the exhaust is turned on,it pulls nice warm air in thru the coil. I used a undersized(judging from the physical size,just a guess)flat plate HX with antifreeze in the coil loop.Wasn't sure if the flat plate would do the job,as it was a ebay cheapie,but it really works excellent. The whole system works better than expected,and really maintains room temp while still exhausting air.The real test will be how it works at colder outside temps. The one drawback is,it takes some extra wood in the boiler if used alot.