I put the diesel heater on high for 1 hour 50 minutes tonight. 8 degree's outside, 14 degrees inside using an old fashioned mercury thermometer on the other side of the garage.
There was a distinct feeling of warmth coming from outside to inside the garage but it still felt pretty cold after being in it for while, but I suppose 14 degree's isnt exactly warm, especially considering I came from inside a very warm house, however if I was doing something other than just standing there it wasnt any where near as cold as it usually is.
So even though I havent sealed the garage up properly yet I am reasonable confident it will do the job and considering it is a heater less than $200 and very efficient I am happy enough so far.
I am going to seal the garage tomorrow and run the heater for a longer time tomorrow arvo/night so that will tell the real story.
But initial thoughts would tell me that it is effective enough for me to be out there working but if I was hosting a shed party for instance I would imagine my wife would be feeling the cold.......but she is a very cold person.....very very cold and nasty and heartless lady like they all are
Just kidding, she isnt a lady.......
So any youtube videos telling you not to run these heaters on high because they shut down is fake news, and anyone telling you not to start them on high is also fake news.....but just like everyone else, this is only my opinion.