I've built two different outdoor boilers now I wouldn't waste my time with a site glass. I can go all winter and only have to add water once matey 3-4gal. The biggest thing is insulation. And be prepared to cut a ton of wood haha.
Not to be the bearer of bad news but If it's blowing hot air it's needs recharged at the least but most likely need a new compressor.hopefully I am wrong but that will make them cycle on and off.