I heat my 3000 sq. ft. house (including basement), it's a log home and in need of sealing , so there are some draft issues. I also heat my 36x26 garage to 45* and an 800 sq. ft. apartment above the garage. My domestic hot water is heated indirectly through my existing system, and I'm going to run the apartment DHW off the coil in the shaver eventually. The boiler is 160' from my house, and 50' from the garage. I used the cheap TSP 1.25", but only go it down 2-3' because of ledge. I had snow melt above the trench for the first dusting we got, but only see a 3-4* drop in temp from the furnace to the house. I'm glad I got the TSP, it saved me over $2000 and I think it will serve me fine.
I've done several mods to my shaver already (you can read my thread) and have a few more in mind. The shaver is poorly insulated in my opinion and d improvement in this area is important to the system efficiency. I bought 1200 lbs of anthracite coal to mix in during the very cold (teens and below) days and it seems to work well. I add 3-4 shovel fulls and then load with wood. I saw a 30* temperature increase in 1 hour mixing in coal on a 10* day, when every zone in my house was calling for heat.