I've been investigating the winterizing requirements for a home sprinkler system. There are many articles referencing CFM and PSI. What seems to be often missing is how much CFM flow is required and what are the maximum and minimum PSI requirements. For 1" polyethylene main pipe 50 PSI seems to be the maximum pressure that should be used and multiple short times should be used at that PSI, rather than one long episode. I read one article from a university extension service that the flow should keep the sprinkler heads up after the water is expelled. That said, here are my observations. I used a 6 gallon 2.6 CFM @ 90 in series with a 6.5 gallon 5.2 CFM @ 90 system (total 12.5 gallon tank) on a 75 foot 3/8" air hose. I regulate the pressure to 50 PSI, although the tank pressure builds to about 75 PSI prior to releasing the air. When released in the open zone, the pressure immediately drops to 50 PSI. With a single 3 GPM sprinkler head zone that is about 10' from the pipe connection source, I can keep the head up at 50 PSI without issue. For a single 3 GPM head that is about 75' away from the air source, I can keep the head up for about 5-6 minutes before it wants to start slowing receding. It seems that once the tank PSI gets somewhere below 30, (maybe 20-25), the heads will not stay up. When blowing out a 2 head 6 GPM zone at about 150', the current system can keep the heads up for about 2 minutes. With a 3 head 9 GPM zone, I can keep the heads up for about 40 seconds. So it seems to me that the horsepower or tank volume needed to maintain a minimum flow of approximately 40 PSI for a period of about 3-4 minutes is required to ensure that sprinkler winterizing is properly accomplished. In my case, I believe my system will not properly handle a 9 GPM or 12 GPM zone. It is also apparent that the longer the head distance is from the air source and also the air connection point of the system, the more horsepower is required. I can see where a 30 gallon compressor with CFM of about 5.5-6.0 at 40 PSI might be able to handle a 12 GPM zone for 3 minutes or so (keeping the heads up during that time), but I am not sure about that. That might be a stretch. But coupled in series to another 12 Gallon of tank space, I believe it may be adequate. I am combining an older system with a new one and the old system has 12 GPM zones in some areas. I have had 5 and 10 HP compressors before, but don't want that bulk at home, so I am trying to keep things portable, if possible, and still accomplish the winterizing task for both my sprinkler system and in-ground swimming pool here in Indianapolis, IN. Prior to purchasing a new and larger compressor, I thought I would see what the experts have to say about it. Thanks for your comments.