i been doin this alot of years and my knees have taken a beating.a shot of cortisone in each knee helped alot,it was amazing,till i changed out two split systems this week,one in a attic the other in a basement,now my knees are right back like they were,my point is while your still young and plan on working in this field for a long time then you should either wear knee-pads or get something to kneel down on,i wish i had started years ago,now i have to use a horse saddle pad on the ground,no more kneeling its lay down and roll around or sit.im tellin you if your still young take care of your knees! you will pay when you get old like me.