i have a system that is 24 ft higher than my boiler and 275 ft away to eliminate any of the same problems your worried about i used a pressurized system and one non press pump just for the stove loop and two press pumps for the house and shop my wasser pumps have check valves built in make sure your pump can handle the head at the gpm you need, put a air scoop and an auto vent,this was more expensive but i have not had any problems,the press of the water being up higher than the boiler will induce a load of about 1/2 a psi for every foot higher than the boiler is so if your 15 foot you will have aprox 7psi of load(on the water tank of the boiler which could actualy cause failure of the system ,split the tank once its heated up ) from the head of water