there's ways to control the pex tubbing temperature to keep it at a constant temperature ( 80 to 100 degrees )with mixing valves very easy and cheap , also i think if you are using D H W tank you will have to be very careful not to have stagnant water in the summer because you a not going to heat the loop , you can get sick if you drink or use this water for cooking or washing ,best is to use boiler water.I work for a oil heating company,as a heating tech.if you have any questions maybe i can help lmk pm.