Modular boiler system with domestic hot water

Chad Pare

Just looking for some piping options regarding a modular boiler setup with domestic hot water. Currently we are using primary and secondary piping. We are setup with two Viessmann boilers rated at 115000 Btuh and 200000 Btuh. The two boilers are setup with a first fed - last return tied into two "legs" dropping down from the primary loop. The two legs are setup so that the tees are 8 inches apart. Recently domestic hot water was added to the system and supply and return were taken off the bottom of the supply and return legs. What is happening is that when domestic is calling for heat, hot water is made no problem, but hot water is also flowing back into the primary heating loop. This is a problem since it is not the heating season!
We are in the process of correcting the the secondary boiler piping to properly pump away from the primary loop and the point of no pressure change, but we are trying to figure out the best place to pipe in the domestic hot water without causing hot water migration into the primary loop.
If anyone has any tips it would be greatly appreciated.
Thanks
Chad

Big Ed

Easy Fix

Install a flow check on the boiler supply above the indirect supply tappings...

You also need one on the indirect boiler supply piping .....If not during the heating season the flow will by pass through the indirect.