What's the effect of introducing bends into a primary-secondary heating loop?
That is actually a very good system you are proposing. My only reservation is that the cost will be excessive for only 2 apartments or zones. This is a new twist for me on a primary/secondary piping system. I have installed them before but this device makes it so much easier.The amount of elbows and other fittings should not be a problem as long as the total piping friction loss is included in the selection for sizing the primary pump. The use of 2 smaller boilers instead of 1 large boiler will lend itself to better efficiency by allowing for a lead/lag boiler operation and will be much more dependable than 1 boiler for each apartment.