What is the difference between a boiler bypass and a system bypass?
The difference is the flow in the bypass. A system bypass a percentage of the water leaving the boiler will go through the bypass and return right back to the boiler. A boiler bypass a percentage of the water will bypass the boiler and go directly back to the system. The name of the bypass is what is bypassed. With a smaller boiler than the connected load I would use a boiler bypass which will keep the flow rate in the system higher. This gives more heat output from the radiation and changes the system water temperature as outdoor temperature changes. With large water volume systems the system temperature will rise quicker with a boiler bypass than a system bypass as the boiler runs at a hotter temperature. A system bypass will reduce the flow in the system which slightly reduces the heat output from the radiation. It will also reduce the air elimination process if the flow is less than 2 ft per second. I would pipe the system as primary/secondary if I used a system bypass so the flow
