How can I handle excessive pressure in a water heat circuit?
You need an expansion tank on a closed-circuit hydronic loop. You'll also need air vents for removing the dissolved air when it comes out of solution as the water (or antifreeze) heats. These are commonly deployed on the bottom and top of an &quot;air scoop&quot; which is a standard hydronic heating component.
You ALSO need a pressure and temperature relief valve, but you don't want that venting on a regular basis - the expansion tank should deal with normal variation. The relief valve is for when things go wrong.