or if the gas flow is restricted

An 11/32" nut driver or socket is the standard size for these external hex drives

and MGB series boiler sizes

This professional-grade backflow tester operates at a maximum working pressure of 200 psi and can withstand fluid temperatures up to 200°F

Grundfos 595916 Pump 321894-706 or if the gas flowThe Grundfos 595916 (Model UP15 10SU7P TLC) is a heavy duty, stainless steel hot water recirculating pump designed specifically for retrofit applications in residential systems lacking a dedicated return line. This 115V single phase circulator utilizes the existing cold water line to loop thermal energy back to the water heater, ensuring instant hot water delivery at the furthest fixture. As a veteran grade recirculation system, it features a 1 25 HP