Challenging the need for minimum return protection valve on boiler with non pressurized storage Pretty much standard practice and required by vendors is a thermostatic return water low temperature protection to a wood boiler. The operation of the valve bypasses return water away from the building heating demands until the boiler is above the setpoint of the thermostatic element. This probably made sense at some point for systems without storage but IMHO its overkill and “belt and suspenders” for those with non pressurized storage. I don’t have pressurized storage so I can’t comment on its needed. Before folks get out the torches and pitch forks let me lay out my reasoning. The intent as I understand it of return water protection is to keep the boiler from operating at a temperature that is below the dewpoint of flue gases to limit dewpoint corrosion on the flue gas side of the boilers heat transfer area. Starting with cold storage and cold boiler there will be a period of time where the boiler will operate below dewpoint. My boiler and other vintage units originally were set up so the circulator pump ran as long as the boiler was powered as the concept of storage wasn’t very popular. With this setup it made sense to have return water protection as during warm up it took a long time to bring the water temperature up. I believe current practice to reduce the electric cost is to use a temperature sensor on the boiler jacket or boiler outlet that senses the boiler water temperature and cycles the pump on and off until the temperature exceeds a set point (I use 140 F). IMO this is effectively doing the same thing as return water protection. The only advantage I can see with return temperature protection is it cuts down on temperature stratification in the boiler jacket from top to bottom. My added on aquastat is at the level of the OEM combined temperature pressure gauge. During initial warm up the pump will cycle on and off for a few minutes until the boiler and the piping out and back from the storage is up to temp. As far as I am concerned during startup, return water protection would actually be encouraging operation below dewpoint on startup as it would be circulating cold water through the jacket while the my pump with aquastat method would not. So what am I missing on why someone should spend an extra $200 bucks to put in return water protection?