Steam indirect

Application: Burnham Alliance indirect off steam boiler. Question: Whats the best way to control the temperature of the water that is leaving the boiler and supplying the indirect? I am trying to keep supply to indirect under 212 to prevent flashing to steam. I can blend return water into supply water. Only concern is due to the fluctuations in boiler water temp. If I get supply water down to 180 when boiler water is 212 obviously I will have problems during non heating season when boiler water is 180. I guess I can reduce supply temp down to 200 when boiler temp is 212 and set aquastat to 190 so that during off season supply temp is close to 180. I looked into mixing valves but they seem to have temp limits off 212 which is too close for comfort(in particular b/c boiler water can reach 215 at 2psi). Any advice or ideas would be greatly appreciated as usual.