Thermal oil heaters temperature control requirements dictate system design. Within the modulation range of the burner provided, most heaters can control temperature to ± 3oC. If the heater cycles off, the system could lose up to 28oC, depending on the system size, quality of insulation etc. If tighter temperature control is required, a primary/secondary loop system may be employed. With the primary loop operating 13oC to 28oC above the secondary loop temperatures, even if the heater cycles off, temperature control of ±1.1oC may be achieved. The use of primary/secondary loop systems also allows multiple users to operate simultaneously at different supply temperatures. Modulating thermal flow control valves also may be used to control the thermal fluid flow to individual users. However, the supply temperature to each user will be identical unless a primary/secondary loop system is used.