As regards the domestic markets,high and low thermostats precede thermistors ,they were like a summer and winter setting, boiler control therefore controlled heating and hot water,high for winter period and low for hot water only
Can not remember settings on apollo,think high limit around 76/78 deg and low around 68 deg,maybe slightly higher,idea made it easy for customer to operate to make more efficient rather than a dial control
The high limit and low limit thermostats were just on /off states when temp reached,with no variable gas rates
Thermistors came a long afterwards,and allowed variable gas rates through one sensor at the burner,originally there was one fitted on a system boiler and two on combi's on heating and hot water,now we have two,one on return and one on flow pipe work to allow really efficient ,fully modulating control of boiler ,allowing range rated boilers and within reason over sizing not such a problem
In between the two ,we went through high and low flame boilers,these have two thermostats ,again, could look at like high and low stats,lower rated stat would reduce boiler from high flame to low flame and the higher rated cuts out the low flame
What ever it has always been accepted two are better than one
imho
