Hey all - can I please ask a quick question thats been bugging me .... why do some boilers have thermistors on the flow AND return ? why not just on the flow like a lot do? what difference does it make? thermistors have repaced thermostats in many modern al electronic boilers nowadays, but why the need for one on flow and return? .........
why 2 thermistors in some boilers?
- Thread starter crazychic
- Start date
Well you seem to have survived the immersion heater!
Modern boilers measure the return and flow temperatures. Combuting from the temperature rise and knowing the heat input the boiler can work out the flow rate and adjust heat input and sometimes flow rate accordingly.
Little difference between the two sensors when the burner is giving heat is an immediate indication of no or insignificate flow.
So simply the boiler uses the two sensors to more closely monitor the boiler operation and also deduce that there is ( or is not ) an adequate flow rate.
Tony
Modern boilers measure the return and flow temperatures. Combuting from the temperature rise and knowing the heat input the boiler can work out the flow rate and adjust heat input and sometimes flow rate accordingly.
Little difference between the two sensors when the burner is giving heat is an immediate indication of no or insignificate flow.
So simply the boiler uses the two sensors to more closely monitor the boiler operation and also deduce that there is ( or is not ) an adequate flow rate.
Tony
The pcb can compare flow and return temps, if the temp diff is too great between the two then the boiler assumes there is a circulation problem and will usually go into a fault mode to protect itself. Much like the vaillants do.
Theres probably other reasons but im tired
Theres probably other reasons but im tired
thanks guys for explaining, i get it now .... agile: yeah i survived the immersion heater situation!!! learned a lot from it though, which was really good!!!! and i have loely hot water now!! 
about immersion heaters - from what i have learned, since 2004, all immersion thermostats now have an overheat cut out device built-in which stops supply to the element if the thermostat sticks shut. they cut out at about 90 degrees celcius. and they have to be reset manually. also, if continuity (learned a lot from seco services about this ) is present with the thermostat turned right down and the water above the setting temp, then the thermostat switch has STUCK shut and is faulty!!
about immersion heaters - from what i have learned, since 2004, all immersion thermostats now have an overheat cut out device built-in which stops supply to the element if the thermostat sticks shut. they cut out at about 90 degrees celcius. and they have to be reset manually. also, if continuity (learned a lot from seco services about this
) is present with the thermostat turned right down and the water above the setting temp, then the thermostat switch has STUCK shut and is faulty!!
as you say if the stat is set to 60 and the water is at 70 and you still have continuity/power to the element then the stat is faulty.
seco - yeah from my course, i learned that if the water was at 60 (common and safe setting) and the stat set at say 50 (lower) and the terminals between the live and through the stat terminal to (T) bleeped when tested for continutiy on a mutimeter, it proved that the thermostat switch was stuck (in the closed position) and faulty. it should be open and not have bleeped! i know it sounds so simple now, but it'[sonly easy when you understand/know!
yes correct if the water is hotter than the stat setting it should be a open circuit (no bleep)
seco-i might be asking you a few questions or for a bit more help / conformation in the next few months .... if i may and more importantly, you have the patience
thanks though, seriously, you have been a star! top man.
thanks though, seriously, you have been a star! top man.
ha!! i like it! seco-a quick q for ya! which i was tested on!! - and i got it right!!
if the hot water cylinder (although my tutor calls it a hot water storage vessel)!! ha - is heated indirectly by a boiler, but the cylinder also has an immersion heater (as back up) - and the boiler heats the water to its setting (ie 60) - will the immersion heater thermostat have continuity when set to 50 degrees, even though the immersion heater has not been switched on?
it's probably such an easy question for you !!!!!
if the hot water cylinder (although my tutor calls it a hot water storage vessel)!! ha - is heated indirectly by a boiler, but the cylinder also has an immersion heater (as back up) - and the boiler heats the water to its setting (ie 60) - will the immersion heater thermostat have continuity when set to 50 degrees, even though the immersion heater has not been switched on?
it's probably such an easy question for you !!!!!
should be a open curcuit. (no bleep) the stat will still detect the temp of water with no power to it as its sitting in the water.
yep, you are right!! told u it was easy for u!! ha!!
ok, one other question i had and got right, although some other class mates debated
- can a part 4 equilibrium (torbeck) float valve be used in a cold water storage cistern, or only in a toilet cistern?
ok, one other question i had and got right, although some other class mates debated
- can a part 4 equilibrium (torbeck) float valve be used in a cold water storage cistern, or only in a toilet cistern?
toilet
you are correct AGAIN!!!! 
even though to prevent water hammer in high pressure areas, it is advisable only to use part 4 float valves in wc cisterns!
even though to prevent water hammer in high pressure areas, it is advisable only to use part 4 float valves in wc cisterns!
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