03-02-2012, 08:39 PM
That's very interesting, and I admit that I didn't know about any of that. Thanks as usual, David 
Interestingly, my boiler controls the fuel-air mixture by measuring the ionisation current. I wonder why this isn't more widespread - after all, all boilers have the electrode in place to sense the basic presence or otherwise of the flame, and it isn't that complicated to extend that binary function into an analogue value back to the flame controller...
Also, this boiler has an NTC thermistor to measure the flue gas temperature. I wonder why? I guess it's a safety feature at the very least - after all the flue is all plastic - but I wonder if it plays a part in determining some operating parameter of the boiler? I guess the flue temperature gives the boiler some indication of the heat transfer into the heat exchanger? I know that there is another NTC measuring the flow temperature at the exit of the heat exchanger, and a bi-metallic switch in the same location which I presume acts as a safety device in the event of the controller getting confused. As far as I can see, it doesn't appear to measure the return temperature.
So, is the flue gas temperature sensor common in modern boilers?
Needless to say, I have a carbon monoxide alarm in the same room. Given their low cost, I wonder why they aren't an integral part of the boiler? Perhaps they will be before much longer - things are always changing...
Talking of which (and forgive me if I said this before), the friendly chap at Viessmann tech-support told me that variable-speed water pumps will become mandatory in a couple of years time. I don't know if he knew this as a fact, or was speculating, but interesting none-the-less. I considered the option because I liked the idea (one more variable that can be controlled to avoid cycling and increase efficiency), but he talked me out of it in the end. It added ~£300 to the list price, and while it saved around 40 watts IIRC, he didn't think it would pay in the long run. I think they use DC brushless motors, which are becoming commonplace in washing machines and other white goods, it seems.
Mark
Interestingly, my boiler controls the fuel-air mixture by measuring the ionisation current. I wonder why this isn't more widespread - after all, all boilers have the electrode in place to sense the basic presence or otherwise of the flame, and it isn't that complicated to extend that binary function into an analogue value back to the flame controller...
Also, this boiler has an NTC thermistor to measure the flue gas temperature. I wonder why? I guess it's a safety feature at the very least - after all the flue is all plastic - but I wonder if it plays a part in determining some operating parameter of the boiler? I guess the flue temperature gives the boiler some indication of the heat transfer into the heat exchanger? I know that there is another NTC measuring the flow temperature at the exit of the heat exchanger, and a bi-metallic switch in the same location which I presume acts as a safety device in the event of the controller getting confused. As far as I can see, it doesn't appear to measure the return temperature.
So, is the flue gas temperature sensor common in modern boilers?
Needless to say, I have a carbon monoxide alarm in the same room. Given their low cost, I wonder why they aren't an integral part of the boiler? Perhaps they will be before much longer - things are always changing...
Talking of which (and forgive me if I said this before), the friendly chap at Viessmann tech-support told me that variable-speed water pumps will become mandatory in a couple of years time. I don't know if he knew this as a fact, or was speculating, but interesting none-the-less. I considered the option because I liked the idea (one more variable that can be controlled to avoid cycling and increase efficiency), but he talked me out of it in the end. It added ~£300 to the list price, and while it saved around 40 watts IIRC, he didn't think it would pay in the long run. I think they use DC brushless motors, which are becoming commonplace in washing machines and other white goods, it seems.
Mark
