29-12-2011, 08:01 PM
Thanks for the replies - I did wonder if it was that simple. As you say, it's much easier to switch off the gas than put out the fire...
This is all very different to my new boiler, which can output anything between 4.8 and 19kW, and varies the flow temperature according to the outside temperature. There is a scary amount of software in there, but so far so good - the weather compensation seems to work very well indeed. When heating water, it does HW priority so that it can up the flow temperature to 70C, meaning the water can be heated to a safe temperature. It would have been possible to retain my Y-plan system, but I would have been mad to do so as the flow temperatures would have been too hot for the house (and for efficient condensing operation) if I wanted the hot water to actually get hot! Of course, the risk of Legionnaires disease is a problem for under-heated stored water...
Regarding the comments about that thermal switch suffering from delay, I have to say that in my experience I haven't come across this. After all, whatever you bolt it to will have a much higher thermal inertia than the switch itself, and providing it's in good thermal contact (silicon grease, etc), then it should work well. I've used them countless times in the past. They are frequently seen inside cookers and boilers (to name but two examples) doing similar things.
However, they might not be the ideal solution for you as they are not adjustable unless you buy a few. An electronic solution would obviously be more flexible as you can vary the hysteresis as well as the set point, but it must be designed with high reliability built in - and I would still include a bi-metallic switch as a fail-safe. The LM35 and its derivatives (of which the LM335 is one) are fairly vice-free, but need a fair bit of external circuitry. I'd consider an NTC thermistor (available in a heat-sink mounting package) and a couple of transistors driving a relay. Again, I've done that lots of times, and it works well. Simple is always best.
Good luck,
Mark
This is all very different to my new boiler, which can output anything between 4.8 and 19kW, and varies the flow temperature according to the outside temperature. There is a scary amount of software in there, but so far so good - the weather compensation seems to work very well indeed. When heating water, it does HW priority so that it can up the flow temperature to 70C, meaning the water can be heated to a safe temperature. It would have been possible to retain my Y-plan system, but I would have been mad to do so as the flow temperatures would have been too hot for the house (and for efficient condensing operation) if I wanted the hot water to actually get hot! Of course, the risk of Legionnaires disease is a problem for under-heated stored water...
Regarding the comments about that thermal switch suffering from delay, I have to say that in my experience I haven't come across this. After all, whatever you bolt it to will have a much higher thermal inertia than the switch itself, and providing it's in good thermal contact (silicon grease, etc), then it should work well. I've used them countless times in the past. They are frequently seen inside cookers and boilers (to name but two examples) doing similar things.
However, they might not be the ideal solution for you as they are not adjustable unless you buy a few. An electronic solution would obviously be more flexible as you can vary the hysteresis as well as the set point, but it must be designed with high reliability built in - and I would still include a bi-metallic switch as a fail-safe. The LM35 and its derivatives (of which the LM335 is one) are fairly vice-free, but need a fair bit of external circuitry. I'd consider an NTC thermistor (available in a heat-sink mounting package) and a couple of transistors driving a relay. Again, I've done that lots of times, and it works well. Simple is always best.
Good luck,
Mark
