CH Thermostats in hallway - why?

[martinalex]

Just getting quotes for new boiler and every plumber wants to put a wall-mounted thermostat in the hallway. I've had many heating systems in different houses over the years and I've always found wall-mounted thermostats ineffectual so I'm hoping soeone can tell me why we have them.
All of my radiators have thermostatic valves (TRVs) so each room temp can be separetly controlled.
I understand that the main difference between a thermostat in the hall and a TRV is that the thermostat can switch the heating on/off according to the ambient temperature and the TRVs cannot.
All the plumbers have said put the thermostat in the coolest part of the house and make sure that the radiator in that area does not have a TRV and is left fully open. So the thermostat will keep putting the heating on because it is located in the coolest part of the house!!! But all the other radiators which are controlled by TRVs will still keep having hot water pushed through to them on the basis that as long as the hallway is cold then the thermostat keeps switching the heating on - and they sell this system as an energy saving plan!!!
Am I going nuts? Where's the sense in that? surely it's more sensible to just control your heating on radiator TRVs?

[martinalex]

If you lower the hall thermostat - it knocks out the room TRVs hence the whole house will be colder and not just the hall.

I think one of the plumbers did imply it was 'building regs' but I'm not convinced - sounded more like it had to be ticked off the boiler manufacturer's sales checklist.

He actually said the manufacturer will look at the check list and ask why a thermostat isn't fitted, then with a bit of whistling through teeth - suggested it might affect the warranty!

Seems unlikely - I would have thought a hall themostat is more likely to result in constant on/offs for the boiler - heating up a single hall radiator causing extra wear on the boiler.

[martinalex]

I'm still confused with this. In my last house we had a combi boiler running about 10 radiators over 3 floors. All radiators, apart from the top floor bathroom, had TRVs. There was no room thermostat. I don't use the timer at all -just switch the boiler on when I'm cold and off when I'm hot. I adjust the temp in each room using the TRV.
There was no room thermostat but the boiler didn't just run on and on. It would run until all the rads reached their temperatures on the TRVs and then cut out. It would then start up again when the temperature dropped. I could hear it going on and off.
What am I missing here? Does the boiler have an internal/integrated thermostat? Otherwise how did it switch itself off and on again?
If the room thermostat is the ignition then a boiler without a room thermostat shouldn't be able to do this - should it?
I still don't get how heating an empty hallway can be shown to save energy when compared to just using TRVs and keeping the doors to the rooms closed.
I really would like to understand this as I'm buying a new boiler/system and at the moment I can't see the need for a thermostat.

[martinalex]

I think what I don't understand is why there is a need for an external thermostat, if there is already one in the boiler? And that if the thermostat is in the hall/coldest space it will make the boiler run more frequently/for longer and use more gas and so not save me energy.
This probably sounds really dense.

[martinalex]

I don't want to become a heating engineer but I do want to understand why heating engineers recommend a thermostat in the hallway and none of the ones saying I should have one can say why - other than you have to have one!
I'm trying to get my head around the necessity for the thermostat per se - (not just specifically in the hallway) - given that the boiler has an internal thermostat activated when the water reaches the required temperature. And why this is an 'energy saving' feature?
When I hear the boiler cutting out - as Ricky says - is the pump still running thus incrring an electricity cost and a wear and tear factor?
Is it also that if I get up and switch my boiler off when I'm hot, and then wait until I feel cold before switching it back on, will the water in the system have cooled to an extent that it requires more gas to heat the water back up again than it would if I kept the boiler activated by a thermostat on a consistent temperature.
I'm not trying to catch the plumbers out - but I simply haven't used a thermostat controlled system before and am struggling to grasp the energy saving aspect.

[martinalex]

If I have a working room thermostat installed in my coldest room - which I don't sit in - (because it's the coldest room) - will I not just be paying to heat unused space, when I could shut off that radiator and just heat the other rooms behind closed doors?
I accept that building regs insist a thermostat is fitted - but I'm questioning if this devise really will save me money.
Would you happily assure anyone buying a new CH system that having a room thermostat will of itself save money? If that's the case, great!

[martinalex]

Thanks Arthur - I understand that you're saying the space temperature measured by the room thermostat will cut off my boiler long before the boiler's internal thermostat - that makes sense, that will save me money. Would it not make more sense then to have the room thermostat (in the absence of one in every room) in the room I use most often rather than an empty cold space. Surely, I would then be able to set the thermostat to more accurately match with my body comfort and set the thermostat to cut off even sooner - saving more money.
Do building regs. insist on a particular thermostat location?

[martinalex]

Thanks Ianz - I was beginning to think I was speaking a different language - I understand exactly what you're saying - you've made the point much more succinctly than I.