Nest 3rd gen query- how to install heatlink into Honeywell ST900 thermostat that has been removed

[stem]

1. You can still have a cylinder stat.
2. As it does on a gravity system with a cylinder stat.
3. Also can be done with a gravity system.
4. Not if there is a cylinder stat.

All true of course. But I think you misunderstood why I said these things. So my apologies for any confusion caused. Of course it you fit thermostatic controls many of the advantages can be gained added to a gravity system, but the question was raised after I posted about a system that was an ancient gravity hot water system that had never been upgraded to have anything other than a room thermostat and that its owner would have done better spending his money upgrading the hot water control rather than on a Nest.

5. Why is less heat emitted to the environment.

With a gravity system without thermostatic control whenever the timer is on, the boiler will continually fire up even when the hot water cylinder is hot. So the pipes, boiler etc will loose heat to their surroundings, also the cylinder will get hotter than necessary and add heat to the environment.

8. So it is often said, but how?

The circulation of the water through the system is much slower with gravity. Prior to converting mine it took nearly an hour to reheat the cylinder, now it's less than 20 minutes. Maybe it's a better cylinder but I can only speak as I find.

 

[ericmark]

Before my dad had the current central heating fitted he had thermo syphon hot water, it needed 22mm pipes and as pointed out boiler was forever cycling and the water got very hot, I fitted a motorised valve in the supply to cistern controlled by cylinder thermostat and it did work of sorts, but because the hot water was not as hot, it would no longer fill the bath, you had to half fill then wait then fill next half, but dad was not worried he used an electric dribble shower and mother had a power shower in the wet room.

Then the cold water tank started to leak, steel tank as originally had a coke fire, so whole new system fitted by a bunch of cowboys from Rhyl. Power shower had to go, and ever since the heating has not really worked well.

The major point with thermo syphon is where the tank is in relation to boiler, with fully pumped the boiler can be anywhere. In the days of solid fuel thermo syphon means water is always circulating so it would not boil or if it did there was some where for the steam to go, it really was noisy when the water boiled you knew what was going on, and plastic header tanks were out, whole header tank could end up with boiling water in it. It was common to run the hot water off to stop it boiling, when the stove was swapped for gas boiler the pipes remained the same. There has never been a need for thermo syphon with gas or oil, you can turn off gas or oil, thermo syphon is a solid fuel system.