Making my central heating and water smart with Shelly - need an electrical relay.

FreeBear

https://forums.moneysavingexpert.com/discussion/comment/82095249#Comment_82095249

My system was an open vented system with gravity fed hot water. No zone or three way valves.

Summer time, I would go round shutting off all the radiators upstairs to stop them heating up when the tank got reheated - Would have been easier to fit a couple of zone valves, but decided against it due to access & cost (and a touch of CBA).

Now that I have a combi boiler, don't have to worry about the old boiler. Should have fitted it years ago, but kept putting it off whilst the old Baxi was still working.

elsmandino

https://forums.moneysavingexpert.com/discussion/comment/82095400#Comment_82095400

Ah right! I know someone who had this system and, whilst I thought it would be massively inefficient, he also said it wasn't as bad as you might think.

Just out of interest, can I check what you used to do?

So do you just forgo a room thermostat and have a single timer to heat the water at set times?

I take it, therefore, that you just turn off all your radiators when you don't need them and the water just runs around the pipes without heating any of them?

I suppose the biggest inefficiency is that when you want the heating on, you are constantly heating the water tank until the thermostat goes off.

Interesting - as you say, the benefit of this system is that it is extremely simple.

Section62

https://forums.moneysavingexpert.com/discussion/comment/82097749#Comment_82097749

So do you just forgo a room thermostat and have a single timer to heat the water at set times?

I take it, therefore, that you just turn off all your radiators when you don't need them and the water just runs around the pipes without heating any of them?

I suppose the biggest inefficiency is that when you want the heating on, you are constantly heating the water tank until the thermostat goes off.

The standard/typical system had a room thermostat which controlled the pump on the central heating circuit. There was no thermostat for the hot water cylinder, the circuit for this would be gravity/thermosyphon. The whole system would have a single on/off timer, usually with two heating periods per day and a 'constant' manual override.

The hot water system was self-limiting, in that once the water in the cylinder had reached the system temperature (the boiler thermostat setting) the thermosyphon effect would cease. This did mean the hot water temperature couldn't be set lower than the system temperature, and you couldn't have just the central heating on. For most people this wouldn't be much of an issue because they wanted hot water available all the time, so reheating the cylinder each time the heating came on made sense. This wouldn't necessarily be an inefficiency of concern.

The central heating would usually only operate if the room stat made the pump run. However, there could be some limited thermosyphon effect which would heat the upstairs radiators, even if the pump wasn't running. Again, for many that wasn't an issue as it meant upstairs bedrooms would get some heat morning and evening even if the central heating was off. To stop this the thermosyphon effect had to be prevented - which could be done by turning off upstairs radiators that you didn't want heated, or by turning off the isolator valve at the pump (remembering to turn it back on before letting the pump run). Typically the bathroom radiator would be t'd off the hot water circuit so it would be heated (by thermosyphon) all year round.

elsmandino

https://forums.moneysavingexpert.com/discussion/comment/82097780#Comment_82097780

Thanks very much for this - really interesting/helpful.

I do love learning about these things, not only of general interest, but it is amazing how this sort of knowledge helps with future troubleshooting.

FreeBear

The standard/typical system had a room thermostat which controlled the pump on the central heating circuit. There was no thermostat for the hot water cylinder, the circuit for this would be gravity/thermosyphon. The whole system would have a single on/off timer, usually with two heating periods per day and a 'constant' manual override.

Pretty much sums up my old system before I started to mess with the controls.

Fitted a thermostat to the DHW tank - That worked fine with the old (crude timer). For Home Assistant, I gave it a temperature sensor (modbus interface) and paired it with a temperature/humidity sensor in the hallway. Using the thermostat component in HA for both heating and DHW, the timing & temperatures were all set by a python script. So could have different temperatures throughout the day/night and variable by the minute.

Still using the python script, but have dropped the DHW side now that I have a combi boiler.