Where did you end up?
Individual rooms are separately controllable, so I can set different temperatures in different rooms at different times of day.
The temperature control is using PID with node-red-contrib-pid and node-red-contrib-timeprop with cascade control, controlling the individual radiator temperatures to adjust the heat output to each room, and hence control the room temperature. This gives me temperature control to within a fraction of a degree with no cycling.
Finally the actual temperature setpoint automatically adjusts based on the outside temperature and wind speed to give consistent perceived temperature. This is because when it is cold and windy outside the inside wall temperatures are lower, so the perceived temperature is lower and the air temperature has to be increased a bit to give the same effect.
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This is a very interesting discussion. One question, how is everyone intending to actually mount
the tank sensors?
Interesting. You presumably have separate temperature sensors for each room. In my system I let the radiator valves control the room temperature (so Node Red doesn't need to interfere beyond setting the temperature - or in fact usually just setting the weekly schedule, which rarely changes anyway).
You have more control but I guess I have more fault tolerance. If my entire automation system goes down, the radiators will carry on doing their stuff independently. I could not do what you do because my valves don't report the report back to the mothership unless they change their valve setting - so I don't have an up to date current temperature. I do have separate thermostats in some rooms (any with more than one radiator or any where I might want to make manual adjustments or switch radiators on or off easily) and they report temperature changes instantly but I haven't felt the need to put them everywhere.
I have yet to adjust the set temperatures according to outside temperatures. That would be useful because you are right, the right setpoint for a windy winter's day has the heating coming on in the early summer when it really doesn't need to because a lower temperature feels comfortable.
Yes, and one measuring the temperature of each radiator (well actually measuring the temperature of one radiator in each room).
Yes, probably. Though I have designed the whole thing so that if I switch off the pi controlling all this then it reverts to the pre-automation system using thermostats. So in the event of something major going down I can just switch it off. I have not had to do that yet though.
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Nice! My wife has made me write down everything I've done to the house so that if I ever get hit by a bus she can get an electrician in and put everything back the way it was!
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Burt
As far as your original question goes, does your boiler have inputs for an external timer controller for hot water and heating. In other words can it be wired with a timer / thermostat to control when the heating comes on and a timer / thermostat to control when the water tank is calling for heat?
That would be normal in the UK. The outputs are usually 240V but are very low current (i.e. they must be controlling a relay or something inside the boiler). On mine to activate the central heating (my hot water is instant so I don't have a separate water timer signal) I have to close a contact which is designed to be connected to the room thermostat. It has 240V across the two wires but when closed it only draws a few milliamps. I control this through Node Red (and ultimately through OpenHab which takes care of all my interfacing with hardware) using a Fibaro FGS-223 Z-Wave switch.
So when my radiators shut down, Node Red tells the boiler to stop pumping the central heating water. The boiler keeps running to heat hot water / circulate water to dissipate heat etc.
It would not be normal to have to turn the power to the whole boiler on or off.
Sorry I realise I am using the UK definition of boiler. What you call the heating system, we call the boiler. What you call the boiler, we call the hot water tank.
Hi Colin, how did you calculate the offsets required? I thought of doing this but the only way I can see to get the required values is by measuring the outside / inside temperatures and wind speed and creating a table, something that seems very long winded.
Or start low and keep upping the temperature until the wife/partner stops bashing you over the head. 
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I use node-red-contrib-comfort to estimate the perceived the comfort level. However, to be honest, I am not sure the time spent experimenting with all the parameters and trying to understand what it was all about was actually worth the effort. Good fun though. If you do experiment with that node then make sure you apply sanity checks to all the values you are feeding in. If you give it silly values (of humidity for example) then it can lock up. It has iterative code inside and if you give it silly values the iterations may not converge so it locks up node-red. Some of the issues have been sorted, but I am not sure they have been completely sorted so now I check all the values are in sensible ranges before I feed them in.
Sorry guys for not responding earlier!
Well it was simply to save on energy. Currently my heating system is running full time, while we only need hot water very often. So I wanted to turn off the heating system and turn it on in the morning and in the evening when we mostly need hot water. But in between I also wanted to turn it on automatically, for example when I see the temperature dropping very fast (which means somebody is using suddenly hot water). And perhaps heating the boiler at night tarif, but not sure yet whether that is useful in my use case: I need to measure first how well my hot water tank is isolated...
Ok thanks, that is useful practical info for me to get started.
Making my drawing a bit more complete:
If possible I would prefer to detect somehow (e.g. by a quick drop of the water temperature or something else) if hot water is being consumed currently. Because - like others already told - I would like to avoid that the hot water is not available, at the moment when my wife has here hair full of shampoo. Seems we have an international agreement about this
Not sure about that. Need to google a bit. But no time at the moment...
Is that important before I start drilling?
Yes would be weird if this logic would not be included in the heating system. It has been running well for 20 years now. But I don't know the internal logic of it. Perhaps by measuring the temperature of the returned water. Don't know yet at the moment.
Yes absolutely true. But much more work involved, so that is already a lot of years on my todo list.
Good question, because you don't want them to start moving when somebody accidentally pushes the cables...
Tip of the day: create a "share your project" discussion 
Yes we have a hardwire thermostat upstairs and one downstairs. But we use that only for the radiators. Don't think the system uses it somehow for the hot water tank: because in the summer these thermostats are at 0°C, and we still have hot water...
That is indeed what I planned to do in the future for my radiators. But now I am going to focus first on the hot water. Step by step ...
Yes short term workaround. Although you know that short term solutions mostly last forever...
Bart
I don't remember seeing anything about how you control the temperature of your hot water at the moment?
Still think you would be better off controlling the pumps in the hot water flow and radiator flow separately and only turning off the boiler (heating unit) when both are satisfied (both pumps off). Turning off the boiler when you do not need to heat the hot water will cause you grief in the winter when your heating shuts down.
I think most of the suggestions so far assume that your boiler (heating unit) has its own control to turn it off when the flow temperature reaches a set value
@bart Is the hot water from the boiler pumped to the hot water tank or is it a gravity feed? You mentioned that it is a very old system so I wonder whether it is not pumped.
@BartButenaers I can't help but think that you are opening a great big can of worms
Most systems in the UK integrate heating & hot water through diverter valves, and which can be fairly complicated, especially if it's a retro fit like what you are doing.
For example, take a Y-Plan system which is common in older installs (and in my home - built 2000), take a quick look at this video which explains the system, it may make you think twice!
A lot of posts in this topic ask about how your current system works, and you really need to know before making a start.... otherwise, it could end with Mrs B being very angry 
Good morning,
In my opinion, from experience and how hot water storage tanks are built, you just need a single temperature sensor and the height at which it must be placed depends on the volume of water you decide to keep at the correct temperature for your hair wife 
For example, you can decide to have 50l (corresponding to 0.05m3) of water at 70 °, when you open the tap it will be enough for a complete shower since you will not use water at 70 ° for washing. As water is consumed, the sensor temperature will drop and activate the burner but you will always have a reserve consisting of the volume of water that is in the upper part of the sensor so you will give the tank time to heat up but you will not run out of hot water.
For out-of-hours consumption I would insert a counter with an impulse output in the cold water inlet so for example you can tell the burner to activate if more than 15l of water are consumed in 15min, but it is only to try to speed up the system because as far as I wrote above the sensor will notice and will turn on the system.
That only works if you only ever need to keep the same amount of hot water. Suppose you have a visitor that likes to have a bath rather than a shower, so you need to heat up a full tank? To do that you need another sensor lower down.
You are right but be careful because for the principle of water stratification if you want the lower part at 70 ° in the upper part you will have a high temperature you could reach 90 ° and more. To have all the water at the same temperature you will need to have a circulator that takes the water from the upper part and enters it in the lower part.
with the probe positioned in the exact point you can vary the temperature threshold by varying the heat capacity of the tank, for example, adjust to 50 ° for maintenance, for occasional use and for immediate availability of hot water while for when you have a viewer or more consumption brings it to 70 - 80 °. It is obvious that we are writing without having an idea of the tank capacity, water consumption and heater capacity.
That is not possible as the water at the top cannot be hotter than the hot water feed from the boiler, which is probably about 70 or 75C max.
The hot water feed to the tank goes through a coil in the tank which extends from near the top to a little above the bottom. Heating the coil induces convection currents in the water, up the middle and down the outside, so that eventually all the water above the bottom of the coil heats up. The hot water feed to the coil goes in the top and the water in the coil cools as it goes down the coil. The result is that the top of the tank heats up first. As I said, though, the water temperature cannot exceed that of the incoming hot water.
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I know the principle of operation and I apologize for the error in the evaluation regarding the circulator.
Hey Paul,
Yes you are right. When I started this discussion, I simply wanted to build a very dump on/off solution. But by starting to know my heating system better, I might perhaps create a better solution with the help of the community.
Until this morning I thought that my heating system was controlling my pumps, because the cables go in on one side and go out on the other side:
So I opened the system and it appeared that those wires just go in on one side, and go out on the other side. But the heating sytem doesn't do anything with it. Which seems pretty normal with this system, after I read today the manual. So now my confusion is solved, because the pumps are simply controlled directly by the wall-mounted thermostat in my living room.
I called to my installer to ask confirmation about a couple of things, and the following picture should be rather complete after I did my homework:
- Cold water arrives from the water company, and is injected near the bottom of the hot water tank.
- The hot water is captured near the top of the hot water tank. The rvs water vessel is a tank-in-tank type: in the inner tank is the water that we consume, while in the outer water vassel is the water that runs through my heating system.
- There is a thermometer somewhere in the tank (don't know at which height). The aquastat is configured to close a relay when the water temparature inside the tank drops below 60°C. Then (see dotted arrow) the aquastat will instruct the hot water pump to start.
- The cold(er) water goes through my heating system to get heatened. There is also an aquastat in my heater that will instruct the burner to start burning oil, when the temperature is below some adjusted temperature.
- The heated water will be injected in the top of the outer tank.
- For the radiators when have wall-mount thermostat in the living room. When the room temperature is below the configured temperature, the thermostat will close a relay that will instruct the heating pump to start.
- The cold(er) water will return to the heating system, via the same pipes as the water that is used for my hot water tank...
Hopefully this is now a bit more clear.
So in fact it is a very simple system. Think I can use Node-RED in the future to make it a bit smarter, simply by controlling relays...
Well as a matter of fact, it seems that my hot water tank has already something like that 
Not sure anymore now whether I could do much extra to save energy. Since it looks like my system already does a lot of the stuff that people were talking about in the above discussions...
Interesting idea!!!!
Do you have a link to an affordable reliable one, that you can recommend? I assume you mention the cold water inlet, because cold water is better for the lifetime of such a sensor?
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