I have a combi boiler, hot water on demand and heat controlled by a Honeywell thermostat, as an electrical engineer could I use a wemo smart plug to someway control the heating ?
Wemo the central heating
- Thread starter markymark2406
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Doggit
How are the boiler time periods being controlled, as this would be where the WeMo might be used to change the functionality of the heating. If the heating were set to be permanently on, then you could use the plug to turn the boiler off, but it'd be a very crude way of doing things, and I suspect it would reduce the life of the boilers circuit board.
The wemo app has rules you set on/off time periods or you can just manually switch it on day for example half hour before getting home, obviously the Honeywell thermostat would have to be closed circuit calling for the heating to switched on
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There are four basic ways to control a boiler.
1 - The return water temperature modulates the boiler (turns down flame height).
2 - The thermostat or hub connects to the boiler ebus and that modulates the boiler output.
3 - An extra low voltage often 24 volt link in the boiler is removed and replaced with a thermostat.
4 - A low voltage 230 volt link in the boiler is removed and replaced with a thermostat.
With number 4 it may be possible to have a socket connected to the boiler supply and a plug could take the line into the boiler to activate it, however the switching device would need to work with a very small load.
However there is an inherent problem, after a power cut most electronic controlled sockets, light switches and the like default to off, they do not resume where they left off, and in real terms the on/off control of a boiler is rather old hat, the idea is to modulate the boiler output so reducing hysteresis and making it more efficient. Every time a boiler turns off the heat retained in the boiler is lost out of the flue, so better if turned up and down rather than on/off.
However modulating a boiler has limits, once the lower limit is reached then it reverts to a mark/space ratio to further reduce the output. In theory one could make a boiler so when the space exceeds a set limit it would turn off, however the problem is once this is done, how could you turn it back on automatically. So we need a device to turn the boiler off and more to the point on again when the demand returns.
So by using wifi connected TRV electronic heads we know when the demand returns, we have a target and current temperature and if any head reports target is higher than current we want the boiler to start up, this is what the EvoHome thermostat does, a little more complex to what I have said, but that is the basic idea. However other TRV heads like the MiHome do send a report to the hub which you can view with a phone, tablet or PC, but to use that information it uses IFTTT protocol and has to work with a thermostat like Nest, but really we don't want to use follow commands as the whole idea is each room can have a different set temperature, we really don't want Nest to measure room temperature direct we want it to simply use the target and current temperatures reported by the eTRV.
So all we really need is a relay, and there are IFTTT relays, so one would think easy just write a IFTTT program, however my level of training I only got to level 5 in Uni is not enough for me to write my own program, it needs some one to write the program so I can select it from the app store. And we return to the problem if you lose power for even a minute the heating will turn off and will not turn back on until the app tells it to.
So we are left with either EvoHome, or some sort of lash up. I went for the latter, so two thermostats if either gets too cool the heating turns on, so the TRV controls maximum temperature and wall thermostats control the minimum temperature. This means the TRV and the wall thermostat need to be set to work together not to fight each other, so you need the TRV in the room with the wall thermostat to have a wide temperature control, say start closing at 17°C and not fully closed until 20°C this means an electronic type is too good, you need to use lowest quality TRV to be able to match it to the wall thermostat, this in turn means a fixed temperature for the rooms with wall thermostats, so any remote control can only switch on/off.
Which returns us to the simple relay. So your an electrical engineer so you show me how you can control heating with a relay where after a power failure it will auto resume. Unless you have a relay where it can have at least 12 on/off times programmed in, so maximum off time is two hours, can't see how it would work, the MiHome relay has only 3 on/off times so not suitable.
1 - The return water temperature modulates the boiler (turns down flame height).
2 - The thermostat or hub connects to the boiler ebus and that modulates the boiler output.
3 - An extra low voltage often 24 volt link in the boiler is removed and replaced with a thermostat.
4 - A low voltage 230 volt link in the boiler is removed and replaced with a thermostat.
With number 4 it may be possible to have a socket connected to the boiler supply and a plug could take the line into the boiler to activate it, however the switching device would need to work with a very small load.
However there is an inherent problem, after a power cut most electronic controlled sockets, light switches and the like default to off, they do not resume where they left off, and in real terms the on/off control of a boiler is rather old hat, the idea is to modulate the boiler output so reducing hysteresis and making it more efficient. Every time a boiler turns off the heat retained in the boiler is lost out of the flue, so better if turned up and down rather than on/off.
However modulating a boiler has limits, once the lower limit is reached then it reverts to a mark/space ratio to further reduce the output. In theory one could make a boiler so when the space exceeds a set limit it would turn off, however the problem is once this is done, how could you turn it back on automatically. So we need a device to turn the boiler off and more to the point on again when the demand returns.
So by using wifi connected TRV electronic heads we know when the demand returns, we have a target and current temperature and if any head reports target is higher than current we want the boiler to start up, this is what the EvoHome thermostat does, a little more complex to what I have said, but that is the basic idea. However other TRV heads like the MiHome do send a report to the hub which you can view with a phone, tablet or PC, but to use that information it uses IFTTT protocol and has to work with a thermostat like Nest, but really we don't want to use follow commands as the whole idea is each room can have a different set temperature, we really don't want Nest to measure room temperature direct we want it to simply use the target and current temperatures reported by the eTRV.
So all we really need is a relay, and there are IFTTT relays, so one would think easy just write a IFTTT program, however my level of training I only got to level 5 in Uni is not enough for me to write my own program, it needs some one to write the program so I can select it from the app store. And we return to the problem if you lose power for even a minute the heating will turn off and will not turn back on until the app tells it to.
So we are left with either EvoHome, or some sort of lash up. I went for the latter, so two thermostats if either gets too cool the heating turns on, so the TRV controls maximum temperature and wall thermostats control the minimum temperature. This means the TRV and the wall thermostat need to be set to work together not to fight each other, so you need the TRV in the room with the wall thermostat to have a wide temperature control, say start closing at 17°C and not fully closed until 20°C this means an electronic type is too good, you need to use lowest quality TRV to be able to match it to the wall thermostat, this in turn means a fixed temperature for the rooms with wall thermostats, so any remote control can only switch on/off.
Which returns us to the simple relay. So your an electrical engineer so you show me how you can control heating with a relay where after a power failure it will auto resume. Unless you have a relay where it can have at least 12 on/off times programmed in, so maximum off time is two hours, can't see how it would work, the MiHome relay has only 3 on/off times so not suitable.
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