I use Domoticz and integration with my alarm system to do this.
When no downstairs PIRs have detected movement for 15 minutes after 11pm the various zones are set to 'nighttime' temperature.
I haven't attempted to look forwards in the Evohome schedule to pick up the set temperature, that could be a future addition. I just send manual commands to the Python library.
I can also run an Alexa command 'Alexa, set house to nighttime' or similar (HABridge)
Should things go wrong then a hard stop in the schedule sets it anyway.
Also it turns off the outside lights (z-wave) and, sometimes, the Christmas tree lights
Here, when my (wired) house alarm is set (either away or night time) I use a zwave sensor to trigger a SmartThings routine which sets Evohome to Economy mode. Before it does that it saves the state, and then it returns to that state when the alarm is unset.
Seems to work well.
I used to use the PIR information in real time too, but realised that it wasn't a great idea. We tend to end each day either on the couch or in bed, both times hardly moving for a long time but not asleep. The PIRs wouldn't detect the small movements, so would shut off the heating and then start up again later and be ineffective. Hence why real time presence based heating systems haven't been effective. So now instead I use a learning based method which shuts off the heating based on a month long working day rolling average of PIR movement, unless ofcourse there is still movement. Works much better that way and now my Evohome schedules are updated weekly too based on this data. It still doesn't work when you visit the guest room or suddenly do your ironing at a different time. For those times a local override works well and I use the big/feature of being able to override even when OFF to my advantage.
Having monitored 15 min gas flows for the past month or so, I have now convinced myself that the benefits of setting Economy for a period of less than 3 to 4 hours is outweighed by the Delta cost in gas of getting the house back up to its set temperatures. The problem is the 2C differential on the HR92s against the 3C drop in Economy. If any or all the zone temperatures have fallen by 3C, then the boiler will crank up to maximum flow when Economy is cancelled. This seems daft to me as there is a marked difference going from a coldish house in the morning, than when going from a warm to a slightly warmer house during the course of the day.Originally Posted by paulockenden
I just go by the laws of physics.
What your heating system is doing is compensating for the heat leakage from your house. And a warm house leaks more heat than a cooler one.
Unless you have significant overshoots when re-heating a cooled house or your boiler is vastly more inefficient when running full pelt as opposed to ticking-over then reducing the zone temperatures when unoccupied should always win.
I think that your last sentence is the issue. My boiler modulates down to 5kWs from 24kWs. Calculations made by the installer suggested that a 18kW boiler would have sufficed for CH. On a cold day, OAT2C, I am seeing steady state gas usage of 0.05 to 0.1 Cm3s every 15 minutes. When I select Economy off, then the boiler cranks up and it can consume more gas in the first 15 minutes than was saved. The other part of the problem is that the zones are not uniform and the boiler demand remains high with just one zone outwith the normal set range and, yes, this does result in some zones overshooting their set temperatures by up to 2C. Opentherm control may be exacerbating the problem is it quickly ramps up the flow temperature. Conversely, in a settled situation, it does get the temperature down. As I post, with 3C OAT, and all zones at their set temperature, the boiler calculated flow temperature is 52C with a return temperature of 42C.
We are talking small margins here as my present boiler/Evohome/Opentherm combination does appear to be very efficient. Gas usage has fallen by 25% whilst, at the same time, I have taken the decision to leave all rooms at a minimum set temperature during the day of 13C - compared to 10C last year. Overnight, all zones with the exception of our main bedroom, are set at 8C. My total gas usage last year was 8700 kWhs compared to 11075kWhs in 2016 for a 5 bed property with 12 zones. The old boiler/BDR combination was replaced with new boiler/OT in April 2017.
Setting it back for a short time say 2-3 hours probably isn't worth it, because to save any gas through reduced heat loss your rooms need to be at a significantly lower temperature for a significant period of time, since heat loss is proportional to the inside/outside temperature differential.
So if they have only fallen 1-2 degrees in that time then it probably won't outweigh the small inefficiency of running the boiler at a high output for a while and (more likely) the overshoots that can result as a system stabilizes - those overshoots are probably the real cause of you not seeming to save anything from short term use of Eco mode.
I don't use the Eco mode to try to save gas, the only time we tend to use it is during a housework blitz where we would otherwise be feeling uncomfortably hot!
On the other hand, we do set all rooms except bedrooms back to 5C over night and let them fall as far as they will, relying on optimal start to get them back where they should be again at getting up time, and likewise when the house is empty during a work day all rooms in the house are scheduled to 5C and allowed to drop as much as they like, and also use optimal start to get them back to comfy temperatures again by 6pm.
I've proven beyond doubt (at least on my system) that letting the downstairs get as cold as it likes at night (it usually doesn't go below 8-10 even in the middle of winter) then heating it back up in the morning uses less overall gas than, say, setting the downstairs back to 15C for 7 hours... a lot less.
Also keep in mind temperature fall with heating off is not linear - it will fall in an asymptotic curve and will level out at some point near or above the outside temperature, but never below it. (Usually it is a few degrees above ambient due to solar gain, even in winter) So say the outdoor temperature 10C when you're at work and solar gain would cause the house to eventually settle at 12C degrees inside.
If you tried to maintain a 15 degree set back all day while you're at work you're constantly putting energy in to maintain that, (once it initially fell to 15) but if you just let it fall to 12 degrees then it would sit happily all day at 12 degrees with no heat input from the heating system. You have further to re-heat for hometime, but the net energy use is still less.
So my takeaway would be that small set backs over short periods of time of less than say 2-3 hours are not worth it if you are going to re-heat again back to where you were soon after, but longer set backs are worth it especially if you allow the temperature to drop down near its natural equilibrium, where maintaining that temperature is "free".
Last edited by DBMandrake; 11th January 2018 at 10:49 AM.
You have major heat loss issues dude.
Kind Regards - Dan Robinson (Jennings Heating Ltd)
I think that the issue here is the narrowness of the set temperature range and the fact that any single zone demanding heat causes the boiler to fire up to maximum flow temperature. For example, I can leave the house for 3 hours on a very cold day and the majority of the zones will be within 2 degrees of where there were when I left. However, I have one zone which sits above an integral garage and this has a greater rate of temperature loss. I am beginning to think that the answer might be just to put this zone back on to a standard TRV.
Feel free to come and rebuild our 1930's fully detached bungalow with raised wooden floors to meet modern insulation standards. For free of course.
The numbers I gave were largely example figures to make the point I was trying to get across anyway. We were away with the heating off for 3 days over the christmas period and none of the rooms went below about 12 degrees.
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