Cost to maintain heat v re-heat home, is occupancy detect worth it?

[ericmark]

Basic the title says it all. I did not get Nest, or Energenie to use geofencing or occupancy detect, but since it was there I have tried using it. However in the morning when heating moves from Eco to Comfort (17°C to 20.5°C) the wall thermostat is programmed to heat to 19°C then 0.5°C per hour until at 20.5°C as observations have shown hall may be at 19°C but the electronic TRV heads have got the other rooms to temperature well before the heat from hall radiator has reached the wall thermostat. So the home heats as wanted, after an over night reduction.

However leave the home and the heating turns down, but on return it goes to 20.5°C and if it has cooled over 0.5°C while out it will over shoot, how much will vary. Over shooting means around an hour latter the hall is still over the target temperature but all other rooms are below target temperature the electronic TRV head can't heat a room if central heating not running.

If I remember I will turn down hall thermostat on arriving home, then turn it up again latter, so it heats in two or more stages, but most times I forget until an hour or so latter when I am cold.

So the question is geofencing or occupancy detect worth it? How much fuel do it really save, OK heating off for say 6 hours, home cooled to 18°C to 19°C, and the boiler has an extra long run to reheat, so not simply 6 hours saving as reheat uses fuel, and since it over shoots loses are increased, sure it must save some fuel, but does it save enough to be worth it?

Since boiler supplied from a FCU can't use a plug in energy meter to monitor how long it runs. The Nest report

Says I didn't use the heating Tuesday and Monday that is clearly not correct. And even if it was, it shows demand for heat, not the boiler running, the boiler switches off/on when heating the mark/space ratio will depend on how many electronic TRV heads are calling for heat.

So is all the geofencing or occupancy detect worth it? Does it really save enough money to be worth having a cold evening because it has over shot again?

 

[^woody^]

Are you using opentherm or external sensors?

 

[ericmark]

Oil boiler so no opentherm, there are 9 electronic TRV heads and 5 liquid TRV heads and nothing in upper bathroom, the Nest does not record DHW heating and since it records zero for some days, it is clearly not showing real use.

But any house takes time to cool, so switching off the heating does not mean a cold house, and over say 7 hours I am out, it may cool maybe 3°C rarely more, so to reheat and raise it 3°C does not take that long. So I would think less than £1 of oil is saved, however hard to measure how much is saved as said you would need to measure the extra time taken to reheat.

So really not sure if worth it, turning the heating off for less than 10 hours.

 

[wwwebber]

Easy one :-

https://www.moneysavingexpert.com/utilities/energy-saving-myths/

 

[ericmark]

Easy one :-

https://www.moneysavingexpert.com/utilities/energy-saving-myths/

It seems they don't know? My engineering back ground tells me I am paying for the heat which escapes from the house, there are two ways the heat escapes, one through the fabric of the building, the other through the flue.

So through the fabric of the building the heat of the walls, and windows and drafts through ventilators and this simply means the warmer the room compared with outside the more heat is lost, so if I reduce the heat in the rooms, I save money.

The second heat out of the flue is much harder to work out, with my oil boiler it is reasonable fixed, but with modulating gas boilers, not so easy, as it is down to the way the boiler modulates, using the ebus some boilers run very cool, so on switch off very little is lost, others are most economic at maximum output, there seems to be a lack of data as to at what point each boiler runs most economic.

Returning to through fabric of building the heat loss is not linear, but more logarithmic, so outside at 5°C and inside a 20°C it could be half the energy required to maintain house at 25°C it does not need to go from 20°C to 35°C to double power required, it is a lot lower temperature than double to need double the energy.

So if the house is maintained at 20°C or allowed to drop to 15°C then returned to 20°C on return there is no question dropping to 15°C is best option, however if after being allowed to drop to 15°C on return the temperature over shoots to 25°C then it is not such easy maths. I suspect it would cost less to leave the heating running, however it the temperature is allowed to drop to 10°C and on return it over shoots to 25°C then likely it would have been cheaper to turn it off when leaving the house.

Now on to that we have to consider the efficiency of the boiler, if a boiler fully modulated at say 10 kW output is 95% efficient, and at full output say 30 kW it is 85% efficient then running the boiler at tick over for 24 hours will cost less than letting the building cool then heat up again, however we don't know that the boiler is most efficient at 10 kW output, its sweet point may be at 20 kW output, or even be best at max output.

And it may be a boiler using return water to modulate is less efficient to one using the ebus, so exactly the same boiler could give different results depending on method of control.

With the boilers at work, we refer to the cost of a match, this is how much it costs to get the boiler from cold to working pressure without it doing any work, and depending on the size of the engine this can be quite high, OK for central heating we are not actually boiling the water, but the same applies, it costs to heat the boiler up and allow it to cool down, even if no water is circulated. In practice water will be circulated, but not through the radiators, if all the TRV's are closed and the wall thermostat calls for heat, the boiler will fire up, and the water goes through the by-pass valve, and as the hot water returns it closes the boiler down again.

So if I leave the house and all my TRV's sense I have left so using the geofencing close down, but for what ever reason the wall thermostat has not sensed I have left the house, then the boiler will cycle on/off loosing energy out of the flue. The big point is I would be unaware of what is happening, as I am of course not at home.

What would be really good is two houses one with all the geofencing and occupancy detection, and one without and monitor the fuel usage, of course two families with similar life style.

I have been in houses where the owner is singing the praises of his control system saying how much it has saved, no wonder, the house is now freezing, that includes my own house, being frank if after being out, I return to a warm house, at say 20°C that's great, but if the house is at 17°C then I am cold, so returning to 20°C is not enough, once you get cold you need a lot more to get warm again, so tendency is to be glad the central heating has over shot, as you have got cold waiting for it to recover.

So even if using geofencing and occupancy detection is saving money, big question is how much money, and is it worth saving, or would walking into a warm house be worth the extra? Some times I remember, and at work ½ hour before leaving I will turn the heating up, but whole idea of geofencing is I don't have to, it should be automatic, and so often 3:30 I will say OK what next, and the guy running to work will say no point starting some thing new now, think we will have an early finish, see you next week, so the idea of turning up heat on the phone does not work, I live within walking distance of where I work, so being lazy often have car, so 5 minutes and home.

 

[^woody^]

If you look at how Opentherm works, the concept is that it keeps water pumping around for extended periods and brings the burner on regularly at low flame to keep the water at just the right temperature to counter heat loss, and it does this on the basis that it's less economical to have the heating tripping on and off in cycles at high flame.

So the answer seems to be to keep the heating on for long periods and on low.

 

[ericmark]

My central heating seems to always cycle, not a large oil boiler, around 20 kW, as to if it cycles because some TVR's are closed, or just too big for system I don't know.

 

[Londoner2]

My central heating seems to always cycle, not a large oil boiler, around 20 kW, as to if it cycles because some TVR's are closed, or just too big for system I don't know.

Same here, had a new combi fitted and evry time i look at it its always recyling or 'on', my previous was a non condensing combi which had a simple on off switch for heating plus freeze protection thermostats.

I now have a crappy salus basic wireless stat that i still cannot understand and too difficult to program and will replace with a user friendly internet based one which would give me more control, in terms of gas usage, despite it being a mild winter, i still have used more gas but not a hugh amount, my electric cost is lower as i don't use the electric heater anymore, the house is more warmer as well.

 

[ericmark]

a new combi fitted and in terms of gas usage,

I would assume then the boiler should modulate not use mark/space regulation, I have questioned a few times using an on/off thermostat with a modulating boiler.

If you had no thermostat the following should be the cast, on turning on boiler it runs flat out, it heats up all the radiators together balanced by the lock shield valves, as the rooms warm up, the thermostatic radiator valves start to close, first pushing more water through those still open, then lifting the by-pass valve, the return water would unless the boiler did some thing would get hotter and hotter, but the boiler does do some thing, it turns down the flame height (modulates) and the algorithms in a good boiler will slowly reduce the water temperature, so by time it reaches the minimum output it is running quite cool, once it reaches minimum output then it starts a mark/space regulation (switches off/on) and will switch on at minimum output so the whole process is gradual.

This has a problem in that it would never switch off, the boiler can only know if the house is warm by the temperature or pressure of the return water, anti-cycle software can reduce how often it switches on to test if required, but it would continue cycling through the summer, so will fit an on/off thermostat in a room normally kept cool to switch off the boiler in summer, in winter it should never switch off, except when the programmer switches it off because house is empty.

The question is what happens if the on/off thermostat is set to a temperature where it will keep turning the boiler off/on? Every time the boiler is switched off any heat in the boiler will escape through the flue, and when it switches back on it will return to max output, so the boiler runs hotter and so more energy is lost through the flue. Also when the boiler is switched off/on rather than up down the temperature hysteresis will increase, and since the heat loss is a logarithmic scale that also means more loses.

Please realise talking about modulating gas not on/off oil boilers here. But in essence with gas everything needs to be analogue slowly increasing or decreasing not on/off. There are modulating thermostats, the OpenTherm is a protocol allowing different makes of boiler and thermostat to work together, unfortunately some manufacturers like Bosch Worcester have not adopted this, so you have to use there special thermostat which will only work in one room, the good quality thermostats Drayton Wiser, EvoHome and Tado work by the TRV telling the central thermostat/hub what heat it requires, and the thermostat/hub in turn tells the boiler, Nest does not connect to TRV heads but does have OpenTherm, Hive has used a completely different method, it simply turns boiler on/off, but if a Hive TRV tells the wall thermostat it still needs heat, then it does not turn off, so it allows the boiler to use its own algorithms by not turning off until all rooms are satisfied.

The whole idea is to let the TRV do the control not the wall thermostat, with the old wax or liquid filled TRV there is only one temperature setting, but the electronic heads start at £10 and you can then program each room independent, the cheap £10 model can't tell boiler to run, it just stops room getting too hot, but the more expensive (Around £60 each) actually tell the boiler when to run.

All very cleaver, but how much money does this cleaver system really save? If I leave house 24/7 at 19.5°C or I set it up so over night bedroom at 17°C warming to 20°C when ready to rise, then cooling over the day back to 17°C until bed time then back to 20°C then cooling again over night, with other rooms also set for example dinning room again sits at 17°C until 4 pm when it heats to 20°C until 8 pm and back to 17°C with similar schedules set for all rooms will this save enough money to be worth it?