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Thread: temperature settings for evohome

  1. #1
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    Default temperature settings for evohome

    Dear all,

    This is more of a generic question rather than one that is specific to evoHome. I've had evoHome running for a good week now, and as of yesterday have now fitted all rads in the house with HR92s, creating 8 zones in total.

    I'm monitoring gas and electricity usage with Loop and use evologger to log actual / target temperatures for all zones. I use a slighly modified version of pyloopenergy to log gas / electricity to a spreadsheet as well, and am now able to offset evoHome demands and gas usage, which is really interesting.

    I'm looking at chrisgare's graphs in envy (here) - my gas usage is jumping all over the place - and I think this boils back down to the fundamental question of "is it more efficient to keep the house at an acceptable base temperature, rather than letting it cool down completely in the night and heating it back up in the morning"...

    I'll use my worst room as an example. The graph attached is the dining room, which was built as an extension to the house many years ago. It has two outside walls, has a big window and a set of French doors, and is generally the coldest room in the house full stop.

    From the graph you can see how the heating goes off at around 8 PM. In evoHome I've set the zone to 10 deg during the night and the room gradually cools down until it reaches 13.5 degrees. The schedule for this room should heat it up to 20 deg for 8AM. You can see how optimum start kicks in at around 7. It then takes a full two hours for the room to come up to about 19 degrees.

    So I guess the question is - would I be better off not letting the temperature drop that low in the night and set it at a constant 16 for instance? Everybody I have spoken to so far has a different opinion I'd be very interested to hear everybody's thoughts on this...

    Many thanks - Martin
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    Automated Home Legend paulockenden's Avatar
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    Imagine a pan of water on your stove one evening, simmering.

    Imagine you need it to be boiling the following morning.

    Which will use less gas: keeping it simmering all night, or letting it cool now, and boiling it again in the morning.

    I think the answer is obvious.

  3. #3
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    Quote Originally Posted by mart1711 View Post
    So I guess the question is - would I be better off not letting the temperature drop that low in the night and set it at a constant 16 for instance? Everybody I have spoken to so far has a different opinion I'd be very interested to hear everybody's thoughts on this...
    If your house is a warm thing in a cold atmosphere it loses heat. The warmer it is the more heat it loses. Letting it cool down will mean less heat lost. Once you reheat it then it will lose the higher amount of heat again, but you will have avoided losing some while it was cooler.

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    There was quite a bit of discussion on this recently:

    http://www.automatedhome.co.uk/vbull...as-consumption

    I also think the answer is pretty straight forward - letting your house cool down as much as possible at night will minimise the rate of heat loss at night thus the total energy loss. When I initially kept downstairs rooms in the house at 14 degrees or above during the night instead of the heating being completely off, my gas usage actually went UP on what it was before I had evohome. I now schedule all rooms except bedrooms to 5 degree frost protect at night and usage has gone back down again.

    With optimal start you don't need to worry about figuring out when to turn the heating back on to get the rooms back up to temperature - if you want your rooms to be ready at 6am you just set your set points for 6am and it will figure out what to do.

    On warm nights when the room only falls to 17 degrees and has to heat back to 20 it might come on an hour before 6am, in colder weather when the room falls to say 12 degrees it might come on two hours before 6am. Because it adapts to starting temperature automatically you're not wasting additional energy over-estimating how early the heating has to come back on.
    Last edited by DBMandrake; 11th November 2016 at 11:45 PM.

  5. #5
    Automated Home Legend paulockenden's Avatar
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    However, there IS a comfort issue. A room which has been kept at 20 degrees overnight will feel more comfortable than one which has just been heated up.

    Because objects, surfaces, etc. will feel cold in the latter, even though the air is up to temperature.

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    Quote Originally Posted by paulockenden View Post
    However, there IS a comfort issue. A room which has been kept at 20 degrees overnight will feel more comfortable than one which has just been heated up.

    Because objects, surfaces, etc. will feel cold in the latter, even though the air is up to temperature.
    I'm not sure about this Paul. Have you confirmed this effect in a room that uses a remote wall sensor, or are you basing this on rooms using the HR92 as a sensor ?

    As I replied in another thread, when the HR92 first thinks a room is up to temperature during the first heat up for the morning the rest of the room can in fact easily be 3 degrees or so below the claimed temperature due to the thermal lag transferring heat from the radiator to the room.

    I've confirmed this effect in our living room with a thermometer standing on the coffee table vs the HR92 reading, back when we used to use the HR92's built in sensor. I noticed exactly what you describe - when the room first "warms up" and reached its claimed 21 degrees it still felt cold, but the separate thermometer (and IR gun checking walls and furniture) confirmed the bulk of the room was in fact still only 18 degrees, and took many hours to gradually creep up to be in agreement with the HR92 reading. We used to "hurry up" this process by temporarily turning the HR92 up a couple of degrees.

    Since we switched to the DTS92 on a wall this does not happen - I can arrive in the room just as the target (as reported by the DTS92) is met in the morning and the room feels comfortable and up to temperature, as reported both by the coffee table thermometer and an IR gun checking the temperature of walls and furniture.

    A couple of other thoughts on this issue:

    1) Radiators don't just heat the air. Even on a convector a significant amount of the total heat input to the room (something like 40% I think I've read) is through direct infrared radiation - this will warm furniture in line of sight directly regardless of air temperature, so you're not only relying on air convection and hot air to warm your furniture, walls, floors etc. On non-convectors a much larger percentage of the heat into the room is direct infrared radiation. Of course this doesn't apply to a radiator that is stuck behind a sofa, so its a good idea to try to have at least one radiator in the room exposed and in plain view.

    2) A wall mounted stat will tend to be influenced somewhat by the temperature of the wall that its mounted on - if the wall itself is really cold and well below the current air temperature the wall mounted DTS92 will read a bit lower than a free standing thermometer on a coffee table - in my room I noticed it would lag a degree or more behind the air temperature during warm up due to the influence of the cold wall. This is actually a good thing because it means the wall mounted sensor will help compensate for the cold wall effect that you describe, as it will sense the coldness of the walls while they lag behind air temperature and apply additional heating but once the room has stabilised air and wall temperature will be equal and the final air temperature attained will still be accurate.

    (As long as you don't mount the DTS92 on an external wall - don't do that. I tried it and the coldness of the external wall does skew the regulated temperature of the room, the effect was quite pronounced)

    Even if after all this it doesn't quite seem comfortable right away, just schedule the room to be ready an extra half an hour earlier so the room has had more chance to reach a thermal equilibrium.
    Last edited by DBMandrake; 12th November 2016 at 10:47 AM.

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    Going back to the original question, there is ABSOLUTELY no doubt that for me, based on my records, it is best to turn the heating off over night. Black and white. Less gas is used. Paul's analogy is perfect.

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    Thank you everybody - much appreciated.

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    Quote Originally Posted by chrisgare View Post
    Going back to the original question, there is ABSOLUTELY no doubt that for me, based on my records, it is best to turn the heating off over night. Black and white. Less gas is used. Paul's analogy is perfect.
    I think that we need to be careful with the term OFF as it applies to Evohome. In the 'olden' days, OFF usually meant that the heating was in an off period as set on the timer. I think what is being referred to here is the setting of low zone temperatures with Evohome heating remaining in an enabled or ON setting.

    For example, I have a bedroom which sits above an integral double garage. The zone target temperature is reduced to 8C overnight. On a really cold night (below zero), I often hear the pump running to maintain the 8C temperature in this particular room.

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    Long ago, many years ago I thought from advice received leaving the heating on overnight was better but it can't be and I stopped doing it many years ago. The analogy of leaving a pan of water simmering is spot on. With Evohome I do always leave it on, 365 days of the year, except that at night it is set to 10c. Even on very cold nights the house rarely drops below 14c or 15c, other than conservatory, the latter is set to 5c, so in effect whilst Evohome is always on the boiler is not firing up. Find what you are comfortable with and forget it, you can always make adjustments from the comfort of your armchair!

    As for optimisation I am not convinced. Yes it works if you leave it alone for some weeks but I still find it starts heating rooms too early and switches off too early so rooms go cold when you are using them. Currently I have optimisation start enabled but have disabled the optimisation off part. Coming on 60 minutes, which it does in one zone, ahead of the set time I find is a waste when the room is comfortable, in a another room it is 30 minutes even though 10 minutes or so would be enough. All more noticeable now that cooler days and nights are here. I have disabled optimisation altogether now. If my heating comes on at 07:00 and that is the time I get up, I find that is fine, pre-heating is not needed. In fact I get up at 05:30 most days and no heating is on other than one room comes on at weekends then. I found optimisation a little chaotic for the bedroom, it brings the heating on too early and switches it off too soon so the bedroom is cooling right at the time you don't want it to.

    Find out what you are comfortable with and stop playing. It works better that way!

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