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Thread: Unexplained high gas usage by evohome/valliant during a weekend away.

  1. #1
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    Default Unexplained high gas usage by evohome/valliant during a weekend away.

    I hope you guys can help me figure out what happened a few weeks ago. I have mailed the manufacturers, but Valliant didnt bother to have someone with knowledge read the mail, and evohome couldnt come up with an answer.

    So I have a reasonably isolated, but older house with 5 heating zone with one HR92/radiator each. Radiators only, no floor heating. 4 year old Valliant ecotec 5.5 gasheater (boiler temp set at 60C) and the first color version of evohome, using opentherm to communicate with the heater. We were a long weekend away, so I manually put all HR92's to 15C continuously. We left on friday, so by Saturday all rooms cooled down to 15C. The evohome system was able to maintain this temperature untill we returned by frequent requests for low heat (see picture with pink graphs for example of 2 rooms).
    1.jpg

    When I returned sunday night, I was shocked that I used about 7m3 of gas that Saturday, which is my daily usage the whole week before, when there is a person at home all day heating the living room to 21C, and a family in the evening.
    2.jpg

    Does anyone has an idea why the usage is this high, while the setpoint temperatures were very low??

  2. #2
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    The key missing piece of information is what was the weather at the time, in particular outdoor temperature and wind.

    So you're saying that you left on Friday (morning ? evening ?) and came back on sunday night, so call that 2 1/2 days, and had your whole house set to 15C, vs one day of usage with a living room at 21C and other rooms at what temperature ?

    Doesn't sound particularly out of the ordinary to me to be honest. While 15C might feel "cold" to us humans, it's still 15 degrees above a 0C outdoor winter temperature, and heat loss and therefore energy to maintain a set point is directly proportional to the inside/outside temperature difference. (With no wind. With wind chill heat loss may be even higher) So a 15C set point is not actually a "very low" set point as you imply, at least not in winter with low outdoor temperatures. (It's still 288 degrees Kelvin!)

    Lets say to maintain your whole house at 20C when it's 0C outside required 5m3 of gas per day. To maintain it at 15C it would take (15-0)/(20-0)*5=3.75m3 of gas a day, or a reduction of only about 25%. Not a lot of gain for a house that now feels cold, but that's the physics of it!

    When the outdoor temperature is higher and closer to the set points, the benefits of reducing the set points increases dramatically. For example say that the outdoor temperature was 10C and set point 20C. Now you need 2.5m3 of gas a day, however if you drop the set point to 15C you actually halve the gas use to 1.25m3 a day as you're halving the temperature differential rather than it being reduced only by a quarter. Now that 15C set point has made a worthwhile difference. (However I would point out that a 10C set point would have used zero gas in these conditions!)

    Take that even further to an outside temperature of 15C and a set point of 20C - this would use 1.25m3 of gas a day to maintain, but a set point of 15C would require no gas at all as there is no differential to cause heat loss!

    In short, if the outside temperature is very low it takes a big reduction in set points to make a worthwhile difference to gas use. A drop from 20C to 15C really doesn't do that much, despite us humans perceiving 15C as being cold.

    When I first got Evohome I was scheduling the downstairs to 15C all night (normal daytime temperature 20-21) and was horrified to discover my gas use was a lot higher than what it was with my old conventional timer and thermostat that just shut everything off overnight. No surprise in hindsight. So I now schedule all downstairs rooms to 5C overnight (the lowest possible in Evohome - basically frost protection only) and let optimal start worry about figuring out for me when to turn things on again to get the house warm again for getting up time.

    I don't care that the downstairs can (and often does) dip below 15C at night as long as its comfy again when I get up. Gas usage went back down and is now below what it was with the old timer/thermostat, despite both bedrooms being on at a low temperature overnight.

    In my opinion the default Away temperature of 15C is too high, at least for leaving the house more than one day. I've reduced my away temperature to 10C - this means in any weather conditions above about 8C the boiler won't come on at all but it does keep the house above 10C no matter how severe the weather gets in our absence, below 10C condensation etc would start to become a problem over a long period of time. 10C I think should be OK for a week or two away on holiday as long as you have the ability to turn the heating on remotely a good few hours before you expect to be home as it could take as much as 6 hours to get back up to temperature.
    Last edited by DBMandrake; 4th January 2019 at 10:24 AM.

  3. #3
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    looks about right. I'd have set it to 5 and possibly warmed it up to something above that for a short time each day. Any reason you chose 15 while away ?

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    Funny how every house is different.

    My night set back is 18c and it uses less gas to get back to 21c comfort temperature in the morning than if the heating was completely off or if I setback to say 10c. The house drops easily to 14c at night so the boiler used more gas the next morning than if it was just coming on occasionally to maintain at 18c.

  5. #5
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    Hmm, that doesn't make sense as the maths says you should be loosing more energy maintaining 18c for 8 hours then in a short boost heating from 14c to 21c, strange.

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    I'm not sure but if I was to raise the house temp from 14 to 21c it would take maybe 3 or 4 hours at 60c flow temp. I have low temp radiators so it's limited at 60c. From 18 to 21 maybe an hour pre heat?

  7. #7
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    hmm interesting..
    Mathematically assuming your boiler has the power to keep the radiator at 60c then
    raising the temperature from 14 to 15c should be a little faster then 18 to 19c as the speed at which it can raise the temperature is related to the difference in temperature between the radiator and the room, the bigger the difference the faster.
    2ndly heat loss is also related to the difference between the rooms temperature and the outside temperature and therefore less heat is lost when going from 14 to 15 then from 18 to 19..
    so in principle the time going doing the 4 degrees from 14 to 18 should be pretty similar to the 3 degrees in going from 18 to 21 so I would have thought 2 hours as apposed to 4.. energy wise they should be about the same too..
    maybe walls being cooled down or something effects things.. do you have any way to graph it?

  8. #8
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    I've not I've just done different gas readings when the outside temp and weather has been similar. The system has to work harder to increase from 14 to 21 than when it's going from 18 to 21. 14 to 21 it sits at 60c for nearly 3 hours whereas 18 to 21 seems to be about an hour and then eventually when at room temp backs right off to about 40c. When it comes on in the night to maintain 18c the flow temp seems to be about 30c so very cool and burner cycling intermittently.

    Deffo uses less as system is just ticking away. Opentherm just wants to get there as quick as it can, so the larger the diff between set temp and current temp the more gas it will use the closer the gap the less gas it will use so provided it backs right there's a fine line between efficiency and comfort.

    The lower your return the better, so yes your in condensing mode but with myself with a flow temp of sometimes 30c that gives a return of 25c, there comes a point where there is no plume outside as the boiler is recovering all that heat from the exhaust gases, this is super efficient.

    It's a difference of comfort for many and different house types etc. But for me having the heating on timed 6 hours a day at a comfort temp its better to have it 12 hours a day at a comfort temp then at night we set it back as we are in bed, but the boiler still comes on to maintain 18c when it needs to, albeit at a very low flow temp.

    Another scenario is driving 100 miles at 70mph your going to use more gas because your going faster but the plus side of that is you get there quicker or you can use opentherm and once you've gotten half way the boiler will lower it's flow temp which means your speed comes down but your MPG goes up because your now going 50mph but it will take you longer to get there, once you get there the boiler modulates down even further and the gas/fuel you've saved from taking your time and going slower can be used to travel more miles or in boiler terms keep the heating on for longer which the non opentherm boiler used by going full pelt but getting there quicker.

    For me it's cheaper to leave the heating on 24/7 with 21c comfort temp and 19c when were in bed. The thermostat keeps the house at this all the time. In the middle of the night the boiler will be running at minimum output with radiator flow temp of 30c this uses very little gas to keep that temp. I don't have to mess with my thermostat as it doesn't overshoot the temp set on the thermostat which anything over the temperature you want is wasted heat.*

    When we get up in the morning and ask for 21c the boiler may put the flow temp up to 45c for a short time then modulate back down. Last week's cold snap was the first time I had seen it go above 50c for a long time but the thermostat knows the outdoor conditions and compensates for this.

    If I turned the heating off fully and the house dropped to say 12c the boiler would come on and produce a flow temp of 80c for several hours till it had pulled the temp up. By the time we leave the house it would be coming up to temp before it would go off.

    This way we are always using gas but the rate of gas being burned is less than two timed periods a day with the heating being off completely in between. I've done the readings and monitored usage two years now and it is less this way for us as the boiler is just putting in a little bit of heat at a time rather than having to to full pelt.

  9. #9
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    Hi guys, thanks for your replies.

    To complete the picture, here are the outside temp and wind that week (the saterday I talk about was the 15th of dec). As you can see, the temperature was about the same for the same on thursday to saturday. Wind picked up a bit, but my house is covered by some trees.
    3.jpg

    The reason I have all rooms at 15C is partly cat/house plant, partly to prevent condensation leading to fungus. I understood that at 15C, you are pretty sure you stay above the condensation point.

    @DBMandrake, I fully get your idea. And it actually is a bit of an eye opener. If you want to safe gas, you should always consider the difference with the outside temperature. Dropping from 22 to 18C doesnt make that much sense if it is -10C outside.

    However, I also have the idea that the opentherm communication isnt working very well. But I havent been able to really proof this, as I dont know how the heater really reacts to it compared to an on/off thermostat. And I dont have a lab enviroment where all factors can be controlled. I do know that have most of the time this really short heating spikes, instead of a slow and steady increase in heat output, where you see the heater doing its own modulation. If this makes sense.
    I have this idea already for a couple of years, so last week I actually changed from an opentherm controller to a BR91 (on/off). The T profile of the heater input and ouput look much nicer (more modulation), but its to early to say if this will improve the gas usage.

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