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Thread: Evohome + intergas xclusive. Low heat demand

  1. #1
    Automated Home Jr Member
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    Default Evohome + intergas xclusive. Low heat demand

    Hello

    I have been using evohome for over 2 years with an old ariston boiler via bdr91. It worked ok, however it was a time to replace a boiler and have chosen intergas exclusive paird via opentherm.

    The system is working fine however there is one problem that bugs me.

    My doughters bedroom is about 0.5-1c colder then the rest of bedrooms. During night we have bedrooms set at 19C. While another 2 bedrooms can retain the heat, my doughters bedroom is at constant low heat demand around 2%-4% during night, which causes the boiler to cycle for around 45seconds every 10min cycle. Its quite annoying as her radiator doesnt even have a chance to heat up for this short period of time.

    Is there any way to restict the boiler/boiler to wait for a higher heat demand?

    I ve tried to change the cycle on evohome to 3 rather then 6 but the setting is ignored. Why is that?

    I ve noticed that boiler has an openthrem parmater to only respond to heat if demand is greater than 30%. Which may be solution.

    Any clues what to do.

  2. #2
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    I found an answer to cycle rate. It is ignored due to ot being installed

  3. #3
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    You are correct that the cycle rate (and minimum on time, etc.) are for controlling the boiler via a BDR.

    With OpenTherm and a low demand situation like you've described, I would expect Evohome to be requesting a low flow temperature - perhaps as low as 30C. Your boiler will modulate its flame down to try and achieve this, but it's likely to require even less energy than the boilers lowest output. In this case, the boiler has no choice but to cycle. Once it's done a short burn, however, I'd expect the pump overrun to keep the water flowing. You say the radiator can't get up to temperature with a 45 second burn, and you're right. But it's not the burn time that matters, it's the flow time. And with a constant demand like that, you'll likely find that the short burn will increase the flow rate to, say, 50C before the boiler cuts out (it will have it's own in-built minimum burn time), and then in order to meet the requested 30C (say) flow rate, it will just keep the pump running and circulating the 50C water which will slowly cool down as the radiator extracts heat.

    Have you checked to see if the radiator is warm to the touch?

    It sounds to me like OpenTherm is doing exactly what it should be doing in this case.

  4. #4
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    For example, here's some data from my (OT-controller) boiler the other morning.

    Screenshot 2019-05-21 at 20.16.00.jpg

    I'm colour blind, so I don't have names for all the colours, sorry! If you look at the period between 07:25 and 07:30, I'll name the lines from top to bottom:

    * The one that's staunchly stuck at 90C is the "requested flow temp". This is what OT is asking for. 90C is "full power". It's asking for this because it's heating the hot water cylinder.
    * One that's hard to see that sticks at 75C. This is the "target flow temp". It's what the boiler is actually aiming for. It's the same as above, except it's limited to what I've set the max temp to in the boiler settings (i.e. 75C).
    * One that very briefly deviates from 75C between 07:25 and 07:30. This is the actual flow temperature. As you can see, the boiler is doing a good job of achieving the target flow temperature.
    * One that's maybe white or cyan in colour. This is the return temperature.
    * The dotted line. This is the hot water cylinder temperature. You can see it moves in steps. This is a function of how the CS92 temperature sensor reports. The target temperature for my cylinder is 60C. You can see it VERY nearly reaches it at about 07:28 before someone jumps in the shower and it falls again.
    * The bold (maybe green) line. This is the boiler output power (measured on the right-hand axis). As the hot water cylinder reheats, notice the power drops away. This is because as the dT in the cylinder gets smaller, the heat transferred is less and so less power is required to maintain the flow temperature. Notice how the return temperature creeps up during this period too. Same deal.
    * The blue one isn't so much a measurement as just an indicator that the pump is on.
    * The red one is an indicator that the flame is on.
    * The final dotted one is dT across the boiler - the difference between flow and return. Since the boiler pump is set to a fixed speed, this is directly related to the boiler power.

    The interesting thing here is to note what happens at 7:53-ish. The cylinder gets up to temperature, and OT drops its temperature demand to 10C (this is "nothing" in Evohome-speak, just as 90C is "maximum"). Then it jumps up to 26C. This is the lowest it ever goes in my experience. Shortly after (at 8:05), it jumps up again to 33C. During this whole period, the boiler decides that it can meet demand by just circulating the already-warm water it has available. The flame stays off for half an hour or so with just the warm water circulating. During this time you can see that the flow and return temperatures are virtually identical. This is partly because the radiator that's demanding heat is extracting virtually none (because dT between the room and the rad is only about 10C-12C at this point), and partly because of my low-loss header setup. But you can also see that the flow and return temperatures are falling. This is partly because of heat loss from the pipes and partly because off heat "loss" from the radiators.

    In summary, the boiler satisfied the 26C and 33C demand just with the pump and without the flame. This is exactly what you'd want to happen. If this were being controlled by a BDR, I expect you'd see a few one minute cycles followed by an overshoot in the room and then a complete shutdown. During the shutdown, the boiler's pump overrun would work for, say, 5-10 minutes and then there would be no flow at all until the room undershot and the TRV called for more heat.

  5. #5
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    Thanks for your input.

    The problem is that radiator doest have time to heat up as entire pricess doest take longer than a minute.

    I will try changing the parameter on the boiler and check if this will cut unnecessary cycles

  6. #6
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    The point is, as long as the pump is on and cycling the water, and the water is warmer than the room temperature (when it reaches the rad), then it will be heating the room. It will just be a low, gentle trickle to replace the heat loss. And this is exactly what you want and what you'd expect from an OpenTherm controlled system.

  7. #7
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    Also, with Evohome, it's often not about heating rooms up - it's about maintaining room temps. That's what that constant heat demand throughout the night will be.

    It's replacing the heat lost.

    P.

  8. #8
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    Ive done some research today. The temp of a room was 20C, using a dial on hr92 i inceased the temp to 25C

    Boiler started to cycle as above. The flow Ch pipe was boiling hot, hr92 open on 100, evohome askes for 100% heat, after 20min the radiator was only slightly warm.

    Never had this problem before with an old boiler surley radiator should be at least hot after 20min its only 8m from the boiler.

    A flow problem? Poor pump on the boiler

    This affected all radiators when i tested them.

  9. #9
    Automated Home Legend paulockenden's Avatar
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    Sounds like a plumbing issue rather than an Evohome one.

    Air lock?

  10. #10
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    Spot on

    The boiler was full of air since i blead it, the water is circulating correctly

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