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Thread: Worcester Bosch maximum flow temperature.

  1. #1
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    Default Worcester Bosch maximum flow temperature.

    I have a GreenStart 24Ri boiler which I had fitted in 2013 and I've generally been very pleased with it. My house is old with solid walls in places. Very happy with Evohome that I have used since it developed.

    I have a question that has puzzled me for years and is particularly relevant when the outside temperature is less than 5 degrees. - the house can take several hours to come up to temperature. I run it at maximum.

    Looking at WB documentation it states that the maximum flow temperature is 82 degrees. However, no matter how few radiators are calling for 100% heat, the maximum flow temperature on the boiler flow pipe never exceeds 60 degrees.

    I have no idea why this should be but I would like to get it up to 70 degrees. I have just measured the temperature of a similar WB boiler and it seems to be 60 degrees as well. I certainly would not want it to be 82 degrees!

    Is this expected explained or could it be considered to be a fault?

    Look at section 3.2, Technical Data: https://www.worcester-bosch.co.uk/pr...g-instructions


    Chris
    Last edited by chrisgare; 6th December 2021 at 06:25 PM.

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  3. #3
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    Bear in mind if the outside temperature drops during the night and if the heating is on low then the room temperatures will be that much lower when you get up and take longer to achieve the set temperature. This morning for example I found nearly all my rooms were 14.5 to 15 c when I got up. But I am sure you will have taken that into account.

    You should be able to set the flow rate in the boiler to 70c. I think 80c is too high as the boiler may not condense properly. Having had a new boiler installed last February I experimented with various rates and settings. My installer left the boiler at 80c and had no idea it would not condense properly and achieve its 90%+ efficiency until I told him. I left it at 65c for some time but we were heading into warming months. Now colder weather is here I find that it takes just a little too long to get rooms up to temperature. I have adjusted it to 70C. That seems about right. However, it remains at that whilst rooms are warming up, but drops back once they achieve temperature. I often see it reading way below 60c. Yesterday I noticed it was 38C yet all rooms were as they should be. The boiler and OpenTherm had reduced the flow rate as it was not needed that high. If I then go and turn up the temperature in a room that has not been used the boiler goes back up to 70c for a time because of the sudden influx of colder water from the radiator in that room. Expected of course.

    You do not indicate how you are measuring the temperature. Could you possibly be measuring the temperature on the return pipe? That should be about 20C lower than the flow temperature plus it should not be greater than about 55c for the boiler to condense. I think the range is 50c to 55c. I have read that many installers know nothing of this. They tell us the boiler is over 90% efficient but it isn’t if the flow temperature is set too high. One independent company that does not install but provides advice, found over 95% of installers did not leave the job with the heating running as it should.

    There is also the kW setting in the boiler. Which installers set too high. Mine is set to 6.4kW.

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    > If I then go and turn up the temperature in a room that has not been used the boiler goes back up to 70c for a time because of the sudden influx of colder water from the radiator in that room

    Just as an aside, this is because Evohome sets a 100% demand when there is around a 1.5C zone temperature differential. The OpenTherm bridge will map this to your max flow temperature of 70C.

    The lower return temperature due to your cold radiator will also result in the boiler increasing its modulation level.

  5. #5
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    Quote Originally Posted by kevinsmart View Post
    > If I then go and turn up the temperature in a room that has not been used the boiler goes back up to 70c for a time because of the sudden influx of colder water from the radiator in that room

    Just as an aside, this is because Evohome sets a 100% demand when there is around a 1.5C zone temperature differential. The OpenTherm bridge will map this to your max flow temperature of 70C.

    The lower return temperature due to your cold radiator will also result in the boiler increasing its modulation level.
    Exactly this. Thanks to the great work that David Bonnes is doing with the RF integration of Evohome with Home Assistant (I think he’s a forum member here too) I can see the below OpenTherm data being returned from my boiler:

    5A7513F3-F3B5-4DD0-B929-CD58F121F770.jpg

    EFF27EA1-B1C8-48F3-B766-D79EE3E121E6.jpg

    6E9DC609-4E73-482B-9BBC-305A668E585A.jpg

    You can clearly see the boiler output modulating as different zones call for heat, but the overall heat demand remains at 100% for most of the heating periods.
    Last edited by guyank; 10th December 2021 at 10:51 AM.

  6. #6
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    Hi I have a clamp-on thermometer about 2M from the boiler flow pipe - no it is not on the return. Even with only one radiator calling for heat it never goes above 60 degrees. This is a fault.

    WP says: "We can confirm that the flow temperature range of the boiler would be from 35-82 degrees C.
    This may be reduced if the boiler is not receiving the correct gas pressure or alternatively if the Co2 settings of the boiler are incorrect. It may also be found that the primary sensor within the boiler is reading incorrectly which would cause a reduced flow temperature from the boiler."

    I have called for a WP visit to correct it. It's been serviced every year. so gas and CO2 are OK.

    This is a fault and I have called for a WP engineer visit to correct it, though they haven't got back to me yes. It's not an emergency so it can wait until the new year.
    Last edited by chrisgare; 10th December 2021 at 03:45 PM.

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    Are you sure there is no max flow setting that you are hitting. On my Vaillant these maxs are set in the installer menus and then the normal user can only wind the dials back from that.

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    There is only the temperature button on the front panel - there is no internal preset that I can see in the WB documentation.

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    I read something about a low Nox plug that is sometimes fitted to reduce the output of the boiler. Might be talking non sense. But it could be your issue too. Yes I think best to get someone out.

  10. #10
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    Quote Originally Posted by chrisgare View Post
    Hi I have a clamp-on thermometer about 2M from the boiler flow pipe - no it is not on the return. Even with only one radiator calling for heat it never goes above 60 degrees. This is a fault.
    Are you sure it is not because, with only one radiator calling for heat, the flow rate through the boiler is too low? I have a WB boiler and I find that, if the number of radiators calling for heat is too few, the boiler will trip out. There is a maximum permitted temperature differential between the inlet and outlet of 30 degC. If the lowest heat output of your boiler, when modulated down to its lowest setting, is still too great and so heats the water more than 30 degC, then the boiler trips with a "2F" fault and waits for the water to cool down before restarting. In that condition, my boiler runs for a long time without getting up to the 70 degC setting for the outlet temperature.

    If I open more radiators, the flow rate increases sufficiently for the boiler to heat the water by less than 30 degC at its lowest modulated output and so stays firing until, gradually, the return water temperature gets up to 40 degC and then the boiler is happy with a 70 degC output temperature.

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