1. Visiting from the US? Why not try DIYnot.US instead? Click here to continue to DIYnot.US.
    Dismiss Notice

Boiler help - flow and return differential or flow?

Discussion in 'Plumbing and Central Heating' started by WiltshireWarrior, 24 Jan 2021.

  1. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    Looking for help/tips from an expert who knows how to diagnose/suggest tips to get the most out of my boiler. Greenstar 40CDi conventional.

    I have mostly ufh now in all rooms, though some circuits are not yet connected. I have a sealed HWC attached too (not a system, just sealed)

    when the HWC calls all is fine. Boiler comes on, stays on, until HWC is satisfied. Then the issues start. I know this is a familiar story....

    when I had rads it was fine. Boiler worked ok. But I know that ufh circuits are different and I’ve read many have the issues I have. The boiler just cycles when it’s just ufh calling.

    the flow from boiler is, for example:

    boiler temp shown: 70
    Flow 50 (measured with infrared on black tape on copper)
    Return is 33 (measured using same approach as flow)

    My question: is it the temp differential causing it to cycle OR is it a flow problem (due to restriction of flow in ufh circuits)

    or is it something else?

    the house is a large 5 bed detached with big rooms

    note when the HWC and UFH are calling here are the temps

    Boiler shows: 83
    Flow: 57
    Return: 38

    and the boiler stays on (no cycling) until hwc is heated.
     
  2. Sponsored Links
  3. ScottishGasMan

    ScottishGasMan

    Joined:
    22 Jan 2007
    Messages:
    7,398
    Thanks Received:
    3,046
    Country:
    United Kingdom
    Firstly if using an IR thermometer, you need to use white masking tape on the pipes, you dont want something dark in colour or shiney like insulating tape.

    Secondly, flow issues cause temperature differential issues, there completely linked, the lower flow the higher the differential temperature.

    Is there an auto bypass fitted to the system? if not then the boilers likley going to damage its self as its already massively oversized for its appliacation by the sound of it and set to run a high flow temp which is the opposite of what UFH is, so the UFH manifolds will have to reduce the incoming flow from the boiler massively to allow them to blend the temperature down to a useable heat.
     
    • Thanks Thanks x 1
  4. ScottishGasMan

    ScottishGasMan

    Joined:
    22 Jan 2007
    Messages:
    7,398
    Thanks Received:
    3,046
    Country:
    United Kingdom
    I dont know any spesifics about Bosch boilers, so dont know what controls are available for them, but in an ideal setting, you would have whats called Priority hot water, so boiler set up with controls that allow it to heat the cylinder at one flow temp (say 75 degrees) then when thats done it would switch to heating UFH using another flow temp, say 35 degrees, much more efficient and healthyer for the boiler and system long term.
     
    • Thanks Thanks x 1
  5. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    yes, I have an auto bypass. Fiddled with it trying to find some optimum setting.

    Didn’t know about white tape. I had always read black tape was better. Thanks

    I have tried lowering the boiler temp, because ufh requires a lower temp than the rads I had before. Didn’t make any difference

    As I say, I’ve read so many people have this ufh conundrum, even when they have smaller boilers.

    My boiler is 13y old and we do plan to exchange at some point for a smaller one. But as it is I’m stuck with trying to get the best out of this. ‍♂️


     
    • Like Like x 1
  6. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    That sounds ideal. but my boiler I don’t think has that functionality.

    I do have it plumbed so that the water gets priority when called for. The tank reheats in <15mins



     
  7. ScottishGasMan

    ScottishGasMan

    Joined:
    22 Jan 2007
    Messages:
    7,398
    Thanks Received:
    3,046
    Country:
    United Kingdom
    It's not so much a conundrum but poor system design and integration. If things are calculated and controlled correctly then they run well.

    For the time being, lower flow temp is best, but you obvuously can only go so low while being able to maintain a good cylinder temp.

    When you do go to change boiler, get one that can do priorty hot water, weather compensation, and that is sized more suitably to the house, newer ones (certain models) can also modulate far lower than older models so they can stay on a low burn longer, but its essential to not go much if any over what the actual house heat loss is.

    And FYI open the autobypass up near fully, it wont help cycling but it will take the stress off the heat exchanger and potentially stop it cracking, you dont want flow and return measured at boiler to be any more than 20 degrees difference at any time ideally
     
  8. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    Thanks. The new boiler will def be sized to the new system. I’ve found some sizing tools online.

    Money etc means not possible right now. Trying to make the best I can in this cold weather. In the summer I think I can retire this one.

    Just one thing I still don’t understand: do you know what is it technically that is causing the boiler to cycle? (What I mean is, to go up to say 70, or whatever, and then switch off and start again)

    Is it the difference between the flow and return temps? Because they seem ok in general - in that it’s in the region of 15-20 degrees. So unless I’m not right with that, what other sensor is driving the software to cause the boiler to switch off and cycle?

    Is it “if delta (diff between flow/return) is > X” then cycle?

    or is there, say, a flow sensor in there that cycles the border if the flow drops?

    “If flow < X litres/min then cycle.....”

    Do you or anyone else know the answers to these specific questions?

    more for curiosity than anything else? ‍♂️

     
  9. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    Update:

    Forgot to add. I now have the boiler working pretty well with no cycling.

    Changes are (some to try and fix cycling, some just as part of house renovation):

    • I added a new UFH circuit to the manifold (I’m adding as we complete rooms in our house)
    • I tuned down the max output power of boiler to 60% (setting 1A, this is a test)
    • I turned UP setting on boiler (yes I remember what you said about manifold blender etc)
    I now have:

    flow: 65
    Return: 40

    and no cycling.

    Ufh blender is set to 60 degrees (we have only tiled ufh)
     
  10. Sponsored Links
  11. MeldrewsMate

    MeldrewsMate

    Joined:
    27 Jan 2010
    Messages:
    1,019
    Thanks Received:
    245
    Location:
    Buckinghamshire, Darn Sarf
    Country:
    United Kingdom
    I think you're wrong there, SGM. The standard for emissivity is the 'black body' with a matt black surface. Having said that, it is up to the OP to follow the user instructions for his IR thermostat to get the best out of it. In any case, to get sensible readings you must have stable temperatures, and the posted readings look like they're anything but stable.

    Your 40kW boiler should be able to modulate down to less than 10kW, and probably 6kW; your boiler manual will state minimum power. If your heating load (and for this I include all loads; radiators, hot water, and UFH) is less than this then the boiler will modulate to its lowest output, but will ultimately extinguish the flame if the flow temperature exceeds the user setting, which I shall assume is 70C. This boiler will have an 'anti-cycling' delay to mitigate the low load short cycling, though it may not help your situation by altering this delay before re-firing.

    A typical copper DHW cylinder will have a load of about 1kW when its contents are approaching 60C, an unvented cylinder will usually have a larger heating coil, so typically 3kW with it's contents approaching 60C (though much more when the contents are at a lower temp).

    As you have not fully commissioned your UFH system, you are likely to have a low load on the UFH, though for the size of house you describe I would expect it to be more than 6kW even without the DHW reheating. I notice that you have managed to achieve a higher load, not surprisingly by adding an extra UFH loop or two. Good. My suggestion therefore, is to wait until you have all UFH loads connected before further action, and those actions will be as follows:

    1. Use the boiler's built-in facilities to read the flow and return temperatures; whether accurate or not, these are the values the boiler controller will use to set its modulation value (its heat output).
    2. Check the differential temperature between flow and return with the boiler at maximum output (which I note you have restricted to 60%). If this is greater than 20C then it indicates too low a flowrate. Investigate pump performance, system cleanliness, or foreign body restrictions.
    3. Underfloor mixing temperature set to 60C !! The UFH design will usually specify the design flow temperature to be in the 40C to 50C range.
    4. Check the primary flow and return pipework orientation at the UFH manifold is correct, and that the UFH pump is in the correct way around.
    5. Check the flowrates in each UFH loop are correct within reason...longer loops need more flow compared to shorter loops. Most manifolds have flow indicators built in; if you can't see the indicator disc the system probably needs a good clean-out!
    6. Balance both the remaining rads, and the hot water cylinder flowrate.

    I hope this helps.
    MM
     
    Last edited: 25 Jan 2021
    • Thanks Thanks x 1
  12. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    Wow - that’s very kind of you to take the time to help. I need to go through it all.

    I shall respond!

    (Ps the 60 degree blender was a test, as I said we have only tiled ufh ATM so I wasn’t concerned about warping wooden floors etc. I usually have it set at about 45)

    thanks again
     
  13. ScottishGasMan

    ScottishGasMan

    Joined:
    22 Jan 2007
    Messages:
    7,398
    Thanks Received:
    3,046
    Country:
    United Kingdom

    Not sure about worcsters software, but typically they will cut the burner off once the flow temperature goes x degrees above your heating flow setting, so set at 70, it may cut out at 70 or 75 etc. It wont base its cycling off of differential temp, although many will shut boiler into a low rate or off into a fault mode if the differential temp spread is too much.

    a 40cdi regular Im sure has a minimum input of 10kw. So anytime your heat load is lower than this the boiler must cycle, when its higher than this, it will generally be able to simmer (if you can call it that at 10kw)

    If your heat load requirement is say 5kw, the boiler will fire up, level out to say 10kw, and the flow temp will rise towards your setpoint (say 70) once it reaches 70, it wants to lower the power output and stay lit but only enough to replace the heat being lost, since the heat being lost is 5kw, and the minimum the boiler can cope with is 10kw, its going to go over the set 70, at which point it cuts the burner, waits for a period, then starts back up again (cycling)

    Its almost impossible to avoid completely in most systems in reality, but the less the better, the better boilers these days can have a min usually between 2 and 3kw typically, so could stay lit at low rate much longer, ie I had datalogger on mine last weekend and burner was lit 5 1/2 hours straight but on very low rate while it just simmered along keeping the place cosy.
     
    • Like Like x 1
  14. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    Thank you! Very kind to reply again.

    And thanks for the answers, that really helps me figure this out. I will check the set max temp (from memory it’s 88 from what I remember in the engineer/installer manual)

    FYI since my earlier post the boiler has been on constantly. No cycling. Woohoo!! And the house is warming much better (though of course it’s mostly ufh so slow to respond)

    Update: meant to include current ufh kw demand:

    3.2 + 2 + 2.4 + 1.2 + 2 = 10.8kw (math is hard!)

    the hwc is a “fast recovery” vented, though the heating system is sealed. Can’t remember the kw
     
    Last edited: 25 Jan 2021
    • Like Like x 1
  15. ScottishGasMan

    ScottishGasMan

    Joined:
    22 Jan 2007
    Messages:
    7,398
    Thanks Received:
    3,046
    Country:
    United Kingdom
    There you go, looks like the maths may be spot on, your demand is over 10kw now and boiler runs stable, (although you may find as the weather gets better it begins to cycle more as the heat load is only that on the coldest days, as the weather gets better, less output is needed from the boiler and it will prob cycle a bit, but again by that time you may have more circuits added)
     
    • Thanks Thanks x 1
  16. WiltshireWarrior

    WiltshireWarrior

    Joined:
    24 Jan 2021
    Messages:
    31
    Thanks Received:
    2
    Country:
    United Kingdom
    Good point.

    the last circuit was our new living room (6.5x5) with 16mm ufh pipe and 2 circuits of ( I think) 160m in total.

    Hence the (6.5x5x~100kw/m) = ~ 3.2kw

    thanks for your help. Greatly appreciated
     
    • Like Like x 1
  17. DIYnot Local

    DIYnot Local

    Joined:
    3 Sep 2019
    Country:
    United Kingdom

    If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

    Select the supplier or trade you require, enter your location to begin your search.


    Are you a trade or supplier? You can create your listing free at DIYnot Local

     
Loading...

Share This Page