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Short cycling at lower flow temperature

Discussion in 'Plumbing and Central Heating' started by ntl142, 14 Apr 2019.

  1. ntl142

    ntl142

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    I have a problem with the boiler short cycling when the flow temperature is any lower than around 75 degrees. It's a Unica 32HE with Siemens RDJ10RF room stat set to 19.5 degrees (baby in the house).

    The thermostat correctly calls for heating if the room temp is outside the range 19 and 20 degrees (+/- 0.5 degrees from 19.5 setpoint). The receiver responds accordlingly, and the boiler fires up correctly. No problems there. Once the call for heat has stopped at 20 degrees, inertia in the system takes it up to 20.5 degrees which is ok. The thermostat works fine. It's not in a draughty cold area, or on a radiator. All TRVs are fully open except a towel rail which is half open. 8 rads in a single storey cottage, all twin panel with fins, with another one being a cast iron thing in a bathroom with no TRVs, just 'tap handles' at each end which are open a hair (rad gets red hot). The system is big copper pipes to a manifold, and then each radiator looks to have it's own microbore flow and return from the manifold.

    The CH flow temperature is set to 65 degrees. When the heating first kicks in at an air temp of just under 19 degrees, the return temperature rises to 55 degrees (just about condensing). Temperatures were measured on the flow and return copper pipes under the boiler with two known good thermocouples. The flow pipe matches the boiler display exactly.
    As time goes on the room temperature increases quite rapidly, the return temperature increases to about 60 degrees (not condensing anymore), and then the flow temperature further increases to 71 (set point + hysteresis) and the boiler cuts out and starts cycling. At this point the room stat is still only on 19.5. The boiler cycles off for 3 minutes, then fires up. Then off again after about 10 seconds, etc etc.

    Before it start short cycling I can hear the boiler modulating to a lower burn to try to maintain the set flow temp of 65 degrees, but it's a losing battle. I imagine it can only modulate so low before it has no choice but to turn off at 71.

    During this cycling it fires for about 10 seconds, in which time the flow goes from whatever it's dropped to (around 60) straight up to 71 and cuts out again, within seconds. I can hear it try to modulate within that 10 seconds, but it's not enough to catch it before it cuts out again. Three minute delay and then repeat again and again. If there wasn't as much intertia in the system it would carry on doing this and the room may never reach the upper limit of the demand temperature on the room stat (19.5 + 0.5). As it is, the inertia in the rads gets the air temp up to 20.5 degrees and the demand for heat stops at a hair over 20. If I turn the boiler off when it starts cycling, the room will still reach 20.5 just on inertia, without the cycling.

    If I increase the flow to 75 it behaves correctly, with no cycling. The downside of this is roasting hot radiators (unsafe for child) and high return tempertures (not condensing). Maybe ok in winter to heat up quickly after work, but not now in warmer conditions when I want a constant 18/19 degrees inside. I want the flow temperature to be around 55 or 60 degrees.

    When I turn the flow down to 60 the above cycling problem is worse. It starts cycling earlier on, and in this case won't achieve the desired room temp. At 75 it works ok. Also, at 60 I get a nice plume from the flue (until it short cycles), which is absent at 65 or 75 degrees for most of the time.

    Where do I go from here to achieve the flow temperature (and suitable return temp for condensing) that I want, without short cycling? I would be happy if the rate at which the house heats up was slower during the summer. At the moment it ramps up fast and cools over a few hours. I would like a more continual 'slow burn' so the rads are warm for longer, rather than scorching hot for a short time, which I think is also more efficient for the boiler too. If possible...

    Because my essay probably isn't particularly clear, I've put some numbers below. I'd be really grateful for some pointers of how to achieve my goals and save a bit of cash on efficient boiler operation! Thanks.



    Flow set at 65 degrees:
    upload_2019-4-14_14-1-0.png
     
    Last edited: 14 Apr 2019
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  3. muggles

    muggles

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    No simple solution to this really - if the boiler's output at minimum is too much for your system, you need a boiler with better modulation. It is an inherent issue with combi boilers that they tend to be oversized for the systems they're fitted to. Some have better modulation than others
     
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  4. ntl142

    ntl142

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    Thanks, I'd feared that. Three questions then, hopefully simple.

    1. Is short cycling bad for the boiler from a wear and tear point of view (short car journeys not good for IC engines etc), or just not efficient to run?
    2. Can I do anything to make the radiators more able to dump heat, and therefore lower the return temperature to the boiler? This may let me run the boiler at a lower temperature before it starts cycling. I have no idea if the rads are properly balanced. All I know is they all heat up at a similar time, and get similarly hot. There are two cold rooms in the house (big windows) and I opened up the lockshields on these to bias flow to those rads. It seemed to work, I think. Anything to be gained by re-balancing the system?
    3. Can the max power output of the boiler be reduced by an engineer? No idea if this is possible, but something like reducing gas flow/jet size or flame size etc.
     
  5. muggles

    muggles

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    It'll both use more gas and increase wear and tear. Bigger radiators could possibly help the situation, especially if you're finding some rooms are a bit cold. It sounds like your system is well balanced if everything is heating up at the same rate. The max output of your boiler may be adjustable, but it's the minimum that is the problem for you and that is fixed
     
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  6. vulcancontinental

    vulcancontinental

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    The 32HE is set for 24KW to heating at max and 6.9KW minimum htg output. Work out the heat loss in your place and get an engineer to reduce the htg fan speed to range rate the boiler's htg output.

    Vokera's fire at 75% of the htg setting for 15 minutes before ramping up to max if demand continues but cannot drop below minimum of course.

    The modulation ratio is only just below 4:1 on that boiler which is why on later models it was increased to 10:1 but there's nothing you can do to improve that.

    You seem to have a low water content system so there will be a rapid heat up of the system water.

    In reality condensing is not such a huge thing but it does improve the efficiency of the boiler slightly; as it is aluminium the boilers conduct heat very well at all temperatures but what is more important is proper servicing of the boiler. The engineer needs to get both ends off of the exchanger off and clean between the coils. This is rarely if ever done and reduces the life of countless boilers. This should be done at least once every five years if not more often although on the good side the hotter the boiler runs the fewer services it needs as it condenses less. Make sure he sticks a blade up the condensate outlet of the exchanger as it can become partially blocked retaining condensate within the shield. There is a cleaning chemical, hard to find, to help clean the exchanger if it is very badly blocked, try to get some as it probably will probably not have been serviced properly (CONS 034 is the part number)

    As long as the boiler is not off the first production run* another option is to fit an OpenTherm thermostat such as Vokera's BeSMART controller or a Honeywell OpenTherm stat (muggles may be able to advise as not all Honeywell stats are suitable, it must be able to control the DHW temperature, some do not) or a Nest. OpenTherm controls do not simply turn the boiler ON or OFF but calculate a lower flow temperature.

    *The first production run did not have the OT software on the control boards, get me the serial number and I should be able to make a guess if that's the route you want to go down.

    As for cycling often just remember the older standard efficiency boilers operated when well installed for 15 to 20 years at high temps cycling on and off far more often than the current boilers and these had less reliable motors than current boilers (yours do not however). Remember, a motor has a runtime life and a start stop life, running longer will shorten the life as will increasing start stops.

    Personally if I wanted to increase the life of a boiler I would spend money on servicing and, especially if in a rural area, a surge protector (current AND VOLTAGE).
     
    Last edited: 15 Apr 2019
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  7. ntl142

    ntl142

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    Bigger radiators might be a solution like you said. I've always felt the one in the cold room was a little on the small size. I'll have a look into a larger one that fits in there and do some sums. In the meantime, it sounds like I need to weigh up wear and tear against efficiency (high return temperatures). If it was you, would you prefer it cycled more, or ran less efficiently without the cycling?
     
  8. ntl142

    ntl142

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    This sounds interesting... How do I work out the current heat loss? Surely that depends on outside temperature/wind? Or is it a sum of the theoretical outputs of the radiators?

    Edit - had a go with a calulator, and got this result, based on some estimates, and internal/external temperatures of 19/8 degrees.

    The annual heat loss from your house is 21,888 kWh.
    https://www.resurgence.org/resources/heac.html

    At 19/-1 degrees it goes to 40,000 kWh.

    The bit you said about range rating the heating output, is that a method of reducing the minimum heating output from 6.9KW to a lower value? Does that also reduce the 24KW maximum value too? In which case I need to work out the heat loss for a cold -10C night up here, as well as a mild 15C summer day where we just need a little heating to maintain 18-20 degrees for the baby.


    The last guy we had out was a Vokera engineer, so hopefully he knows what he's doing. I felt confident after he left as he seemed genuine and obviously clued up on those boilers as you'd expect. Now I've got a few more questions to ask him next time, thanks. I have no idea of the history of the boiler though - it was fitted when I moved in 3 years ago.


    I'm not sure it's worth investing more cash on the boiler. I'll have a look into it. You said that the OT controls don't simply turn it on/off, but modify flow temperatures. If the boiler is already cycling at a flow of 65 degrees, would this actually aid in this respect, or would the cycling issue still be there?


    What is the surge protector for? Does it only protect the PCB for spikes, or are there other benefits too?
     
    Last edited: 15 Apr 2019
  9. ntl142

    ntl142

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    Another problem this morning: After reducing the flow to 60C to experiment yesterday, I put it back up to 65. It fired up correctly on the thermostat a few hours after that, and again another few times overnight. Then come around 6am it failed to fire up. The thermostat was calling for heat, the receiver had the light for heat too, but the boiler was doing nothing. No errors codes/lights, just sitting there showing the current flow temp of something around 30odd degrees. I flicked the room thermostat on/off and it made no difference. Then I put the boiler dial switch to off/standby (rather than DHW or DHW+CH etc). Waited 10 seconds, put it back to DHW+CH and it fired straight up and has been happy ever since. It did this a few weeks ago too, just the one, so I changed the room thermostat batteries. However, seeing as the receiver showed the light then it's not a wireless/RF issue. Any ideas? Is it possibly related to the short cycling issue or more likely unrelated? Thanks again.
     
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  11. ianmcd

    ianmcd

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    You cant reduce the minimum, you can reduce the maximum but the minimum is set
     
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  12. vulcancontinental

    vulcancontinental

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    There are other things that can be checked but not by you I'm afraid.

    As Ian says the minimum htg output cannot be reduced but if max htg is 75% of 24kw that's a lot for a property. Where I live in the south a 7 rad 3 bed semi needs 10-ish at -1 degree.

    Not precise at all but size your rads, look in an online catalogue and you'll know roughly what heat can be emitted. That's not what you might need, just what may be put into the house. That's a starting point..

    The surge protector, voltage, most are current, is to protect against spikes. I believe many failures are caused by spikes moreso in rural areas where variance is greater.
     
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  13. ntl142

    ntl142

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    Thanks for the confirmation, I understand now.
     
  14. ntl142

    ntl142

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    I've looked up all my rads online, looks like they are all from screwfix which makes things easier. In total I have 7,235W of emitters. That seems to fit ok with the 6.9kW minimum boiler output. Perhaps it's not so far out?

    Also used an online calculator to work out the size of radiator needed for the cold room - approximately twice the output of what's currently in there, so there is room to add another 1,000W with that if needed. Is that a significant difference?
     
  15. vulcancontinental

    vulcancontinental

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    That seems good but remember without adjustment your boiler will output18kw when starting get hot very quickly, too quickly, before ramping down
     
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  16. ntl142

    ntl142

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    Do you say that because of the "75% of 24kW for 15 minutes" bit? I now understand that the minimum power (6.9kW) can't be reduced. But can that 75% or the 15 minutes bit be modified?
    I'm slightly puzzled though, because when it fires up after a short cycle I can hear what I'm relatively sure is it trying to modulate. Surely it wouldn't fire straight back up to 75% for 15 minutes after each 3 minute gap when cycling? Maybe it's not that clever though!?
     
  17. A gas boiler is only clever when correctly specified & installed by a cleaver installer. if the system is altered after installation any level of being cleaver has to be dismissed.

    Back in the day vokera (gr claudio (y)) sourced a retro fit on site adjustable timer to prevent rapid boiler on-off cycling for its dmcf,rs turbo,flowmatis & others appliances. Doubt that will be helpful but boiler/system adaptation is possible.

    But

    got a unica condensing combination boiler in a northern europe holiday home,working on an open therm system. During winter the boiler runs all the time no on/off cycling (winter with 30cm snow on roof,more on the ground) the boiler runs at a variable reduced gas rate resulting in excellent comfort conditions. actually very impressive :!:

    Many uk properties leak heat although the home owner will think their property does not leak heat but they do,the boiler and its bespoke installation could just be doing its best :idea:

    edit-just to mention on your original post,you had found a higher boiler thermostat heat setting will prevent more frequent on/off cycling but you have edited your original post.

    happy heating :D
     
    Last edited by a moderator: 16 Apr 2019
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