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Vaillant VR66 wiring control centre causing heating modulation woes?

Discussion in 'Plumbing and Central Heating' started by Matthew B, 16 Jan 2021.

  1. Janko32

    Janko32

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    guys I have some questions regarding modulation:

    1. are we absolutely sure that by "modulation" is meant modulation of flow temperature during one cycle or only modulation of flame power output once it reaches eBus target? or different flow temperatures in different cycles (eBus targets changes only in another cycle)?
    2. do you guys who have experience with the vSmart really see the boiler changes the flow temperature once the room temperature comes close to target room temperature? (for example it's now 20 in the room and you set the target at 22, does he really start with lets say 60C flow and once the room temperature reaches 21.7 the flow is changed to 50 and then again when the room temp is 21.9 it further "slows" down to 45)
    3. what oscillation in room temperature is considered as normal? -0.1 to 0.1? or -0.1 to 0.3?
     
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  3. Harry Bloomfield

    Harry Bloomfield

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    It definitely modulates the flame and the air fan in the boiler and thus the flow temperature, dependant on the heat loss/ outdoor temperature. I see flow temperatures varying from 35C, upto 80C, when it is really cold outdoors. As the set temperature is approached, the boiler modulates down so as not to overshoot. I have never seen it overshoot the set temperature.

    My open vented system only has the ability to turn the pump on or off, so no flow variation is possible.

    Yes, see above.

    That is called 'hysteresis' - I have not really been able to put a value on this, other than in certain circumstances [1], I have not been able to spot any error between the set and actual temperatures.

    [1]Outside doors opened, release warm air and the indoor temperature can fall quite suddenly.
     
  4. SoftTopGeek

    SoftTopGeek

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    I have a Vaillant ecotec plus open vent boiler, and I wonder if that would be able to run at a low output temperatures under ebus control?
    I ask this, as in 230 v operation, each time it recieves a call for heat, it runs at around 60% power for approx 1 minute, before it modulates, and often exceeds the flow temperature, thus shutting off, and never allowing the low power output.
    When under ebus control, does the ignition to run procedure missout that fierce blast of heat does anyone know?
    It is a 418, s plan, 1 CH circuit and 1 DHW cylinder. Both with their own motorised valves.
    Currently (looking to up grade) with a programmable room stat, but I like the idea of a ebus controller if it will be able to keep the temperatures under control well.
     
  5. rbranco

    rbranco

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    Sounds like your d.00 range is too high, you need to set it correctly based on the total output of your radiators, if its too high, pump speed is too low or you have too much bypass happening then you can overshoot before it has time to scale back. My 418 has been ranged to 12 and it does not overshoot any longer.

    There is a time lag between when the boiler sees increased flow temp and when it scales back.

    If you want full modulation then you will need to get a VRC programmer with VR66 wiring centre, it can still fire to full temp, it depends on multiple variables such as outside temp, inside target and current flow temps.
     
  6. SoftTopGeek

    SoftTopGeek

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    Thanks.
    My D.00 is set to 9 kW. I manually change it according to the weather (I call it manual weather comp!)
    It doesn't seem to matter what I set D.00 to, the boiler always starts up at the same output power rating, maybe 60% of its maximim. After 1 minute, it then runs at D.00 value, so I hear it either change up or down, depending on what I have D.00 set to.
    My worry is that ebus will request a low flow temperature (say 50 Deg C), but within the first minute after firing the flow temperature exceeds that (because all the water is around 40 Deg C say, and many of the TRVs have closed off because we are about the right temperature), then I'm going to be no better off with the ebus controller.
    I was considering the vSmart or the sensoHome / sensoComfort, with the wiring center.
     
  7. rbranco

    rbranco

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    I actually find on my boiler on eBus it starts low for about 15-30s then ramps up to max (d00) then once it comes close to flow temp requested by VRC it scales back and just keeps running until the flow temp rises too much where it can not scale back any further and turns off, pump keeps running, flow temp goes down, anti-cycling stops it short firing and then the whole thing repeats.

    I have the VRC720f + VR66 + VR10 (cyl temp NTC, 60c but flow temp of 80c). HW request runs at much higher KW value than CH but you still have to be careful that it does not overshoot, so needs a bit of tweaking based on your cylinder. A VR66 will best run in HW priority mode else you can not operate two different flow temps properly.
    Its possible on the VRC to set the minimum flow temp and then it will start and stop boiler correctly, ive had to do that as trying to maintain a 35c flow temp is not easy when you have a ABV loop, some radiators very close to the boiler and also not very high efficient convector radiators.

    Have you calculated the total KW requirements by all your boilers or just guessing a figure?

    When I last had issues with system temp rising to quickly even when scaled back I ended up finding a problem with my pump as it was failing, I ended up draining, filling and putting in some MC3 and cleaning out the whole system and now it runs far better, also have magnaclean on and agitated the radiators to get sludge moving.
    Problem is that most systems have a slow decline in performance and people look away from basic system maint.
     
  8. Harry Bloomfield

    Harry Bloomfield

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    A proper VRC ebus system will be much better, more economical on gas, provide better comfort - because it can sense and calculate the amount of Kw output needed from the boiler and directly modulate the boiler to suite the requirement. The result is much more stable room temperatures, no over shoot and slightly better economy.

    All your programmable stat can do is tell the boiler there is a demand for heat, or no demand, with no clue as to the size of the demand - all or nowt, leaving the boiler to modulate as best it can based on flow temperature. When I had my new Vaillant installed, I had it installed with the old basic controls and it pretty much operated the same way as my old boiler had, with much wider variations in room temperatures and obvious by the creaking noises, when the boiler stuck up, but I was able to have both CH and the cylinder heated at the same time.

    A few weeks later, I decided to investigate the VRC ebus controls, bought a full kit and installed it. I lost the ability to have it provide HW and heat the cylinder simultaneously, it heats the water as priority now, but it holds the room temperatures perfectly steady, never over shoots and modulates perfectly.
     
  9. jackthom

    jackthom

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    I use an ebus controller purely to call for DHW, with the CH still controlled by a conventional thermostat.

    In DHW mode the boiler no longer runs for a while at around 60% of full power, which used to frequently cause the flow temp to rise very rapidly and trip off the boiler. It now starts very gently at the lower d0 level I have set for DHW and the cylinder is usually reheated with no cycling and at a much higher flow temp than would be ideal for central heating.

    I’d also like to know the answer to SoftTopGeek’s question. I don’t really know if ebus control of heating would avoid the higher boiler output during the initial run up but suspect it won’t. It was only ever an occasional problem for me because I opted for the 28kW model when the 18kW would have been just enough.
     
    Last edited: 3 Feb 2021
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  11. Harry Bloomfield

    Harry Bloomfield

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    Starting from cold, or if there is a sudden increase in the 'desired temperature' the boiler will run at higher output trying to meet the new demand. What it will not do is over run that desired temperature because it is completely blind to its progress it heating the house.

    Presuming you drive, I would assume you don't go charging up to traffic lights then bang the brakes on hard when they turn red, simply because you can see what is happening and predict ahead? The VRC does the same, it modulates down, when it sees it is getting near and has a learning process so it is aware of its operating environment.
     
  12. jackthom

    jackthom

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    Having reread the thread I’m getting the impression that with ebus control of heating the boiler no longer runs up to around 60% of full power in the first 30sec or so, but instead it won’t ever ramp up beyond the power set by d0. Is that a fair summary?

    I already know that this is the case for DHW mode which has its own independent max power setting.
     
  13. rbranco

    rbranco

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    D00 is the max the boiler ever goes to regardless of ebus, there is another setting for max kw for HW when using ebus.
     
  14. SoftTopGeek

    SoftTopGeek

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    My D.00 can be set from 5 kW to 18 kW.
    If I set it to 5, it still runs at around 60 % of its max (around 11 kW) for around the first 1 minute after ignition. That is the annoying part.
    Mine (being 230 V control) doesn't know if it is running CH or DHW, as it only has a 'demand' input.
    I like the idea of the weather sensor adjusting D.00 value, and the internal sensor adusting the flow temperature, hence the questions!
     
  15. ScottishGasMan

    ScottishGasMan

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    Weather sensors don't adjust d00. Only the flow temperature.

    D.0 should be set to match what the system has been designed for, the issue comes as always that the system wasn't designed, just some radiators and pipe connected together in the hope it will all run ok
     
  16. wannocks

    wannocks

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    The 400 series ecotecs light at around 2/3 of the appliance output regardless of the D0 setting. This is to establish a stable flame on the burner before it can ramp down. If the boiler is oversized for the load it can overshoot the demand temperature while still in the ignition phase.
     
  17. SoftTopGeek

    SoftTopGeek

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    I thought that the weather sensor tweaked the power (how quickly the water heated) and the inside actual to required temperature tweaked the water temperature set point (how hot the water could get).
     
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