Worcester Bosch Greenstar Modulation

Discussion in 'Plumbing and Central Heating' started by gazm2k5, 3 Nov 2021.

  1. gazm2k5

    gazm2k5

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    I'm trying to learn how my boiler and thermostat deal with modulation.

    I have a WB Greenstar Compact CDi ErP Combi boiler. I believe it uses the EMS Bus for actual modulation (ie. controlling the flow temperature), but the manual suggests it can modulate without mentioning a requirement for EMS (in fact the manual doesn't mention EMS at all):
    upload_2021-11-3_16-11-24.png

    It says the boiler is regulated according to the demand for heat. My question is, how does the boiler know the demand for heat, and choose to turn the flame low? The thermostats (Drayton Digistat +3) just have a live supply and neutral supply, and live switch;
    [​IMG]
    So surely they can only tell the boiler to turn on/off? Which would mean the thermostats are in charge of any "modulation" by way of PWM signal, cycling the boiler on and off and not controlling the flow temp.

    Is that correct, or is the boiler capable of something smarter that I'm missing?

    PS I actually have multi zone, so I believe the thermostats will be wired to a zone valve, which when open will send live return to the boiler.
     
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  3. ianmcd

    ianmcd

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    your thermostats are simple on/off controls, they wont modulate the boiler, the boiler will modulate by sensing the flow and return temperatures
     
  4. gazm2k5

    gazm2k5

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    Ooh interesting.

    From what I've read the thermostats I have do do modulation through PWM, which I will now call fake modulation.

    So the boiler raises the gas if the shower/taps are on high, or lowers gas if they're on low?
    And also raises gas if the return temperature from central heating is low?
     
  5. ianmcd

    ianmcd

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    your boiler if installed properly will be rated for the heating requirements, the HW will start a full tilt, then modulate back to keep a constant heat from the boiler, the boiler uses thermistors to monitor everything, and modulates up or down as required, your thermostats are simply opening or closing zone valves and never directly communicate with the boiler in any way
     
  6. gazm2k5

    gazm2k5

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    I'm confused as to how the boiler would know how to modulate. If it's 16°C in my house and I turn up one of the thermostats, which tells one zone valve to open, thus turning on the boiler - you say the boiler goes full on to begin with and then modulates back a bit. But how does it know if I'm trying to reach 18°C or 23°C?
     
  7. denso13

    denso13

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    The boiler doesn't know, the room stat knows in this case. The boiler modulates to keep an acceptable heat output, the room stat shuts things down when the selected room temp is reached.
     
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  8. ianmcd

    ianmcd

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    As @denso13 says the boiler only knows the temperature of the water leaving the boiler , and the temp of the boiler returning to the boiler. it has no Idea what it is actually heating, it does its on board configuration of how much energy is required to heat the water, it then modulates the burner to suit this, there are far more complicated control systems that make claims of gas consumption savings, but not with your setup
     
  9. muggles

    muggles

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    It doesn't. Your thermostat is a basic on/off model with no feedback loop to the boiler, and certainly no PWM. It is a switch with a timing function. You can purchase BUS controls for your boiler, but this isn't one of them
     
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  11. gazm2k5

    gazm2k5

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    Ohh I see, so it's just adjusting the flame to achieve whatever I set as the flow temperature on the boiler based on the temperature returning from the loop (amongst other things maybe).

    How do you know the thermostats have no PWM? Just from my googling, the second page from a post on this forum suggests otherwise https://www.diynot.com/diy/threads/drayton-digistat-3-accuracy-issue.210304/
    I mean, I was surprised because I'm inclined to believe you - my thermostats seem very basic.

    Thanks for the info though guys, much appreciated. I have more questions but better suited to a new thread.
     
  12. muggles

    muggles

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    I think what's being described there is TPI rather than PWM, and some stats have adjustable cycle rates too. Your situation is slightly different anyway though - the post you linked to relates to the operation of a single zone combi boiler, whereas your stat operates a zone valve. There is no direct connection between the power being fed from the stat to the zone valve, and the power from the zone valve to the boiler. The Digistat is supplied with a permanent Live in, and when it activates the heating it powers the brown wire on the zone valve head to open it. The zone valve is also supplied with permanent Live in via the grey wire, and at the end of the motor's travel it depresses a microswitch which links the grey wire to the orange wire. The orange wire supplies 230v to the boiler's switched live terminal, which causes it to fire up. The boiler then monitors rate of temperature change and flow/return temperatures, and modulates the flame accordingly. The orange wire connected to the boiler's switched live terminal will only ever have either 0v or 230v on it, according to whether the valve is open or closed. The thermostat will keep the valve open all the time that it calling for heat.
     
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  13. gazm2k5

    gazm2k5

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    Yeah I agree, PWM is a poor name, but seems to be what they call it.

    Isn't the end result the same though? The thermostat cycling on/off the valve indirectly cycles on/off the boiler in the same way.

    I think what you have described of the wiring is what I figured out from my googling. Behold my crude ms paint wiring diagram:
    upload_2021-11-4_11-59-3.png

    Wire colours are wrong, but when the thermostat returns live through the black wire, it completes the valve circuit which opens the valve, which mechanically presses the micro switch, completing the switch live circuit for live return on central heating.
     
  14. MJN

    MJN

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    Be careful about mixing up modulation (generically 'to change something to better suit the situation') that the boiler itself can perform and that which external controls can apply.

    Taking the former, even with just a binary 'gives us heat!' call from your thermostat, the boiler heats the flow up to the configured temperature and sends it round the radiators that in turn dissipate heat into the cold house thus lowering the temperature of the water returned to the boiler. As the house heats up and TRVs close down the amount of heat extracted from the flow reduces and thus the temperature of the return gradually increases (particularly so if/when an automatic bypass valve subsequently opens) and thus the boiler is required to perform less work to maintain the desired flow temperature and hence will modulate the burn rate down so as not to exceed the target flow temperature. That is *internal* modulation made possible by variable gas valves.

    *External* modulation - or rather modulation of the wider system that the boiler is only part of - is something that external controls can bring to the party - whether that be by sending a proportional 0-100% demand signal to say anything from 'We're miles from the our target room temperature, give me 100% output!' through 'We're starting to get close to our target room temperature so throttle back to 30% so as not to overshoot' all the way down to 'We've reached the target so don't send any more heat (0%)' and this requires the likes of Opentherm or EMS to communicate that demand. There is however also a 'poor man's' binary approximation to proportional linear control that can effectively modulate a demand signal by varying the ratio of 'demand' to 'no demand' within a given time period. As you've said, this is a form of Pulse Width Modulation - varying the width (time) of the pulse (call for heat) in order to change something to better suit the situation (modulation). That's exactly what 'Time Proportional and Integration' is - the proportioning of the on: off duty cycle based on the difference between the measured and target room temperatures as well as the measured response rate of the system under control. Thus, a 100% demand is effected through calling for heat for the entire period (say, 10 minutes), 70% what call for heat for 7 minutes and rest for 3, 30% would call for 3 minutes then rest for 7 and so on. Note that whilst I might have described is as 'poor man's' approximation to proportional linear demand signalling it can work very well indeed in practice although does have the downside of potentially introducing greater stresses relating to starting and stopping than would otherwise be the case from either linear control (which is rather more elegant with its ability to ramp up and down) or even the most basic of thermostats (whose high hysteresis means much larger amounts of over- and undershoot hence less stability and comfort).
     
  15. ericmark

    ericmark

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    With a gas modulating boiler the TRV is king, however the TRV has a problem, it can't tell the boiler to turn off completely, the boiler has to fire up and circulate water to know if any TRV is open. So we have a wall thermostat to basic tell boiler the weather is warm your not required.

    The whole idea is the TRV which is a type of zone valve is not on/off, but can slowly open and close, the whole system is analogue except for the thermostat on the wall. As each TRV closes more water is forced though those open until they can't take any more when the by pass valve opens, so as they close the return water gets hotter and the boiler modulates.

    As to why two wall thermostats and two motorised valves that allows you to turn off the heating in one section of the house, but retain frost protection, so if like me you only use the upper floors the lower floors will not freeze. If a programmable thermostat is used it also allows you to turn off a chunk of the house when not required. However with a small house it does seem pointless, as the TRV heads could do the same, and to have as with my main house nine programmable TRV heads gives clearly far more control to having two motorised valves, but the old wax heads have a large range. So set at say 2.5 it will start to close at 18°C and fully close at 21°C and are a pain to set up correctly.

    And really that is why we use zone valves, if I turn the thermostat in the flat under my house to 10°C I have turned off flat in one action rather than go around four TRV's adjusting every one, often we use programmable wall thermostats so a section of the house can be turned off in one go, and I have to admit I have nine programmable TRV heads but I rarely change their settings.

    What when I had a Worcester Bosch boiler I found was a problem was to set the lock shield valves, the radiator must not heat up too fast, as the TRV is slow to close, so when the time clock (programmer) turned on boiler, the radiators heated up to full temperature before the TRV could close and once hot even with TRV closed it was still heating room so temperature would over shoot.

    My TRV shows the current and target temperature TRV_report.jpg so if current over target closed lock shield a tad, and once set there were spot on, and the boiler modulated as it should, the problem was Energenie had an OTT anti hysteresis software installed, so set to 20°C at 7 am it was 10 am before it got there, so I would set at 22°C at 7 am and 20°C at 8 am to speed up the reheat. Drayton Wiser claims to use algorithms to stop that.

    Although the electronic TRV heads worked well with a modulating Worcester Bosch boiler, they have not worked very well with an on/off Worcester Bosch boiler, they are not always open when boiler is running, and opening when boiler not running clearly does not work, but cost of an oil modulating boiler if you could get one is prohibitive.

    But while it is cold and you want heating the modulating boiler should never turn off, the wall thermostat is only to switch off boiler in summer months, it should not really do anything in Winter.
     
  16. gazm2k5

    gazm2k5

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    Yeah I would like the external modulation for the supposed increased efficiency it provides. I'm not sure how much more efficient it is, but the thought of turning down the flow temperature to something like 30°C to maintain a 21°C thermostat temperature sounds pretty good. And there's also the benefit of not starting up the pump and other components so many times, to reduce wear on the boiler.

    Interesting, I never though of how the TRVs work with the modulating boiler to achieve this. The trouble is one of my radiators in each zone is a bypass, installed without TRVs which is annoying. That means I need the zone valve to fully turn off downstairs.
     
  17. ianmcd

    ianmcd

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    You just have to accept, that you have an S plan system, installed correctly and your model boiler can only do boiler modulation
     
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