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VR65 with on-off heating and hw

Discussion in 'Plumbing and Central Heating' started by fezster, 2 Jan 2014.

  1. fezster

    fezster

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    I thought I'd start a separate thread for this to avoid confusion.

    I've found that heating my hot water (megaflow 250l indirect) at 80 degrees seems to provide the fastest heating time, and reduces (if not eliminates) cycling on my boiler (Vaillant 438).

    I have a two heating zones and one DHW zone. The VR65 does not officially support two heating zones, but understand that two zone valves could be used with it. Putting that aside for a second, and if I only had the one heating zone - can the VR65 be used to adjust the flow temp on the boiler to 80 degrees for DHW, whilst reducing to whatever is set on the dial for CH, and still function with standard programmers and programmable room thermostats. Im not (currently) interested in weather compensation. Or would I have to change my programmers / thermostats to vaillant controls ?

    Many thanks.
     
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  3. ALEC1

    ALEC1

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    in this configuration you need the zone thermostats to open the valve and the orange/grey to make at the boiler

    all quite doable... but rate the boiler to the biggest zone...
     
  4. The831Bunny

    The831Bunny

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    A VR65 can be used with on/off controls for heating only with the SL connected to T4. The HW in this case is not timed, but works on HW priority if D.70 is set accordingly at the boiler. Flow temperature, priority window, KW rating of the Megaflo coil, and the HW pump over-run can then be adjusted indepentanly at the boiler D codes. The heating load, pump over-run, re ignition time delay and flow temperature can then be set to suit your heating circuits.

    In my opinion you'd be better off with a VRT 350 and a VR66 which can work with 2 heating zones and a HW zone, AND do all of the above plus more.
     
  5. fezster

    fezster

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    Thanks very much. Yes, i saw the VR66 soon after posting. I am reluctant to use vaillant controls as I hear they are quite unintuitive. I like my setup, I just want to be able to control HW flow temp independently.

    Am I right in saying that the DHW kw rating can then be set independently in the boiler ? Is this what d77 is used for (not sure if this applies to the 400 series open vent boilers).

    Would you mind just elaborating a little ? What is SL ? I assume T4 refers to the terminal on the boiler - does that mean the T3-T4 connection should be removed (and probably already is, in my setup) ?

    Thanks.
     
  6. ALEC1

    ALEC1

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    its not that they are intuitive, they technology approaches the heating conundrum differently from what you expect.

    Thats because in the controls are designed to maintain a programmed stable temperature by varying the flow temperature.

    in some ways they are very intuitive, raise the target room temp and the flow temp rises, turn it down and the flow temp lowers...

    the VR 66 is best used with a mixing valve for floor circuits, which is what it is designed for, but it also works on two heating zones..
     
  7. fezster

    fezster

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    Whats the minimum required to have the vr65 control the flow temp for dhw? Can the ch connections be left as is, and just route the dhw through the vr65?

    I plan on getting an electrician in to do this, but I want to ensure they are doing it right. I dont think this is a common setup as information is scarce.
     
  8. fezster

    fezster

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    So looking at the wiring diagrams in more detail, and reading again from above, I can't use a standard programmer with DHW if the VR65 is wired in. This is what was confusing me, as I assume the vaillant controls signal the boiler to come on via ebus at their pre-programmed times of the day.

    How then, without a programmer, would you signal the DHW to heat up ? Would it simply be on "always", and be controlled by the cylinder stat ?
     
  9. The831Bunny

    The831Bunny

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    The VR65 will control the HW in priority mode if D.70 on the boiler is set to 0. i.e. if the Megaflo is more than 5C less than the set point on any ebus control (VRT350) OR in absence of that, the set point of the DHW temp dial on the boiler. the VR65 will close the heating zone valve, open the HW zone valve, and the boiler will fire at 80C and at the Kw rating set in D.77 to heat the cylinder.

    The software attempts to heat the cylinder in 'one hit', applying the correct heat input depending on the temperature of the cylinder and even if its dropping, like when a bath is being run. This is a completely different 'logic' to the radiator heating. After the cylinder has reheated to the set point the pump overruns around the cylinder for around 2mins, the HW zone valve closes, the HTG zone valve opens, and the boiler operates for heating if there is 230V SL (switched Live) to Terminal 4.

    I would not use the Megaflo stat, use the VR10 NTC sensor which comes with the VR65. It communicates the exact temperature making HW reheating more efficient. The NTC sensor can be put in the dry pocket where the Megaflo phial goes. The system will still meet G3 regulations as the boiler now forms the overheat protection.

    Otherwise download the VR65 instructions from the Vaillant website. Just make sure they are followed by the electrician ;) You will need an ebus cable from the VR65 to the boiler. Two core 0.75mm cable is fine, don't be tempted to run ebus along spare cores in a cable that is 230V mains!

    A VRT350 can be added anytime in the future if you wanted to time your HW reheating. Personally its a matter of timing it to be off for noise if thats an issue, heat loss from a Megaflo is negligible.

    Hope that helps!
     
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  11. fezster

    fezster

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    That is extremely helpful, many thanks for taking the time.

    I'm contemplating going full WC controls. I'd like to talk to people with the 400 series boiler to see how it benefits and if there's any reduction (or increase) in heating bills. I had a vaillant engineer out today, and he was raving about it (maybe he would!). He said he had it installed in his house. The vaillant cycling blog worries me as it's the high KW 400 series boiler which had problems with the WC controls.

    I think, as you say, investing in the VR65 and rewire without the vaillant controls is probably a little pointless.
     
  12. Norcon

    Norcon

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    Disregarding the megaflows control thermostats contravenes G3!
     
  13. Agile

    Agile

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    Using a little bit of electronics you can use a simple time clock to prevent the boiler heating the HW at certain times if using an NTC temp sensor.

    Simply you use the time clock to apply a low value resistor in parallel with the sensor so the controller "thinks" the water is already up to temp. ( About 2k ohms with a 10k sensor ).

    Tony
     
  14. Steelmasons

    Steelmasons

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    No it doesn't´............and in any case main voltage can be applied through Megaflo overheat stat contacts.
     
  15. Norcon

    Norcon

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    Yes it does. The stats are there for a reason.
     
  16. Steelmasons

    Steelmasons

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    As said main voltage (end switch) can be applied via Megaflo overheat contacts.
     
  17. The831Bunny

    The831Bunny

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    This may have been done elsewhere, but did you REALLY need 38Kw?? Pity about the standard S Plan you have - Sizing a boiler with a HW priority system (VR65) means you only need to size for the largest load, possibly the cylinder, at around 18-20Kw for a Megaflo.

    The main problem with the 438 is the high output on minimum output (about 6.5Kw) and the start up output (about 26Kw) for 50 seconds REGARDLESS of the setting in D.0. This has been addressed in later release PCBs AFAIK, but is not helped by over zealous installers/customers thinking 'bigger is best'. The best and most efficient boiler is the correctly sized one!!!!

    Lastly, don't think a 5M or even a 6M pump will do, it probably won't. Due to the need to shift 27 L/min through the boiler to extract 38Kw than a larger pump than a 'domestic' model is sometimes needed.

    WC on a 400 series boiler that is oversized with the wrong pump is asking for trouble. Its like getting a Bugatti Veyron to nip down the shops... No sooner has it come on then the 26Kw has overshot the required temperature of 47C and its turned off. Size the correct pump and bypass and with the later PCB there should be no problems.

    Not seeing your system first hand, I'd still say there are benefits of going WC, but beware, AFAIK you can't run a 470 with a VR66, only a VR65. A VRT350 will run with both, but no WC, just room modulation.

    Good luck ;)
     
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