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Thread: EvoHome Hot Water Kit >> Megaflow CL210

  1. #1
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    Question EvoHome Hot Water Kit >> Megaflow CL210

    Hi, I am fairly competent DIYer, but the evoHome hot water kit slightly concerns me and I could do with a sanity check.

    Background:
    I have three heating circuits., When I updated the boiler a few years ago the plumber put in three BDR91s (in a row, 2cm apart) to control the circuits with remote thermostats. The BDR91s control the valve on each heating circuit. It took some fun and games before I worked out (RTM...) that my assumption that the controller just "needed to know" which BDR91 belonged to which zone was wrong. Eventually I put them all in binding mode and then they were all bound to the controller when setting up "Boiler Control". I expected them all to come on every time any room asks for heat, but weirdly they don't. But the ones coming on don't always match where the heat is required, although in the end the heat always arrives (e.g. circuit one is demanding heat now, and the BDR for circuit 1 and 3 are on, but circuit 2 is off). Thinking this was the nearness I have separated them by 30cm, but it has made no difference. I do wonder about binding them all to one BDR91, but, it works... so I will leave it as it is. The BDR91s have power all day now because I changed the timer controls to ON for those circuits. evoHome is in charge.

    Anyhow... I just purchased a hot water tank kit to fit to my Megaflow tank. Mine is a CL210. They are very much sealed but my model has an immersion heater half way up with a pocket for a thermostat. I intend to remove the thermostat and isolate it, then stick the remote sensor in to the pocket. I can refit the immersion heater thermostat if I want, though have only needed it once in 20 years, and that was when the boiler was replaced.

    I have talked through what to do about wiring with a friend who is a commercial heating engineer (not domestic), but wanted a sanity check, so if anyone sees this a metaphorical thumbs up would be appreciated. The guarantee on the Megaflow is 25 years and I don't want to interfere with it (though it is 20 years this year!).

    This is the plan:

    1. Isolate heating system from mains.
    2. Tap L and N from the junction box where all the other BDR91 come from and take to the L and N in the new BDR91. BDR91 will now be ready to rock and roll.
    3. Wire the A and B sockets in the BDR91 in SERIES with the ORANGE (or grey) wire to the inlet valve to the megaflow.
    4. Tweak up the Megaflow stat to higher than the set point on the evoHome (I have not taken the cover off the control point on the megaflow, it is in an unbelievably awkward position, it will be possible but I am not going near it until the power is off. I only want to do it once).
    5. Remove and isolate immersion thermostat, freeing up a void half way up the tank.
    6. Push remote probe in to the void, seal lead in with something sensible, I have a vast array of rubber grommets, one should be right.
    7. Connect probe to hot water unit, pair it all up with the controller...
    8. Power on.

    As I see this, the megaflow will always think the water is too cold, so will be powering the valve. The BDR91 will then have full control over whether the valve opens or closes. If the tank gets too hot, the megaflow will cut the power to the valve.

    I have not touched anything on the megaflow to do this, so I think the warranty will be fine. (If anything goes wrong and the BDR freezes open, the wiring is identical to what it was before), I will just get hotter water.

    Does this make sense??

  2. #2
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    Quote Originally Posted by omdaddi View Post
    3. Wire the A and B sockets in the BDR91 in SERIES with the ORANGE (or grey) wire to the inlet valve to the megaflow.
    No this would be the brown wire to the Motorised valve you need to tap into. So L to A, B to MegaFlo Stat Common, MegaFlo Stat to Motorised Valve Brown. Essentially you replace what would have been your switch live from the clock with the BDR91. Don't mess with any Orange or Grey wires as these are just S/Ls for the boiler.
    Quote Originally Posted by omdaddi View Post
    4. Tweak up the Megaflo stat to higher than the set point on the evoHome (I have not taken the cover off the control point on the megaflow, it is in an unbelievably awkward position, it will be possible but I am not going near it until the power is off. I only want to do it once).
    Yes that's fine.

    Quote Originally Posted by omdaddi View Post
    5. Remove and isolate immersion thermostat, freeing up a void half way up the tank. - Wouldn't do this, the stat will be in the wrong place. Should be about 1/3 up from the bottom. Sounds like your MegaFlo is the same as mine, when you remove the cover for the wiring where the normal stat goes theres a pocket where the traditional probes are. You can fit in your EvoHome probe in there too. Will give better accuracy for HW readings.

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    Make sure the overheat stat on the megaflo still closes the Zone Valve when activated. This is a very important safety feature.

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    I think there's a lot of scaremongering about Megaflow and other similar tanks.

    IF the tank was being heated by an immersion heater, and IF there wasn't a pressure relief valve then yes, it could become a big wet bomb.

    However.... When heated via a loop of water from your boiler it's never going to get over 80 degrees or so. The only risk is a slight one of scalding from any hot taps that take a direct feed.

    As for trying to fit the sensor in, the official Evohome sensor is quite bulky. But you can buy 10k NTC thermistors which are much smaller. I use one of these: https://www.ebay.co.uk/itm/122775246996 - works perfectly, and easy to slide into a pocket alongside any existing sensors.

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    Quote Originally Posted by paulockenden View Post
    I think there's a lot of scaremongering about Megaflow and other similar tanks.

    IF the tank was being heated by an immersion heater, and IF there wasn't a pressure relief valve then yes, it could become a big wet bomb.

    However.... When heated via a loop of water from your boiler it's never going to get over 80 degrees or so. The only risk is a slight one of scalding from any hot taps that take a direct feed.

    As for trying to fit the sensor in, the official Evohome sensor is quite bulky. But you can buy 10k NTC thermistors which are much smaller. I use one of these: https://www.ebay.co.uk/itm/122775246996 - works perfectly, and easy to slide into a pocket alongside any existing sensors.
    I've seen on 3 separate occasions where the high limit stat has kicked out (So 90oc) when other components have either failed, or been installed incorrectly when been heated by a boiler. It's there for a reason. Also removing it won't comply to any regs.

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    Quote Originally Posted by paulockenden View Post
    However.... When heated via a loop of water from your boiler it's never going to get over 80 degrees or so. The only risk is a slight one of scalding from any hot taps that take a direct feed.
    I have a Megaflo 210 cylinder and it is pressure that bothers me much more than temperature. As I understand it, there is no physical diaphragm to separate the air bubble at the top of the cylinder from the water. So the air gradually dissolves into the water reducing the ability of the cylinder to act as an expansion vessel. This is different from the heating circuit on an unvented system, where we tend to use expansion cylinders with a physical diaphragm. I failed to top up the air in my Megaflo and found that the pressure in my domestic hot water plumbing had got close to the 8 bar of the cylinder's relief valve setting, without any failure on the part of a thermostat. This was just heating the cylinder from 5oC to 60oC. I found it hard to operate my kitchen hot tap when it had 8 bar behind it! I now make a routine of adding air into the cylinder, by running a bath tap, shutting the inlet valve to the cylinder and opening the relief valve to admit air. (The cylinder is at the top of the house.) I do this about once a month and it keeps the air sufficiently topped up to prevent significant over-pressure events.

    I don't mind doing this as I understand how all this works. I'm not sure my granny would be too keen on needing to top up the air in her hot water cylinder though, so it does seem a poor design.

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    I find my bubble needs recharging every 6m or so. I have a potable expansion vessel which I'll have installed at some point so I can do away with the megaflo's poor solution.

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    Quote Originally Posted by Edinburgh2000 View Post
    I have a Megaflo 210 cylinder and it is pressure that bothers me much more than temperature. As I understand it, there is no physical diaphragm to separate the air bubble at the top of the cylinder from the water. So the air gradually dissolves into the water reducing the ability of the cylinder to act as an expansion vessel. This is different from the heating circuit on an unvented system, where we tend to use expansion cylinders with a physical diaphragm. I failed to top up the air in my Megaflo and found that the pressure in my domestic hot water plumbing had got close to the 8 bar of the cylinder's relief valve setting, without any failure on the part of a thermostat. This was just heating the cylinder from 5oC to 60oC. I found it hard to operate my kitchen hot tap when it had 8 bar behind it! I now make a routine of adding air into the cylinder, by running a bath tap, shutting the inlet valve to the cylinder and opening the relief valve to admit air. (The cylinder is at the top of the house.) I do this about once a month and it keeps the air sufficiently topped up to prevent significant over-pressure events.

    I don't mind doing this as I understand how all this works. I'm not sure my granny would be too keen on needing to top up the air in her hot water cylinder though, so it does seem a poor design.
    Without going too off topic from the OP question....

    The MegaFlo has a baffle in the top of it along with the air bubble, i'm guessing that's what you mean. The issue is people don't realise/haven't been informed that the cylinders should be serviced annually and the internal or external expansions need to be checked and recharged annually along with the safety devices. They also don't seem to think that when they'e got water dripping from the discharge pipe that there is something wrong.

    To be honest the external expansions ones aren't much better, the still usually need recharging every 2-3 years and i've had a few vessels start leaking at 5 years old which isn't good.

    If you're topping up the air gap every month then you've got an issue, you need to have an external expansion fitted to the Cold inlet of the cylinder (After the pressure reducing and Non Return valve). This would prevent you having to do this so regularly. I wouldn't want to be over working the PRV on such a regular basis.

    I had one customer with a MegaFlo, 17 years old. He'd never recharged it and the air gap was still working as it should (Admittedly it maybe didn't much use)...I wouldn't of expected an External vessel to have lasted so long.

  9. #9
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    Quote Originally Posted by mtmcgavock View Post
    No this would be the brown wire to the Motorised valve you need to tap into. So L to A, B to MegaFlo Stat Common, MegaFlo Stat to Motorised Valve Brown. Essentially you replace what would have been your switch live from the clock with the BDR91. Don't mess with any Orange or Grey wires as these are just S/Ls for the boiler.
    I just checked this thread - found that the notification had gone in to spam folders.

    I did all the wiring today, but had not seen your excellent reply. I did break in to with the orange wire, I was wondering why the water temperature was staying so constant. I am hoping effectively disconnecting it for 12 hours won't have broken anything.

    What I have done now is simply put the BRD91 relay in series with the brown wire from the valve.

    Quote Originally Posted by mtmcgavock View Post
    So L to A, B to MegaFlo Stat Common, MegaFlo Stat to Motorised Valve Brown.
    The junction box had two brown wires joined. I am hoping that one of these is the L you mention and one is the MegaFlo stat common. All I did was pull one brown wire, connect it up to A, then bring B down to where the brown wire used to be.

    I have not yet looked at the MegaFlow settings at all. For historic and annoying reasons the access panel is about 150 degrees away from me, 20cm from a wall. It is very awkward. I have not looked at it. My cunning plan was to set the evoHome temp 1 degree below the temp that the MegaFLow holds it at (58 degrees) then use the run on setting on the evoHome, which I assume lets the heating overshoot for a few minutes. Assuming I do try and look at it tomorrow, I am hoping there is a big dial I can just twiddle up?

    Quote Originally Posted by mtmcgavock View Post
    Wouldn't do this, the stat will be in the wrong place. Should be about 1/3 up from the bottom. Sounds like your MegaFlo is the same as mine, when you remove the cover for the wiring where the normal stat goes theres a pocket where the traditional probes are. You can fit in your EvoHome probe in there too. Will give better accuracy for HW readings.
    I will check this tomorrow, but as I say, the access is bloody difficult.

    I am very grateful that you took the time to correct my error.
    Last edited by omdaddi; 14th November 2018 at 02:13 AM. Reason: Noticed the extra comment about the stat pocket!

  10. #10
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    Quote Originally Posted by fezster View Post
    Make sure the overheat stat on the megaflo still closes the Zone Valve when activated. This is a very important safety feature.
    How would I do this?

    Thanks for taking the time to help me out.

    (I am slowly working this out. Once I put the hot water relay in I realised I had made the (common) error of setting up the zone valves as boiler control. I have just set them up (I have three zones) by binding them at the same time when setting up the guided configuration. I worked out that there is < 10 litres in each circuit with all the TRVs closed downso am not going to worry too much.

    All a bit of an adventure really, though not sure it is going to save money. The kids are delighted they can crank up the temperature in their rooms!

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