Vented to unvented project with Evohome using Megaflo cylinder

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  • Ubernoob
    Automated Home Lurker
    • May 2016
    • 7

    Vented to unvented project with Evohome using Megaflo cylinder

    I've had Evohome for about 12 months now on my Y-plan vented system (heating and water controls). It works ok most of the time but I do get some phantom switching on of the heating shortly after the heating program has finished - I've put this down to the heating BRD91 having not great reception which I will fix with this project.

    What I'm doing (with the aid of a couple of plumbing friends) is moving to a Megaflo unvented cylinder system, and moving the cylinder to the loft whilst I'm at it, allowing a bigger bathroom. We start work tomorrow.

    I upgraded the Evohome system myself, being reasonably ok with electrics and will be doing the electrics on the new system. But things are a little more complicated as the Megaflo cylinder comes with its own 2-port valve for the HW control, and the manufacturer wants installers to use the Megaflo's water thermostat with the HW 2-port valve, this is a safety requirement (to stop the cylinder overheating) - this all seems sensible. But, this creates a problem for my Evohome installation in two ways: 1) Evohome wiring likes to control the HW itself; 2) I can't see anywhere to fit the Evohome HW thermostat on the Megaflo cylinder. I've spoken to Megaflo who confirm there is not an auxiliary pocket to fit a 2nd thermostat. Their recommendation was to fix the sensor to the HW outlet at the top of the cylinder.

    So I thought I'd ask the forum if they have any ideas (I have done some searching but couldn't find any definitive guidance, and I don't really want to drill a hole in the outerskin of my nice new water tank)! I get that the Megaflo sensor should be fine for controlling the internal water temperature, but I like seeing what temperature the water is in Evohome. Also, I want to use Evohome as the programmer to control when HW turns on / off (I don't want it on all the time)! Can I use the HW BRD91 without a HW sensor attached and have it acting just as a programmer? And one last point of complexity, I have pv solar running off a solar switch (that diverts spare electricity to the immersion) - I don't see this as a problem as the solar switch is on the immersion supply.

    Can anyone offer any words of wisdom on how best to control HW with the new system?

    Thanks - Adrian
  • g6ejd
    Automated Home Guru
    • Oct 2016
    • 153

    #2
    You could fit the (ATF500DHW) supplied sensor head to some exposed tank or strap it (cable tie) to the output pipe and adjust the temperature on the Evohome to compensate for the slightly higher water temperature at that position, or lower if losses are giving a lower reading than water temperature.

    As the manufacture insists on their thermostat being the safety switch, then wire theirs in series with the A (ABC) terminal of the BDR91, then if the manufacturers thermostat goes open on over temperature, then that removes the supply from the BDR91 boiler feed and there can be no more water heating until the tank has cooled. That's what I would do. Well all this assumes the supplied thermostat is a simple on/off switch. Set the in-built thermostat to some maximum 'safety' temperature and let the Evohome control water temperature within normal limits.

    Controlling the in-built valve is no different from an external one, so direct to A and C on the BDR91.

    Comment

    • Ubernoob
      Automated Home Lurker
      • May 2016
      • 7

      #3
      Originally posted by g6ejd View Post
      You could fit the (ATF500DHW) supplied sensor head to some exposed tank or strap it (cable tie) to the output pipe and adjust the temperature on the Evohome to compensate for the slightly higher water temperature at that position, or lower if losses are giving a lower reading than water temperature.

      As the manufacture insists on their thermostat being the safety switch, then wire theirs in series with the A (ABC) terminal of the BDR91, then if the manufacturers thermostat goes open on over temperature, then that removes the supply from the BDR91 boiler feed and there can be no more water heating until the tank has cooled. That's what I would do. Well all this assumes the supplied thermostat is a simple on/off switch. Set the in-built thermostat to some maximum 'safety' temperature and let the Evohome control water temperature within normal limits.

      Controlling the in-built valve is no different from an external one, so direct to A and C on the BDR91.
      Thanks for the quick response. Using the supplied sensor to the outlet pipe sounds like a great idea (it's probably not going to be 100% accurate but providing it allows the switching of the boiler I think it will be fine), wiring in series sounds sensible too, it maintains the OEM's safety feature and adds another. I'll let you know how I get on.

      Comment

      • bruce_miranda
        Automated Home Legend
        • Jul 2014
        • 2307

        #4
        I'm not so sure about putting the sensor on the outlet. That pipe will get hot only when hot water is actually flowing out. You want the temperature of the water sitting inside the cylinder.
        I had the same problem with my vaillant unistor. The way I over came this was the dry pocket is actually deep enough to take both sensors. I still have my unistor sensor that is set at 65C, my evohome sensor is pushed into the same pocket and that is set to cut off at 60C. So i retain both the safety of the vaillant controls that will shut the valve if it ever gets to 65C. But because evohome never lets the temperature get anywhere close the valve is under control by the evohome only. You could try either the dry pocket of the main NTC or the immersion thermostat.

        Comment

        • DBMandrake
          Automated Home Legend
          • Sep 2014
          • 2361

          #5
          Originally posted by g6ejd View Post
          You could fit the (ATF500DHW) supplied sensor head to some exposed tank or strap it (cable tie) to the output pipe and adjust the temperature on the Evohome to compensate for the slightly higher water temperature at that position, or lower if losses are giving a lower reading than water temperature.
          This won't work well at all because the Evohome won't know that your hot water needs heating until you have completely run out of hot water. This is why the strap on sensor is mounted only 1/4 to 1/3 of the way up, so that it detect cold water entering the bottom and fires up the boiler long before the hot water is all used up. Also if no hot water is used for a while the pipe at the top will cool down and cause it to falsely think the hot water is cold and bring it on until the megaflows own safety sensor cuts out. Not recommended!
          As the manufacture insists on their thermostat being the safety switch, then wire theirs in series with the A (ABC) terminal of the BDR91, then if the manufacturers thermostat goes open on over temperature, then that removes the supply from the BDR91 boiler feed and there can be no more water heating until the tank has cooled. That's what I would do.
          As long as the current limits of the BDR91 are not exceeded this should be OK. If it's just supplying a fairly standard motorised zone valve it will be fine.

          Comment

          • bruce_miranda
            Automated Home Legend
            • Jul 2014
            • 2307

            #6
            Originally posted by g6ejd View Post
            As the manufacture insists on their thermostat being the safety switch, then wire theirs in series with the A (ABC) terminal of the BDR91, then if the manufacturers thermostat goes open on over temperature, then that removes the supply from the BDR91 boiler feed and there can be no more water heating until the tank has cooled. That's what I would do. Well all this assumes the supplied thermostat is a simple on/off switch. Set the in-built thermostat to some maximum 'safety' temperature and let the Evohome control water temperature within normal limits.

            Controlling the in-built valve is no different from an external one, so direct to A and C on the BDR91.
            In fact this is exactly what I did. But I took a short cut. My wiring was a typical S plan wiring where the dual channel timer clock was wired in series to the cylinder thermostat for HW and room stat for CH (as is always the case). I just connected my HW and CH BDR91s in parallel to the dual timer clock. Set the timer clock to OFF. But it's there, just in case the evohome decides to pack up one day. Turn the old room stat to 25C and the cylinder stat to 65C. That way they are both normally always on. Set the evohome room temperatures to below 25C and the HW kit to below 65C and everything works nicely with the least wiring or plumbing changes. Given that nowadays most electrical wiring boxes are protected with a 5A fuse, the BDR91s are more than capable to driving most things within the heating system. Just don't try and run the immersion heater directly with the BDR91 relay.

            Comment

            • g6ejd
              Automated Home Guru
              • Oct 2016
              • 153

              #7
              Megaflow have a scare tactic on their page: http://www.megaflo-advice.co.uk/Mega...a_Megaflo.html

              It looks like the cylinder has a metal jacket, I've seen these being manufactured on the Discovery Channel 'How do they do it?' and can see that adding a sensor to it will be a challenge, perhaps use the heat exchange pipe for sensor attachment, the copper will be close to the water temperature at a low point in the tank, then it needs calibrating for actual temperature.

              Comment

              • g6ejd
                Automated Home Guru
                • Oct 2016
                • 153

                #8

                Comment

                • Ubernoob
                  Automated Home Lurker
                  • May 2016
                  • 7

                  #9
                  Originally posted by g6ejd View Post
                  Thanks. Unfortunately my Megaflow (which I purchased May 2016) does not have a sensor dry pocket (I have contacted Megaflow tech support who confirmed this).

                  The system is in and working, I placed the Evohome DHW sensor behind the safety pressure valve (behind the polystyrene insulation foam). It's not perfect, the sensor reads 38 degrees here where the tap water is 50 degrees. I wired the DHW from the BRD91 to the 2-port valve in series with the Megaflo thermal cut out and that worked fine. I validated the cut off is working by calling for hot water and turning the cylinder thermostat down, the 2-port shut and the pump / boiler cut out. The design is slightly compromised as I'm not getting a true DHW water temp. but then again I wasn't in my old vented cylinder (as this was close to the base of the cylinder). I am having to rely on the Megaflo thermostat to control DHW temperature, but I don't think that's a major problem (I still get an indication on Evohome).

                  Comment

                  • DBMandrake
                    Automated Home Legend
                    • Sep 2014
                    • 2361

                    #10
                    Originally posted by Ubernoob View Post
                    I placed the Evohome DHW sensor behind the safety pressure valve (behind the polystyrene insulation foam). It's not perfect, the sensor reads 38 degrees here where the tap water is 50 degrees.

                    [...]

                    The design is slightly compromised as I'm not getting a true DHW water temp. but then again I wasn't in my old vented cylinder (as this was close to the base of the cylinder).
                    Are you sure that your reading is incorrect ? It's to be expected that when you measure the temperature of the hot water towards the bottom of the cylinder (honeywell recommends 1/4 to 1/3 of the way up from the bottom for a strap on sensor) that the reading will be considerably below the output temperature of the cylinder as soon as you start using hot water and thus draw cold water in through the bottom.

                    This is due to stratification of the hot water and was just recently discussed in the thread below, where I was caught out by the same misunderstanding:



                    This lower reading after using hot water is as it should be, and it's correct to measure the temperature this low down the cylinder so that the system can detect the cold water entering the bottom and start reheating well before you actually run out of hot water. In an ideal world it would reheat the cold bottom section of the tank before that volume of water reached the top - thus you get uninterrupted hot water coming out the top at the correct temperature the whole time. (If your cylinder can reheat fast enough to keep up with demand)

                    The only time the temperature reading taken down the side will equal the actual hot water output temperature is during heating (when there is ongoing convection in the cylinder to break the stratification) and after heating has finished but before any hot water has been drawn off. As soon as you start using hot water and before reheating commences the reading may drop as much as 20 degrees or more below the actual cylinder output temperature due to cold water entering the bottom.

                    If your reading is still a lot lower at the end of a reheat cycle then you may indeed have a problem with the thermal conductivity to the tank. I found with my vented copper cylinder that even scrubbing the copper "clean" with a scourer was not enough - the reading was about 2 degrees below actual hot water temperature immediately following a reheat - I found there was a very thin invisible lacquer layer over the copper insulating the strap on sensor slightly, after sanding that off with sand paper to get good metal to metal contact my hot water temperature measured at the tap is within about +/- 1 degree of what the Evohome says immediately after a reheat has finished, but when using hot water within just 2-3 minutes of running the tap the reading can drop down into the 30's even though the temperature at the tap has not changed, due to the bottom part of the cylinder now being cold but the top still being hot.
                    Last edited by DBMandrake; 27 October 2016, 11:05 PM.

                    Comment

                    • originaltwist
                      Automated Home Lurker
                      • Sep 2015
                      • 3

                      #11
                      I wonder if Megaflo gave you the option of having extra sensor pockets when you ordered the tank? These days I always add a few extra sensor pockets to the Xcel tanks I use. There are some nice tricks for tanks combined with home automation; for example you can de-stratify the tank when it gets really hot at the top and thus effectively increase the tank capacity.

                      Comment

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