Electric hot water cylinder control with ATF500DHW ?

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  • DBMandrake
    Automated Home Legend
    • Sep 2014
    • 2361

    Electric hot water cylinder control with ATF500DHW ?

    Hi All,

    Can anyone tell me whether the Evohome hot water kit can control a standard electric immerser element for a small hot water cylinder, or is it only capable of controlling boiler based heating of stored hot water, that will result in a boiler demand on the boiler relay ? (eg controlling a 2 way or 3 way valve)

    Also can anyone point me towards a copy of the detailed installation manual that comes with the kit ? I can't seem to find anything online other than the general Evohome install document itself, which devotes about one page to the hot water kit. I need more detail than that to see exactly what the kit contains and what it is capable of.

    The situation is as follows:

    Conventional gravity fed boiler with external pump and ABV for central heating, no 2 way or 3 way valves. Boiler controlled via a BRD91 configured for heat demand, and pump controlled by a pump overrun timer also from the BDR91.

    Off the boiler heat exchanger are two separate tappings for a non-pumped thermo-syphon loop to the vented hot water cylinder located above the boiler. The cylinder also has an electric immerser element with an insertion thermostat and a manual switch in the kitchen to turn the immerser on or off.

    Originally the boiler was wired to come on with the timer and only the pump was switched via the wall stat, however since changing to Evohome and a BDR91 (which replaces timer and wall stat functionality) I have rearranged things so that the BDR91 does turn on/off the boiler and also via a pump overrun timer I added, the pump. (I won't go into all the details of why I added the overrun timer just now, but suffice to say I found it necessary for the system to run smoothly with TPI switching)

    Whenever the heating is on and the boiler fires the hot water cylinder automatically "piggybacks", and in the colder months the hot water is adequately hot during the day. (Or potentially too hot if you set the flow temperature above 70 degrees as there is no way to stop the thermo-syphon flow except a manual valve)

    The problem is when the weather gets warmer like it is now, the boiler is on so little that we need to manually switch on the immerser for hot water, but of course we always forget until it's needed. As we have a dishwasher that heats its own water and an electric shower we don't very often need hot water from the cylinder except for washing hands or running the occasional bath but when we do need it its annoying to discover it's cold.

    So what I was thinking about is whether the hot water kit could allow me to schedule the control of the immerser rather than the boiler. The idea would be for the relay that comes with the kit to indirectly switch the immerser via another high power relay (I assume the one that comes with it is just a BDR91 designed to control a 3 way valve, and therefore not up to the job of switching a heating element of that wattage) and in addition I would take the live supply for the relay contact of this BDR91 from the normally closed contact of the heating BDR91 instead of direct live.

    The idea behind that is any time the Evohome fires the boiler for heating it would disable the immerser, and only allow the immerser to come on when the heating was not currently running.

    Therefore in the cold months when the heating is on during the time hot water is scheduled the immerser would not actually come on because the switched live from the first BDR91 would not be passed through to the second BDR91 - the hot water will "piggyback" on the heating demand via the boiler loop as it does now, however if the heating was not currently on when hot water was scheduled the element would come on instead.

    TPI modulation of the boiler for heating would inversely modulate the immerser - eg if the boiler demand BDR91 was on 20% of the time for a low heat demand, the immerser would have power for 80% of the time when hot water was scheduled to allow it to still reach temperature, while if the boiler demand BDR91 was on 80% of the time the immerser would only be on 20% of the time. This gives the boiler a chance to heat the water first (over the immerser) if it is active but allows the immerser to do the job if the boiler is not going to be on enough to reach the required hot water temperature.

    The problem I can see with this plan is I suspect the hot water control of the Evohome has been designed only with boiler control and 2/3 way valves in mind, thus it would always fire the boiler relay for hot water demand even though I was only wanting to turn on the electric element ? Can anyone advise if this is the case ?

    If I can't do this, then I guess I'd either have to get the system re-piped as a pumped hot water system with a 3 way valve, (which is probably not worth the effort on an old system with an uncertain remaining life time) or give up and put up with what I have now.
    Last edited by DBMandrake; 24 April 2016, 11:49 PM.
  • bruce_miranda
    Automated Home Legend
    • Jul 2014
    • 2307

    #2
    The hw kit is a simple temperature sensor (albeit a special kind) which triggers a bdr91 based on time or temperature.

    You can use the bdr91 to do whatever you want. Except dont try to run your 3kw immersion directly via it.

    When you add the hw kit you get an option for 1 or 2 zone valves. This is different from Boiler Demand.

    So you can use the hw bdr91 to run your immersion and the ch bdr91 to fire your boiler.

    Comment

    • The EVOHOME Shop
      Site Sponsor
      • Dec 2014
      • 483

      #3
      Originally posted by bruce_miranda View Post
      The hw kit is a simple temperature sensor (albeit a special kind) which triggers a bdr91 based on time or temperature.

      You can use the bdr91 to do whatever you want. Except dont try to run your 3kw immersion directly via it.

      When you add the hw kit you get an option for 1 or 2 zone valves. This is different from Boiler Demand.

      So you can use the hw bdr91 to run your immersion and the ch bdr91 to fire your boiler.
      As DBMandrake has a 'WIRELESS RELAY BOX' for his 'BOILER CONTROL' any BDR91 controlling an immersion heater (via a contactor) for hot water would fire the 'BOILER CONTROL' relay whenever there was demand for hot water also. There are ways to do gravity hot water systems using a 3x BDR91 relay system and a normally open motorised zone valve (which I have done in the past) and this works well giving independant control of hot water.

      On a system without 'BOILER CONTROL' then yes, you can have hot water control of anything - in my house I have a second evohome Controller controlling our heat pump and buffer cylinder with the hot water kit.

      Comment

      • DBMandrake
        Automated Home Legend
        • Sep 2014
        • 2361

        #4
        Originally posted by The EVOHOME Shop View Post
        As DBMandrake has a 'WIRELESS RELAY BOX' for his 'BOILER CONTROL' any BDR91 controlling an immersion heater (via a contactor) for hot water would fire the 'BOILER CONTROL' relay whenever there was demand for hot water also.
        Thanks for confirming that, that's what I was afraid of. Yes the BDR91 is currently configured as "Boiler Control".
        There are ways to do gravity hot water systems using a 3x BDR91 relay system and a normally open motorised zone valve (which I have done in the past) and this works well giving independant control of hot water.
        Can you elaborate on this ? I think I know what you're driving at here but would like to be sure.

        If I were to configure the system for "S plan", what if I had two BDR91's, with the CH "Zone valve" BDR91 actually controlling the boiler directly in place of a "Boiler Control" relay, (eg rebind the existing BRD91 as CH instead of Boiler Control) and the HW "Zone Valve" controlling both the contactor for the immerser and the motorized zone valve that blocks the thermosyphon loop from the boiler ?

        When configured in this mode I'm aware that you can't adjust the minimum on time and cycle time for the HW and CH relays, however can you confirm whether the CH relay is still TPI modulated and if so with what minimum on time and cycle period ? If its just the default 6 cycles per hour and minimum on time of 1 minute I can live with that even if I can't adjust it.

        Or do I lose TPI modulation completely if I directly fire the boiler from the CH relay ? If it just comes on continuously even when there is a small heat demand then that would be unacceptable for me.

        I'm curious to know why a normally open motorised valve would be required instead of normally closed ? If I understand what you're suggesting properly when the system is off (heating and hot water off) the motorised valve would be permanently powered to keep the valve shut ? Do they use much power ?

        I'm also curious to know what the 3rd BDR91 is doing in your suggested configuration ?
        Last edited by DBMandrake; 27 April 2016, 09:17 PM.

        Comment

        • paulockenden
          Automated Home Legend
          • Apr 2015
          • 1719

          #5
          You still get TPI in s-plan mode. I reckon the minimum on time is possibly less than a minute, as sometimes it isn't enough to fire my boiler (because it doesn't seem to allow for the delay in the motorised valve opening before the boiler gets power).

          P.

          Comment

          • DBMandrake
            Automated Home Legend
            • Sep 2014
            • 2361

            #6
            Originally posted by paulockenden View Post
            You still get TPI in s-plan mode. I reckon the minimum on time is possibly less than a minute, as sometimes it isn't enough to fire my boiler (because it doesn't seem to allow for the delay in the motorised valve opening before the boiler gets power).
            Ok that's good to know. In my example I wouldn't have the delay of an actual zone valve for CH as it would be powering the boiler directly. And my boiler starts very quickly - it's an old pilot light type that ignites instantly and is producing useful heat in about 10 seconds...

            Comment

            • The EVOHOME Shop
              Site Sponsor
              • Dec 2014
              • 483

              #7
              Originally posted by DBMandrake View Post
              Can you elaborate on this ? I think I know what you're driving at here but would like to be sure.

              If I were to configure the system for "S plan", what if I had two BDR91's, with the CH "Zone valve" BDR91 actually controlling the boiler directly in place of a "Boiler Control" relay, (eg rebind the existing BRD91 as CH instead of Boiler Control) and the HW "Zone Valve" controlling both the contactor for the immerser and the motorized zone valve that blocks the thermosyphon loop from the boiler ?

              When configured in this mode I'm aware that you can't adjust the minimum on time and cycle time for the HW and CH relays, however can you confirm whether the CH relay is still TPI modulated and if so with what minimum on time and cycle period ? If its just the default 6 cycles per hour and minimum on time of 1 minute I can live with that even if I can't adjust it.

              Or do I lose TPI modulation completely if I directly fire the boiler from the CH relay ? If it just comes on continuously even when there is a small heat demand then that would be unacceptable for me.

              I'm curious to know why a normally open motorised valve would be required instead of normally closed ? If I understand what you're suggesting properly when the system is off (heating and hot water off) the motorised valve would be permanently powered to keep the valve shut ? Do they use much power ?

              I'm also curious to know what the 3rd BDR91 is doing in your suggested configuration ?
              Sorry, firstly I was supposed to say 'normally closed' for the valve type which is fine to go on the gravity return pipe of gas and oil boilers on gravity circuits. Then use 1x BDR91 for the hot water motorised valve, 1x BDR91 for the heating pump (most gravity systems have just a pump for the central heating and not a heating motorised zone valve) and 1x BDR91 just to fire the boiler as a 'BOILER CONTROL' relay.

              You could use an S Plan configuration of 1x BDR91 for the boiler 'heating' switching on (but would have the standard 6 cycles per hour and 1 minute min on time which is not good for oil boilers) and 1x BDR91 for the immersion heater through a contactor. This would work on a gas boiler fine.

              The problem you have with S Plan/Y Plan configuration on a gravity system with a 'pumped' heating circuit is the backfeed of power to the heating circuit pump on hot water demand, hence we put the pump on its own BDR91 (comes on whenever there is HR92 demand) and the boiler on its own BDR91 (fires the boiler for any demand).

              Hope that makes sense?

              Comment

              • paulockenden
                Automated Home Legend
                • Apr 2015
                • 1719

                #8
                Is a 3x BDR91 setup officially supported Richard? I might be wrong but I seem to remember Top Brake saying not to do it some time ago. Perhaps I misremembered.

                Comment

                • The EVOHOME Shop
                  Site Sponsor
                  • Dec 2014
                  • 483

                  #9
                  Originally posted by paulockenden View Post
                  Is a 3x BDR91 setup officially supported Richard? I might be wrong but I seem to remember Top Brake saying not to do it some time ago. Perhaps I misremembered.
                  Good memory Paul but thankfully this was 'fixed' in the last firmware update!

                  Comment

                  • paulockenden
                    Automated Home Legend
                    • Apr 2015
                    • 1719

                    #10
                    Originally posted by The EVOHOME Shop View Post
                    Good memory Paul but thankfully this was 'fixed' in the last firmware update!
                    So does that mean an S-Plan system can now use a 3rd BDR91 for boiler control, rather than using the microswitches in the two port valves?

                    If so, I think that's big news - surprised it hasn't been shouted from the rooftops.

                    Comment

                    • The EVOHOME Shop
                      Site Sponsor
                      • Dec 2014
                      • 483

                      #11
                      Indeed, hence you can also have the R8810 OpenTherm Bridge and S & Y Plan now too!

                      Get shouting...

                      Comment

                      • paulockenden
                        Automated Home Legend
                        • Apr 2015
                        • 1719

                        #12
                        Originally posted by The EVOHOME Shop View Post
                        Get shouting...
                        Or perhaps, "get re-wiring". ;-)

                        Comment

                        • DBMandrake
                          Automated Home Legend
                          • Sep 2014
                          • 2361

                          #13
                          Originally posted by The EVOHOME Shop View Post
                          Sorry, firstly I was supposed to say 'normally closed' for the valve type which is fine to go on the gravity return pipe of gas and oil boilers on gravity circuits. Then use 1x BDR91 for the hot water motorised valve, 1x BDR91 for the heating pump (most gravity systems have just a pump for the central heating and not a heating motorised zone valve) and 1x BDR91 just to fire the boiler as a 'BOILER CONTROL' relay.

                          You could use an S Plan configuration of 1x BDR91 for the boiler 'heating' switching on (but would have the standard 6 cycles per hour and 1 minute min on time which is not good for oil boilers) and 1x BDR91 for the immersion heater through a contactor. This would work on a gas boiler fine.

                          The problem you have with S Plan/Y Plan configuration on a gravity system with a 'pumped' heating circuit is the backfeed of power to the heating circuit pump on hot water demand, hence we put the pump on its own BDR91 (comes on whenever there is HR92 demand) and the boiler on its own BDR91 (fires the boiler for any demand).

                          Hope that makes sense?
                          Yes your description of the 3 BDR91 configuration for gravity hot water makes perfect sense to me. CH BDR91 controls the pump, HW BDR91 controls a zone valve in the gravity heating loop (to prevent hot water getting too hot during CH demand) and Boiler demand BDR91 controls the boiler for both.

                          I'm not going to use that approach though as my system is not happy with the boiler running with the CH pump stopped. Yes I know that's how gravity fed hot water systems were originally designed to operate, and when we moved into the house the conventional timer powered the boiler whilst the wall stat only switched the pump off - hence when the (single) timer was on the boiler would continue to cycle on and off for hot water.

                          The problem with that is I found it made the system very noisy due to steam bubble generation in the boiler when the pump stopped - every time the pump came back on there would be a swath of noisy steam bubbles rushing around the radiators for a minute or so. In fact at higher flow temps (over about 70) it would sometimes vapour lock the system some time after the pump stopped due to the heat rise in the boiler with the sudden loss of water flow. When the wall stat came back on the pump would be chewing on steam for up to a couple of minutes until it could eventually get itself primed. Not good for the pump!

                          Limescale build up in the boiler is probably a good part of the problem causing steam generation at lower than normal temperatures, but I've already done what I can to clean it (chemically) and while it improved matters a lot it didn't completely solve the problem.

                          The rapid TPI cycling the BDR91 does compounded the problem greatly (since the original wall stat would only switch about once an hour) causing radiator noise and vapour lock to be a big problem so I decided to rewire things so that the BDR91 switched the boiler and the pump ran from the boiler power via a pump overrun timer. So the boiler never runs without the pump now and the pump runs on for a few minutes each time TPI turns off the boiler which prevents temperature rise in the heat exchanger when TPI cycles off. (It also helps distribute the heat to the radiators during the off part of the cycle) No more vapour lock even if I run the boiler at its maximum 80 degrees and no more noisy radiators unless I go right up to 80, (which I never do) and even then there is only a little bit of occasional noise. The system has been running really smoothly and quietly like this for the last 6+ months.

                          A non-standard configuration for an old gravity fed system perhaps but pragmatic as it makes the best use out of what I have. Not ideal for hot water, as that now just piggybacks on CH use - but in winter that's all I need, and the hot water actually still used to get too hot in winter despite the boiler never running without the pump.

                          So the approach I want to take is to use two BDR91 in S-Plan mode, with the CH relay controlling the boiler directly (and also the trigger input for the pump overrun timer) and the HW relay controlling both the contactor for the immerser and a two port zone valve which I will add to the gravity hot water loop. The reason for the zone valve is that in winter with the flow temperature high (70-75) the hot water actually used to get too hot - scalding hot, so the zone valve would let the Evohome block the gravity loop when the cylinder is up to temperature and CH is still on...

                          I've worked out that with one extra (standard) relay I can wire things so that the gravity loop zone valve is always controlled by the HW BDR91, however the immerser contactor is only powered by the HW BDR91 if the CH BDR91 is off.

                          This will give priority to gravity hot water over the immerser, thus avoiding unnecessary use of electric hot water heating when the gas central heating is already running. EG if CH is on, the immerser is never allowed to run, but the HW Zone valve is allowed to open in the gravity loop. However if CH is off, the contactor will be powered via the HW relay.

                          In fact if I want to be a cheapskate I can do it without an extra relay at all - making use of the otherwise unused limit stop switch in the zone valve. The HW BDR91 supplies power to the zone valve, then I take the normally closed output from the CH BDR91, (live when CH is off) feed that to the limit stop switch in the zone valve and then from there to the contactor... Thus when the zone valve is open and the CH is off, the contactor gets power. (Of course if the zone valve fails I don't get electric hot water either, whereas a separate relay would allow this to still work...)

                          So can you clarify a couple of things for me about S plan mode with two or three BDR91's ?

                          In S Plan with only a CH and HW BDR91 (no Boiler control BDR91) you say the CH relay still does TPI at 6 cycles per hour and 1 minute min on time - if so that is fine. Although I would have liked to keep the ability to adjust these parameters I have already experimented with them before and ended up back to the defaults.

                          I assume the HW BDR91 does not do any sort of TPI and simply turns on when hot water is scheduled and the cylinder is below the cut in temperature, and turns off when above the cut out temperature ?

                          If there is a 3rd, Boiler control relay configured in S-Plan mode, I presume that will also do TPI, however does the CH BDR91 still do TPI in a three BDR91 configuration like this, or does it revert to just on for any heating demand and off for no demand and let the Boiler control relay do the TPI ?

                          If the former (both do TPI) what would happen if you adjusted the minimum on time and cycles per hour for the Boiler control relay - would the CH relay continue to do TPI at 1 minute and 6 cycles (thus be out of time with the other) or would it follow the same cycling timing that you configured for the boiler control relay ?
                          Last edited by DBMandrake; 12 May 2016, 10:29 AM.

                          Comment

                          • DBMandrake
                            Automated Home Legend
                            • Sep 2014
                            • 2361

                            #14
                            Just an update to this.

                            Although this hybrid zone valve and immerser approach should in theory work, I decided against it due to the relative complexity and the fact that I would still have the slow reheat speed of gravity hot water circulation when the central heating is on (and thus inhibiting the immerser) and the greater expense of heating hot water with the immerser on a single rate electricity plan when the boiler is not in use for central heating.

                            So in the interim I'll be converting the gravity circulation loop to a "C Plan" arrangement in the next couple of weeks by adding a 2 port Honeywell zone valve in the indirect return loop, controlled by a strap on cylinder temperature sensor. This will solve the problem of scalding hot water in winter when the central heating flow temperature is set high and the heating is running at a high duty cycle as the zone valve will be able to close the indirect loop when the cylinder is up to temperature.

                            I haven't decided yet whether I will use the Evohome hot water kit to do this or just make use of a spare digital temperature controller which I already have on hand.

                            It doesn't solve the issues of hot water not being heated when central heating is off (except by manually switching on the immerser) nor the expense of running an immerser when central heating use isn't sufficient to get the water hot enough, so I've decided the longer term solution to be implemented next summer is to convert the existing cylinder from the C Plan that it will shortly be to a fully pumped S plan arrangement, by adding a second 2 port zone valve and doing a bit of re-piping around the pump and the cylinder, which is directly above the pump.

                            When I do that I will add the Honeywell hot water kit to control it if I don't already have it by then. I currently use an external digital flow temperature thermostat for the boiler, I'm thinking that I will add a second identical flow temperature thermostat so that when the call for heat from the boiler is for hot water, I can independently set the flow temperature for cylinder re-heat - for example in warmer weather I could have the central heating flow temperature set to 55, which is insufficient to heat hot water, but the cylinder reheat temperature set to for example 70 - that way I get a minimum flow temperature set by the re-heat thermostat when hot water re-heat is required, but it will then go back to the central heating flow temperature (lower or higher as the case may be) when only central heating is required. If both hot water and central heating are demanded at the same time it will run at the higher of the two set temperatures.

                            I may get a plumber to come in and do the actual re-piping work for me as it will require some fiddly re-arrangement of several sweated joints due to the somewhat cramped physical location of the piping and short distance between the pump output and the T joint that feeds the upstairs and downstairs radiators - which doesn't leave much room for a zone valve without moving the pump further back and relocating the automatic bypass valve. I've looked at it and I think I probably could do the work OK myself but might not feel confident tackling so many sweated joints in one go unless I've had a bit more practice with them by then. I'll be using one right angle sweated joint to add the C Plan zone valve when i fit that shortly which will give me some practice - but I can make that joint away from the system clamped in a workbench with nice easy access, which is not quite the same as doing it in cramped conditions with other pipes and wires getting in the way...
                            Last edited by DBMandrake; 5 October 2016, 12:12 PM.

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