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.
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.
Comment