The Efficient Home Forum

Having re-installed mum's S Plan Evohome kit into her new house that has a combi boiler, I'm left with an unneeded hot water kit.

My Evohome setup is on an older oil-fired system boiler, which isn't plumbed to the hot water tank. Tank is immersion only, controlled by an old timer.
I'm wondering if I could use the hot water kit to replace the immersion timer? I'd set the immersion element's built in stat to 70 degrees (as a safety device), and use the Evohome hot water stat to keep the water at 60 degrees.
Obviously the BDR91 can't handle the load of the immersion element directly, so I'd use it to switch a 240v relay (20A plus) which would then power the element.

I'm OK with the electrical aspect of the installation, but I'm wondering if the Evohome controller can be configured to use the existing BDR91 "boiler relay" setup, and also a hot water kit?

Unfortunately you can't configure evohome to run an immersion heater when using a BDR91 as a 'boiler demand' relay with an oil boiler.

OK thanks. That would explain why I couldn't figure it out from the installation manual

I have managed to install the hot water control on a immersion hot water tank, you will need a few more bits but it isnt bad, you need an extra temperature sensor (any type) and a high power relay like this one http://www.heatingsave.co.uk/product/im ... ter-relay/

so you have to install the hot water thermostat on the tank just like boiler heated tanks. the relay will activate the high power relay which is a switch for the power into the immersion heater. so then set up stored hot water in evohome, after setting up hot water we need to unbind the bdr91 so on the bdr91 hold the button for 15 sec to unbind, then with the other temperature sensor just create a new zone with the bdr91 that is connected to the high power relay and this is your control

by doing this you can see the temperature of the hot water tank on the evohome and control it as a zone. this will mean the boiler dosnt fire when you turn o hot water.

Howard King - (or anyone)

Thanks for your solution.

Can up advise in your set up do you have a boiler option set up as well?

Many thanks to Howard for this info which has allowed me to integrate control of a hot water immersion heater into my evohome set up.

I have a slightly unusual set up in that my central heating radiators are fed from a dedicated oil boiler whilst my hot water is supplied completely separately via a vented/gravity fed cylinder which is heated by an Aga in winter and an immersion in summer.

The central heating is controlled by evohome and by using the above instructions I have been able to configure a separate bdr91 to trigger a relay which turns on the electric immersion when needed.

As advised I set up hot water using the guided configuration - binding a cylinder temp sensor and a bdr91 which is used to trigger a high current relay and thence the immersion heater. The problem with this is that whenever you switch on the hot water the system Not only correctly triggers the immersion bdr91 but also calls for heat from the central heating boiler Bdr91 causing the heating to fire up. Obviously evohome assumes that the hot water bdr91 is activating a valve not an immersion and that you also therefore need it to turn on the boiler to heat the water. Clearly perfectly reasonable in most set ups but it resulted in my central heating firing in the middle of summer whenever I asked for the immersion to go on.

As suggested to get around this I unbound the immersion bdr91. Doing this leaves the hot water temperature display on the evohome controller so you can still see the cylinder temp. To control the immersion I then created a new zone called “hot water” using the evohome controller as the sensor and bound the immersion bdr91 to this zone.
By setting the zone temp to 5C when I want the immersion off and 30c when I want it on I have easy control of my hot water. Obviously the system cannot switch off the immersion when the cylinder is at temp so I have to rely on the immersions integrated thermostat for that.

I should add that as I am supplied by octopus energy on their agile tariff my electricity costs vary every half hour. By using IFTT I am able to pull the current electricity Price point from octopus and use this data to Automatically trigger the immersion via evohome whenever the cost of electric drops below a set point. This means I can heat my hot water using consistently the cheapest electric. Indeed over the past weeks the cost of electricity has become negative for several hours a week so in effect I get paid to heat my hot water !

Hi guys, I ordered my relay months ago off the back of this thread, and it's been sitting in the house for a couple of weeks, finally decide to go about installing and there's a new comment only yesterday, what are the chances

Anyway, I'm here because I'm unsure if the relay I purchased is actually fit for purpose :https://www.google.com/url?sa=t&source= ... 1906230368

There is a wiring diagram on the second picture, it differs from the relay linked above, I'm hoping I can still use it, am I correct in thinking I wire the BDR91 to the Solar Controller part of the rely, or is this not up to the task?

It looks up to the job. Only question is that the listing specified it as being 110v. Clearly if in the uk you will need 240v.
I’ll post a link to the relay I used - it came with a BDR91 already integrated.

You would wire the BDR91 up to the solar controller terminals. It’s more than up to the task as the bdr91 (or the solar controller for that matter) won’t be drawing big current - it is just a trigger for the high amp part of your circuit from mains to immersion.

I’m not an electrician but a few points (sorry if sucking eggs etc)

You will need thermally rated cable for the relay to immersion run. As the relay will be pulling a heavy load for many hours you must make sure that all terminal connections are 100% - no stray strands of copper or loose contacts which could overheat. If you google immersion fire you will scare yourself into being paranoid on this one!

I used bootlace ferrules to make sure that contacts were all 100% and then left on for 6 hours while checking the temperature with a thermal camera to be 100%. You will also need a decent isolation switch similarly wired up but if you already have an immersion I imagine you will already have this in place.

Here’s the relay I used.

Doesn’t look like it is available now but I suspect if any interested party messaged then could be made up.
It’s expensive for what it is but it took the guess work out for me as a non sparky.

https://www.ebay.co.uk/itm/113854324728