Evohome and Resol Solar Thermal Hot Water Tank merging

[MrB]

Just interested to know if anybody has done this.

Our (new) Evohome system has the remote HW tank temp sensor for the indirect heating of the HW tank, but the HW tank is also heated by a Solar Thermal installation controlled by a Resol box. I can link to the Resol box to get it's temps and status in realtime.

The obvious problem is that the gas boiler will potentially run at the same time as the Solar because the Evohome controller gets the low temp from the remote sensor and fires up everything (which for us is just opening the HW Tank 2-way valve, which in turn gives the heat request to the boiler) at the same time as the Solar Thermal is heating the HW tank. This is obviously a seasonal issue but it does exist.

It's pretty straightforward to use the relay the Evohome controller is connected to be just a simple zero-volt relay and attach that to a Raspberry PI (or similar). Then I can merge the signals from the Resol and the Evohome and use my own software to decide if the gas boiler should be used (via a new mains relay) or to let the Solar Thermal heat everything. Which would, of course, be during the daylight hours when the hot water usage is low.

Anyone?

[paulockenden]

You could use IFTTT to switch off the evohome water heating on sunny days. Not perfect, but doable out of the box with no wiring changes.

P.

[MrB]

Paul,
I have looked, in depth, at IFTTT but I find it a bit simplistic in what can be acheived. Bit like the Controller - it does the basics, but could do an incredible amount more with the data it has.
I think that most people installing an Evohome are looking for a very granular, but integrated, control of their entire house and that is why there appears to be so many people 'rolling their own'.

[paulockenden]

You can use Maker to give IFTTT a bit more complexity. Multiple inputs and actions, for example.

It isn't what Maker is meant to do, of course, but that's no reason not to!

[electronicsuk]

I suppose it depends on your work schedules, but is it really a problem? On the average day, we only use hot water early in the mornings before work and in the evenings for washing up and the occasional shower. If this is similar to your lifestyle, I would think that having evo bring on the hot water in the mornings would be a must anyway, as there won't yet have been enough solar energy to heat the tank. You could then schedule the hot water off during the day, and on again in the evenings, by which point the solar system will already have brought the tank up to temp, which evo should sense, and the boiler won't need to fire. If there hasn't been enough solar to heat the tank, the boiler will top it off.

[MrB]

Thanks for your thoughts.
It's too broad to try and use a weather based trigger. Has to be tied to what is actually happening with the Solar Thermal system in realtime.
So I'll be going down the Rasp.Pi route...

[MrB]

Just a footnote to the above.
There are 2 basic cost justifications for installing Solar Thermal.
1) Government quarlerly RHI payment.
2) Savings from primary hot water heating, which in my case is a Gas Boiler. Therefore reduced gas usage.

That's the real reason for trying to switch of the gas boiler when the Solar Thermal easily creates 65c water temp. The max temp I've seen on the roof panels is 140c. Very seasonal of course, but worth the effort.

Shame the Evohome Controller does not have an input for this. Only needs to be able to receive a S.T. 'running' signal.

[paulockenden]

If you could somehow get your solar thermal system to produce an IFTTT trigger when heat is available (and not), it would be trivial to use this to switch the Evohome controlled hot water on and off.
P.

[emmeesse68]

Only needs to be able to receive a S.T. 'running' signal.

Is your S.T. able to drive an additional relay when active? If so, you could insert that relay to be normally closed and in series with the BDR91 that your EvoHome is using to drive the HW tank 2-way valve. This way your gas boiler would not heat your HW cylinder when your S.T. is doing so.

And, since your gas boiler gets the signal from the valve, it won't fire up at all.

Not as smart as a Rasp.Pi but damn simpler...

[MrB]

Emmeesse68,
Good lateral thinking ....

The S.T does simply use a relay to control it's own standard circulation pump so easy to take a feed off that to control a simple mains relay in parallel. No problem with extra power requirement either. Then wire it as you describe.. Nice...
Now the only problem is that S.T is upstairs and the boiler stuff is downstairs... So will have to see what is available otherwise it's back to Wireless/WiFi.
The RPI2 really is a sweet bit of kit - just £40ish for everything and I use Lazarus/FPC to create Services etc (aka Daemons). All updating a SQL server in realtime as well.

Thanks for idea...

Paul: IFTTT - you are right - that would work, but it does all the become a bit of a string, glue, plasters, Aunt Matilda's knicker elastic, etc doesn't it...

Edit:
Just be thinking about the relay.
It really gets quite good because it would be possible to have a little box next to the BDR91/Wiring Junction Box that has a mains led, mains relay and override switch. Then it's easy to see what is going on from just 1 place and if you need a boiler boost it's a simple switch flick.