We had our Vaillant Arotherm Plus system installed last week and I have been spending the weekend familiarising myself with the data available in emoncms. British Gas did the install including the emonpi kit.
All the data makes sense except the DHW data and perhaps the error value.
Other than one period of about 15 minutes duration the DHW value has remained at 0. The 15 minute active period occurred at 11:00 am approximately 24 hrs after the system was powered up. The tank is meant to be heated each night between 00:30 and 04:30 to take advantage of our Octopus Go tariff.
We have definitely had a number of DHW heating cycles given have had several hot showers from the tank.
My plan is to trigger a manual DHW boost and see if the emonpi picks this up but I suspect it won’t so wondered if there are any tips to help me diagnose the problem?
If it is something on the low voltage side of things I will fix it myself but if to do with the 3 port valve I will have to get British Gas back in.
I have tried to find information on the DHW sensor on the site but found nothing helpful.
A CT clamp used to be used, clipped around the switched live of the 3 port diverting valve. but recently a optoisolator has been adopted for DHW cycle detection and is connected to the switch live and neutral of the 3 port diverting valve. Do you know what type/make of diverting valve you have?
Hi Robert,
Great to hear you’ve got a heat pump installed! Thanks for installing our monitoring.
It looks like the optical sensor has not been connected correctly, it should be connected to the switched live terminals which control the diverter valve. The instructions for installing the sensor are in the printed installation guide: https://files.openenergymonitor.org/emonhp-pi2.pdf
On a Vaillant this is UV1 (Auf) :
This is an optoisolated 240V sensor
Just to help clarify and hopefully not confuse! There are basically two types of 3 port diverting valves - Spring return, these have a neutral and switched live, which switches the valve to the DHW position, then the spring returns it back to the HTG position when switched off. The other type is a MOMO valve (motor on/motor off), these have a neutral and two live wires and could be either 2-point or 3-point controlled. The 2-point has one switched and one permanent live, the switched live will generally be used to move the valve to the DHW position and the permanent live will return it to the HTG position. The 3-point has two switched lives, one for the DHW position and the other for HTG position. In all of the above the optoisolated 240V sensor would be connected to the neutral and DHW switched live. As Glyn said it’s probably been connected incorrectly!
Thanks for the very detailed suggestions.
All I can tell you about the 3 port valve is that it is a Honeywell Home unit which is wired to a spur point along with the DHW sensor.
I will take the cover off the spur point later and have a look once the British Gas people leave… they are coming back to finish lagging and sort a few other things out but the electrician won’t be there as he is on holiday this week 
I would not be surprised to find that he has not connected it correctly as he struggled to follow the installation instructions and initially never routed the power through the electric meter and had no idea where the DHW sensor connected. In hindsight I should have worked with him on it but thought once I had explained everything he had got it.
The electrician was a really nice guy and has done a great job with everything else as far as I can tell just totally unfamiliar with this kind of thing.
Once again thanks for all the help and all the kit which has already let me verify the knocking sound I heard this morning is due to loads of air in the system 
The Honeywell will be a spring return valve. The DHW optoisolator sensor may be connected at the heat pump control board (usually) as per the diagram on Glyn’s post, or at the spur box.
The OEM kit comes with some instructions for things to do to help remove the air from the system, I’m sure it’s available on their website and dare say numerous posts on the forum.
Good luck.
Finally got a chance to look at this.
I took the cover of the spur point and there are only three wires neutral, switched live and earth. The DHW optoisolator is connected between neutral and switched live as it should be.
I then took the cover of the DHW optoisolator box. The green LED was off. I told the system to do a hot water boost and the green LED came on. This clearly indicates that the switched live signal is being connected.
I checked the data on emoncms and saw the DHW flag was still zero. I then got my trusty multimeter and measured the voltages going into the emonPi. Both the red and blue connections were at 3.3v with respect to the green wire. When I subsequently switched the hot water boost off the voltages all remained the same but the green LED did go out. Presumably the blue wire is the signal wire and has a pull up resistor tied to the 3.3v supply. I would guess a phototransistor is meant switch on when the switched live is active and draw current through the pull up resistor to drop this voltage.
If my assumptions on how this is meant to work are correct then I guess it is a question of where the pull up resistor resides. If it is in the optoisolator circuitry then the fault lies in the optoisolator side of things but if the pull up resistor lies in the emonPi then there may well be a break in the signal cable.
What do you guys reckon?
I can’t find any documentation regarding this item, so it’s a question for @glyn.hudson or @TrystanLea .
Excellent! That means it’s wired correctly, it could just be plugged into the wrong port on the emonPi? It should be plugged into the ADC input which is next to the USB power
The Red wire is VCC, Green is GND and Blue is signal.
Sorry deleted my last post as I realised it included my QR code which was probably a bad move. Anyway photo attached that I hope shows the DHW is connected to the correct socket
You mentioned in your original post that the DHW sensor did trigger once when first installed and have confirmed that it all appears to have been connected correctly, which would suggest that a component may have failed, if you’ve ruled out a loose connection / broken wire.
*or could something of changed on the data feed?
Yeah I think you are correct as I just checked the continuity of all three wires and all were good and I think you are correct I think the optoisolator has failed.
I’ll contact the relevant people to organise a replacement and report back once I have had a chance to try it.
Replacement DHW unit arrived yesterday.
Excellent service by the Open Energy Monitor team.
I fitted it today and did a quick manual hot water boost to check the signal was picked up in Emoncms and everything looked good.
I will update this post tomorrow after I see how it goes after a full hot water cycle as the original worked for about 15 minutes.
Fingers crossed 
Left this for a few days just to be sure and all good 
Now to arrange to ship the faulty unit back.
Thanks all for the help.
That’s great to hear, sorry for the hassle.