Heat flow sensor for ASHP

I think this is a question for @TrystanLea

I’m building up an ASHP / Battery / PV Diverter setup (having had an EMON PV Diverter for several years). Slowly building up the sensor collection, which are rather scattered around a largish house…

I was going to use an EmonTx for the Heat metering input - till I realise this might not be possible…

Is there a way to connect a flow sensor to an EmonTx, or do I need to make up one of your Heatpump boards, and if so will you be selling any again in the near future?

Thanks very much (and thanks to all you guys for amazing kit and support - all quite humbling!)


Thanks @dlongson, do you know which flow sensor you will be going for? Will it be a MBUS meter or a voltage output sensor?

I think it will be a voltage output sensor, insofar as it definitely ISNT an MBUS… (I realise the MBUS needs the dedicated interface, and they are expensive!)

Thanks @TrystanLea

The other option of course is a pulse output. If you can find out the name of the sensor you will be using I can try to advise on the best approach. A pulse output would work well with the EmonTx if that is an option…

Or we now (or soon to be released) pulse interfacer for emonhub.

It’s a pair of Mitsubishi-branded VVX20 on a couple of Ecodan ASHP

(there’s another post from Jan this year from George (GCK303) assembling the same kind of project. In the other post there’s reference by Robert to “There’s a diagram in section 5.1.1 of the manual”, but I’m not sure what he is referring to). I’ve asked George today if he has progressed…


12 months nearly later, I’ve no idea now either. Presumably, I found a manual on-line and read it, and found a diagram that looked helpful.

The Sika VVX flow sensor has a 50% pulse, like this:

I’m not sure if I am getting anything sensible out of it, as it’s a Mitsubishi one, where the face plate states “do not use” for the pulse.

However, I’m not clear of the config setting I need for the Continuous Monitoring firmware for the EmonTx3.4. If my max flow is 50 litres / min, and the sensor is one pulse / litre, is it correct to set the minimum period as 60/50 = 1200ms?

(TBH, I’ve tried a range of numbers, with no impact - on the serial port I’ve just been getting a slow increment in steps of 1, from 1 to 6 in about 20 mins - so I suspect the ‘do not use’ info on the sensor means just that, though why would they also print a calibration on the faceplate?)

@TrystanLea - any ideas? Thanks!

Hello @dlongson if the Ecodan is already using the pulse output, Im not sure that you can use it with the EmonTx as well, the voltage supplied by the EcoDan might be different and the ground of the EmonTx will not be the same ground as the EcoDan control unit… I would be weary of interfering with the EcoDan’s measurement system as it may affect the heat pump operation…

Hi @TrystanLea
No, the sensor is no longer connected to the ASHP in any way - the installer disconnected it as it was no longer needed, and so that I could use it in this project.

Alternatively I could use the voltage output, but am I right in understanding that I need the Heatpump Monitor board rather than the EmonTx3.4 as there are no analogue I/p on the EmonTx that I can use for this purpose?


Full details are in the emonLibCM documentation.

Essentially, the minimum pulse period is a lockout in an attempt to inhibit spurious counts arising from mechanical contact bounce. An electronic output won’t have contact bounce, so you should be able to set the setPulseMinPeriod to zero.

“weary”? I think he meant wary.

@TrystanLea Damien says the sensor is no longer connected, so I don’t see why it won’t work with the correct interface, assuming that he has the “open collector frequency output” option.

Having found the manual now…
You will need to have a 5 V d.c.version as well as the frequency output.

You can connect pin 3 (GND) to the emonTx GND, and pin 1 (UB) to the emonTx 5 V. You will need to use a 5 V d.c. USB adapter to power the emonTx.

The pulse output (pin 4) goes to the emonTx pulse input.

If you have the “NPN” version, the internal pull-up is not strong enough given the possible 10 kΩ load of the sensor, so you will need to add a “RL” (Fig. 5.1.1) of say 4.7 kΩ between the pulse and the 3.3 V output of the emonTx.

If you have the “PNP” version, then “RL” will need to be a pair of resistors to divide down the 5.5 V so that no more than 3.3 V is applied to the emonTx pulse input. I suggest 1.8 kΩ connected to the meter pulse output pin 4, 3.3 kΩ connected to the meter GND (pin 3), the junction of the two being the pulse output to the emonTx.

Correct, there is no analogue d.c. input available.

Just to put a close on this thread, for anyone else trying to use the Mitsubishi Sika Vortex Flow Sensors as a pulse input for an Emon project. Push Pull, PNP and NPN didn’t work for me, and looking directly for a pulse on the pin didn’t work. @gck303 informs me that he put an oscilloscope on the pin, and also got nothing. So I think the the pulse pin (Pin 4; ‘Do not connect’) really doesn’t work.

Which leaves me with a voltage sensing solution- which for me will be the Emon Heat pump board…

At least, that’s one avenue closed. Thanks to Trystan, Brian and Robert for chipping in.


That’s good to know @dlongson thankyou for the update. We do have the heatpump monitor board in stock at the moment and I do have one pre-assembled here as well if that is of interest.

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Already purchased a few weeks ago. Decided to get some 10k resistors for the CT sensor voltage dividers, so good to assemble now. Thanks for the help though.

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Hi. An update on the project to get my Sika to work.

I am in the process of giving up and taking a different approach:

  • getting sufficiently accurate readings from an analogue feed is fiddly. IMO using a digital feed is easier, it removes many calibration, voltage, wiring and other issues
  • the sika flow meter supplied with the ecodan does not have a digital output. It only provides the analogue voltage

I think a mbus approach is likely better with from a dedicated and accurate heat meter.

As I am going to open up the heating system to add in a dedicated meter I am unsure what to fit. Likely a Kamstrup heat meter with temperature probes that are fitted into pockets.


Hi George,

I don’t think what I’m about to say will help, but I’m sharing just in case it does.

I have a 14kW Ecodan with the FTC5 control unit. My installer fitted the EMP3-M which allowed me to claim the MMSP rebate. I’m not as brave as you so I’m using their MELCloud system to monitor (and in fact control) my system.

Anyway, here’s my SIKA unit that they use to monitor the flow. Feel free to ask if you want any other photos, there are lot of bits and pieces in my system.

Bit of a late response! But I’ve just piggy-backed the Sika vortex meter output (which is still connected to my Ecodan controller) onto the analogue input of a ESP8266 NodeMCU board. It’s being powered from the 5V supply to the flow meter and so far seems to be working OK.

Ooh that sounds like something I need.