I’d love to see some “old fashioned” heating systems added to heatpumpmonitor.org 
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Thanks for the replies all. Point taken Billt, makes sense.
Nick and Mike thanks for the extra information about the Mbus interface on the Sontex 
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On the Sontex 789, the MBUS does indeed provide power to the meter, so battery life is infinate. I’ve tested this by removing the internal battery and connecting the meter via MBUS, and it powered up and continued working 
. I’ve not tested this with other brands, but it’s not a big issue the battery is easy to replace, it’s just a standard 3V lithium, I think they’re about £6
I would definitely recommend using MBUS communications rater than pulse since lots more data is available
I’ve used both the 749 and 789 the body material seems to be the only difference. I don’t think it makes any difference, I would just go for whatever is in stock and cheapest. Sontex seems to be having supply issues atm
Yes, definitely go for the 2.5m^3 version, the 1.5 m^3 version has a high pressure drop so it would negatively effect performance of the system. Pipe size is less critical, the 2.5m^2 meters come with 3/4" flat face unions, you can then a 3/4" BSB to 28mm compression iorn to install on your primary pipework.
Good luck with the installation, I look forward to seeing the data on heatpumpmonitor.org
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Ah, amazing Glyn, thanks for this more detailed information and the pictures, very good to hear!