We have one of these, displays flow in m3/hr to 0.1 accuracy.

https://www.bes.co.uk/wilo-yonos-pico-25-1-8-130-central-heating-pump-27551/

@ian33a 

link to heat meter

https://shop.openenergymonitor.com/axioma-qalcosonic-e4-heating-cooling-meter-m-bus-dn25-qp3-5/

and Mbus dongle

https://shop.openenergymonitor.com/m-bus-to-usb-converter-heatpump-monitoring/

I would imagine you can integrate this to home assistant - however I would recommend OEM as it will just work out of the box - and if you then subscribe to heatpumpmonitor.org there are good tools available for calculating heatloss based on real data over a heating season.

Welcome to to the sad retired engineer club by the way.

Thanks - that's certainly very useful.

I'll need to think carefully about these options in the context of our dual circuit UFH system with a thermal store. 

I can see another DIY project coming !

Note this version is suitable for up to 17kw heatpump - which is practically all heatpumps you are likely to install.

Note also it is not calibrated for use with glycol in the system.

@ian33a 

suspect thermal store and heatpump may not play nicely but not sure about that- do you have solar thermal as well?

By the way we are very pleased with our heatpump in a 250 year old stone house. The thermal mass is massive so long low and slow works well - we just let it run 24/7 during the winter. We have mostly rads but if you have UFH even better providing you have good insulation underneath. Thermal mass is huge and you really see it when first switch on heatpump - takes 24 hours to reach steady state. I would imagine cob even more so.

I've heard that heat pumps and thermal stores are not ideal too. We don't have solar thermal.

The system was installed by a previous owner. I've gradually modified controls for it to improve efficiency. I'm reasonably confident that it was decently installed and, yes, thermal mass really makes a very positive difference. We run the system 24/7 in the winter too - 18 degrees during the day and a 2 degree set back at night using a flow temperature of around 45 in each circuit. In the summer I turn it off as the mass of the house keeps things nice.

The front third of our house is about 300 years old (cob), the rear third is victorian stone and the two ends are 20th century block cavity. Insulation is reasonable but I don't want to fit more and ruin the character of the house. 

@ian33a 

if you have done what is practical insulation wise then best to focus on draught proofing.

Posted by: @ian33a

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I've heard that heat pumps and thermal stores are not ideal too. 

They aren't.  The problem is that thermal stores generally need high temperatures to store sufficient usable energy for typical hot water needs, much more than just heating the hot water directly in a UVC. 

Modern r290 heat pumps are capable of reaching the required temperature, but it will compromise the dhw cop.  That said and depending on how much dhw you use and whether you can schedule it to heat on a cheap tarrifs, the payback period may well be far too long to justify changing an existing system.  Do the math and make a rational decision!

Got thinking about an easy way to measure heat output of an existing boiler without having to do surgery on the old system.

found these connectors

https://www.amazon.co.uk/Ribimex-PRRACRPVC-RIBILAND-Outlet-Fitting/dp/B075Z1J2QK

so you could isolate the circulation pump - remove the pump and connect into circuit with a heat meter and the existing pump on some bypass pipe work with some flexibles. Then leave it to collect data. 

could be a solution for difficult to evaluate situations