Thank you OpenEnergyMonitor: Octopus Daikin ASHP monitoring

[markocoheat (Marko Cosic)]

W

@matt-drummer

Looking at my data it appears the heat pump can only continue to increase the flow temperature, it cannot maintain a flow temperature.

I really thought it would heat to the flow temperature requested by the WD curve and then maintain it until the desired room temperature has been achieved.

What it actually does is heats to the WD curve flow temperature, shut down, cools down, and then has another go (and so on) until the desired room temperature is achieved assuming the flow temperature and emitter design is enough to achieve that.

A heat pump CAN run continuously and maintain a set flow temperature IF the radiators can put out at least as much heat as the heat pump generates at minimum output.

Here’s a load of rough workings:
Knoll Pipe Calcs.xlsx (2.1 MB)

On the “heat pump” worksheet you can see the min/max output at various supply/return temperatures for various outside tempreatures.

Point 1)

The datasheet excerpts aren’t in English becuase English installers are considered f**king muppets by the manufacturer so they dumb down the English manuals to avoid person-in-blue-van or -erson-in-purple-van or person-in-red-van having heart failure.

You do want to know what the min and max outputs are if you’re designing though!

It’s also notbale that peak COP is NOT at minimum or maximum output but somewhere in-between. (that’s why they startup and shutdown instead of turning down furhter - it’s more efficient)

Point 2)

The “7 kW” unit is good for a max of 9 kW at -2C and can turn down to 3.5 kW at +12C

On the “Emitters” worksheet you can see each radiator and what the power output is / flowrate needs to be at certain temperatures.

At 45/40 they kick out 9.6 kW

At 35/30 they kick out 4.5 kW

So the heat pump can happily tick away constantly from flow temperatures of 45C down to something below 35C.

You can also see - if the balancing is correct and all that water is equally in motion - that there is 103 litres of water to heat up and cool down. At the minimum heat pump output (say 3.5 kW) the time it takes for that water to go from say 25C (with the radiators putting out maybe 1.5 kW) up to 30C (with the radiators putting out 3.5 kW) will be plenty (2 kW kettle boiling 1 litre of water 100 times? You’ll be there a while) The cycles will be long

Takeaway?

These heat pumps should have REALLY long cycle times IF all the water in the system is actually in equal effect.

Chances are it isn’t. Chances are that a significant proportion of the water is short-circuiting through some radiators. The house still gets warm but your effective volume in circulation is small and the heat pump cycles away even when it shouldn’t be.

There’s also the possibilty that the 9 kW heat pump isnt’ conencted to 9 kW worth of radiators, so it’s haveing to “turn down” even when it’s fully cold out and then quickly runs out of “turn down” capability at part load.

So back to the question: what is the output of the radiators that this 9 kW Daikin heat pump are conencted to? If it’s less 9 kW that will be contributing to startup and shutdown; or the unit generally running away from it’s best efficiency point.