Confused by Flow Rates

phil bell · 2025-11-12T17:56:52Z

So I've watched the latest video on youtube, and there was a lot of talk about flow rates, a pretty confusing episode and i've watched it twice now. My Daikin unit literally makes every effort in the world to crash my flow rate down to 7ltr pm, it can happily run much much higher when it first runs, circa 25ltr's pm but will actively look to dive that down low. Also at those high flow rates it drinks juice. So we have nice fat pipework, big rads etc. They all seemed to suggest that a system had some kind of mythical flow rate that sounded high, hence more pumps as a lazy solution. Daikin seem to have designed their heat pumps to crash flow rates. I've seen loads of Daikin Altherma users who see this behaviour. So has Daikin literally built really bad heat pumps that are incapable of peak efficiency? We run a system that is on all day every day and we see low power draw so it seems to be efficient. The Dainkin energy reporting suggests we are comfortably seeing a COP of 3.9 With a DT target of 4 Any more thoughts on what i'm clearly not getting.

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Air Source Heat Pumps (ASHPs)

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Rikiarn

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03/12/2025 10:34 am

Posted by: @jamespa

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Posted by: @simonwig

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I have a related query, feel free to move it to a new thread if preferred.

I have an Ideal Logic Air 10kW.

The flow rate showing on the Halo was 25 – 30 l/min, but often quite a low dT, maybe 2 - 3C.

I’ve noticed that it is possible to set the pump to modulate to maintain a dT of a variable number so as an experiment I’ve set it at 5C, (default is off for this setting, 5 if it's on).

It’s also possible to set the minimum pump speed %, I’ve set it to 50% (default is 70%), it’s unclear when this actually operates if the Modulation is off.

The pump is now generally running slower, currently 16l/m. House still maintaining temperature.

I’m really asking if there is any value in doing this

Not particularly unless you have a problem with noise in the pipework

Posted by: @simonwig

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it seems logical to run the pump slower and maintain the dT.

No, but there is a bit of a trade off and its not straightforward.

The heat pump will be more efficient at lower deltaT. However the water pump will consume less energy if its operating at a slower speed, which will mean less flow so higher deltaT. Since the heat pump is likely to bne consuming 100s of W and the water pump 50W max, the heat pump will usually dominate.

The sweet spot is dependent principally on your house loss. The folks on openenergymonitor did a calculation and, for their particular set of circumstances, the optimum deltaT at the design OAT was ~2C (which implies that deltaT will be less at higher OATs). Im pretty certain this was a ~5kW house. The upshot of this, given that normally heat pump systems are designed for DT5 or thereabouts, is that for most cases the optimum is going to be to run the water pump at the highest comfortable speed, which likely will give a DT between 2-5 if its been designed correctly.

In short if its operating at 2-3C most of the time dont worry about it.

Hi, I have a similar situation as Octopus changed the downstairs radiator based on the heat loss but left the oversized radiators upstairs. I tried to restrict the flow Upstairs to almost close, as I think the flow valve are not that gradual in shutting down, and had to limit the temperature with the trv to not over heat. They are tado, that we still have from the gas boiler and seems to restrict the flow not shutting the radiator completely. I left downstairs fully open. You mentioned to not leave a towel rail radiator with flow completely open, can I ask why? Sorry trying to learn as much as I can.

For us the towel rail is the first in the heating circuit and Octopus changed it to a bigger one to meet the heat loss of the bathroom. Especially when the flow rumps up you can hear flushing noise from it like water running, mainly when the heat pump is switched off. Is there anything that we can do to to limit the noise. So far has been left fully open, based on the point above should we restrict the flow and will it potentially reduce the noise?

Thanks for the help

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