Have a look at John Cantor's excellent online calculators here (specifically the flow rate calculator):

https://heatpumps.co.uk/calculators/

Heat (kW) = Flowrate (Litres/sec.) x 4.2 (Specific heat of water) x temperature rise (dt)

So for a given heat output from your ASHP, flow rate is directly proportional to dT (between flow and return).  As the flow rate drops, the dT will increase, and vice versa.

Some systems use fixed flow pumps, so your flow rate will be static (some are adjustable with maybe 3 settings). Others will use PWM pumps which are able to adjust their flow rates, and the heat pump will attempt to control the flow to meet a target dT.

Posted by: @old_scientist

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Heat (kW) = Flowrate (Litres/sec.) x 4.2 (Specific heat of water) x temperature rise (dt)

So for a given heat output from your ASHP, flow rate is directly proportional to dT (between flow and return).  As the flow rate drops, the dT will increase, and vice versa.

Using 6kW as power ('heat'), 6 / 4.18 / 8 * 60 (to get lpm) = 10.77lpm, or the 10.8 I quoted - nice.  (The same equation gives the 17.2lpm if I use deltaT of 5)

Some systems use fixed flow pumps, so your flow rate will be static (some are adjustable with maybe 3 settings). Others will use PWM pumps which are able to adjust their flow rates, and the heat pump will attempt to control the flow to meet a target dT.

Nothing is straight forward, delta T will change depends on which way you heat, for example radiators will take a wider dT than UFH.

Example

Target room temperature 

Well insulated house UFH. Return temp from floor does not go below room target temp. So no lower than 21. If you set dT at 8 your min flow temperature becomes 29. Plus your compressor restart hysterisis is dT plus a further temp delta. So now your Min flow temp is about 29.5. if you operate with dT 4 your min flow temp becomes 25.5 which is better for CoP and not over heating the house.

So dT is horses for courses. You get a better CoP with the widest dT you can operate. But you can easily short cycles at lower loads.

My 6kW heat pump ramps from 14 l/s to 18l/s doing UFH over a 10 min period.

Yes - do you know if using a Grundfos (with three modes + three speeds in each mode) capable of / using PWM would be continuously-variable, or just step through the three speeds the pump mode would be set in at the time?  Can you share any info as to how the FTC controls pump speeds (settable separately for DHW + heating between 1 and 5) on its cylinders?

I don't I'm afraid - you could try looking up the specs of the Grundfos pumps online.

My own system uses fixed flow rate Wilo pumps, with 3 different curves or modes, and 3 different speeds. I've just left them in the standard fixed mode, and have tried the 3 different speeds to see the effect on flow rates, and hence dT. These are not PWM pumps.

I think systems that do use PWM pumps, will attempt to vary the pump speed to keep dT constant at whatever it has been set to - so if you've set the system with a desired dT of 5C, and the heat pump is producing 5kW of heat, the system will attempt to adjust the flow rate to be 14.3L/min. If the heat pump output should rise to 6 kW, the system would attempt to increase the flow rate to 17.1L/min to keep the dT at 5C.

I wouldn't become too fixated on it - as long as you have sufficient flow rate to transfer the required heat around your system, dT will be what it is. My dT is low at ~2.5C with a fixed flow rate of ~25-26L/min when the heat pump is just idling putting out around 4.5kW but rises to 4C as the heat output rises to 7K and again to 5C at 9kW output. I could turn my pumps down slightly (to around 21L/min), and dT would increase a whole half degree to 3C at low (4.5kW) output, but I wouldn't want to run a flow rate much below that.

Edit: I need slightly higher flow rates for the same dT due to use of glycol in my system.

Posted by: @rhh2348

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Thirdly, it would not be surprising if the flow rate needed to be different for just DHW compared with demand of two heating zone pumps, though it could be said that the heating pumps should have been chosen/configured to work optimally at the same flow rate and therefore it isn't a consideration.

Optimising flow rate for DHW heating is something I'd been thinking about over the summer, as we are only heating DHW at that time.

My simplistic thinking is that you'd want to try to keep dT quite narrow/small so would favour a higher flow rate. My reasoning, equally simplistic, is that if I want to heat my DHW to 50C, and I have a dT of 5C, to achieve that I may need a final flow temp (LWT) of 55C and return of 50C. If dT were 10C, I'd need a flow temp of 60C and return temp of 50C, so I'd have to heat the LWT higher to deliver that same final DHW tank temp?

I tried running some tests but there were too many other variables (OAT, DHW tank starting temp etc) making it difficult to compare truly like for like runs.

Happy to hear other's views and to have my thinking corrected / adjusted as appropriate.

Posted by: @rhh2348

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Can you share any info as to how the FTC controls pump speeds (settable separately for DHW + heating between 1 and 5) on its cylinders?

Hi I assume you have found the settings for pump speeds in the settings menu as you mention settings 1-5. if you have a newer FTC 6 then you can set the pump speed separately for heating and dhw.

When the heatpump is running you can go into the running information menu and type in code 540 this gives you the current flow rate.  You can then see how changing the settings 1-5 affects that flow rate.  It is fixed and the heatpump can’t change it dynamically you just have to set it at an appropriate speed for your system

Posted by: @old_scientist

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Optimising flow rate for DHW heating is something I'd been thinking about over the summer, as we are only heating DHW at that time.

There is a video from heat geek about optimal settings for DHW. To get best CoP it more about the cylinder coil being able to strip away as much energy as possible so return temp stays low, for best CoP. Worth finding and watching.

Mine setting are preset and not adjustable, so pump runs at full speed for DHW, so about 1.5m³/h, heat pump adds heat and keeps dT at about 3.5. Seem to get a CoP of about 2.5 to 3.5.

Posted by: @johnmo

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[snip]Example

Target room temperature 

Well insulated house UFH. Return temp from floor does not go below room target temp. So no lower than 21. If you set dT at 8 your min flow temperature becomes 29. Plus your compressor restart hysterisis is dT plus a further temp delta. So now your Min flow temp is about 29.5. if you operate with dT 4 your min flow temp becomes 25.5 which is better for CoP and not over heating the house.

So dT is horses for courses. You get a better CoP with the widest dT you can operate. But you can easily short cycles at lower loads.

In the example above, if my flow was 30.5 rather than 25.5, would the floor get the room to target temp quicker, or would it get there at the same time but overshoot and make the room too warm?

If I want my flow at 25.5, I assume I then adjust the w/c 'curve' so that the flow remains at 25.5 depending on the OAT?

Using 6kW as power ('heat'), 6 / 4.18 / 8 * 60 (to get lpm) = 10.77lpm, or the 10.8 I quoted - nice.  (The same equation gives the 17.2lpm if I use deltaT of 5)

Posted by: @rhh2348

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In the example above, if my flow was 30.5 rather than 25.5, would the floor get the room to target temp quicker,

Yes, but that is thinking like gas boiler logic, not heat pump logic, you need as low a flow temperature as possible. Also the further you are away from ideal flow temperature the more likely to overshoot, then depending on your thermostat, screed thickness, insulation levels outside temp you could also undershoot as the system can't keep up.

Watching the discussion on dT. An observation.

As you add heat to the floor the return temperature will rise, the heat pump should let this occur and add more heat to the flow, keeping your dT stable. Once dT and target temp are at point where your settings say is correct, the heat pump should modulate down to keep target and dT stable. If your heat required is below min modulation, the dT will get smaller, and the flow temperature increase, until flow hits your upper limit. Mine can come down to about 3 dT.

Posted by: @johnmo

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Posted by: @rhh2348

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In the example above, if my flow was 30.5 rather than 25.5, would the floor get the room to target temp quicker,

Yes, but that is thinking like gas boiler logic

On the basis when OAT gets lower, more heat is lost from the room, would the floor flow temp need to go up (perhaps even to 30.5C) to keep up with losses or if it's kicking out 21C heat all the time the room should broadly stay at 21C?  Would the flow temp need to increase just to maintain the floor's own temperature at 21 C, or does it need more to then maintain the room's temp?