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Significance of delta betweeen flow and return temperature

Air Source Heat Pumps (ASHPs)

Last Post by HydroS 4 years ago

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greekgoddj

(@greekgoddj)

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Joined: 4 years ago

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Topic starter 23/01/2022 2:01 pm

Hi everyone,

I have been wondering about the following. Does the delta between the flow and return temperature have an impact on the efficiency/energy use of the ASHP? For example, assuming a ASHP that is focused on heating the flow to 40C, does it matter if what returns is 20C or 30C? In other words, if it is heating 1 room or 5 rooms, as long as the ASHP is aiming for a certain flow temperature (whether said temperature is manually set or driven by compensation curve), is the workload on the ASHP the same?

Thanks

Mitsubishi Ecodan 11.2kW ASHP

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Derek M

(@derek-m)

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23/01/2022 4:11 pm

Posted by: @greekgoddj

Hi everyone,

I have been wondering about the following. Does the delta between the flow and return temperature have an impact on the efficiency/energy use of the ASHP? For example, assuming a ASHP that is focused on heating the flow to 40C, does it matter if what returns is 20C or 30C? In other words, if it is heating 1 room or 5 rooms, as long as the ASHP is aiming for a certain flow temperature (whether said temperature is manually set or driven by compensation curve), is the workload on the ASHP the same?

Thanks

Hi,

The simple answer is yes, the delta T of the flow and return water does impact the efficiency and energy use of a heat pump.

It takes a defined amount of energy to heat a specified amount of water by 1C. So if the water is entering the heat pump at 20C and exiting at 40C, then it will have required 4 times more energy to heat the water than if it was entering at 35C and exiting at 40C.

👍

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greekgoddj

(@greekgoddj)

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Topic starter 23/01/2022 6:48 pm

@derek-m Thanks!

Mitsubishi Ecodan 11.2kW ASHP

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HydroS

(@hydros)

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Joined: 4 years ago

Posts: 59

03/03/2022 10:37 pm

Picking up this significance of deltaT topic again but from the opposite side.

My system is an Daikin Altherma 11kw Monobloc, described in other threads. I have it working with weather comp, with the internal controller modulating the leaving water temperature. It's working alright, COP about 3.2, but I notice after the overnight setback, the pump runs nicely with a deltaT around 5C, and if I record the meters (fitted as part of a trial) the COP for this period is up above 3.6, but as the house reaches temperature a bit of modulation happens reducing the leaving water temperature gradually, but then leaving and return water temperatures end up being very similar, often the same for several hours, so no deltaT. The house is warm enough, so there is no heat demand in the property but the pump keeps running in this tick over state, and this drops the COP drastically because the ASHP is using electricity but not putting heat in to the property. So that's the scenario. Should I be trying to achieve a greater deltaT through the radiators? I could wind in the locksheilds on my rads, whilst keeping the system balanced, to reduce flow and allow a greater temperature drop across them? This is counter intuitive though as I've read that high flow rates are better for heat pumps.

Thoughts appreciated about whether I'm over thinking this one.

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adaml

(@adaml)

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04/03/2022 1:57 pm

@hydros I see similar behaviour with my Daikin with rads as well, COP for Feb was 3.6. I wonder what value you are using for Leaving Water Temperature Maximum Modulation? What weather dependent heat curve are you using?

This post was modified 4 years ago by adaml

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HydroS

(@hydros)

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Posts: 59

04/03/2022 4:07 pm

@adaml I don’t have the option to set a maximum modulation value, for some reason that menu option doesn’t show. It’s the 11kw Altherma monobloc, the last gen, not the latest one.
The curve is 44C/-5 and 23C/15

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adaml

(@adaml)

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04/03/2022 4:56 pm

@hydros Are you comfortable accessing Installer settings > Overview field settings?

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HydroS

(@hydros)

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04/03/2022 4:58 pm

@adaml yes.
I’ve had to learn because the installer didn’t know what he was doing…

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adaml

(@adaml)

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04/03/2022 5:26 pm

@hydros

What value do you have for [8-06]?

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HydroS

(@hydros)

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04/03/2022 5:49 pm

@adaml I don’t know how to get to that value. It’s not in a menu option. Is it only available via laptop access? The installer manual has this

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HydroS

(@hydros)

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07/03/2022 6:57 pm

@adaml I worked it, I scrolled through the overview settings, I’d left this lot alone until now.
8-06 looks to be 3. Have I read that correctly? Is that a sensible number? I’m guessing it’s not enough. Today with the solar gain the house is up at nearly 23C, with the LWT at around 30C.