Heat output , science and practice

Air Source Heat Pumps (ASHPs)

Last Post by BobTSkutter 12 months ago

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Terry1812

(@terry1812)

Estimable Member Member

Joined: 12 months ago

Posts: 50

Topic starter 21/02/2025 6:53 pm

Hi all. Have been viewing the site for a while very interesting discussions. My heart goes out to @anca1808 and heri ssues. So I have been trying to work out the heat output from my heat pump. I came across the formula flow rate*cp ( specific heat capacity of water) * delta T, which should give the heat output from the heat pump. I have a Daikin Altherma 3, 8kw nominal. I am running it on pure weather comp, with one zone only, all Trv’s fully open. Not that it really matters but I have the curve set for LWT 40c at -2c and LWT 25c @ 18c, though I would have expected to turn the heating off once it gets to a steady 16c. The Daikin has a minimum flow rate of 7L/minute, and it seems to be there most of the time. So here is my query it seems that once you get to steady state the key variable is delta t. From my calculations at steady state with a max delta t of 10 ( which is the Daikin max setting ) the heat pump can only produce 4.88kw. So in theory, if the outside temp drops to -2c , running weather comp I can only get 4.88kw, which given I have a heat loss of 7.3kw, isn’t enough. I might not be explaining this very well, so let me put an example.

assume outside temp is fixed at -2c, assume inside temp starts the same at -2c. Heat pump goes on. Now the key factor seems to be the delta t, also whatever system constraints there are, one being the max output is 7.9k . So initially the delta t will be massive, so you have to assume the heat pump will deliver its max output of 7.9kw. It will do that until it reaches its steady state with LWT 45 and RWT 35, giving max design delta T. But at that point it will only be delivering 4.88 kw. According to my calculations, I might be able to get up to 14c, before it balances out. So the system is capable of more but steady state , when using weather comp, seems to lock it into max 4.88kw. To get more, I need the flow rate to increase , the only way the flow rate might increase is if I move over to thermostat control.

Am I understanding this correctly or am I missing something obvious.

This topic was modified 12 months ago by Mars

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Toodles

(@toodles)

Illustrious Member Contributor

Joined: 3 years ago

Posts: 2542

22/02/2025 9:40 am

We have a Daikin EDLA03 8 kW Altherma and on that, I believe that the flow rate can be set via the installer access settings on the MMI controller. If your setup has the MMI, you can set the user profile to ‘Installer’ by entering the code 5678 and then go through the settings via the menu selector knob on the left.

We have Homely smart control over ours so I am not able to run through the setting procedure you might need but you should have the manual with that information. Homely modulates our flow rate and sits at an average 15 lpm with occasional excursion to 30 lpm or so. Regards, Toodles.

Toodles, heats his home with cold draughts and cooks food with magnets.

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BobTSkutter

(@bobtskutter)

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Joined: 1 year ago

Posts: 94

22/02/2025 10:27 am

@Terry1812 welcome, you're going down a thermodynamics rabbit hole 🙂

Long story short, yes you're correct.

At a fixed flow and a fixed DT your power is also fixed.

Q = rate * Cp * DT

Q = power

rate = L/s

Cp = specific heat

DT = delta temperature, C

If rate is fixed (0.12L/s in your case) and DT is fixed (at 10C in your case) then power is also fixed (at 4.9kW), because the only parameter left in the equation is Cp which is constant. (If you really want to be detailed Cp does change by very very very small amounts as the water changes temperature but the changes are so small as to be negligible).

Fundamentally you need to increase the flow to at least 0.2L/s or 12L/min to get the max power out of your heat pump.

Regards

Bob

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