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In general - are shorter hotter cycles more costly or longer cooler cycles?

Air Source Heat Pumps (ASHPs)

Last Post by JamesPa 10 months ago

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Crimson

(@crimson)

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Topic starter 06/11/2024 6:11 pm

I'm trying to get to a balance with my system. I'm starting a new topic as this is quite specific.

Essentially with my system a downstairs zone doesn't perform as well as others due to very short in frequent cycles. Unless I keep upstairs zone open, cycles are more frequent/longer, but then the challenge with that is upstairs overheats/I have to really balance down upstairs which is proving complicated due to different use cases on the first floor (unless I crash down TRVs in bedrooms, bedrooms overheat etc etc).

What I'm finding is that upping the minimum flow temp on the ASHP, lets downstairs get more to temp with these very infrequent cycles (and they seem to be at the moment maybe 4x-6x a day - outdoors ranges from 10-12C here atm).

What I'd like to ascertain is, in theory what would be cheaper to run:

more frequent cycles at a lower temp, but a zone heated that doesn't need it
far less frequent cycles at a higher temp, with just the problem zone calling for heat

Difference so far is option 1 with a min temp of 29C, gets to around 19.5-21C (rooms performing differently).

Option 2 is seems to push towards 21C, but the min temp is 32C, so there's a 3C difference in min temp with the WC atm it's a 2C flow temp difference. Min temp is set to run when outdoors is 20C. However am seeing the rooms are closer performing downstairs.

Ideal situation would be to rip apart the plant room and optimise the actual temp hitting rads but currently not an option.

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JamesPa

(@jamespa)

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06/11/2024 6:27 pm

In purely theoretical terms the cooler the flow temperature the more efficient the heat pump is at delivering any given amount of energy, so longer and cooler is better than shorter and hotter - this is a function of the basic thermodynamics (the Carnot equation/cycle if you want to research it)

However your flow temperatures are low, so the thermodynamic efficiency difference may be 2% or less per degree C increase in flow temperature. You are suggesting only a modest increase of 3C and its quite likely that the thermodynamic difference, particularly at moderate outdoor temperatures, will be dominated by engineering factors. I think you are going to need to experiment, although it may be difficult to make an accurate determination because you cant control the key variables!

I know that's not very helpful but even with quite long setbacks of several hours its proving difficult to reconcile experimental results (with all their problems) with simple (albeit robust) theory. Particularly at moderate temperatures the 'engineering' factors, which are difficult to take into account/aren't actually declared by the manufacturers, seem to have a material influence.

This post was modified 10 months ago by JamesPa

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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Crimson

(@crimson)

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Topic starter 06/11/2024 6:34 pm

Thanks James. I'll keep experimenting. I suspect a spreadsheet that supplements the WC reckoner someone made (apologies every time forget who it was, and it's specific to the Grant ASHP), I'll have to note which temps at ambient temps get me to desired room temps depending on running upstairs along side or not.

Probably a long haul.

Next year I think I'll finally give in to allowing EON to install a smart meter and map against running costs also.

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JamesPa

(@jamespa)

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06/11/2024 6:41 pm

Posted by: @crimson

↑

Thanks James. I'll keep experimenting. I suspect a spreadsheet that supplements the WC reckoner someone made (apologies every time forget who it was, and it's specific to the Grant ASHP), I'll have to note which temps at ambient temps get me to desired room temps depending on running upstairs along side or not.

Probably a long haul.

Next year I think I'll finally give in to allowing EON to install a smart meter and map against running costs also.

Good idea. You will probably find that the cycling 'problem' disappears as it gets cooler. Its probably best not to get too intense about this, you could probably drive yourself mad! Think of it as a slow long walk not a sprint or marathon.

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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