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Ecodan - is there actually such a thing as a 'minimum' flow temp to avoid cycling, and why if so?

 
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Air Source Heat Pumps (ASHPs)
Last Post by rhh2348 8 hours ago
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 rhh2348
(@rhh2348)
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Topic starter 15/04/2025 10:29 am  

it’s clear you are wanting to run your heat pump at a low flow rate ie 30c. But you are also aware of some of the limitations of operating at low flow temperatures.

however one significant behaviour of operating at at low flow temperature is the HP will only be able to continue operating at the WCcurve set flow temperature if the return temperature water has dropped enough to allow a reasonable DT. (this also applies to the fixed flow temperature mode

The low flow temp has been arrived at by a combination of repeated balancing of the rads such that the one furthest away has sufficient minimum output to satisfy the required room temperature at the various OATs, tweaking the comp curve, and setting an appropriate flow rate required to keep the RWT sufficient so that it doesn't cycle further.

 

So in my experience of the ecodan it is not the failure of the HP to modulate lower, as your comments have stated but the operating requirements of the system to operate at a workable DT.
As I mentioned before, I've seen my dT < 3C and it not cycling, and I've mentioned elsewhere a few times Mitsubishi's 'recommended' 8K, which I still don't understand!

 

volumes...
See earlier post.

general controls, plumbing variants, pumps LLH, cylinder type etc all helps to give a fuller picture.
Compensation curve, one pump, no LLH, cylinder not applicable here (DHW works well)

If the radiator cannot lose the required heat then the return temperature comes back too high, squeezing the DT until the Lowish flow temp cannot be maintained any more. So it has to cycle.
Invariably it's the opposite - the LWT always rises too quick.

 

I also attach a graph showing the minimum output of your heat pump which shows its minimum output at different outdoor temperatures. When you are operating close to the minimum output small variations in gas or construction may result in earlier or later shutdown times.

I also draw your attention that the graph only goes down to 35c flow temperature so at 30c you could assume the minimum output threshold will be higher than the graph.

I did comment on this previously.  From 4C - 7C, it seems the minimum is 3.2 - 2.9 at 35C, and (as you say) increasing with lower flow temps, so as I concluded, I don't think it's a modulation issue; do you still?

 


   
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 JamesPa
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15/04/2025 10:51 am  

Posted by: @rhh2348
↑

- how can I know that larger always means flow temp can be reduced more/rather than flow volume/rate? or by asking the question, I suspect that confirms the way forward is only with a volumiser rather than larger rads...

The radiator output at any given ft will increase if the radiator is larger.  So larger rads will translate to lower ft (for the same house temperature) and higher COP. 

 Larger rads will also increase system volume and thus should reduce cycling frequency, however the min output of heat pumps often increases with reduced ft, so the improvement may not be quite as great as you expect. 

A volumiser will reduce cycling frequency but probably won't have a major effect on cop (there will likely be a small effect due to the reduced cycling, but not as much as upsizig the rads).

It's possible to estimate the size of volumiser needed to give a particular run time if you collect the right information.  Likewise it's possible to estimate how much change uprising the rads will make.

Does that help? 

This post was modified 14 hours ago 5 times 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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 rhh2348
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Topic starter 15/04/2025 1:38 pm  

Thanks again for a response, @jamespa - will come back to it in a bit but wanted to highlight something happening at present first.

The house has had the heating off for two days but others have reported they are cold so I have put it back on today.  To test some of the points made here I fired it up with 12C OAT and curve temp of 27C.

  • It cycled after 6 mins as the dT hit 6C on a flow of 11L/min - this is with everything cold.
  • I then put it onto flow temp and 25C with 15L/min flow to test the min output theory
  • It waited the 5 mins before starting again
  • At 6 mins in and a dT of 4.5, FT went 2C above flow setpoint (FS) and cycled
  • It waited the 5 mins
  • It did the same again but got 1.5 mins with FS +2 of FT before cycling

Each time the output was higher than the minimum and the house demand was definitely there.

Am I missing something obvious as to why it is cycling, other than 'the flow setpoint is too low'??


   
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 JamesPa
(@jamespa)
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15/04/2025 1:52 pm  

@rhh2348 have you got graphs to illustrate this, if so can you post them please (or were you sitting and watching?)

This post was modified 11 hours 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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 rhh2348
(@rhh2348)
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Posts: 97
Topic starter 15/04/2025 4:34 pm  

We have had quite extensive discussion on this
...which I assume started shortly after this post. I actually found that thread a couple of months ago but had yet to go back and digest fully.  Doing this now!

 


   
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