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?

 

Posted by: @rhh2348

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- 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? 

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'??

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

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!

 

Posted by: @jamespa

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

Just to clarify this was a serious question, its going to be easier to explain your observations above if you have plots.  If you dont then just say and I will see if I can work it out (others may also).

 

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

Yes, can do that, though I ran the heating again from cold-ish this morning and got a different set of behaviours so will analyse and put up relevant bits.

 

Posted by: @rhh2348

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Posted by: @sunandair

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First thought is that cycling is a designed in process. And Mitsubishi appears to to have a 20 minute minimum run time for normal operation with 3 to 5 minute off time between cycles.

I know why you say that but, having a quick look back over the past few weeks of data only, I can see cycles as little as eight minutes in length.

this means that at 30c flow temp the temps are gentle on the compressor. And if the restart is only 3 to 5 minutes away the restart temp is still up at around 26c. So it is generally believed that cycling at these low temperatures and low temp differences is less stressful than perceived.

Logically, the lower frequency = more gentle, yes, but isn't the main wear/damaging issue here the stopping then restarting, and possibly the 'burst' stress on other components during the restarting?

it may not adjust in a conscious way( if it were you or I in control.) but the algorithm appears to be searching for the best flow temp based on WCC is asking for. And if it can’t sustain operation at the flow temp requested it deliberately raises its flow temp for 6 or 8 minutes before shutting down. To me this is consistent behaviour.

From memory, I think I agree, however I'd say it needs significant refinement as to how it does this.

But you’re right- total radiator volume is crucial to operating at lower flow temps because you need to be able to ditch the minimum kw of 3kw or 3.2kw in your HP size case.

It'd be useful to know what that minimum volume is and why Mitsubishi doesn't state it, as they do other operating parameters

 

Can you list your total system volume and DT50 rating of all your radiators/UFH? This might help clarify if you’re light in capacity.

From previous calculations, I believe I have over 150 litres in the system - rads alone hold 35 litres and their DT50 output is 31kW, though at a flow temp of 33C, I calculate 1.4kW. I don't know how to calculate the equivalent for UFH.

 

Thanks, Need to clarify do you have any UFH. Since your rads only equate to 1.4kw capacity at 33c. Obvs. With a HP chart showing min output of 3.2kwh at 35c that would be well below min capacity. If you have UFH it would be possible to calculate an approx figure if you know sq m area.

Also I note on comment replies to James you have been operating at 28c and 25c flow temps and seeing particularly short cycles of only 8 minutes. Presumably that’s because the system is well below min capacity. (There are other reasons for that kind of short cycling)

I think you may benefit from finding out what flow temperature you get a minimum of 20 minutes cycle on time and ~5 mins off time consistently then establishing that as the minimum temperature on your WCcurve. You can easily do this by operating at fixed flow temperature of 35c, then 34c, 33c, 32c etc until you find consistent operating duration. And of course you may also find that you may find you have to go even higher than 35c to find settled consistent operation.

I also note your comment that it is flow temperature rising and not the return temperature rising. I have stated that that is usually the last event of the forced cycle where the HP control is about to stop and it rises the flow temp a few degrees in what appears to be a boost before it shuts down briefly.

However that is a different function than the return temperature rising because the emitters haven’t offloaded enough heat. The return temperature is monitored by the system and adjusts the delta T based on the target flow temperature. But once the system cannot satisfy  the target target flow temperature and minimum DT the system appears to revert to the cycle to pause the heating process, but maintain the circulation pump operation to allow a few degrees loss of heat before commencing the cycle again. When there is adequate drop in the return temperature  and adequate deltaT then the cycle becomes extended and may even operate continuously.

So to my mind the best course of action initially, would be to find that balanced point where temperatures are offloaded to satisfy the minimum operating output of your model of hp. 

PS

In another post you wrote: 

“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?”

I think I would like to see a few charts of you system operating in steady state to have a view on that. But, yes it could be as simple as one thing…. Operating below the minimum output requirement for example. The ecodan is very flexible in adjusting its DT. It can have a narrow DT followed by higher varying DTs all in the same cycle. Which all appear normal. Other HP brands favour varying flow rates to maintain a certain fixed DT. 

You’ve also stated previously that you’re aware the Ecodan 6kw is a variant of the 8.5kw model. This may dictate how the minimum output has been defined.