@jamespa not sure what you mean… is that a theoretical comment or a comment from experience.?

Aren't you potentially confusing two things here.  OP could of course increase rad size/ volume and reduce ft but that won't change the minimum output required because that's determined by the house.  So unless the min output of the heat pump reduces with reduced ft,

@jamespa The first thing I’m trying to achieve is to reassure that cycling is not necessarily harmful at these low temperatures.

Regarding low flow temperatures that’s exactly what im saying - lower flow temps -  that means the home has a greater potential to operate at a lower flow temperature.  It would still need to operate at or above its minimum output but there is a greater chance that the heat loss of the house would match the heat gains of the lower flow temperature. I’m well aware that we’ve been here before. We have had quite extensive discussion on this and based on your more recent posts I thought you had now accepted that radiator size does help overcome cycling without overheating the home by simply allowing the whole system to operate slower and lower in line with the heat losses. And still be within a nominal DT5 and fully within the control of the accurate WCcurve.

If the HP is sized correctly the scope of modulation should be able to cope with this minimum output. Obviously this would not work if the HP is oversized to such an extent that the minimum output would still overheat. But we are talking about a 6kw HP. So of course…. I have then asked for clarification on the  system, volume and capacity being used.

I’m sure @rhh2348 will find these comments useful!

Posted by: @sunandair

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@jamespa The first thing I’m trying to achieve is to reassure that cycling is not necessarily harmful at these low temperatures.

Very much agree.  I think OP is over-concerned about this because, as you say, cycling is 'designed in'.

Posted by: @sunandair

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If the HP is sized correctly the scope of modulation should be able to cope with this minimum output. Obviously this would not work if the HP is oversized to such an extent that the minimum output would still overheat. But we are talking about a 6kw HP.

Also very much agree

Posted by: @sunandair

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We have had quite extensive discussion on this and based on your more recent posts I thought you had now accepted that radiator size does help overcome cycling without overheating the home by simply allowing the whole system to operate slower and lower in line with the heat losses. And still be within a nominal DT5 and fully within the control of the accurate WCcurve.

We may be splitting hairs.  Larger radiators = lower flow temperature and larger system volume.  The latter certainly would be expected to reduce cycling frequency, but (unless you heat the house to a different temperature) doesn't change the heat loss of the house and therefore doesn't change materially the outside temperature at which cycling starts to occur (there will be minor changes because the minimum output of the heat pump will vary a bit with flow temperature, usually upward as FT decreases but not on all spec sheets I have seen).  Although it will reduce the cycling frequency the on/off ratio should remain pretty much the same at any given demand, because the ratio of demand to minimum output is not materially changed.  I would expect there to be some detailed variations however because of the way heat pumps appear to modulate their compressors during the start up phase, which is obviously significant if cycling is occurring with a short period.

Posted by: @jamespa

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We may be splitting hairs.  Larger radiators = lower flow temperature and larger system volume.  The latter certainly would be expected to reduce cycling frequency, but (unless you heat the house to a different temperature) doesn't change the heat loss of the house and therefore doesn't change materially the outside temperature at which cycling starts to occur (there will be minor changes because the minimum output of the heat pump will vary a bit with flow temperature, usually upward as FT decreases but not on all spec sheets I have seen).  Although it will reduce the cycling frequency the on/off ratio should remain pretty much the same at any given demand, because the ratio of demand to minimum output is not materially changed.  I would expect there to be some detailed variations however because of the way heat pumps appear to modulate their compressors during the start up phase, which is obviously significant if cycling is occurring with a short period.

Hi, This set of comments all appear to be set in theory rather than making specific answers. 

for instance when you say “lower flow temperature and larger system volume… certainly would be expected to reduce cycling frequency”

How specifically might it reduce cycling? What do you know about how volume “helps” with cycling? This is a sentence I’ve seen you use before without a how attached to it.

I also find that some (not all) of your interjections on these forums seem to kill conversations rather than enabling and encouraging OPs to continue to discuss issues. It’s as though you insist on having the last word.

in my post above I set out some basics to give some tangible support the purpose being to open up the way for Mor information and hopefully more helpful comments. However, I believe your doubting opinions which are not supported with hard fact just serve to put people off.

maybe it would help if there was less interjection and see what transpires from discussions.

Posted by: @sunandair

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Hi, This set of comments all appear to be set in theory rather than making specific answers. 

for instance when you say “lower flow temperature and larger system volume… certainly would be expected to reduce cycling frequency”

How specifically might it reduce cycling? What do you know about how volume “helps” with cycling? This is a sentence I’ve seen you use before without a how attached to it

The full sentence was

"larger radiators = lower flow temperature and larger system volume.  The latter certainly would be expected to reduce cycling frequency". 

This is theory but is frequently stated by many, not so far as I am aware disputed, and has a simple basis namely: with a larger system volume it will take longer (at any given set of conditions) for the heat pump to heat the water in the system from the lower hysteresis limit to the higher hysteresis limit.  Thus the cycling period will increase (or equivalently the cycling frequency will reduce).  Obviously one can do a calculation of the predicted effect and thus be more specific, but going into that seemed like too much detail at this satge.

Posted by: @sunandair

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in my post above I set out some basics to give some tangible support the purpose being to open up the way for Mor information and hopefully more helpful comments. However, I believe your doubting opinions which are not supported with hard fact just serve to put people off.

I actually said 'Very much agree' to two out of three of your tangible suggestions, with the intent of amplifying them for the benefit of OP.

The one I contested was the benefits of increasing radiator size given that this is an extensive undertaking and OP appears to be concerned about cycling not COP.  The simplest way that I know to improve cycling in that situation is surely to add a volumiser.  I am sorry if agreeing on two points but disagreeing (in part) on one was offensive.

Posted by: @sunandair

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maybe it would help if there was less interjection and see what transpires from discussions.

I agree with this, however it is the nature of this forum that multiple people like or choose to offer help/facts/opinions and this inevitably generates discussion, particularly where there are contradictory views or, more likely, just an (apparent) lack of clarity.  I cant realistically see how that can be avoided. 

I try only to intervene in three cases

• I have something new to add which I think is of practical use
• I believe someone else has said something which is either not clear or might mislead OP
• Sometimes, I I think it will help, I want to amplify what someone else has said for the benefit of the OP, in which case I generally either give the whole post a thumbs up or make a short comment like 'very much agree'

Perhaps I spend to much time on this forum!

Posted by: @jamespa

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I actually said 'Very much agree' to two out of three of your tangible suggestions, with the intent of amplifying them for the benefit of OP.

The one I contested was the benefits of increasing radiator size given that this is an extensive undertaking and OP appears to be concerned about cycling not COP.  The simplest way that I know to improve cycling in that situation is surely to add a volumiser.  I am sorry if agreeing on two points but disagreeing (in part) on one was offensive.

So James  I’m sure you might realis that my first 2 points were giveaway comments - ones that you couldn’t really object to. So your agreement is somewhat expected.

The third point was obviously the central point of my post. And your reason for querying it was that you didn’t think the OP was interested in COP. Well that was not my view.
You appear to have overlooked that the OP was concerned about being able to operate his system at 30c…. That, to my mind is someone very interested in operating as efficiently as possible. I gave due consideration to the OPs post and made what I consider to be the right call. 

So perhaps your interjection, as I have already asserted was premature and perhaps unnecessary.