I think I would be asking myself ‘ Why is my gut instinct so far out of kilter with the science and the maths!?’ Impressive COP though😉 Regards, Toodles.

@aaron Welcome to the forum.

With a heat loss of 5.1kW for the main part of the house, I'm not surprised you are seeing cycling as the weather is currently a little milder. My assumption is the heat pump is producing more heat than the property needs at these milder temps, even when modulating down to minimum output. Bringing the additional building online will help in milder weather.

My own heat pump is oversized for our heat loss (we have radiators, not UFH), and we are only able to run constantly at low flow temps when the outside temp reaches ~5C or below, and even then we tend to turn off for a few hours overnight to achieve 18C for sleeping. At the current ~10-12C ambient temps we are seeing, we tend to be on for an hour or so and then off for an hour or two, depending on the hysteresis. As long as the system isn't short cycling (cycling very quickly), I don't think this is overly inefficient compared to constant running.

It's our first winter, but so far my impression is that an oversized heat pump isn't the worst thing in the world and can be operated efficiently. Like you, we are able to achieve good COPs. The last couple days we have got COPs over 5 running 32C flow temps in our radiator system, and have had a worst case daily COP of 3.0 on the coldest day, with the coldest month averaging in the high 3's. If you are achieving COPs of over 5 in winter, I'd be delighted.

EDIT: Not sure about ECO mode on the Vaillant, but I use Samsung's Quiet Mode for DHW runs which limited the output (and hence the noise) to around 70%. This has the effect of ensuring the heat pump is operating in it's most efficient range (i.e, compressor not running flat out), and narrows the dT allowing the heat to transfer into the DHW tank at a slightly lower flow temp. Last summer I ran some tests and found it consistently used less energy each day to reheat the tank than in 'normal' mode.

Its good to hear that you are pleased with the outcome.  I agree with @old_scientist that cycling is to be expected (and is unavoidable) in milder temperatures with a heat pump that has a max output well in excess of the house demand.  Its cycling slowly so not really an issue, unless you are uncomfortable with any temperature variations in the house, in which case there are some adjustments that could be made.  My 7kW Vaillant is pretty well matched to the house and the onset of cycling is about 11C, which is more or less precisely what one would expect.  

The detrimental effects of cycling arent particularly well quantified.  At higher flow temperatures such as might be typical with radiators there is a quantifiable detrimental effect on COP (because when the system is running it must do so at a higher temperature than if it were running permanently), but with your very low flow temperatures (resulting presumably from the closely spaced UFH) this is very much less.  Soft start compressors mitigate the potential damage to the compressor that was the bogeyman when compressors were fixed speed, so its really a bit unclear overall when you are operating at such low FTs.

Posted by: @aaron

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The older building we’ll be linking to adds another 80m², with similar U-values. However, it won’t match the airtightness or thermal mass of the main house and will require radiators. This brings the total footprint to 380m², with a calculated heat loss of 6.86

Im sure you already know that but you will lose the COP advantage of UFH if you end up having to operate your radiators at a higher flow temperature and 'mix down' for the UFH.  You may want to give some careful thought to the emitter design/selection (if you havent already/its not too late).

Posted by: @aaron

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id consider installing an MVHR system, especially since our home is fairly airtight, and recovering up to 80% of the heat from outgoing stale air sounds like a great idea. But every home is different, and how it performs depends on how it’s actually used by its occupants.  Instead, we stick with good old-fashioned Stoßlüften (shock ventilation)—turns out, simply opening a window at the right time works brilliantly, especially when combined with the building’s thermal mass! Our windows have always been, and always will be, wide open first thing every morning while the beds are made, and again, probably kept on the night latch every evening. Fresh air, no faff!

There is a reason why windows are deigned to open!  One air change per day is well below the recommended value of typically 0.5ACH but as you say the effect on energy is nothing like as much as one would imagine, because it is just that, one air change per day, perhaps 2, as opposed to 12 from 0.5ACH.  With MVHR 0.5ACH is equivalent to perhaps 2-3 air changes per day in terms of energy loss, so about the same.  Obviously you have some natural infiltration on top, but if the house isnt getting mouldy, damp or musty I can see nothing wrong with the low tech solution personally!

@jamespa Opening windows works for us too. Regards, Toodles.

I wonder if those charts could be redrawn to make the LWT and RWT trends easier to read?  Was the system in eco mode during the night when it was cycling?  Can you add compressor speed to the chart?

A heat pump works by condensing the refrigerant against the water flowing through the heating.  As the RETURN water temperature increases, the compressor discharge pressure must increase otherwise the refrigerant will not condense.  This is why the COP usually goes down as the LEAVING water temperature goes up, because the return water temperature also goes up.  However, if you have a house with a huge thermal mass (lots of concrete underfloor heating) the return water temperature doesn't go up very much, because all the energy is removed from the water.  So houses with huge thermal mass and underfloor heating should be able to get very high COP, at least upto the point where the slab is "fully charged".

Anyway...assuming its the RETURN water temperature which sets compressor power and therefore COP, you would get the best efficiency at the lowest return water temperature.  It looks to me like eco mode is trying to limit the return water temperature.

I would say engineering is about knowing the theory of how the system should work and then adjusting the design based on what you know actually happens.  I strongly suspect this system is benefiting from the oversized compressor not being run flat out, but it will cycle at low power demands.  My opinion would be to have two heat pumps, so you could achieve very low power outputs from 1 pump, and high outputs from 2 pumps.

Regards

Bob

Posted by: @bobtskutter

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My opinion would be to have two heat pumps, so you could achieve very low power outputs from 1 pump, and high outputs from 2 pumps.

The Mitsubishi R29O pump has 2 compressors to achieve exactly that.  Sofaik the only domestic one so far to do this 

@jamespa we will design the emitters/radiators to suit the same flow temperature. If this can't be achieved with radiators or they take up too much space we will consider in  ceiling heating.

@old_scientist

Posted by: @old_scientist

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EDIT: Not sure about ECO mode on the Vaillant, but I use Samsung's Quiet Mode for DHW runs which limited the output (and hence the noise) to around 70%. This has the effect of ensuring the heat pump is operating in it's most efficient range (i.e, compressor not running flat out), and narrows the dT allowing the heat to transfer into the DHW tank at a slightly lower flow temp. Last summer I ran some tests and found it consistently used less energy each day to reheat the tank than in 'normal' mode.

That is interesting, I read an article recently advising against the use of "Quiet Mode" for heating, it is only for use when the heat pump is in cooling mode in the summer. This is so the pump does not create too much noise during a warm night.

How long and to what temperature does your set up operate to?

Posted by: @technogeek

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@old_scientist

Posted by: @old_scientist

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EDIT: Not sure about ECO mode on the Vaillant, but I use Samsung's Quiet Mode for DHW runs which limited the output (and hence the noise) to around 70%. This has the effect of ensuring the heat pump is operating in it's most efficient range (i.e, compressor not running flat out), and narrows the dT allowing the heat to transfer into the DHW tank at a slightly lower flow temp. Last summer I ran some tests and found it consistently used less energy each day to reheat the tank than in 'normal' mode.

That is interesting, I read an article recently advising against the use of "Quiet Mode" for heating, it is only for use when the heat pump is in cooling mode in the summer. This is so the pump does not create too much noise during a warm night.

How long and to what temperature does your set up operate to?

You could be correct (I'd love to see a link if you can find the article). My Samsung unit does weird things (does not behave as expected) if I use quiet mode when space heating is in operation. When running with weather compensation, the LWT does not stay at the set point, but continues to rise, so there is a bug there (or as you suggest, this is just not it's intended usage). I do not use Quiet Mode when space heating for this reason.

However, during a DHW run, where the LWT does just continually rise until the DHW tank set temp is reached and it switches off, activating Quiet Mode has the effect of limiting input/output/compressor speed? (I'm not sure what it actually limits). If quiet mode were not activated, when reheating the DHW from 30-45C, I would typically see the input power gradually rise over the 20-30min run, starting at around 2kW and maybe peaking at 4.7kW. The LWT may finish at around 9C above the DHW tank set point of 45C (so maybe 54C).

If I turn on Quiet Mode, the peak power draw is limited to around 3.3kW, and the difference between the final LWT and DHW tank set point is lower, with LWT finishing 6-7C above the set point (LWT 51-52C, DHW tank 45C). The run takes slightly longer but uses less energy input overall, and obviously we are not having to heat the LWT as high. I also benefit from longer runs at lower power, as more of the power used is covered by my 3.6kW solar array in summer.

This also has the added advantage that the system is not running flat out annoying the neighbours for 20-30mins at 4am.

@old_scientist here is the link

https://renewableheatinghub.co.uk/never-use-this-heat-pump-setting

I don't believe your approach is purely gut-feel.

You've used knowledge/experience to build in some features which allow you greater flexibility to control the heating efficiency.

Posted by: @aaron

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We’ve got underfloor heating (UFH) throughout with 100mm pipe centres buried in 98mm of beautifully polished concrete.

Somewhere you've picked up the concept that pipe-spacing makes a difference when embedding them in a concrete pad.

You've also decided to use several interpersed pipe-loops within the same concrete slab.

That allows you to turn off a loop if you wanted to lengthen the heating-time.
Equally you can turn on loops if you wish to increase the total volume of fluid in the system.

Those two actions will affect the amount of Heat-Pump cycling.

The vast majority of the respondents arriving here on the RHH Forum are lacking this sort of background knowledge.

They don't know enough to query the proposed solution being offered by an installer.
By the time they get here, we are often the "last gasp" port of call in a dispute with the installer due to the very high electricity bills they now have.

Informing the public about the underlying concepts is an important part of our role,
either through this forum or talking with friends & neighbours.