Posted by: @morganr

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But am I right in thinking a LLH isn't as bad as a buffer tank..?

I've not really thought about low loss headers much, and so I asked @heacol if they were as evil as buffer tanks today, to which he replied: "Just as evil, with the same or more distortion." 

@morganr I agree the table is quite confusing. I'm not sure if it is intended to cover a range of heat pumps rather than just one model. 

The first line suggests it is a 3 phase unit and I'm assuming your property is single phase like 99%+ of domestic properties. You won't need a 3 phase heat pump.

In the shoulder season for heating you will need to be able to get down to about 1.5 kw output (300 watts input at a COP of 5) to avoid excessive cycling so I think that is the minimum output i would be looking for. I would ask in writing for clearer data on this before committing. 

Posted by: @editor

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"Just as evil, with the same or more distortion."

@editor Hah, interesting! Thank you for asking! I assumed they weren't as bad just due to the smaller size, and the way they're spoken about in some of the Heat Geek videos I watched (here I think) sounded not AS evil. Partly because they have at least one upside that buffer tanks don't: filtering of air and particles.

@bontwoody The table I showed was only covering one specific 6kW heat pump (I cropped the rest of it which refers to their 10 and 14kW versions):

Posted by: @bontwoody

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The first line suggests it is a 3 phase unit

Oh that's very interesting! Would a 3-phase unit not work (correctly) in a single phase property? If not, I wonder if that just hasn't been updated when they presumably copied the data over from their European website (where I think 3 phase is more common?).. cos they're definitely installing these in the UK atm. I'll ask about that and the minimum output for sure, though!

you will need to be able to get down to about 1.5 kw output

Can I ask where the 1.5 kW comes from, just so I understand what I'm asking them? 🙂

I did a little thinking on it myself! In February this year we used an average of 31 kWh of gas per day for heating + HW. Assuming a generous boiler efficiency of 92%, that’s about 28 kWh of energy actually used.

If we use the same energy to steadily heat the house with a heat pump over the course of 24 hrs (rather than our reasonably low & slow boiler), that’s only 1.16 kW per hour. If we only run the heat pump for 12 hours a day (eg let it switch off at night), that’s 2.3 kW per hour.. 

Which, if a HP can only modulate down to 2.5 kW, is still too low. 😮

And that calc is including HW usage too..

And February isn’t exactly the mildest of the months that we’d want heating on..

So yeah.. unless my logic is wonky, it does seem like we’d want a heat pump that can happily modulate down to between 1-1.5 hW! Even if COP is 4 not 5, that’s 250W.

Does anyone know which (if any) HPs can actually go that low..? 😲 

Rather than looking at average energy usage for Feb, do you know what your gas usage was for the coldest day last winter (or the day with your highest usage). Assuming a 'normal' winter, this should give you a good approximation of your heat loss. You will need a heat pump that can deliver this amount of heat as a minimum at a temperature of -2C (or whatever the design temp is for your area).

In terms of modulation, a reasonable estimate is that most heat pumps will modulate down to 25% of their rated output, so an 8kW heat pump may be expected to modulate down to around 2kW. When considering this rule of thumb, beware heat pumps that are actually of a larger capacity but have been restricted in software so the manufacturer can offer a 'range' of sizes (e.g, my 12kW Samsung unit is actually a 16kW unit under the hood, so only modulates down to around 4kW [25% of 16], not the 3kW we may expect).

In answer to your question as to which heat pumps may modulate down to 1-1.5kW, by my rule of thumb I'd expect a 4-5kW model may achieve that, but that's not much good if your heat loss is 6kW at -2C.

Installers often favour one or two brands, and often client's heat loss will fall between two models within the range. I am a classic example of this. My installer used Samsung. My heat loss was calculated to be 7.4kW, which the 8kW model could not quite deliver at design temp of -2C, so they installed the 12kW model (which is really a software limited 16kW unit). In reality, my heat loss is actually lower and the 8kW unit would have been fine. Our 12kW unit cannot modulate anywhere near low enough for our property, and we are unable to run continuously until the temperature approaches 0C. Trying to find a heat pump that will meet the heat loss at design temperature with a little room to spare/safety margin AND that will modulate low enough for continuous running in milder weather is a challenge, particularly when an installer is picking from one one or two brands.

Hello @old_scientist!

Yep looking at the coldest day(s) is exactly what I showed in my spreadsheet on the previous page, here: https://renewableheatinghub.co.uk/forums/postid/46778/

You can see on the worst day in 2 years (18th Jan 2024) our boiler used 78kWh total (with it set to low flow temp and allowed to run all day if it wanted to, so pretty representative of heat pump style heating), which suggests a worst case heat loss of only 2.5-3 kW maximum.

So looking at the average day in February was my attempt to find a more ‘normal’ winter day, to see what our commonly required power might be. And with less than half the heat demand of the ‘outlier’ cold days, it looks like we’d need a HP that can regularly drop down to only 1.2 kW, and presumably much less than that on milder days that still require heating.. IF we want to run continuously on more than just 2 days a year!

Which sounds like we’d need to convince someone to put in a 4 kW unit really.. 🤔

Hello @old_scientist!

Yep looking at the coldest day(s) is exactly what I showed in my spreadsheet on the previous page, here: https://renewableheatinghub.co.uk/forums/postid/46778/

You can see on the worst day in 2 years (18th Jan 2024) our boiler used 78kWh total (with it set to low flow temp and allowed to run all day if it wanted to, so pretty representative of heat pump style heating), which suggests a worst case heat loss of only 2.5-3 kW maximum.

So looking at the average day in February was my attempt to find a more ‘normal’ winter day, to see what our commonly required power might be. And with less than half the heat demand of the ‘outlier’ cold days, it looks like we’d need a HP that can regularly drop down to only 1.2 kW, and presumably much less than that on milder days that still require heating.. IF we want to run continuously on more than just 2 days a year!

Which sounds like we’d need to convince someone to put in a 4 kW unit really.. which would mean totally disregarding MCS’s guidelines.. 🤔

@morganr 

Firstly congratulations on your understanding of this and background work.  You have clearly done due diligence.

Cutting to the chase on the various matters:

Everything you have said suggests your house is 3-4kW and no more.  You will struggle to find a 3kW heat pump, there are several badged as 3-4kW but most if not all are just downrated 5-6kW devices.  The down rating reduces the max output but not the min.  I'm not saying they don't exist (and hopefully somebody here can name a model), but they are at best few and far between.

This being the case you are almost certainly oversizing and will have spare capacity, so why on earth would you want a second heat pump to do hot water?  Heat pumps can happily switch between flow temps and are designed to do so.

A 6kW pump will likely modulate down to 2kW.  Whilst most pumps do achieve 4:1 at any given ft and oat, the most interesting comparison is max output at low OAT against mine output at higher oat (and correspondingly lower ft,). This is typically more like 3:1

Don't allow any form of system separation, llh buffer tank or phe, into your property unless there is a really sound reason, which so far there isn't.  It will simply add 15% to your running cost and hide problems which should be addressed.  System separation is a tool to benefit poor installers, commercial installations or very unusual homes, it's not a tool for householders of normal domestic properties.

Provided the heat geek table shows you can get sufficient heat through the rad tails, which it does, you should be fine unless radiators have been daisy chained.  You will only find that out by trying, so I would try it and fix problems if they exist rather than fixing problems that almost certainly don't exist.

If you want occasional cooling in one or two rooms (I think you mentioned that) its worth considering fancoils in those rooms, but make sure your chosen heat pump can monitor and operate above the dew point. 

Hope that summary helps.

@morganr Hi, like you I did an awful lot of my own calculations to ensure I knew what was needed. So I appreciate how much time you’ve put into it, well done. Having done that don’t left any potential installer pull the wool over your eyes.

Yes like James above I concur that your house needs a 4kW pump but you’ll probably have to have a 6kW because of availability. You absolutely don’t need a separate pump for DHW that is 5*BS.

In particular the lower efficiency on DHW is the higher temperature needed it has virtually nothing to do with the rate of change of temperature, that is simply the heatpump power, cylinder size and end temperature needed. 

When an electrical water pump tries to overcome the resistance of pipes it gets to a pressure limit sometimes expressed in units of height but correctly is in Pascals Pa. Then no matter how much more electrical power goes in nothing more happens. This is totally separate to the heat output power of a heat pump. Some heat pump models have an internal water pump so it is external to the house, others use a separate water pump and it might be fitted inside the house. There is no significant difference between these approaches.

When a water pump is trying to push more flow through a pipe the flow becomes turbulent and hence noisy, otherwise known as ‘singing’, tuneful or resonant. It is unpleasant to hear and eventually (as stated above) literally wears the inside of the pipe.

A low loss header is just one version of  buffer, no matter what else you get told. The only sensible version of a buffer to accept is one in series with the heating pipe on the return side, since it helps during defrost cycles.

In shoulder season your ashp will switch off when it has reached the correct temperature to heat the house, and you need system volume to keep these cycles nice and long. A series buffer will help with this.

@jamespa & @judith Thank you for giving me confidence in my logic! It’s great to hear I’m kind of on the right track, and it’s super helpful to hear about your own experiences. 😀

Adlar responded to my questions today:

Power:

The lowest under part load condition 15% is 2.030kW, with effective power input of 0.266kW. It is single phase 230V, that is an error.

266W sounds fairly reasonable and standard for a 6kW unit then, from what you guys have said? And it seems we’re unlikely to find much smaller!

LLH / Pump

Happy to remove this based on what you’ve put forward. I am not sure why our HL is so off but I will send the feedback back to the survey team.

Good news here then! 

So on the topic of having 1 external HP only providing heating plus 1 internal unit which is a hot water cylinder with its own HP (similar concept to the Mixergy iHP), can you see that being significantly detrimental vs a single HP system, or just superfluous for our case?

I ask because we’d effectively have to pay a lot more to get a single HP system.. 😣

My main reservation is that the heat pump cylinder is slow to reheat. Which means we’d probably go for a 300L tank to make up for that.

We could otherwise pay a bit more to go with a Cosy 6, but the only place Octopus will install is on the lawn of our tiny garden, which I think I’ll regret for many years to come..

Or we could pay a Lot more to go with a local HG Elite installer and probably put in something like a Samsung 5kW.

But according to my calculations, even if a very bespoke install achieved a SCOP of nearly 5, and Adlar’s system only achieved 3.5 or so, it’d take around 15 years for the difference in the running costs to offset the difference in the install costs! 😳 I think because our usage is so low..

Posted by: @morganr

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So on the topic of having 1 external HP only providing heating plus 1 internal unit which is a hot water cylinder with its own HP (similar concept to the Mixergy iHP), can you see that being significantly detrimental vs a single HP system, or just superfluous for our case?

Not detrimental just superfluous and something else to go wrong, but I cant understand why its more expensive, cylinders with integral heat pumps arent cheap!  Also, check the noise spec given its going to be indoors. 

I managed to find a bit about Aurora online and I note that they are claiming an advantageously high COP for their DHW heating, but of course thats because they are (presumably) drawing heated air from the house so they should be factoring in the COP to produce that heated air - which I dont think they are doing.  Some of their other claimed advantages for the two pump system are equally suspect.  Im suspicious when people claim things apparently incorrectly, but then most marketing teams do this!  On the plus side they make a feature of the fact that they dont use a LLH (albeit that they falsely claim that 'conventional' systems do).  The real question is why on earth do they effectively insist on a two heat pump system - is it something to do with the controller I wonder 9see comment below)?

Im presuming the heat pump proposed is the Aurora/AdlerCastra one.  This is, I think, the first time they have appeared here - it looks like a British Start up using industry components - nothing wrong with that inherently, CoolEnergy have been doing it successfully for a while.  It might be worth finding out about the control unit (particularly given their insistence on a two pump system) - heat pump controls vary quite a lot in their flexibility/capabilities/simplicity of UI and some heat pumps ideally need an add on like Homely if you want to get the best out of them and be user friendly.

PS here is a genuine 3kW heat pump, rather odd as its entirely internal and ducted (but I suppose no different to the heat pump cylinder in concept!). 

As to more conventional brands a quick search shows:

• Riello do a 4kW but Im not sure if its a derated larger model, I think this is also available with alternative branding (The Baxi may be a Riello, Midea and Riello seem to share pumps)
• Vaillant's smallest is 5kW (actually nearer5 6).  The one they brand as 3kW is just the 5 derated. 
• Mitsubishi do a 5kW.  I think it might be a 'genuine' 5 but all of Mitsubishi's heat pumps are software limited at the top end for some reason, that said they do publish very comprehensive data
• Ideal also do a '4' but its closer to a 5 in reality.

Further to the above I found the Aurora/AdlerCastra user Manual

ADLAR.CO.UK "https://www.adlar.co.uk/wp-content/uploads/2024/05/Adlar-Castra-User-Manual-Guide.pdf"

Page 4 concerns me, the WC curves are stepped as a function of OAT not smooth and the steps are, in some cases, quite large.   I have no idea why you would do this and it might suggest that the intention is that the actual house temperature is controlled by a thermostat/temperature sensor not principally by the WC.  Im not saying its wrong, just unusual, and it begs the question why?  It smacks perhaps of a product in development rather than one which is 'finished' but at the same time there may be a sound explanation.

Personally I would want to understand this product and the company a bit better before purchasing it.  They seem to be doing things differently for reasons that aren't clear and I might also be concerned about long term support.  Of course different is sometimes very good! 

PS It looks like they are the British arm of a Dutch Company.  The Dutch and the Germans are generally reckoned to be 'ahead' when it comes to heating.

https://find-and-update.company-information.service.gov.uk/company/15579925/officers

https://find-and-update.company-information.service.gov.uk/company/14758039/officers

https://www.creditsafe.com/business-index/nl-nl/company/castra-groep-bv-nl06261143