Just updated my signature so my current situation might help. I do have solar panels with a 10kwh battery and drive an electric car so on Octopus Go tariff. 

@chansug 50C is probably the top end of what you would want to live with. Its not standard, all houses are different but for efficiency you want to get your flow temperature as low as you can. Also it wont be 50C all the time if you use weather compensation as you should. This article is also useful:

https://www.heatgeek.com/what-to-do-with-microbore-pipework-on-heat-pumps/

@chansug Also the solar, battery and EV will all help a lot with bills if you utilise a tariff such as Octopus Intelligent Go

I was looking for the link about what heat could go down each microbore pipe. But I’ve been beaten to it. Your heat loss document should give the loss for each room so you could check each radiator off against that. 
A number of people have used Octopus and then later as a diy task changed to bigger radiators so that they could run at lower temperatures. Which reduces your running costs.

Although there aren’t huge differences between heat pumps the only combination which works is the one the installer is used to fitting. So beware of someone who says no problem I can install any brand you want.

The other big difference is the human interface and degree of control available and how effective they are. You will read several threads on here of people struggling with theirs. The Octopus own brand one is too new to comment on, Clivet more unknown but Vaillant is well known across the industry.

Your gas usage has a wide range, the upper figures imply the upper range of pump power. Have you discussed this usage with potential installers?

My answers to your initial questions:

1. They are all close enough and match your usage figure to be right. I’d say the not so local firm is wrong to then specify a machine that is 50% oversized. 
2. The not so local firm doesn’t make sense, 10mm pipework is good enough from all I have read. I don’t put much trust in this kind of insurance, always tastes like curry upsell to me. I’d put more trust in octopus not going out of business in the next ten years and indeed heatgeek still having a good name to protect too, then the insurance not making excuses. Octopus is probably more likely to fix any problem free of charge after the fact. 
3. Personally I’d find something more fun to do with the extra £5.5k then get a similar specified heat pump, unless the extra service you might get from the heat geek is worth it to you. So a choice between octopus and heat geek.  

Good luck!

Hi @chansug, a warm welcome to the forum.

A few points if I may on your quotes recieved.

1. The heatlosses are all (relatively similar). You may wish to have an independent heat loss run or obtain the design parameters from the companies. I would imagine the key influencers are the air changes and the outside air temperature they have used. Outside air temperature should be adjusted depending on the property's elevation, exposure levels etc and taken from a local weather station. MCS have specific guidelines about how these can be calculated but they should be calculated by the designer and not pulled straight from the database.

2. Clivet/Midea are good units in my personal opinion if you are going for a lower-cost heat pump. The controls and operation are very basic if we are talking about the R32 unit (often referred to as M-Thermal or similar). The warranty is less than most major brands, iirc it's 10 years parts only but this may vary depending on the installer and their relationship. The Midea controller is, in my opinion, not great but the heat pump itself is reliable, quiet and efficient. It's the controller, intelligence of the system and the warranty and breakdown support that lets it down. The actual heat pump is just fine. It is pretty much a 'does what it says on the tin' heat pump. If someone was looking for low cost heat pump systems, this would be one of the reccomendations from me.

3. You'll hear a LOT of debate about microbore but the boil down of it is, you want your heat pump to operate at as lower flow temp (lower dT) as possible to achieve the highest efficiency. 50deg is high for a heat pump. Really 45 should be a max in the most brutal operating conditions if you want an optimally designed system. Is 50 the max flow temp permitted or a mean water temp? Do they propose the system will be room stat driven or weather compensation driven? These are all worthwhile questions.

I am personally not one to reccomend microbore on retrofits because it canes the efficiency of the unit by requiring you to set a higher flow temp because of decrease in mass flow rate and therefore increased dT (should be dt5 ideally but systems like this can creep to dt10 or higher). Even if you change all the radiators the pipework will choke the flow or you'll end up turning the heating circ pump to max with a really high flow rate and possible issues with noisy pipes/increased heating circ pump consumption- that is, if it even functions and doesn't go out on alarm.

There is a real risk that the system may not perform well because fluid flow becomes unpredicatable once you start pushing flow rates close to the pipework's max.

When having a heat pump fitted, going for a long-term solution, it's best to do the work upfront and change the pipes so you know that the whole system is heat pump ready and you avoid the risks described above. I would not personally feel comfortable signing off a heat pump on microbore save a replacement heat pump for an existing one that ran on microbore and where the homeowner was ok with the poorer efficiency and just wanted a like for like swap without changing pipework or rads.

Summary: Realistically the pipework needs to be swapped out for you to have an efficient, well-functioning low temperature heating system. A good technical surveyor will be able to calculate how undersized your pipework/rads are and show if and why it won't work.

PS the Vaillant Arotherm has a higher output than what it is badged for and the Clivet has lower. This is why there is a discrepancy in sizing. It's to do with the conditions in which the units are tested and is provided in a big table in the unit datasheets.

Posted by: @chansug

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I thought all heat pumps are designed to 50c specification, am I wrong in assuming that? All quotes i received so far are designed to 50C specs so I thought that is standard.

It might be 'standard' in that it complies with the way MCS-approved installers are required to undertake calculations... 😥 
... but the lower that maximum flow temperature, the greater is the efficiency (COP).

Have a look around the forum, and you'll see the there's a tendency amongst the better-informed members to operate HPs at 45°C

Equally if your total heat-loss is only 9kW, then there's no point installing an ASHP rated at 12kW !

Having a heat-pump which is 'over-specified' means that the installer is expecting some sort of thermostatic feedback to switch it off when your house reaches the set-temperature.
But heat-pumps are meant to run all the time.
Slow and steady is the right approach.

So the heat-pump rating should only be marginally greater than the heat-loss measured in the survey.

One other point about microbore, or any system where the pipework isn't adequately sized for the required transfer of heat...

To obtain the desired Δt (difference in temperature between flow and return) there needs to be a sufficiently large body of water to dissipate that energy.
When the pipes and/or radiators aren't large enough, the overall amount of water in the system is inadequate.

To compensate for that, installers will add another cylinder, a volumiser or buffer, and hence reduce the temperature in the return pipe to the heat-pump.

.

That cylinder costs you money...
... which would've been better spent in having the right-sized pipes.

Installers can make a lot of extra profit from adding volumiser and buffer tanks.
If the heat-pump and pipework is correctly specified in the first place, then that additional cost isn't required.

@chansug

Posted by: @rob_cirrus_energy

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3. You'll hear a LOT of debate about microbore but the boil down of it is, you want your heat pump to operate at as lower flow temp (lower dT) as possible to achieve the highest efficiency.

Hi Rob, im sure its just a typo but for the sake of not confusing the OP, you are not saying that they should be reducing the dT just the flow temperature. The dT should ideally stay around 5C.

This thread really got me thinking about my own understanding of the situation and I think its worth talking about it to hopefully explain things to the OP (at least as I understand them).

Having 10mm microbore limits the amount of heat energy per second (Power) that can be put through it at a certain flow velocity (usually 0.9m/s) and dT (the difference between incoming flow and outgoing flow temperature - usually 5C for a heat pump). Using water without antifreeze this power is 1.15 kW.

As long as the dT and flow velocity stay the same, it doesnt matter what flow temperature is used with respect to how much heat is dumped by the radiator into the room. However using a higher flow temperature means less efficiency from the heat pump but also a bigger difference between the radiator temperature and the room temperature. This makes it easier for heat to be moved into the room and so its possible to lower the flow velocity and still get the same result.

Lowering the flow velocity means you can use smaller pipework.

Hence microbore can be more easily used if you have a higher flow temperature  but will lead to lower efficiency.

If I am misunderstanding anything here then anyone, please feel free to jump in and correct me.

@transparent @bontwoody 

Hi both,

Thank you so much for your input really appreciate it. Some of the discussions are going well beyond my understanding. I do understand the relationship between microbore and the velocity of flow now and how it impacts the efficiency of the heat pump. So far I have had five quotes but only one of them have stated that they will change the microbore for their installation. According to them it is not a huge job for the size of my property. Google AI puts the cost of upgrading microbore for a 4 bed property at £2500 to £5000 but they are charging me (itemised) £1500. Why is that the rest of them including HG trained surveyors not recommending changing to 15mm pipes? 

@rob_cirrus_energy thank you for reassurance re Clivet. There is literally no feedback anywhere for this unit just like octopus cosy heat pumps. These are two options I am considering at the moment.

I totally understand the need for changing the microbore if not for the company who have been in trade for only about five years but seem to have excellent feedback on trust pilot. I am going to ask for a few more quotes with microbore change and go from there. 

@judith what is wrong with Clivet and where is the evidence for suggesting they are rubbish? They seem to have a decent reputation amongst installers (source- Heat Pumps UK facebook group where a lot of installers post) and ours was over 2k cheaper than the Vaillant. We are perfectly happy with ours and it is achieving decent COP figures. The equivalent Vaillant was also a huge double stack unit.

I'd look at payment terms, and proceed with caution if those terms are to pay the full balance minus the grant prior to installation. I've since had a read up on some of the companies that quoted us and asked for this, and sure enough the negative reviews have started to emerge with people having all sorts of problems they can't get fixed. Having some leverage in this regard has proved invaluable for us. Obviously less of a risk with the likes of octopus etc. without knowing their specific payment terms.