Thanks for those insights @eggnchips

For others who come across this in future, let me point out that the higher the concentration of glycol, the lower the (external) air temperature at which the heat pump can operate.

However, glycol has a lower heat capacity than water. So a neat glycol system can't transfer as much heat as the HP is designed to operate at.
That makes it difficult for the control electronics to handle. It will be programmed with a curve which expects a certain range of temperature rise based on the flow-rate, but the 100% glycol system will be outside the range of the curve 😖 

A local heating engineer should understand the lowest likely air temperature which is likely to be met by a house in the area.
So they will adjust the percentage of glycol appropriately.

There's nothing to be gained by adding more than is necessary.

@transparent

While a heat pump is powered and operational, there should actually be no reason for anti-freeze within the system, since the thermal energy produced by the heat pump will keep the water within the system well above 0C.

Anti-freeze is added in case the electrical supply is lost during below zero weather conditions, when there could be the possibility of the water freezing and causing damage within the system.

The percentage of anti-freeze that would need to be added to provide complete peace of mind is therefore dependent upon the lowest ambient air temperature that could occur.

@eggnchips welcome to the forums. If you have questions about your system specifically please feel free to start a new topic.

HI @transparent. Thanks for the info. In our case the high concentration of glycol was just a mistake when installing the system. I was also told by the Mitsubishi engineer that excessive glycol damages solenoids and pumps and can reduce their working life. We do need something though as it gets down to -6 on occasions where we live.

If you're facing possible nighttime temperatures down to -6°C then you'd avoid pipes freezing with just 15% glycol.

I use a 20% solution in the pipes which run to my rooftop solar-thermal array. That protects down to -8°C
I'm south of your location in an 'old Devon farmhouse'.

That's a useful observation from the Mitsubishi engineer about possible damage to mechanical components in contact with pure glycol.

In the presence of oxygen, ethylene glycol breaks down to form organic acids which can cause corrosion of metals.
So the level of damage is also dependent on how much oxygen is available within your closed system.

We've had other examples on this forum where installers need to have a broader understanding of the chemistry behind the work they're doing.
Simply taking a course to become an MCS-approved heat-pump installer isn't sufficient.

As we move further towards technologies using renewable energy sources, it is ever more apparent that the level of basic science knowledge in the wider population is insufficient.
I will shortly be discussing that issue with engineers at National Grid.
Ofgem is also aware that the UK's progress towards Net Zero is constrained by lack of knowledge on energy matters.

@eggnchips according to @heacol, glycol can add 20%+ to your heating bill: https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/are-antifreeze-valves-essential-for-a-glycol-free-ashp-system#post-27610

Posted by: @transparent

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If you're facing possible nighttime temperatures down to -6°C then you'd avoid pipes freezing with just 15% glycol.

That's really interesting. I didn't realise there was a formula for this.

Hi,

Been making use of all the great information in various posts in the forum already but thought I should sign up and say hello. I'm considering an ASHP as a replacement for our current mains gas boiler as we are doing a renovation that will mean the boiler will have to be moved. My initial quote for the boiler move was astronomical so that triggered "for that money could an ASHP be a better option long-term?". Though since then I've had more realistic boiler quotes that muddy the water further when it comes to the financial aspect of this. 

The biggest challenge (not uncommon it seems from reading posts on here) is finding a competent installer!

As things progress I might create a thread but just wanted to say hi and thank you to all who've already shared their experiences - lots to read and think about.

Posted by: @gunboatdiplomat

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The biggest challenge (not uncommon it seems from reading posts on here) is finding a competent installer!

Welcome to the forums @gunboatdiplomat – where are you based in the UK?

@editor I'm in south Essex.

I've had some constructive conversations but a few usual niggles come up - the use of a low loss headers (especially when a preplumbed clyinder is proposed) for example. I've put myself down for an Octopus survey as well but who knows when that might happen as it sounds like a bit of a backlog there. 

@editor Hi, we are in Kinross-shire and have just completed installation of a hybrid ASHP/oil system. House is off-grid for gas, 4 bedrooms, single storey and built between 1970 and 1990. ASHP is a Vaillant AromaTHERM Plus 12 kW with a new Bosch Danesmore 18/25 oil boiler as topup via a buffer tank. This all replaces a 23 yr old 50-70 kW oil boiler.  We added 150mm of loft insulation to what was there, replaced decaying 197x vintage double glazing and plugged most of the draughts. We have cavity insulation but there is no access under the suspended floors and only 50 cm clearance so underfloor insulation would involve serious ripping up of floors.

Heat calculations indicated about 17 kW heating load on the coldest days hence the small auxilliary boiler so why the original boiler was this large is a mystery. We installed a Hive system 2 or 3 years ago which recovered its cost by allowing me to keep on top of the system but is now redundant except for controlling one smart TRV which we have left in.

8 out of 20 radiators needed upgrading but the deeper K3 radiators just went in one-for-one without trashing decorations or needing to move pipes.

I have a software engineering background so of course my instinct is to climb all over and inside how the new system works, not easy.  Like other contributors here I have a bunch of Govee thermometers recording the room and external temperature and I'll be installing a bunch of WiFi temperature sensors on the various flow and return pipes and possibly a couple in the fan unit airflow. I can monitor it all with HomeAssistant. 

This week I'm getting my head around it all, reading the forums :-), struggling with the MyVaillant app and I'm waiting for OvO to come back to me to move forward with their heat pump tariff. Costs otherwise look a bit high. Conscience not cost was the main motivation since our old system was generating about 10 tonnes of CO2 per annum. We had grant support for this and a condition was to get a new EPC certificate. The house has gone from 'E' to 'C', mostly because of the heat pump which is good news.

My grandly renamed plant room (formerly the boiler room) is now full-on Monty Python with pipes everywhere.

@dr_dongle If you wish to consider underfloor insulation, Q-Bot can use a ‘robot’ to spray under the floor with just 200 mm of clearance ( https://q-bot.co). Not cheap but we had ours done about 3 years back and have found it beneficial. They can gain access through a small hatch or create one for you if required and they are very tidy workers. They created an access trap in our understairs cupboard and left everything very tidy afterwards. Regards, Toodles.