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. 

Good Morning! Just wanted to jump on and introduce myself. I'm Bec, and I work for a UK-based company called Fogstar. I'm interested in emerging strategies to make better use of renewable energy sources - battery storage in particular.
I live in the West Midlands region, for which NGED is the DNO. Always happy to answer questions on battery storage or battery systems integration, I have quite a bit of experience in this area.

@fse_bec welcome to the forums, and it’s very generous of you to offer your expertise and experience. There are quite a number of Fogstar users on these forums already.