I went a completely different route VDI2035, no antifreeze valves.

System run WC mode all the time, circulation pump always on.

Have battery so minor sub zero power outage no issue. Battery drive the whole house.

Have a generator, so prolonged outage of power not an issue.

Issues I have with antifreeze valves, they dump the water out of the system, so when power does come on again you have no heating. Someone needs to fill and bleed the system before heating can restart.

Likelihood of system freezing up even in a few hours is very low.  Likelihood of antifreeze valves dumping the water from the system is reasonably high.

The village hall situation, where you really don't want to be calling out a plumber, need it fool proof, glycol filled to protect to -10.

Posted by: @johnmo

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Issues I have with antifreeze valves, they dump the water out of the system, so when power does come on again you have no heating. Someone needs to fill and bleed the system before heating can restart.

Do they dump all the water or do they just drip slowly?  I thought it was the former but the altenic video suggests the latter.

It seems to me this is an area where a properly engineered solution is needed, perhaps involving a battery.  Presumably only the heat pump itself needs protecting (I am guessing that the flexible pipework will stand freezing?). It surely isn't beyond the realms of possibility to come up with a solution which dumps enough to protect the odu, doesn't admit much or any air, and can auto restart, perhaps giving 24 hrs protection before taking more serious action needing manual intervention to restart.  A 100Ah car battery stores 1kWh, which can provide background heating for a fair while.  Heat pump manufacturers could build the logic into the controller and provide a terminal to connect a battery if desired.

I agree with your analysis of the village hall (having for many years been responsible for 4).  Glycol so no intervention guaranteed.

Welcome to the forums @toucan 

I’ve addressed this topic several times with experienced heat pump installers. There are quite a few differing opinions out there, and the general consensus from many in the industry is that VDI2035 can be a bit of snake oil. The idea behind it is that if there’s no oxygen, there’s no corrosion. However, for this to work, all pipework within the system must prevent the absorption of oxygen, which isn’t always the case – some plastics, for example, allow oxygen to pass through. Additionally, any top-up water needs to be fully treated to remove limescale, minerals and correct the pH to neutral – you can’t just top up the system with tap water.

A key benefit of using central heating inhibitors is that they typically include oxygen scavengers, which remove dissolved oxygen from the system water and help reduce the risk of corrosion. Some inhibitors also contain dispersing agents that break up and remove existing sludge or limescale, further helping to protect the system and improve efficiency.

While inhibitors can’t completely prevent corrosion, maintaining the correct levels can slow the rate of galvanic corrosion and prolong the life of your system. From what I’ve heard, the best holistic approach tends to involve using a good-quality inhibitor.

I also reached out to a few installers to get their specific thoughts on your case, and here’s what they had to say:

Damon says that, “First and foremost, you need to follow the manufacturer’s instructions. I’m not sure what Mitsubishi’s are, but my personal preference is no antifreeze or inhibitor – however, only Nibe and Viessmann allow this as far as I’m aware. If you go with VDI2035, it’s essential that the installer fully understands it and implements it correctly. It needs to be checked annually with all parameters recorded, and even with VDI, you should still use inhibitor or glycol. If you’re switching to a cold-water fill system, you’ll need to ensure the system is flushed properly if glycol was used before.”

Rob says, “This is just the tip of the iceberg when it comes to system design. Even VDI2035 will fail if other design and commissioning elements aren’t done correctly. Expansion, for example, is often misunderstood or incorrectly calculated. Get it wrong, and you can create a vacuum in the system, which will allow oxygen ingress, ultimately defeating your corrosion prevention efforts. Oversizing expansion won’t hurt, but you need to work on 10% of the total system water volume. For instance, if your expansion vessel is at the same level as the fill point and you charge the system to 1.0 bar, your charge rate will be 1.2 bar. If the vessel is in the loft, you need to account for the static height difference and adjust accordingly.”

Posted by: @jamespa

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Presumably only the heat pump itself needs protecting (I am guessing that the flexible pipework will stand freezing?).

Think you need a lot of hours to cause any issues. There is an article on here suggesting 12 or hours in a freezer.

Posted by: @jamespa

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drip slowly

A slow drip overnight is quite a bit of water lost from system, as soon as circulation restarted air pocket has to move somewhere, usually not a good place.

Posted by: @editor

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VDI, you should still use inhibitor or glycol

Not correct, if you use inhibitors or glycol then VDI 2035 is not applicable to your system. You are using a different system. My heat pump instructions has all the water properties identical to VDI, but you are allowed to 40% glycol. So it's not called VDI.

Posted by: @editor

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Additionally, any top-up water needs to be fully treated to remove limescale, minerals and correct the pH to neutral – you can’t just top up the system with tap water.

It really depends on the properties of your tap water. In most areas of the UK tap water isn't far off VDI spec.

Posted by: @editor

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key benefit of using central heating inhibitors

And one of the downsides after a few years when the chemical is depleted, you top up, with more chemicals, meanwhile the exhausted chemical can become toxic to the system if not flushed out. 

All systems fill methods have plus and minuses, key is they all need maintenance and maintenance of the heating media is rarely done. Hence lots of threads where black sludge is mentioned.

I don't like using glycol unless absolutely necessary.

The specific heat capacity of water is 4200 J/kg°C, whereas Glycol is only 2433 J/kg°C.
Adding glycol reduces the heat you're able to push through the system.

Or, looking at the other side of the coin, it takes longer to heat the building...

Or, on the third side of the coin(?!) you need a larger UFH circuit.

Here's the classic glycol/water mix graph.

To protect to -10°C you'd need to have 24% glycol in the circulating fluid.

That equates to 3770 J/kg°C, which is a loss of 10% of the heat capacity of the fluid.

What's the issue you're trying to solve @toucan ?

Are you switching off the village hall heating for days when it's not being used?

Or are you wanting to protect the system from damage during a power outage?

Interesting article by Graham Hendra on the subject: https://renewableheatinghub.co.uk/heat-pump-freeze-protection-are-we-overreacting

Indeed.  Several of the manufacturer installation guides I have read say that freeze protection must be fitted.  Installers tend to cling to what the manuals say like a limpet, however doubtful.  So notwithstanding Grahams arguments, which are very reasonable, they will likely be fitted.   

@transparent    Protection of the system from corrosion & possible freeze. Hall is in use all the time so perhaps risk of freeze is very low. However it feels irresponsible not to protect the system. It feels like reluctantly in our case Glycol is the most appropriate method.

Thank you for all your thoughts   

OK @toucan  That's helpful.

But you won't get any corrosion prevention from adding glycol.
To achieve that you require an Inhibitor which prevents oxidisation of the metals within the pipework.

Fernox F1 and Sentinel X100 are the most common.
Both companies offer pre-sales advice.

If your water is slightly acidic (as mine is), or if your area gets limescale in kettles from dissolved carbonates, then please ask us for further advice.

Longer term, I'd recommend that the Village Hall Committee considers installing storage batteries, and operate the heat pumps (and the entire hall!) in an "off-grid" fashion.

That would provide you with a reliable 240v supply even during a power-cut.
You could choose a time-of-use tariff to recharge those batteries at cheap-rate.
Reducing electricity bills is usually pretty attractive to a Hall Committee!

Optionally you can run lights from the same batteries.
That leaves the entire hall illuminated in the event of a power-cut, rather than just 'emergency lights'.

The price of lithium has dropped to one-third of its 2022 value.
Now is a good time to prepare yourselves for the future grid.

Posted by: @transparent

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But you won't get any corrosion prevention from adding glycol.
To achieve that you require an Inhibitor 

Sorry that my be true for straight glycol but not the correctly specified antifreeze for heating systems. One example being from Screwfix (many others available). Adding too much chemicals or mixing can be as bad no chemicals or worse.

Flomasta Concentrated Central Heating Inhibited Antifreeze 5Ltr (4409)

Effective frost protection, limescale and corrosion control. Suitable for all heating types, including those containing aluminium. Compatible with No Nonsense products. Suitable for use with underfloor systems and ground source heat pumps (GSHP). For use in accordance with BS 7593 code of practice for treatment of water in domestic hot water central heating systems. Dilute at point of use. Addition rate: 25% of system volume.

Of note never use Glycol specified for cars in your heating system, as it is poisonous. Only heating grade specifically for heating systems as it's a different type of glycol which is not poisonous.

The performance downgrade and heating efficiency losses are generally overstated, so take with a pinch of salt. But for reference here are attached the actual information from an ASHP manufacturer for reference.

 KISS, works every time. Just keep it simple. Add antifreeze of your choice, check data sheet to ensure it meets the required frost protection required and has corrosion inhibitors. Get plumber to check effectiveness of the frost protection every couple of years.