@kev-m @scratchyman

The Daikin DHW Manual was vague in that it just said "Glycol", the Monoblock manual has these handy hints...

The installer has left this, none been added yet, its says its non-toxic and biodegradable 

its here:  

SOLARANDHEATSTORE.COM "https://solarandheatstore.com/product/e600-heat-pump-thermal-fluid-25l/"

E600 Heat Pump Thermal Fluid 25L - Solar and Heat Store

25L Pre Mixed to treat 25L System to minus 10 degrees. E600 Solar Thermal Heating System Fluid is a blend of Mono Ethylene Glycol, Inhibitor and Biocide, simple add E600 to your system without any dilution in order to receive the full minus 10 degree protection.

contains  Mono Ethylene Glycol ? not propylene

@saf1973,

if it contains ethylene glycol, it's not non-toxic.  

EN.WIKIPEDIA.ORG "https://en.wikipedia.org/wiki/Ethylene_glycol_poisoning"

Ethylene glycol poisoning - Wikipedia

The reason Daikin says to use propylene glycol in a system that includes DHW is that there is a small chance that the flow water could leak into the DHW and someone could drink it.  Propylene glycol is much less toxic.    

Anyway, that E600 says it's pre-mixed and doesn't need diluted.  That suggests it should be used to fill the whole system.  25% or so of that in your system isn't going to do a lot.  On the label it says "add HTF50 for lower temps". HTF50 is the full strength concentrated stuff, which is what the Daikin table refers to.  

An alternative approach to using glycol is to use anti-freeze valves.  Basically you put these at the lowest point of the ASHP outdoor circuit, and they open when the temperature in the pipe gets close to freezing.  This allows the water to drain out and air to get in, preventing a burst pipe.

One question I have about glycol: if you want to make modifications to the plumbing, perhaps moving a radiator, if there aren't suitable isolation valves what do you do?  With a normal system you might drain and refill, or use a pipe freezer, but you can't freeze the glycol and you'd rather not throw it away.  Do you have to pump it all out, store it and and pump it back again afterwards?

@prunus 

I saw them mentioned somewhere else but had never heard of them.  When I had 3 rads taken off to decorate I wasn't sure whether to bother topping up the glycol.  In the end I did; the plumber closed the valves, removed the rads then put the glycol into one of them when he replaced them. 

The installer is coming back next week to fix my MMSP, which will need the system drained.  I think they said they would drain it into a container and re-use but we'll see.

My system has the expansion valves on both external pipes  meaning only water is flowing. If the system is off and the pipes freeze, then the valves allow water to flow out before freezing. You then refill via the hydro box, in my case in the garage.

I asked for the glycol level to be checked for my ASHP as it's so frequently topped up due to endless pressure drops. Using a refractometer the guy said it should be safe down to -5°C. I told him that I am starting to see outdoor temps of -7°C but he wasn't concerned even though he said HP's are normally topped up to provide protection down to between -10°C and -12°C. I also live in an area with frequent power cuts, although these generally last only max 3-4 hours. Should I be concerned?

@saz The issue I had was with getting the ASHP manufacturer to provide breakdown cover. They said they wouldn't if the glycol level was too low. But they relented and would cover the system if we switched on the anti-freeze function on the controller. I have done that, and now, at the present very low temperatures (-6C at the moment), the recirculating pump is constantly running.

A chinese copy of the Celeffi anti-freeze valve, £19, screwed into a 28mm press T. All the anti-freeze protection you need, without a drop of glycol.

@scratchyman Yeah, the defrost cycle is enabled. I wouldn't want my circ pump running 24/7 to be honest with you. Getting crazy low night time temps here at the moment!. We get alot of power cuts. I'm trying to work out a balance between risk and reduced efficiency due to the glycol.

@hughf I've heard about these but unfortunately I wouldn't be allowed to make any mods to my system.

Posted by: @saz

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@hughf I've heard about these but unfortunately I wouldn't be allowed to make any mods to my system.

are you a tenant? If not, it’s your system, you can do with it what you please. 

@hughf Sadly yes.