Welcome to the forums. It’s generally recommended to premix antifreeze before adding it to ensure a consistent concentration in the system, especially when adding a specific volume like 5L. This also helps prevent any undiluted glycol from potentially “pooling” in certain areas. Premixing with distilled water is ideal to prevent introducing any impurities, although tap water can work if that’s not an option.

I only recently discovered that the concentration of glycol is indeed crucial for both freeze protection and corrosion prevention. Most ASHP systems need a concentration between 25% and 35% glycol. If the mixture is too diluted, it may not provide adequate freeze protection. Most antifreeze suppliers provide guidelines on concentration levels, so consulting the product documentation might help.

This is why installers use glycol refractometers.

Posted by: @swooned2437

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Hi. I have an ASHP system with glycol added at installation (not sure exactly how much). For this reason I have the freeze stat function turned off (although the installer left it on).

I drained a couple of standard sized dual fin radiators during the summer since I was decorating and needed them off the wall. When time came to connect them again I just topped up the system with the cold water valve at the cylinder.

My question is: I've bought 5l of antifreeze/inhibitor. Is there anything to stop me just draining 5l from one of the larger radiators and then directly adding the mixture at the radiator? Is there any reason to premix the glycol? I'm presuming the quantity is not super critical although I don't have an effective way to test the concentration.

Any advice appreciated!

Before you put glycol in the radiators, do you have a heat exchanger to separate the external plumbing from the internal.  Thats quite common (but not universal) when glycol is used, in order to avoid the (very small) risk of contaminating DHW and reduce the amount of glycol needed.

Thanks for your responses.

@editor That's interesting about "pooling" - I just imagined that there's no way the glycol would remain undiluted as it went round the system, being mixed at various points. Seems a stretch if I'm honest.

@jamespa That's a good point. I'm not entirely sure. It's an Ecodan with pre-plumbed cyclinder. I was under the impression that there was a heat exchanger to heat the hot water in the cylinder and a diverter (?) was used to change the circuit for the water being heated by the outdoor unit (but ultimately the water going to and from the ASHP ends up being the same water that is used in the radiator circuit). I've probably not explained that very well - maybe someone else with an Ecodan can chime in.

Posted by: @swooned2437

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@jamespa That's a good point. I'm not entirely sure. It's an Ecodan with pre-plumbed cyclinder. I was under the impression that there was a heat exchanger to heat the hot water in the cylinder and a diverter (?) was used to change the circuit for the water being heated by the outdoor unit (but ultimately the water going to and from the ASHP ends up being the same water that is used in the radiator circuit). I've probably not explained that very well - maybe someone else with an Ecodan can chime in.

Your description of the Mitsubishi pre-plumbed cylinder is what I understand also.  If there is a (separate) heat exchanger to separate glycol from non-glycol it will be before the diverter valve, possibly with a pump either side.  It sounds like probably not but it would be sensible to trace the pipework back and check!

Posted by: @swooned2437

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That's interesting about "pooling" - I just imagined that there's no way the glycol would remain undiluted as it went round the system, being mixed at various points. Seems a stretch if I'm honest.

I've just had your question answered in a podcast I finished recording. As it's coming out in 9 days, I wanted to reiterate that the experts said you should not do this the way you described.

@editor Hi, yes that does seem to echo the application guidelines on the product itself so I expect I won't just add at the radiator.

This feels like a 'best practise' versus 'good enough' question to me.

I've up-sized several of our radiators and just filled them with neat glycol through the top before opening them to the system. The sky hasn't fallen... yet. 

But after the upstairs loop was drained for more fundamental surgery I got them to top the glycol back up at the annual service. They did this with neat glycol using an electric pump via the filling loop. And the sky still remains in place. 

To monitor the concentration of glycol in the system I bought a cheap refractometer on eBay- item number 325240420511. 

It would be interesting @mars to hear what the argument is for pre-diluting the glycol. 

I imagine its the risk that a slug of it gets stuck somewhere/coats some component (eg the heat exchanger) before it fully mixes.  If that could happen then its potentially nasty.  I believe that the viscosity of glycol reduces as it heats up so, if put into a cold system (which is quite likely) then there could conceivably be a slug of quite viscous liquid for some while.  Propylene glycol is more viscous than ethylene glycol (but less toxic), Im not sure which is used.

How likely any of this is I have no idea!

I've recently topped up by adding a few litres at the towel rail - Fernox HP-5c (which was used in the original install). It's Propylene based and not massively viscous. I feel it maybe took a little while to fully mix in but nothing to write home about. No negative effects to report!