Welcome to the forums Sally. To answer your query, it’s important to know that there are two different types of system additives that might be in your heat pump circuit and we need to know which one he’s talking about:

1. Inhibitor: this is a corrosion inhibitor used to protect the metal components inside your system (pipes, rads, heat exchangers, etc.). It doesn’t provide any protection against freezing… it’s purely there to reduce the risk of rust, scale and sludge build-up.
2. Glycol (antifreeze): this is used to lower the freezing point of the water in your system if any pipework is outside or in unheated spaces. Most glycols also contain inhibitor as part of the mix, so you often get both in one product.

There are two main types of glycol:

• Ethylene glycol: slightly more efficient thermally, but toxic, so more commonly used in sealed systems.
• Propylene glycol: less efficient but non-toxic, and often preferred for domestic settings.

Now, on to testing and answering your question.

If your engineer said the antifreeze level was low, the correct way to confirm that would have been with a refractometer which is a small handheld tool that reads the glycol concentration by placing a drop of fluid on the lens and checking the reading. It tells you roughly what the freeze protection level is (e.g., down to -10C or -20C). It’s a quick and reliable way to check whether you actually have glycol in the system and if it needs topping up.

Did you see him use one? If not, it’s worth asking what he based his assessment on. To my knowledge this is the only way to verify glycol concentration. If you didn’t observe him doing this, he didn’t check for it.

Thank you for your detailed response.
So if I don’t know what type it is, is it necessary to drain the system before topping up?

@sallyl that I don’t know. 

@heacol can you please cast some light on this? 

Posted by: @editor

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Ethylene glycol: slightly more efficient thermally, but toxic, so more commonly used in sealed systems.

Sorry that isn't correct, it may be used in vehicles, but has zero place in a domestic heating system. An issue with a cylinder heating coil you can end poisoning the house in a worst case.

Bigger question is do you need antifreeze? My answer is no. You don't even need antifreeze valves.

I really wouldn't worry to much.

I changed to VDI 2035, which is very clean water, no additional additives. No anti freeze valves or antifreeze in water. We get -9 outside temperature in winter. No issues to date.

Posted by: @sallyl

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but unless he knew what type it was he would have to drain the whole system, because there might be a chemical reaction.

Pretty unlikely. Just top up you wish with heating system antifreeze from Screwfix. To test drain a little (half cup) from heating system into a cup, add same amount of Screwfix antifreeze mix and leave overnight. If its looking the same it's ok, if it's changed colour, gone thick or any other change maybe not compatible.

Posted by: @johnmo

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Sorry that isn't correct, it may be used in vehicles, but has zero place in a domestic heating system. An issue with a cylinder heating coil you can end poisoning the house in a worst case.

It may have no place in domestic heating, but it has been used.

This is what is used in nearly all ASHP system that have glycol.

If they have used the wrong antifreeze you would want it taken out, if they used one of the 3 most popular monopropylene glycol's they can likely be mixed but there is no way to test which one you have without lab analyses and all of the manufacturers state do not mix them as this is a cover all for mixing chemicals and how the responsibility carries.

Its simply not worth the risk as the chem reaction could be caused over months and may do long term damage to your system.