From what I understand you mostly don't need the antifreeze in the pump. When it's cold, it's going to be running so it's not going to get to the point that it freezes. Even if it was off, it's not going to freeze in a couple of hours.

Mitsubishi recommend 25% glycol based antifreeze in the system, which (I think) protects down to -16C.  I recently had to remove 3 rads.  I asked the supplier if I needed to top up. He said if I topped up with water it would reduce the protection to about -10C. In the end, I bought 5L of heating glycol for £26 from screwfix and my plumbing guy topped it up.   

It's a bit fiddly to top up; the easiest way is to drain a radiator, remove a bleed valve and  and pour it in.  This is a lot easier with a towel rail than a radiator.  You could DIY or get a plumber to do it like I did.  

To drain and refill the whole system will be expensive as the antifreeze alone will cost hundreds. 

I wouldn't risk protection only being to -3C; that will happen and if there is a power cut you might be in trouble. 

My freeze stat is set but it only comes occasionally when it's below 5C.  I'll have a look at the settings later, not at home now.

@kev-m Thanks for the reply.

I was told by someone that it wasn't good practice to top up these systems via a radiator. No idea if that is correct or not.

Our property was a new build in 2019. The commissioning document states 25% antifreeze added. A year later, at the first service by Mitsubishi, they measured the protection as down to -6C. So my guess is that the 25% originally added was just guess work on behalf of the installer. They did not even mention this after the service, as an issue. I have no idea how much water the system contains but the quote from Mitsubishi specifies 35 litres of antifreeze required for protection down to -13C for installs in England. Plus a ridiculous amount for the labour. Hence the ENORMOUS quote total. 

@scratchyman 

I'd never heard not to add through radiators.  Too late now for me! 

My freezstat settings are 20C at 5C and my cold weather function is set to -15C.  I don't really know what these mean to be honest.

Posted by: @kev-m

@scratchyman 

I'd never heard not to add through radiators.  Too late now for me! 

My freezstat settings are 20C at 5C and my cold weather function is set to -15C.  I don't really know what these mean to be honest.

Hi Kev,

What is the actual description, and where are they in the manual? I may be able decipher what they mean.

When we were looking at your MMSP system, I noticed that it should have been calibrated for an antifreeze mixture of 25%. I have also read in some of the manufacturers manuals that their internal energy measurement systems do not work if antifreeze has been added to the system. When they say do not work, I think that if you set the parameter to indicate that antifreeze has been added, it does not produce any data because it knows that it will be incorrect.

@derek-m, they are on p35 of the FTC6 manual.

My flow meter has a sticker on it saying "25% glycol" so the installer at least aware that it needs to be set up that way. Whether it is, is a different story.

Posted by: @kev-m

@derek-m, they are on p35 of the FTC6 manual.

My flow meter has a sticker on it saying "25% glycol" so the installer at least aware that it needs to be set up that way. Whether it is, is a different story.

 

Hi Kev,

I think that the sticker means that the flowmeter was calibrated with a 25% glycol mixture, so should give accurate readings provided that your system contains a 25% glycol mixture.

This is a very interesting subject, and when it comes to heat pumps, the experts are yet again divided.

Our system, for the record, has glycol in it which GES were insistent on. Heat pump purists (guys like RED) will tell you that a properly designed and installed system has no requirement for antifreeze or glycol.

Derek, however, has a great point – what happens if the power goes off after a storm and it's -5C outside and you don't get your power back for 2-3 days (over over 10 days like some households after storm Arwen). 

Interesting subject and conversation. 

Posted by: @editor

This is a very interesting subject, and when it comes to heat pumps, the experts are yet again divided.

Our system, for the record, has glycol in it which GES were insistent on. Heat pump purists (guys like RED) will tell you that a properly designed and installed system has no requirement for antifreeze or glycol.

Derek, however, has a great point – what happens if the power goes off after a storm and it's -5C outside and you don't get your power back for 2-3 days (over over 10 days like some households after storm Arwen). 

Interesting subject and conversation. 

To which Derek do you refer?

@editor I should imagine that damage to your property from lack of heating would be covered by your insurance as its "storm damage" in that scenario. I am sure they might try to argue differently, however if the power goes down it doesn't matter what heating system you have - your pumps wont be working so they would all potentially suffer the same fate.

Posted by: @batalto

@editor I should imagine that damage to your property from lack of heating would be covered by your insurance as its "storm damage" in that scenario. I am sure they might try to argue differently, however if the power goes down it doesn't matter what heating system you have - your pumps wont be working so they would all potentially suffer the same fate.

Hmm, If I were an insurance assessor and the ASHP I was inspecting froze and broke because it hadn't been filled to the correct spec, I might recommend not paying out.