Great question. @heacol will know the answer.

According to Samsung, you either have to have antifreeze valves or glycol. As glycol will add 20%+ to your heating bill, it is not really an option. In reality, if you want top install them, 1 will suffice on the lower pipe, however, it is nearly impossible for ice to damage the Samsung unit as the internal connecting pipes are rubber, and will expand on freezing. 

It is up to you, I have about 600 units in the field, of all makes, and no glycol or antifreeze valves, and have not, in the last 15 years, had a single unit damaged by ice.

@heacol Thanks Brendon, that is very helpful.

@heacol I’ve also wondered how that worked. Will the system remain protected if there’s a big winter storm with temperatures below zero and power lines get taken down, and we’re without power for a few days. Wouldn’t that do some damage and wouldn’t stuff that shouldn’t freeze, freeze?

@editor I am not saying the units will not freeze in some conditions, and I have seen many, however it is the design of the unit it'self that will determine if there is any damage. Units that have standard circulator pumps in them such as Valiant, Midea/Clivet and others are vulnerable as standard circulators can easily be damaged by freezing, some units with pumps like Panasonic are not affected as they have taken the time and effort to make their own pump. Units that have no pump, like Samsung and Mitsubishi, are not usually affected.

As I said, it is your choice. My choice is Panasonic hands down, it is so far ahead of the pack for many, many reasons.

Posted by: @heacol

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As glycol will add 20%+ to your heating bill,

Really? That is an enormous claim, citation required.  I am aware of talk around a small increase,  but 20% plus ( by how much plus) is astonishing.

@abernyte Difficult to get specific figures but I work on

Water 4.184kj/kg.K. 

Monopropylene glycol 3.884kj/kg.K (@30%)

Monoethylene glycol 3.712kj/kg.K. (@30%)

Pump at say 50W will cost 1p/hour to run, with glycol added 1.1p/hr.

@david-i interesting. Thanks for sharing. So do you avoid glycol in systems too?

@editor No Glycol in our Daikin installation but we do have two antifreeze valves. When the ‘Boss’ came to do a little ‘snagging’, he was surprised (and I think, a little miffed due to their cost) that his installers had fitted them in both pipes rather than just one. I was rather pleased that they had fitted them in both as surely, if the pipes are both very cold, isn’t better that they both drain? (Or is not how this precaution is supposed to work perhaps?). (Daikin are coming today to carry out the first annual service) Regards, Toodles.

Anti-freeze valves provide some guarantee of a fail safe if the system is unpowered for a long period and all other forms of frost protection such as the unit pump circulating water and/or frost protection heating in the circuit fail. You are into double jeopardy however - a long power failure coincident with extreme prolonged low temperatures. Anti-freeze valves should be on both outdoor unit supply and return pipes at the lowest point. Glycol isn't really required in the UK provided external pipework is properly insulated and protected.

Glycol lowers the specific heat of water with increasing concentration, it's the renewable parallel of adding hydrogen gas to mains gas methane, which lowers the calorific value of the resulting blend. That's why using H2 for domestic heating is a dumb idea, aside from the cost of producing it and the safety issues it creates.

Which does not get us anywhere near a 20% plus energy cost increase caused by using glycol.