I have anti freeze valves. Due to complications of my install this past week in the cold snap, my anti freeze valves kicked into action and drained my system. The system was filled early afternoon, but they couldn't get the electrics to work so were unable to turn on that day. The temperature outside was below 0C all day and bottomed out at about -5C, so well below the design operation temp of the valves at 3C. The system started draining at about 11pm and was still going the next day. I'm unsure why it took so long, but I suspect a mixture of the valves being slow releasing and no air getting in from anywhere else to release the water. 

I would suggest if this is going to happen to anyone else to isolate the exterior pipe run from the interior pipe run so you don't have to refill the whole thing when power returns after a long power cut. I was going to do this, but had assumed that the installers didn't do so for a reason (since we discussed the possibility of this happening) so didn't want to mess with it, but they said the next day that I should have. Do NOT isolate the heat pump from the valves. 

As you can see from my timeline, you have a lot of warning that this is going to happen, and then it takes a long time to fully happen. Only if you were away from your house could this fully drain your system in my opinion, otherwise you will catch it, and if you are away from your house for so long in such conditions then you should be glad that the system did empty your interior pipework because that could burst too along with anything else that holds water in your house. Moreover inhibitor is cheap and easy to add through a rad, so if your whole system does dump then it's an easier fix than anti-freeze.

I suspect I'm one of a very small group of people this has ever happened to, but at least we now have proof the valves work. 

@oswiu - very interested to hear your account of what actually happens. Am I right in thinking that once the anti freeze valves have decided to do their thing, you have no heating until you refill the system? I assumed this is the case, and your account seems to confirm it is the case, but isn't that the Achilles heel of using such valves, they do protect the system, but in a way that makes it incapable of operating, possibly when you need it the most? And furthermore, how many residents are standing by, possibly in the middle of a freezing winter's night, with all the supplies and kit needed to refill the system, which in turn may be easier said than done?

@cathoderay yes I suppose you are right that you have to refill. I think if the outside is isolated from the inside this shouldn't be too difficult, but I think one should be mindful of the filling instructions in the manual eg mine says to run an air purge on the controller if I remember rightly. 

I doubt anyone is standing by in the middle of the night. My point was once there's a power cut then you should isolate outside, and you probably have in excess of 12 hours before anything happens which is long enough for power to come back in most circumstances, but if it isn't then I personally wouldn't worry about adding anything but water when it does come back because my exterior piperun is less than 10% of my system volume, so I'd think about inhibitor another day. That's not advice of course. 

My old gas boiler was in my unheated garage with only thin lagging on the pipework, so under such a power cut that too could have frozen along with the heat exchanger in the boiler but with no protection. 

@oswiu - I hadn't joined up the dots, and read your post to mean - I think I have now got it - it is as if you had a power cut. Bottom line was, for whatever reason, you didn't have a functioning heat pump that could protect itself from frost damage. On anti freeze additives vs anti freeze valves, I suppose both have pros and cons, and so it is horses for courses. Anti-freeze additives offer 'fill and forget' protection, at a cost, and until they need topping up, anti freeze valves also work, and I gather are cheaper (though whether that should be a consideration when the cost differential is £ while the cost of repairing a frost damaged heat pump will be ££££), but that add in the hassle of refilling the system, albeit a simpler process because only water is used.

@cathoderay yes that's right it was as if I had a power cut. This was the installation of my heat pump, so when the plumbing guys finished the pipework they filled the system up and went home, but the controller was showing an error and the ASHP wouldn't turn on. In the end they had to replace a cable the next day after it had emptied itself.

I discussed glycol vs the valves with my installer who gave me the option. I was keen to get the valves because they're cheaper to buy and you don't have to contaminate the water with less efficient glycol.

Posted by: @oswiu

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don't have to contaminate the water with less efficient glycol

True, I meant to mention that in the pros and cons but forgot. As I understand it, the dilution of heat carrying ability isn't huge, but neither is it trivial. I forget the exact figures, but even if is it only 10%, that means 10% less heat power delivered to the house, 9kW when an all water setup would deliver 10kW. That might well be enough to take marginal installations below the building's heat loss.

Posted by: @cathoderay

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Posted by: @oswiu

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don't have to contaminate the water with less efficient glycol

True, I meant to mention that in the pros and cons but forgot. As I understand it, the dilution of heat carrying ability isn't huge, but neither is it trivial. I forget the exact figures, but even if is it only 10%, that means 10% less heat power delivered to the house, 9kW when an all water setup would deliver 10kW. That might well be enough to take marginal installations below the building's heat loss.

The antifreeze in my ASHP has a Specific Heat Capacity of 3655 J/kg/K measured at 30% dilution at 25 deg C. Compared with water at 4184 J/kg/K that’s a 12.64% reduction, which seems pretty significant. Do antifreeze valves only operate if there is a prolonged power cut and low temperatures, assuming the ASHP has its own frost protection system?

@mike-h - I don't have anti-freeze valves, so don't know the answer. Maybe someone who does have them can answer.

Posted by: @mike-h

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Posted by: @cathoderay

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Posted by: @oswiu

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don't have to contaminate the water with less efficient glycol

True, I meant to mention that in the pros and cons but forgot. As I understand it, the dilution of heat carrying ability isn't huge, but neither is it trivial. I forget the exact figures, but even if is it only 10%, that means 10% less heat power delivered to the house, 9kW when an all water setup would deliver 10kW. That might well be enough to take marginal installations below the building's heat loss.

The antifreeze in my ASHP has a Specific Heat Capacity of 3655 J/kg/K measured at 30% dilution at 25 deg C. Compared with water at 4184 J/kg/K that’s a 12.64% reduction, which seems pretty significant. Do antifreeze valves only operate if there is a prolonged power cut and low temperatures, assuming the ASHP has its own frost protection system?

 

Yes. Read my post above for a fuller explanation, but mine operate when the valves themselves are below 3C. This requires rare cold temperatures and a power cut several to many hours long. 

A heat pump's frost protection should kick in a few degrees above the design temp of the valves. Mine is set to do frost protection from 6C, and it runs the water at 25C.