Hi @Tufty Insulation looks adequate, it's a little short on one of the external pipes to the unit, and a little untidy inside, and missing over isolation and control valves. All is easily rectified. It's only the external stuff you need to concern yourself for frost risk with and there's hardly any of it. Frost protection valves on the ASHP bulkhead connections might be another option, if Mitsubishi recommend them as an alternative to glycol. That would need the ASHP isolating at the service valves and draining off to fit, then refill the drained section with water/glycol at required concentration. For me, I doubt a sheltered corner such as the one you show really is going to see -15degC and if it ever did, only very briefly. My subjective opinion, I would not be concerned about a -4degC frost protection level on pipework that is external but insulated. You can always double up the insulation on the external pipes, adding a second layer over the first, or get it all renewed and better fitting, neither will cost much and are simple DIY jobs. Use insulation that is external grade with a water repellent outer skin, as wet insulation doesn't insulate!

If the contents did freeze something will eventually give, an external copper pipe at best, the plate heat exchanger inside the ASHP at worst. A burst pipe is an easy fix, but if it happens in deep winter and leaves you without heating and hot water, and maybe waiting for tradesmen to repair, it's more the inconvenience. The plate heat exchanger failure is a very expensive fix, probably writing off the unit, as it can contaminate the refrigeration circuit with water. If the glycol content of your system meets the manufacturer's minimum requirements however, you could probably claim on your home insurance in that extremely unlikely event. Check your policy includes ASHPs, as some exclude them.

@tufty

I suspect the main item that would likely be damaged would the the condenser, which is a Plate Heat Exchanger (PHE). Whilst I don't think that they are extremely expensive to purchase from a stockist, I suspect the heat pump manufacturer/installer would insist upon you replacing any damaged item with their very expensive 'official' spare part, which will probably be from the same PHE manufacturer, and that you could get much cheaper elsewhere.

I wonder if anyone has considered battery storage and trace heating, to help prevent the water from freezing for a reasonable period of time? This could be combined with water dump valves, in case the battery storage runs out before the mains electricity supply is restored.

A further method would be battery storage and inverter to run the primary water pump. This would have the effect of taking thermal energy from inside the building and helping to keep the water flowing through the heat pump from freezing. The water pump would probably draw less than 100W, so could be kept running for quite a number of hours even with a modestly sized battery storage.

The other alternative would be a 'split' system rather than a monobloc, that does not have any water outside that could possibly freeze.

Thanks so much guys, very helpful.

Youve put my mind to rest (I tend to catastrophise!). I will ask the supplier to quote, and then make a decision.

I will certainly look at doubling-up the external pipe insulation. Seems a simple enough job that even I can do it. 🙂

Cheers!

Posted by: @allyfish

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Hi @Tufty Insulation looks adequate, it's a little short on one of the external pipes to the unit, and a little untidy inside, and missing over isolation and control valves. All is easily rectified. It's only the external stuff you need to concern yourself for frost risk with and there's hardly any of it. Frost protection valves on the ASHP bulkhead connections might be another option, if Mitsubishi recommend them as an alternative to glycol. That would need the ASHP isolating at the service valves and draining off to fit, then refill the drained section with water/glycol at required concentration. For me, I doubt a sheltered corner such as the one you show really is going to see -15degC and if it ever did, only very briefly. My subjective opinion, I would not be concerned about a -4degC frost protection level on pipework that is external but insulated. You can always double up the insulation on the external pipes, adding a second layer over the first, or get it all renewed and better fitting, neither will cost much and are simple DIY jobs. Use insulation that is external grade with a water repellent outer skin, as wet insulation doesn't insulate!

If the contents did freeze something will eventually give, an external copper pipe at best, the plate heat exchanger inside the ASHP at worst. A burst pipe is an easy fix, but if it happens in deep winter and leaves you without heating and hot water, and maybe waiting for tradesmen to repair, it's more the inconvenience. The plate heat exchanger failure is a very expensive fix, probably writing off the unit, as it can contaminate the refrigeration circuit with water. If the glycol content of your system meets the manufacturer's minimum requirements however, you could probably claim on your home insurance in that extremely unlikely event. Check your policy includes ASHPs, as some exclude them.

The installers have quoted £450 to top up the system with glycol. This is quite a bit, to protect myself from something that is very likely not to happen. In life I usually dont mind paying a bit more for 'peace of mind', but not usually £450 for it(!)

Im certainly going to add insulation to the pipework - both external (obviously) and internal.

Theres a whole range of pipe insulation out there. Theres cheap external grade pipe insulation on Amazon which looks like it will do the job. Can you recommend any? (brand, thickness of insulation etc... )

Thanks