if you are having to top the water pressure up more than 2-3 times a year it’s leaking somewhere at a rate that is not insignificant. You want to catch that with the installer whilst you still have a valid installer warranty as that water is going somewhere. 

I'd picked up that there was a leak coming from around the water pump when I topped up a few weeks ago (the first time I'd noticed this) and had been in touch with the installer.  I'd also left the system at low pressure pending a visit, so as to prevent any ongoing leakage (which is only evident when the system is refilled).  The service overlapped with this and the service engineer addressed it by tightening the nuts above and below the pump (he said he'd got 1 and 1/4 turns on one of them, which made sense in the circumstances as to why there'd been an issue).

As far as I can see, the issue was isolated to that area, so looks to now be resolved.

I'd half-expected that the service engineer might have bounced the issue back to the installer during the visit, so was pleasantly surprised that he addressed it to the extent that he did.  He also confirmed that he'd checked for any other signs of leakage around the installation too and found none, but I'll continue to monitor the pressure for any signs of an ongoing issue and still have a further 12 months of workmanship warranty with the installer to address this if so.

if you are having to top the water pressure up more than 2-3 times a year it’s leaking somewhere at a rate that is not insignificant. You want to catch that with the installer whilst you still have a valid installer warranty as that water is going somewhere. 

@old_scientist 

if you are going DIY then you may want to check the outcome of your efforts, I’d get yourself a handheld combination TOC/COD/conductivity/spectrophotometer. 
Id bet there are formal protocols for doing this, the Germans are very good at generating these. I do know they have a protocol for refilling and purifying the fluid that involves recirculating it through a mixed bed ion exchange resin until a threshold purity is met. 

just checked and it is VDI 2035

if you are going DIY then you may want to check the outcome of your efforts, I’d get yourself a handheld combination TOC/COD/conductivity/spectrophotometer. 
Id bet there are formal protocols for doing this, the Germans are very good at generating these. I do know they have a protocol for refilling and purifying the fluid that involves recirculating it through a mixed bed ion exchange resin until a threshold purity is met. 

Many many moons ago I had exposure to glycols in biochemical manufacturing. Its physiochemical properties are quite complex and related to its molecular weight and changes thereof. It will change from being a greasy like insoluble molecule to one that is fully soluble in water. It might be something best left to a professional as below certain concentrations it’s a rather nice substrate for bacterial and fungal metabolism and you probably don’t want those organic acid by products in your system if you haven’t adequately flushed it all out. 

Thank you. Yes, getting it all out of my system is going to be quite the headache given the volume (inc flushes) and safe disposal thereof.

Many many moons ago I had exposure to glycols in biochemical manufacturing. Its physiochemical properties are quite complex and related to its molecular weight and changes thereof. It will change from being a greasy like insoluble molecule to one that is fully soluble in water. It might be something best left to a professional as below certain concentrations it’s a rather nice substrate for bacterial and fungal metabolism and you probably don’t want those organic acid by products in your system if you haven’t adequately flushed it all out. 

We'd paid for the ASHP and tank service, and the service engineer was great - very happy to show me what he was doing and seem genuinely pleased I was interested and engaged.

Outside he stripped the casing and thoroughly cleaned. We talked about the vibration from the compressor which I thought was a little noisy. We briefly fired up the unit for him to hear, and we agreed it was at the top end of normal. He's asked me to record a video when it's running hard for him to send to Samsung. The compressor is mounted on rubber feet, and just moving the compressor slightly reduced the vibrations, so we will see how things go.

Due to vibrations, he explained that cable connections can work loose over time, so tightened all the screw down cable clamps to ensure they were tight and had not worked loose.

He examined cabling for signs of burning and explained some cables can sometimes start to brown, so just visually checking all the cabling looked good and no signs of electrical faults.

He showed me where to check for refrigerant leaks. He mentioned the circulating refrigerant gas contains a small amount of oil so you can feel if the joints have leaked. He showed me the common failure points to check and all looked good. He did not otherwise check refrigerant levels.

Inside I am happy with all the controller settings, so we skipped checking those at my request.

We did have an issue with our immersion heater cutting out too early to complete the legionella cycle (cuts out at ~57C). He had a spare thermostat for the immersion, so swapped that and just quickly checked it powered up without tripping anything - I didn't want to fully test the cut off temp of the new thermostat as we were on peak rate by then, so I'll test that later.

We cleaned the Y strainer and the mag filter. I'd already cleaned the Y strainer a couple months after installation, but I now know how to clean a mag filter, so that's good. Checked pressure relief valves and expansion tanks.

Overall I'm pleased we did the service. The engineer was great but I do feel the price is a bit steep (part of the issue will be the amount of travelling these guys are doing to service nationally, so I expect he's done more travelling than servicing in a day). I've learned quite a bit and know more things to keep an eye on and check. Of course the main issue is the warranty. Next year I think I'll be happy to maybe just pay for the ASHP service, and take care of the tank side myself.

Well, another year has passed and this year I decided to service it myself.

Today I stripped down the outside unit, checked for refrigerant leaks, inspected and tightened all electrical connections that may have come loose from vibrations and thoroughly cleaned the unit including the heat exchanger and fan blades.

Tomorrow I will turn my attention to inside, and clean the Y strainer and mag filters.

One question - I've currently got glycol in the system, which I'm eventually going to drain and replace with water (as we now have battery backup and experience few days below freezing / power cuts). Can anyone recommend some litmus paper or similar, or other simple tests I can perform to check water quality. I'm not really interested in checking glycol concentration but am more interested to ensure the glycol has not degraded and become acidic. Should I use specific litmus paper for glycol solutions or can I use any regular litmus paper?

When he went, I fully cleaned the filter which was disgusting, combed the fins on the pump, fully drained down the system, flushed and refilled with new chemicals. Checked the pressure release valve (something he didn't bother doing) and a few other things.

All in the name of a warranty.