@dak51, what brand is your heat pump and do you know what's gone wrong with it? Five months is not very long for it to have gone on the fritz.

Hi Mars, thanks for the response

Failure is the compressor, extremely noisy and little heat output before system trips out. It was running fine until it wasn't! No prior warning of oncoming failure but with the bad weather we hadn't been outside recently near the unit so it could have become noisy over time. The system has been inspected and interrogated with a fine tooth comb during two separate engineer visits with nothing wrong identified due to the design or installation. I checked the inline filter, very small amount of debris, as this is a completely new system. 400 litre DHW tank, 90 litre buffer. Heatmiser zone controls for the underfloor heating, now realise these are not the way to go from reading the various posts, weather compensation is enabled with recomended settings from supplier and system was set to run continuously, not timed. We have a hotel loop system for the hot water, plant room is in the middle of the house, but runs are still long to bathrooms. Underfloor heating only, no water based radiators, just electrically heated towel rails in bathrooms on timers. I would rather not state the brand or other specifics at this stage, I am "hopeful" that it will be fixed next week at which stage I can comment constructively on the customer service and start getting seriously into optimising the system.

@dak51, very interesting and I look forward to hearing what the outcome and level of support will be. Please keep us updated.

I've been a bit quiet as the situation has developed and it's too soon to go fully public on all details. 

The compressor was replaced on site, by a contractor to the manufacturer under the warranty. I wasn't happy that they took this route, but I have little choice. I would have preferred a replacement of the whole unit at 5 months in service, however, it has been on site since Dec2020, more on this later.

I was quite happy with the contractor, RJC ltd. Their engineer, Clive carried out the work which required brazing in the new compressor and refilling with the working fluid, he was knowledgeable and helpful. This was completed on the 19th so we had three weeks with no heating. We had a week or so with the heating running and then turned it off because the warmer weather came, but I left the ASHP running the DHW. In the last week I have noticed (you cant miss it) that the compressor has become noisy again, so I have lost faith totally in this system. I turned on the heating this morning as I need to use the slightly colder temperatures to stress the system in case it is really failing again, before we find out the hard way next winter.

There are a number of lessons learnt:

1. Because this is a self/ new build, we installed the ASHP significantly ahead of the commissioning date because you need it at 1st fix. Not sure how you avoid that, perhaps hold off the ASHP purchase/ installation until just before moving in? Of course that requires trades back on site well after they have finished everything else. Hence 12 months sale of goods act doesn't help me.
2. The design/ supply company used was chosen based on recommendations online. As I was project managing the whole build and doing quite a lot of the work, along side my engineering consultancy I relied on their guidance. They only recommend one manufacturer, doing my own research (had I found the time) would possibly have raised concerns that there are better choices of machine especially as my ASHP does not provide return flow temp or COP for instance.
3. Because this is a new build we also had all plumbing of the whole house to do, MVHR, electrics, dry lining, etc. luckily I have a family member who is a very competent Vaillant trained heating engineer, so he installed all the 1st fix plumbing, I did the 2nd fix. Hence we had an installer for a system supplied by a local supply/ design company, but realised once the kit arrived that they source from an agent for the manufacturer, but ultimately the manufacturer (who is not Vaillant) holds the warranty and calls the shots. So far no dispute, but it's quite a supply chain to deal with. Will have to see how this all pans out. I think the lesson here is carefully read the terms and conditions assuming there will be a failure.

@dak51, thanks for the update. Which brand heat pump is it, and I can’t believe the replacement is now already making a noise. As you said, the optimal solution would be to get the whole unit replaced. 

Good learning points and advice.