Go for new rego box. It’s true they are hard to get but we’ve replaced a lot of them and no issues. 

if you do replace the heat pump then it’s madness to put in an Air to water when you have a geo collector. 

geothemal heat pumps last much longer and are generally more reliable than air to waters. This is from 25 years of fixing both. 

@greenlight I actually called you and spoke to you to ask if you had a work instruction or could give me an idea of how much it should cost to replace these. You said you would call me back, but I guess you have been busy.

Just leaving an update to my issue.

The pump completely shut down on the 26th Jan. I had ordered the internal flexible hoses from Alto Energy and they turned up in 2 days, (see below), brilliant service. With nothing to lose, together with a friendly plumber, who was willing to help and had the right tools, we shut off the heating circuit at the filter and the pumps, and began to drain the system at the bottom connection to the electrical cassette. (see below also). I could not believe how much water was in this circuit, it is obviously a large water heating loop around the tank.

After a fiddly few hours (due to the fact that we could only access the pump from the front, see photo) we removed the hoses x3 and replaced them. We then refilled the loop, which also took a long time. With anticipation, after checking there was no air in the system, we opened up the heating loop and turned the heat pump on. It immediately gave a GT3 alarm, same as before, but then sprang into life. Continually checking GT8 vs GT9 temp delta, which remain within 5 degrees for the first time in a while, the pump remained heating the water to the set temp, still set at 35 degrees, and then switched to rad heat. Everything seemed find. My friendly plumber left me to clear up, and I was feeling rather pleased, especially as I was about to leave my wife and kids for a week on business in Orlando. 

I then thought I would try out the electrical cassette. Switched on 'add heat' and BANG! All our electrics tripped. I reset the master trip and switched the pump back on and BANG! Main trip went again. I was not in the best frame of mind at this point and thought I had jumped the gun. However, I waited for 10 minutes and tried again. The pump came back to life, and continued in Rad heat mode. 

Reflecting at this point I thought we must had got moisture into the electrical cassette controller which sits below the cassette, just to the left in the photo attached. I put a dehumidifier next to the pump and left for the US, with fingers cross that the following Sunday (5 days away), when the pump tried to go into legionnaires mode, the control box would be dry. 

My wife complained that the water was not hot enough, still set at 35, so she discovered the 'add heat' function 2 days later. and NO bang, the pump heated the water to 60 degrees. 

So in summary, now after 7 weeks and some really cold weather, the pump is still operating (touch-wood) and in fact using less than a 1/3 of the electric it did when in HTF alarm mode. So @greenlight it worked!!

Sweetest of all my friendly pumper only charged me 1/2 a days labour at £150. So why do so called renewable heating experts expect you to pay over a £1,000 plus VAT? The extortionate costs and lack of knowledge of heat pump maintenance are the reason I still believe heat pumps are not yet a viable option in the UK. 

I am still looking at putting in a secondary gas heating system. I just need to work out how I can switch between the 2 depending on kWh prices and very cold weather. 

So a stay of execution for the time being for the heat pump.

I hope this post may help others with an IVT heat pump, as I gather that these flexible hoses are an issue on all of their models.

Posted by: @abbote

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Sweetest of all my friendly pumper only charged me 1/2 a days labour at £150. So why do so called renewable heating experts expect you to pay over a £1,000 plus VAT?

Somebody has to pay for their ignorance!

In fairness there are some very good ones out there, as well as some poor ones.  That's true of much of the building trade, it's not special to heat pumps at least based on my own experience.  Don't forget that many, possibly most, of us have been paying about 10% more for our heating than we needed to for about the past 20 years, and 'enjoying' a reduced level of comfort, courtesy of the (gas) heating industry.