Question to all: Has anyone ever been told they must shut down their ASHP hot water cylinder due to danger or compliance?  Now we all know someone that had their gas shut down on a boiler due to a leak or a failing gas boiler as it burns wrong so yes it could explode but a hot water cylinder connected to an ASHP? 

Well today we did just that, can you think of a reason why and how bad it must have been for us to do that? I will tell all once I have a couple of guesses.

There is a 'live' 'mixergy' thread on the forum at present where op is @bontwoody and exactly this happened due to a very obviously faulty cylinder.  In fact not only did mixergy tell him to shut it down, two people on this forum did also.  

Four weeks later mixergy have still not fixed it.  Minus douze points to Mixergy.

@ashp-bobba, oooh (rubbing my hands) this could be very, very interesting.

@jamespa oh I didn't hear about that. So in this case it was even worse as this is a retro - retrofix 

@ashp-bobba, I’d imagine it must have been something fairly serious, but I'm guessing it wasn't... nevertheless, I'll play along: maybe a failed temperature sensor causing uncontrolled heating? Or perhaps a faulty mixing valve or immersion running wild?

By no means an expert here, but I can’t think of any scenarios where an ASHP-fed cylinder would present a genuine danger on par with a gas leak, so now you’ve got us all intrigued. Go on then... what happened?

Our client had an Ecodan with an official Mitsubishi cylinder installed 7 years ago and never had a problem until 5 weeks ago, to add a bit of context they only met us today and we did not know them until now and today was the 1st time we have ever seen this system.

So, they have a 14kW Ecodan on 28mm primaries (bit small) leading to a 120ltr cylinder in the airing cupboard and my lead engineer is calling me today and her exact words are "you may have to come to site Simon as I cannot work this one out!" Now, for Rae to say this either she is looking at the enterprise and need help fixing the quantum flux engines or this is one messed up system. 

Posted by: @ashp-bobba

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@jamespa oh I didn't hear about that. So in this case it was even worse as this is a retro - retrofix 

Here's the case James is referring to: https://renewableheatinghub.co.uk/forums/domestic-hot-water-cylinders/mixergy-tank-failure/paged/4/#post-50094

This system has a 120ltr Gledhill IND vented cylinder with no expansion vessel on the water side and a tank still connected in the loft, so ok, its an open vented system and yes not ideal on an ASHP due to heat expansion migrating up the cold fill and slightly higher risk of legionella but I have seen one or 2 before on ASHP and there are some who argue its fine. Anyway, so Rae says no but the open vent is capped off and there is a 3 barr pressure relief valve cut into the now sealed off open vent pipe? This PRV is inside the loft tank and aimed at the wall of the tank so if it lets go it aimed into the body of the tank, what I dont understand is why? if the pressure built up it would just reverse back up the fill line, flood the tank and overflow which is also wrong.

There are also 2 pumps on the primaries, one on the flow and one on the return, the 3 port is reduced down to 22mm and the client is complaining about the noise. Also there is no G3 tundish or any pipework but the system is sealed off as you can isolate the cold water down main. So this is the point that we shut it down due to regulation, nice electrical element in the cylinder, client has been using that to heat the water, it not wired through the Mitsi so no control and there is no G3.

So, Rae reposts that the water is running round the non high gain 1m2 coil inside the cylinder that it only heated the water from 18-23 in 90 mins, the DT is 1 and the noise is about 40db just from that pipework, the initial complaint is the heating is not working and it worked for 7 years. So poor Rae is trying to work out how did this system ever work as its just not possible you see whats happening here is the tank is so badly undersized and an incorrect coil inside that the ASHP at 14kW is short cycling every 3 mins.

Then the final part of the puzzle lands, Rae says hold on there is one thing, this cylinder looks new, wait there I will ask the client. Ok client says yep 5 weeks ago a company came in due to a leak on the cylinder connections, said it needs a whole new cylinder, removed the Mitsi cylinder, fits a £400 Geldhill vented in place of the unvented, re-establishes the tank, says open vented works better due to lower water pressure and that the reason for the leak on the old cylinder, return 2 weeks later and fits a brand new shower boost pump on the DHW outlet to fix the now poorer pressure for the hot water all over the house, returns again a week ago to look into why there is no hot water and heating, says its the ASHP and need a specialist ASHP engineer.   

So this system is piped like an open vent and unvented at the same time is missing an expansion vessel, G3 discharge line, cold fill pressure regulation, incorrect 3 port diverter, un-required boost pump, un-required extra primary pump, low gain cylinder, lack of controls connections, PRV illegally sealing off an open vent with a system fed by an open tank now boiling over and through the ceiling as well as no DHW and No heating, all from a little leak 5 weeks ago and a thank you very much £2,800.00

Conclusion: So the only way we can fix this is a new complaint cylinder, regulators and place it back on unvented to allow the Mitsubishi to actually reach the 50 Deg C on the cylinder so it will actually start to heat and the Ecodan was trying to prioritise the DHW and never reaching the temp. We need to remove all that unsafe valves and piping and get this system back to something normal.

For a company to remove the G3, incoming water regulation, expansion system and an unvented system then half change it back to a vented system in the name of lower pressure = less leaks. This make me want to pack up and pretend I work in another industry. I am glad I am just an AC engineer, perhaps that my cover when someone raises their eyebrows at me like last week and say 'Ah yer, ASHP's Eh, but they don't really work do they eh?!" 

Hard to defend my industry when I am wading in this stuff.

KR

@editor So the risk was an uncontrolled electrical element half wired through the Mitsi controller with no safety stat as it would just boil over into the tank above if needed but then was sealed off by a valve on the incoming and a prv on the open vent. weirdly piped and controlled like a vented and unvented at the same time.