It is not good practice to lay slabs on soil; good practice would have been to lay a concret base in the first place! As to the Eddi dilemma, at least you can just turn off the Eddi to try out the suggested routine. Sounds to me as though that first installer is clutching at straws for excuses to me. Regards, Toodles.

1. Was the installer right to charge us for making good the foundation under the heat pump? I did query it and his response was "Well this has never happened before and I've installed a large number of heat pumps" and "well its like building a house, if it suffers subsidence the house owner is responsible for repair." Thoughts please?

2. Was the installer correct to tell us to remove the Eddi solar diverter?

To use the installers analogy of subsiding foundations, if I had bought a 4 year old house and it was starting to subside, I would expect the original builders to rectify it at no cost to me. He should have ensured his original base was up to the job. 

The Eddi issue I am not sure about. I think the main issue is why is your system producing lots of black gunk. Heatgeek have a YouTube video on the need or not of an inhibitor in a system. 

As an aside, it would probably be more cost effective to use the heat pump to heat your water and export your excess solar. I have an Eddi too but it is turned off for just that reason. 

@jennyross your installer charging for the foundation repair under the heat pump is debatable. On one hand, it’s reasonable to expect that a foundation should be properly prepared initially to support the heat pump without future issues. If the installer didn’t account for the soil conditions adequately in the first place, it might seem unfair to charge you for rectifying their oversight. In our case, the foundations for the heat pump were our responsibility, so if the installer undertook that as part of his work, it’s probably unfair you were charged again.

Regarding the Eddi, it’s quite odd to blame it for the system’s dirty water problem. The diverter’s role is to manage surplus solar energy, directing it to your immersion heater rather than the grid. This should not cause contamination or sludge in your heating system. I’m not following his logic at all.

The black gunge you’re describing is usually a sign of corrosion or sludge build-up in the system, which might suggest that the system wasn’t properly flushed and treated initially. It could also indicate ongoing issues with the system’s inhibitor levels or water quality.

I’ll put this forward to Myenergi as a question.

ok first there is no need and never has been a need for a concrete plinth, as long as your unit is level ish. Mine is on the grass, guess what it works fine.

Next the dirty water in the coil is the same as the dirty water in the heating circuit, they are not separate. The dirt is biological growth in the water, it needs draining, flushing, it doesn't need a power flush you can just use the filling loop, fill it with clean water run it for a few mins, drain repeat till clean. Finally fill with new water and a decent inhibitor or biocide, mr google will help here. You could put a splash of bleach in crazy idea but cheap, its what i would do. Once clean its done. The eddi has literally nothing to do with this problem. 

the growth is caused by the low run temperatures in the heating circuit. 

But like everything on the internet, this is free advice, I'm feeling generous this morning.

but hey! what do i know about heat pumps. 

The Eddi has nothing to do with the black gunk - this is either caused by the system not being correctly flushed and/or the system not being tested and then dosed correctly with inhibitor, or the water treated, demineralised, etc without inhibitor. You may also have different metals making up the system which may be compounding the problem - again nothing to do with the Eddi.

The foundation should be suitable for the installation - as it's a new job remedial work should be at the installers cost imo. Did they also incorporate a soakaway for the defrost water? I imagine not... 

As others have said the foundation issue is debatable.  You don't actually say whether the installer fitted the slabs initially or whether they were provided for him.  In the latter case I would say they are justified in charging you.  In the former case then as others have said he should have provided an adequate base, but that would have cost more which you would have had to pay for earlier. So in this case I would say he can charge you for laying the concrete but not for removing and replacing the heat pump.

Regarding the eddi the installer is simultaneously talking a load of rubbish, and at the same time there is possibly a grain of logic.

The eddi is just switching on the immersion, and that isn't going to cause black sludge in the heating water that it is not even in contact with! 

However it's likely that some sludge is anyway accumulating in the heating water due to slow corrosion etc.  If you heat your dhw in summer using the heat pump then at least some of this gets flushed through the filter as the flow water circulates.  If you heat your dhw in summer with the immersion, then because the water pump doesn't switch on, the heating water doesn't get flushed through the filter.  So when you do drain it all the sludge comes out in one hit.  But it's the same amount of sludge so it's incorrect to say that the eddi/immersion is 'causing' the sludge.

The installer who commented about poor servicing may be right.  With correct treatment (additives) of the heating water corrosion is reduced. It may be that the treatment isn't correct and you could reduce corrosion and hence sludge build up with better treatment.

This is nothing to do with heat pumps btw.  All central heat systems are subject to corrosion and need to be treated and normally filtered, which is why we fit mag filters and the like.

Hope that helps.  If you want to continue using the eddi I would do so (noting comments above about cost).  Maybe switch on the heat pump occasionally in the summer season (eg on dull days) to flush the water through, and get someone specifically to check the water treatment when it's next serviced.  Incidentally I believe that modern practice (again not heat pump specific) involves measuring and adjusting pH not just throwing in a litre of fernox or whatever treatment fluid you use.

@jennyross a couple of thought on this one:

Vaillant ASHP monobloc support: If the installer used an existing base, such as a patio, then subsequent issues eg: settlement of paving slabs, can happen due to the added dead load of the ASHP. They would not be responsible for that, as they cannot reasonably determine the state and condition of the existing support surface. If they provided the base as part of the their scope, then the fitness for purpose of the base is absolutely their responsibility. If the installer set two concrete pavers on uncompacted garden soil, that is poor workmanship and an inadequate foundation for an item weighing 125kg. The dead weight of the base should weigh considerably more than the unit being supported, this is basic stuff! 600x600x50mm concrete pavers weigh about 40kg each, so two pavers weigh much less than the ASHP being supported. That is inadequate support. If the installer has 'done it many times this way' they have done it many times wrong!

Black sludge: Hopefully the post-install flush was carried out with all 2 port or 3 port zone control valves energised, all radiators in circuit open, and also the hot water tank in circuit. Valves may need energising and de-energising during the flush to ensure all parts of the circuit get good flushing for sufficient a time, the HW cylinder coil is unusually a lower hydronic resistance to the radiator circuit, and water takes the path of least resistance. That being so, if sludge is coming out of the cylinder coil, maybe the original flushing done was inadequate and might not have included the cylinder coil part of the circuit. If it was a new cylinder there should not have been any sludge.

The Eddi solar diverter is just a thyristor controller which provides a varying amount of power to a 3kW immersion heater. It cannot be responsible for any sludge, nor can it create any. How old is the water tank? It's a system tank, hopefully, compatible with ASHPs. Some older heating systems used open vented thermal stores, which give mains pressure hot water on demand, but are not suitable for ASHPs. They are prone to sludging due to the very large volume of static hot water they hold.

It's sometimes difficult the get all the sludge and sediment out of older systems even with power flushing. ASHPs have higher flow rates than fired boilers, and that increases the velocity in the pipes and radiators. This can dislodge old sediment over time that the original power flush may have left behind. It depends on the condition of the system before the ASHP was installed and how thorough the flushing was. For retrofits, the condition of the magnetic filter is a good tell-tale indicator for the condition of the system. If it's collecting a lot of residue and the water is heavily discoloured, a flush and refill, plus a clean out of any strainers in the system, would be wise. The sludge is certainly not the fault of your Eddi!

Hot water, ASHP or Eddi? You have a battery and solar PV, so the most cost effective way to generate hot water is with the ASHP and not via the Eddi. In summer, for every 1kW of free solar power via your battery, you'll get at least 2kW for free from the outdoor air. You can turn 1kW of solar energy into 3kW of heat energy, all for free. With the immersion heater, you'll only get 1kW out for every 1kW in. Also, it gives your ASHP a run for an hour or so each day, rather than it being switched off for a long period over the summer. That circulates the water in the system, rotates the fan motor/s, energises the circulating pump, energises the compressor and circulates the refrigerant and refrigerant oil. All this is good for ASHP longevity and reliability and reduces the chances of a fault of breakdown when the unit is first energised in the heating season having sat idle for several months prior.

Set the ASHP to generate HW for you around 1pm each day when the outdoor air is warmest for maximum efficiency, and only only use the Eddi for timed weekly Legionella boost and as a back-up. I have an I-boost and do this, my ASHP provides all the HW I need for free at 300% efficiency c/o Solar PV+battery in summer. If you generate surplus solar power, sign up Octopus Agile outgoing fixed export tariff at 15p/kWh (other tariffs are also available) and make some money on your surplus. 😉

Thank you everyone for your thoughts and responses. Very helpful.

For clarification, we did not lay the foundation in the first place. One of the reasons we chose this particular installer is that we did not wish to undertake any of the physical tasks (my father's funeral was the same month as the installation). At least one other installer quoting for the job had asked us to lay the foundation. I didn't wish to be responsible for this precisely because, in good faith, I expected the installer to consider the need for a long-lasting foundation and to choose the right materials for this. The climate is changing, rainfall patterns are less predictable. In this part of eastern Scotland we have had the wettest autumn (2023) on record and an extremely wet spring and it was raining yesterday and today - when other parts of the UK have been basking in 30 degrees 🙂 . I didn't question the installer about the choices he was making. I expected him to know what he was doing. I did want to question him at the time but didn't. I wish I had. 

Not surprisingly I agree with those who feel that we have been let down. Thank you Allyfish, toodles and others. If, Graham, you have installed a pump on grass and you have no problem with movement (yet) then in my view you're lucky. Sorry - I know you're one of the heat pump gurus around here. In this part of the world it will move; there is a lot of sand below us. There is often heavy rain. Soil moves. 

The entire ASHP installation (without battery and solar panels which were installed the following year) cost us nearly £20,000. We found we had bought a house that had single piping throughout and we needed to have the piping in the radiators set up in parallel for the heating to work properly.  So our ASHP retrofit required new pipework being laid in every room in the house. Until the last month, when the heat pump was moved, we had had no problems with the heat pump. 

My commentary around the Eddie diverter was tongue in cheek. I was simply repeating what I had been told - although I couldn't believe that it was responsible for the black gunge because, as others have said, there is no logic here. James' explanation about the ASHP not heating the hot water in the summer months and the effect on build up of sludge is the one the installer has given me. I can see some logic in that.

Interestingly this year we have had such a cold spring here that we were still using the heat pump for heating through May and into June. It also heats the hot water from 4-7am every morning. So it's not sitting not being used.

I'm left with the issue of whom to trust. My installer is trying to make himself irreplaceable - he tells me he is the only way who can fix the problem because he is the only one who knows the system. 

Thanks for all the feedback. I hope I've not offended anyone. I'm definitely not an expert - just seeking a solution to a problem. I'm grateful for any help.

@jennyross I hope you are able to come to an amicable agreement over the foundations and the sludge problem. Regards, Toodles.

Posted by: @jennyross

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I'm left with the issue of whom to trust. My installer is trying to make himself irreplaceable - he tells me he is the only way who can fix the problem because he is the only one who knows the system. 

On the basis of that attitude I wouldn't trust your installer!

In order of decreasing trust I personally go for

Logic based on sound facts

Myself

Others who are prepared to explain their reasoning and have no vested interest

Others who are prepared to explain their reasoning but have a vested interest 

Anybody who won't or can't explain their reasoning or tells me I won't be able to understand it 

Given that it was his job to supply an adequate foundation I would say that your installer should either have charged you nothing to rectify the foundation, or max say £200 for a concrete base (but not the removal and refitting) if he could legitimately argue that he could only tell what the ground conditions were after he had quoted.  Personally I would lean towards nothing, but this is the building trade!

@jamespa Yes, these tradesmen go for special training - in how to whistle through their teeth as they are about to provide an estimate (that IS JUST AN ESTIMATE and will be lower than the actual price charged).😉 If you are old enough, think of the Arthur Haines characters where he was always fleecing Nicholas Parsons for a building botch err.. job. Regards, Toodles.