Descaling ASHP heat exchanger

[joth]

- Probably nothing ASHP specific about this question, but I'm specifically asking in relation to an Mitsu ecodan 8.5kW ASHP

- This is the next instalment of my ongoing debacle about flow rate errors, link below. I now know this is not a sensor or plumbing error, but the plate heat exchanger is completely blocked and needing clearing.

 

My plan is to get pickup some Fenox DS3, mix it up with hot water and pour into the PHE. Obviously with the whole lot disconnected from the primary circuit, and using appropriate PPE.

Then flush it all with mains water.

 

If that doesn't clear it up, then I probably need to get Mitsu in to replace it.

 

Interested if anyone has had similar problem and how they attacked it??

 

 

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More background:

1/ original post 2 years ago, thinking it was flow sensor error, then thinking it was kinked flexi pipes.

 

2/ Subsequent update:

I bought a cheap submersible pub (Makita LXT powered) and did some simple flow tests into a bucket:

- without the PHE inline: 35 l/min

- with the PHE inline: 4.5 l/min ---- just a trickle, and critically less than the 5l/min minimum required

 

While flushing it through I initially saw plenty of white-ish sand-like granular dirt come out. Odourless. Presume this is limescale debris:

- We're in a very hard water area; originally (2021) the system was filled (by others) with softened water, but since then I heard that's not advised so on subsequent fills I used mains water and Sentinel X100. It originally had glycol but I've never bothered to refill that. It's been emptied and filled numerous times due to FCU and volumizer additions, as recorded elsewhere on here.

- It has a Fernox TF1 filter, I've regularly cleared and flushed that but never had significant dirt come out - but perhaps a bit of grains of limescale thinking back.

 

My hypothesis now is that this was always the issue, but every time I messed about replacing flexi-pipes and blowing out the airlocks, I was also dislodging a bunch of limescale each time, but it's still had a constant build up that needs to be properly descaled. 

 

[JohnMo]

Once you get descaled I would be taking care of the full water and chemical dosing.

 

I filled mine following VDI2035 spec, but later added inhibitor and biocide.

 

The parts I used were on the whole pretty cheap, have read-through of the below thread.

 

[Dillsue]

Dont forget everytime you change or add water you're adding to the scaling potential. I'd be tempted to buy a pump that will alow you to return any drained water back into the system rather than adding fresh water.

[joth]

Thanks @JohnMo that's super useful info

 

Today's project will be just to descale (the second batch of DS3 is currently resting in the PHE), flush, flow rate test, lob in a bottle of Fernox F1 and set the system going for a few days to validate 

I'll definitely look into the VDI2035 for a follow up project once the basics are back working 

 

[joth]

OK early signs are (very) promising!  the "bucket test" delivered 24 l/min, and I've refilled the system (adding a bottle of F1) and it's now doing a DHW cycle at 16 l/min. Still not quite where I'd like it to be, but far better than the 10 l/min  or less it's been hobbling by on for the last 24 months ....

 

4 hours ago, Dillsue said:

Dont forget everytime you change or add water you're adding to the scaling potential. I'd be tempted to buy a pump that will alow you to return any drained water back into the system rather than adding fresh water.

Interesting... I'd always thought using such occasions as an opportunity to flush out the crap and refill with clean. But yes, or course ... if scale is the problem refilling it is introducing a whole load more calcium carbonate ontop of what's already built up.  I guess running the drain off through a fine filter would help sift off the worst of the mess coming out. 

 

Challenge is where to put the 100l of water when temporarily draining down the system ... although in most cases I can isolate the volumizing tank and leave the UFH loops untouched, so probably a quite doable.

 

I'll read the heatgeek page and ponder how to attack this next time I do some service on it.  Good news is I do now have this nifty wee pump so cycling water through it is much simpler now. 15 quid well spent https://www.ebay.co.uk/itm/187759301764