Ecodan SIKA Flow sensor failure

[joth]

Since July I've been getting periodic L-9 "Low flow rate" system error on my FTC6 (for MITSUBISHI Ecodan PUHZ-W85VAA)

 

I've checked all filters etc and everything is clean and appears to be free running. The circulation pump is set to maximum flow rate (factory default, as it always was). I'm currently using it in cooling mode only (disabled DHW, doing that with immersion) and still happening. (When I had DHW enabled it happened more often in the DHW phase, but even happens without at a couple random times per day)

Using the controller "commissioning wizard" I see the flow rate was flickering on and off at 5l/min, which is about the lowest value it can sense.

 

Looking back through grafana, I see prior to May it would peak at about 12l/min but then steadily declined month on month until the errors started.

 

 

 

I've flipped dip switch 2-8 to say "no flow sensor" and the unit has been working fine for the last week. (Very needed as it's been very hot and the cooling is very desired by our guests!). Only downside is the output energy monitoring now gives bogus values (as it estimates it based on flow rate and temperature drops).

 

 

So questions

1. Does this sound like the flow sensor itself has failed? Any suggestions how to verify it is at fault? (Is this a reasonable thing to fail after 3 years use?)

2. Aside loss of energy monitoring, do you see big risks of further damage by using it without a flow sensor?

3. Any tips where to buy a new one? It's a SIKA VVXC9SNBUC00242P. (Mitsubishi won't talk to me as I'm not MCS + mitsu trained installer)

 

 

Edited August 16, 2024 by joth

[ProDave]

I had similar flow rate issues with my ASHP (different make and model) when first installed, and the only way I solved it was to buy and install an in line flow meter so I could see what flow rate was being achieved regardless of what the sensor said.  In my case it was only an on off sensor (enough flow or not enough flow) and I needed a real reading to see what was going. on.

 

I solved it by adding a second external pump to increase the flow rate above what the inbuilt pump could achieve.

 

The fact you have an actual measure and it has been decreasing, suggests there is an actual problem.  Which again is where an independant flow meter would help.

[JohnMo]

44 minutes ago, joth said:

I've flipped dip switch 2-8 to say "no flow sensor"

The heat pump will almost definitely have a flow switch which is activated on a low flow condition in order to self protect. Mine (not mitsubishi) doesn't measure actual flow, I can hook up an external flow meter, if I felt the need, but if it senses min flow via the switch, it shuts down.

 

I have an external heat meter to see what the actual flow is.

 

Are you on glycol? Either way have you checked condition of what is being circulated? If glycol based is it turning into a soup m

 

Looks like steady decline in flow rate, for both the high and low peaks - do you have zones switched off, or heating fluid getting more viscous?

[joth]

1 hour ago, JohnMo said:

The heat pump will almost definitely have a flow switch which is activated on a low flow condition in order to self protect. Mine (not mitsubishi) doesn't measure actual flow, I can hook up an external flow meter, if I felt the need, but if it senses min flow via the switch, it shuts down.

 

I have an external heat meter to see what the actual flow is.

 

Are you on glycol? Either way have you checked condition of what is being circulated? If glycol based is it turning into a soup m

 

Looks like steady decline in flow rate, for both the high and low peaks - do you have zones switched off, or heating fluid getting more viscous?

 

Was on glycol but added anti-freeze valves a year ago and I believe all glycol will be out of the system by now. I've had to drain and refill it several times due to other things going on. (Adding FCUs, adding volumizer, adding more bleed valves, probably something else).

When I washed through the system filter (fernox tf1) the discharge was perfectly clear looked just like tap water.

 

When I last filled it up last (Nov 2023) I put some misc inhibitor in (Sentinel X100) as there's no glycol to act as inhibitor. That's rated down to -2.5°C. the water never drops below 10°C so can't see that would be causing an issue. But, the gradual decrease in flow since May does corelate to the start of cooling season. If I can limp through to autumn it'd be interesting to see if heating-only mode undoes the problem.

 

Otherwise I'm wondering if there's a strainer somewhere else in the system that has got clogged. Maybe on the external flexi pipes.

 

Also to clarify the flow sensor (and the L9 error) are just on the primary circuit, which consists of: ASHP, pump, filter, 3-port valve*, LLH/UVC

So issues in the zones (UFH and FCU) shouldn't impact this flow.

 

* - the 3 port valve is a mid-position valve - yuck. I bought a diverter valve ages ago to replace it with, but when I went to fit it last week I found out I'd ordered 22mm but need 28mm. (The UVC is in 22mm which I measured, not spotting the reducer right where it enters the valve).

I've just received the correct size 28mm diverter and plan to install, but this was mostly to reduce my suspicion that DHW/cooling switch over was losing efficiency due to a sloppy mid-position valve change over.  I don't think it physically could be the cause of a reduction in flow rate.. But who knows.

 

 

 

 

 

[joth]

Hmmm well 10 weeks running it with the flow sensor disabled it seems to be performing just fine, so I'm still inclined to think it's a sensor failure. Ideally I'd borrow a replacement to test - they're not cheap, and a bit of a pain to get hold of.  I only really need it to get more accurate energy delivery estimates from the controller.

[joth]

Bit of an inconclusive update.

Got a replacement flow sensor today, but was unable to install as the pipework is clamped so tight around the existing one I can't open the threaded connectors out enough to remove it. Decided that's a job for after the holiday. But then noticed the instructions say it shouldn't be mounted up downward flowing pipe, only upwards or horizontal, which perhaps is the cause of it failing.

Firing back up the FTC controller then failed in error code EE, comms failure with outdoor unit. Checked all wiring, all seems fine. Did many power cycles to no avail. Eventually re enabled the flow sensor dip switch 2-8, and it boots right up. But now I'm back to the L9 error whenever it tries to do any work. 

It's four years since install.

[joth]

Finally got some real progress on this. I decided to check the flexi pipes again, and after removing the insulation jacket discovered this

It was incredible how hidden this was under the insulation. The fact the flow decreased over time suggests it was slowly twisting itself into this shape, and May - Aug is cooling season so perhaps these flexi pipes are more prone to doing this when cooling rather than heating. (I think the issue was introduced when I had anti-freeze valves installed the year before).

 

Disconnecting the pipes from the outdoor unit and running mains water through the circuit, the (existing) flow sensor started reading 1-2V (so 10-30 l/min) again, but with the flexis connected to it (open to air far end) it would stay stuck at 0.3V which seems it's base output for flow rate below its minimum sensitivity.

So The existing hose seems permanently kinked, doesn't  seem like it can be salvaged.

 

I've had a minor heart attack looking at replacement prices. Over 200+VAT at City Plumbing! The "MCS compliant" tax is strong.

These looks better. https://www.bes.co.uk/inta-pre-insul-heat-pump-hoses-1-bsp-f-swivel-elbow-x-28mm-comp-x-750mm-pair-26190/

Longer hose to avoid the sharp kink, and I'll cut back the feed pipe a bit, to try and reduce the increase the radius.

[joth]

Finally fixed this tonight. New flexi installed Monday (what a bloody pita the rubber washers are, in the cold and dark) but took two days to get the system bled such that the flow sensor can detect flow again. 

(I needlessly replaced primary circ pump yesterday thinking it was pooched, but it was just hard to extract trapped air. Venting it out via the outdoor connections eventually was the trick. Lesson learned: don't run ASHP up and over the ceilings, without figuring a high point bleed valve for them. Wish my architect , builder, plumber or  MCS installer had mentioned that... 🤦‍♂️)

 

[Russdl]

Well done for fixing it but

 

On 05/01/2025 at 18:03, joth said:

The fact the flow decreased over time suggests it was slowly twisting itself into this shape

That can’t happen can it? Seems utterly bizarre. 

[joth]

@Russdl thanks! And yes, utterly bizarre.

 

All I can think is the compression joint to the 28mm was not quite tight enough so over a number of expansion and contraction cycles it was pushing against it and managed to rotate on it, a bit like a ratchet.

The fact I do DHW reheat and space cooling on higher outputs during cheap over night electric (and this happened exactly in cooling season, starting May) perhaps compounding it.

It must have had a pretty bad kink to start and then this just collapsed it.

 

The other thing I noticed is the pipe seems to have lost a lot of its "flex", almost like it has perished and seized up a bit

 

Anyway for a part costing so much (for what it is) it's very disappointing to have this happen 

 

It'd be interesting if there's industry figures on failure modes and reasons for breakdown call outs. These do seem a prone part (having installed new ones and seen how easy it is for bad bend radius to kink them, but hidden under the insulation)

Edited Thursday at 08:26 by joth

[joth]

I generally dislike doing plumbing work, and this project was no different, but it is a massive help having this chap's monotone voiceover echoing in my mind whenever I do make pipework changes. 

https://www.facebook.com/reel/1692555494842644?mibextid=rS40aB7S9Ucbxw6v