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Vaillant Arotherm monobloc error f.788 - ASHP is not working!

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(@bobshopsupreme)
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Hello. I hope someone can give me some help after an exasperating two weeks with our plumber.

We have a Vaillant Arotherm Plus 155/6 ASHP that was installed a few months ago.

1 day after we left the house for the Christmas break and following a power cut in the house, I got a fault code through the app of f.788 and the system is completely shut down.

I checked the manual (and another post about the same error), and it is something to do with the building circuit pump. In our case (unlike the other poster), turning it off and on has no effect.

The installer went to the house a few days ago and said 'the pump is completely stuck and he cannot unstick it and it's dead', and contacted Vaillant for a new one. The lead time for this is 6 weeks so we have a cold house until the end of January 2023.

Reason for posting is I'm not sure the installer knows what he is doing. The manual talks about the the system 'switching off and locking' the pump. Is it possible that the pump is 'locked' rather than dead? If so, what 'unlocks' it? I don't know if he has done the other troubleshooting of filters etc. The other possible clue is the pressure of the heating circuit dropped to 1.2 bar when it got cold during the power cut. Could that cause this fault code? There has been no separate warning about low pressure.

Thank you very much for any light you can shed! Reasonably desperate given possible 6 weeks with no heating or hot water with a three-year-old child.

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This topic was modified 1 year ago by bobshopsupreme

   
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(@derek-m)
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I would suggest that you start by topping up the water in the system and bring the pressure up to approximately 2 bar. If that does not rectify the problem then switch the power to the heat pump and controller off for at least 10 minutes, then try again.

If the problem still exists then locate and clean any water filters.

I assume the water pump is internal to the heat pump unit. If you feel confident then remove the access covers and check to see if there are any lights showing on the pump. Check around the system for tripped circuit breakers or blown fuses.

 


   
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(@bobshopsupreme)
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Thanks very much for such a quick reply. I'm not at the house but the installer is calling past this afternoon and I'll pass on the suggestions. If he doesn't get anywhere I'll ask him to turn on the immersion heater in the Unistor cylinder so we have some domestic hot water and then I'll take a detailed look at the other troubleshooting steps when I get back this weekend. I do struggle to believe that a pump is dead after 3 months and that it happens to die just after a power cut, so I'm hoping this is either a pressure issue or random electronic issue that can be cleared.

Will post back here if/when I find a solution so it can help others. Thanks again.


   
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(@bobshopsupreme)
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Quick update as the despair continues.

 

Installer was there 5 hours today (I was not), and he has got the pump working, which he said was completely seized.

All error messages are now cleared and the pump turns, but the compressor doesn't start at all even when there is a call for heat from the central heating circuit. No error messages or evident problems.

He has gone home baffled and will try and come back tomorrow. I'll tell him to call Vaillant to see if they can help diagnose. Otherwise it looks like we're coming back to a cold house on Sunday. 

If anyone has seen anything similar I'm grateful for ideas. Hard to diagnose now there are zero fault codes.


   
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Mars
 Mars
(@editor)
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I’ve had a look and the error code f.788 and outside the published codes cited above it could indicate a problem with the flow temperature sensor. This error can occur if the flow temperature sensor is not functioning properly or if it is disconnected - maybe it was damaged in the power outage.

I’ve no experience with your machine or Vaillant but flow temperature sensors on heat pumps can be troublesome - ask your installer if he checked it. It’s a long shot, but worth asking.

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(@bobshopsupreme)
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Thanks for the suggestion. I'll add it to the list. The strange thing is that the unit no longer shows this error or any others. 

The app shows a FlowTemperatureVF1 of 13.6C which is a plausible value given it has been broken for 2 weeks. I guess there may be multiple sensors and another one is cause the compressor to not start. An error message would be quite useful!

I'll make sure the installer checks other things like fuses, filters etc. It still seems like a minor problem that is just preventing operation. If the compressor itself is completely dead after a power cut that would be surprising, and you'd hope that in itself would generate an error message. 

Was much easier when we just threw lumps of fossilised plants on an open fire.


   
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Mars
 Mars
(@editor)
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Posted by: @bobshopsupreme

If the compressor itself is completely dead after a power cut that would be surprising, and you'd hope that in itself would generate an error message. 

From experience, the ‘intelligence’ in the heat pump controller will prevent the compressor from starting as a safeguard to protect it from damaging itself if things in the system are out of kilter. Things like flow will cut it out immediately - the unit will be on, but the compressor won’t start.

It’s just weird, as you’ve said, that all your error codes are gone.

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(@alec-morrow)
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Posted by: @bobshopsupreme

 

Reason for posting is I'm not sure the installer knows what he is doing.

 

 

Please do not demonise installers. That code would throw many an installer, and support from manufacturers for installers is woeful, as is the training.

 

Although the book is clear it doesn’t explain why the fault developed..which is where understanding comes in

 

I mean how do you test fir under voltage or over voltage when the pump has shut down?

 

 

 

 

 

Professional installer


   
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(@bobshopsupreme)
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@alec-morrow That’s a completely fair point and I didn’t mean it as a slight. The installers have done a great job on the initial installation but by his own admission he is baffled by this and doesn’t know what the solution is, either to the initial error or this new problem of the compressor not starting. So I was posting in case there was an simple answer based on previous experience that I could use to help him out. It sounds like he’ll have to just go systematically through the various possibilities until he finds the problem.

 

Fully take the point that these are complex machines and the codes don’t always pinpoint thr problem.


   
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(@ken-bone)
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@bobshopsupreme do not have an immersion you can switch on or bypass, I know alot of the older heat pumps (experience with old Daikins) if the system water temperature is below 18⁰ the unit would not start, as it needs to grab some energy from either the ambient, which could.be too cold, or the heating. You have said the system water temperature is 13⁰.

We would get hot water into the cylinder via immersion, then manually open the 3 port valve, this hot water in the cylinder would then go back to the outdoor to give it enough energy to get it going.

I'm concerned that you unit has not been running through these really colder weeks we recently have had, I hope they is gylcol in the system.

 

 

Heat pump installer


   
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(@bobshopsupreme)
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Thanks for all the suggestions.

Quick update - system still not working. The installer had cleared the pump error by replacing it with another pump that is not an exact match (don't know exactly which model). This cleared the F.788 error, but he thinks that it is not sending the data expected by the controller and that is inhibiting the start up of the compressor. So the only solution to get the compressor working is to wait until end of January for a new pump from Vaillant.

Equally frustrating is that he has tried to activate the back up heater in the hydraulic station the MEH 97/6, but that is not providing any heat at all (flow temperature still 14 degrees). The way it is described in the manual is as a supplementary heater in case of low outdoor temperatures, so I'm not sure it is capable of providing heat independently of the heat pump during a breakdown.

The final problem is that our Unistor 200 DHW cylinder does not have an immersion heater. Our house is in northern France, and it seems like the UK models have integrated 3kW immersion heaters but the French equivalents do not.

We come back to the house tomorrow to begin our one month without heat or hot water. I've asked the installer to fit an immersion heater in the cylinder (assuming it has the opening to accept one) which would at least get us hot water, and then we'll muddle through with electric room heaters.

Not too happy with my £18k so far! I know there are supply chain problems, but they have been going on for 2 years now and Vaillant should really be able to replace a water pump in less than 6 weeks for system in complete shutdown.


   
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(@hughf)
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@bobshopsupreme I don’t want to be Mr Pessimism, but all domestic heating circulation pumps comply with a standard specification for pwm control and pwm feedback.

If your plumber has fitted a new pwm pump, it should ‘just work’

My suspicion is that the power cut/surge/whatever has clobbered the vfd that drives the comprsssor.

Assuming the Vaillant is all custom stuff (not built with off-the-shelf parts like my machine), you’ll probably need a new power pcb/vfd.

Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.

My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.


   
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