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Power cut and no heat

Air Source Heat Pumps (ASHPs)

Last Post by Mars 57 minutes ago

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Brazilguy67

(@brazilguy67)

New Member Member

Joined: 4 hours ago

Posts: 1

Topic starter 12/11/2026 8:37 am [#3221]

We had a power cut for 36 hours and our Valliant 12kw needed it's FM5 configured after. There was also a water flow issue as told by our sensor interface.

The plumber came out checked the filter, configured the FM5 and everything seemed fine. However it's struggled to raise the temperatures in the house to 17 degrees centigrade.

We've tried various things including changing it from time schedule for each circuit to manual. The actual flow only gets to 26 degrees and the target is 35.

We did originally try getting the rooms back degree by degree but it's struggling.

Can even hear and see the heat pump and conduits in the water lighting up and flowing water.

Out of ideas.

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Mars

(@editor)

Illustrious Member Admin

Joined: 6 years ago

Posts: 4967

12/11/2026 11:17 am

Welcome to the forums. That's a frustrating situation, especially after being without power for 36 hours and now struggling to get the house warm again.

The thing that jumps out at me is the difference between your target flow temperature of 35C and the actual temperature of just 26C. I appreciate that after such a long power cut the house will have lost quite a lot of heat, particularly if you've got underfloor heating, and it could take some time to recover. But I'd have thought the heat pump should still be capable of getting its water temperature to the 35C target, even if the rooms themselves take considerably longer to warm up.

I'm wondering whether something hasn't quite returned to normal following the power cut, especially as you've already had an issue with the FM5 configuration and a warning about water flow. From what I understand, the FM5 is involved in managing the heating circuits, so I'd be curious whether all the original settings and configurations have been restored correctly. The flow warning is also something I'd be reluctant to dismiss entirely, even though the filter has been checked. Could there still be an issue with circulation or trapped air somewhere in the system? We had an issue two winters ago, where there was trapped air in a circulation pump (ironically after a power outage). We changed the pump settings to 1 (lowest) and then ramped up 3, and we heard a distinct "whoosh". Worth doing this as it only take a minute.

One other thing I'd be interested to know is whether the outdoor unit is actually running its compressor and producing heat, rather than just circulating water around the system. The fact that you can hear water moving and see circulation indicated on the controller doesn't necessarily mean the heat pump is generating the heat you're asking for. Is your domestic hot water working normally?

Personally, I wouldn't keep changing schedules and settings at this stage because you could end up chasing several different issues. Given that everything apparently worked before the power cut, I'd be inclined to get the installer back to investigate why the actual flow temperature isn't reaching its target, and perhaps involve Vaillant's technical support if the answer isn't immediately obvious. The latter could lead to a far faster resolution.

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