Can you tell us what the weather compensation settings are please and also is there a buffer tank/LLH or (separate question) a volumiser.  Have you got any kind of low noise/night mode/noise reduction set?  Also whats the system volume

Backup heater isn't 'standard practice' but is a known way to deal with a heat pump that is either undersized or operating beyond the system, design envelope (eg my system is designed for -2, its -5 today).

@johnd, I'd hesitate before saying that's definitely wrong, but I'm certainly sceptical. Defrost cycles are certainly a big hit on performance but they're far more affected by damp weather than cold weather - that's why heat pumps work perfectly well in Scandiwegia.

Just to start the ball rolling before better brains than mine come along here, are you able to tell us what heat pump you have, what size of house it's supposed to be heating (with any heat loss info if you have it, but don't worry if you haven't) and whereabouts in the UK your house is?

**edited to add that said better brains than mine definitely includes @jamespa with whom I cross-posted**

@jamespa There isn't a volumizer or buffer tank. Weather compensation is enabled although it doesn't seem to alter the flow temp automatically based outdoor temps.  Low is 0 degrees and high is 15 degrees. I do not know what the system volume is, there are 19 radiator with a mix of 10mm and 15mm plastic pipework. No noise reduction enabled. Octopus did the heat loss calcs at -3 @ 50 degrees = 11KW heat loss. The 11KW heatpump which was installed has been upgraded to a 16kw as the manufacturer deemed it too small when we initially had issues with getting the flow temp above 32 when the outside temp dropped below 3 degrees.

@majordennisbloodnok Its a Peak Energy Products V therm 16KW unit. The house is detached, 3 storey and 5 bedrooms based in Lancashire. Heat loss according to Octopus onsite survey is 11KW.

Posted by: @johnd

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Weather compensation is enabled although it doesn't seem to alter the flow temp automatically based outdoor temps.  Low is 0 degrees and high is 15 degrees

That doesn't make much sense as it should alter the flow temp, that's what WC is.

0 and 15 are presumably the outside temps, what are the corresponding flow temps set on the curve?  Also with 0 being the low OAT point the ft will flatline at this temp so as soon as oat drops below it won't be warm enough.  This needs to be changed.

Can you do some screenshots of the WC config screen?  Also how often is defrost occurring and is there anything about where it is sited that would restrict airflow or make the air particularly humid?  According to their website/manual "Our unique defrost process means we get better performance in the critical temperature range of between -2°C & +2°C because 
our units go into defrost mode less often."!

The online manual is pretty short on detail (I think they have edited out all the useful bits), hopefully the UI is reasonably self explanatory!  Is this an Octopus install or someone else?

@johnd, what you’re experiencing is common with air source heat pumps in colder weather in the UK where it’s humid during cold snaps. When the outdoor temperature drops below around 3/4C, frost tends to build up on the outdoor coil, which reduces the heat pump’s ability to extract heat efficiently. To clear this frost, the heat pump initiates a defrost cycle, during which the flow temperature can drop temporarily, often from +40C down to the low 30s. This cycling between heating and defrost is normal and necessary to maintain system performance.

The backup heater is designed to support the system during these cold spells when the heat pump alone can’t maintain the desired flow temperature continuously, especially during defrost cycles. So yes, it is standard practice for some units for the backup heater to come on during cold weather to ensure your home stays warm. Even upgrading the heat pump size doesn’t eliminate the need for defrost cycles or backup heating because the physics of frost formation and heat extraction limits remain.

Having said that, one thing to consider is whether your system has sufficient water volume and proper flow rates. Insufficient system volume can make defrost cycles more noticeable inside the home due to rapid temperature drops, and get stuck in the cycle of flow temperatures not getting above 40C because you’re persistently battling defrosts. Devices like volumisers can help by maintaining stable flow temperatures during defrost, preventing sudden cold blips in your radiators.

I therefore wonder if you have sufficient volume from what you’re describing.