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Peak Energy Products V therm 16kW unit heat pump not reaching flow temperature

johnd · 2026-01-05T15:48:07Z

Since the UK outdoor temperature has dropped below 3 degrees our heat pump is unable to go over 40 degrees flow temp. It reaches 40 degrees for shortish period before starting a defrost drops back to 33ish and starts again in a loop. The installer has enabled the back up heater which resolves the issue but why is this happening? Is it standard practise to use the backup heater for this during cold spells? It is unlike to be an issue with the size of the heat pump as this has already been upgraded to a 16KW from an 11KW to resolve the a similar issue where the flow temps where only reaching 32 degrees before getting into a defrost cycle.

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Air Source Heat Pumps (ASHPs)

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DaveC

(@davec)

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04/02/2026 10:48 am

@johnd just had a message from the Peak engineer; they are currently working on a software update that will be automatically installed once approved.

No timescales attached though.

Have you had the same?

11kw Peak Energy ASHP - SunSynk 3.6kW Solar Inverter with 3.5kw panels and GSL 10kWh home battery

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johnd

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Topic starter 04/02/2026 11:01 am

@davec thanks for the update Dave! No I chased the engineer for an update but he hasn't responded to me. Sounds promising though.

Are you using a heat pump tariff? We are intelligent go but we don't do alot of miles so I'm considering moving to cosy to reduce my bills. We spent £320 on electricity last month including car and house usage.

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DaveC

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04/02/2026 11:22 am

@johnd we’re also on intelligent go and charging up the house battery (10kWh) during off peak, along with the car charging. Not that the battery lasts that long once we start discharging it with the AHSP running.

The battery was discharged in 3 hours this morning (5:30am - 8:30am) so back into the peak rate.

I’d have to get the whiteboard of truth and the calculator out to see if the cosy tariff would work out any cheaper.

11kw Peak Energy ASHP - SunSynk 3.6kW Solar Inverter with 3.5kw panels and GSL 10kWh home battery

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DaveC

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21/02/2026 4:41 pm

With complete radio silence from Peak and no sign of this software update, we booked a full survey through HeatGeek to have the house and system reviewed.

We had a lovely chap round from Ribble Valley Heating yesterday and with the HeatGeek iPad app he scanned the house and has provided us with a complete heat loss report, which wasn't provided originally. With also lots of head shaking when he looked over the current ASHP and as we discussed the operation and available controls.

Simply put; the initial heat loss calculation of 7kw is completely inaccurate explaining the poor performance that we have been experiencing.

The revised heat loss is actually 10.3kw, with a recommendation for a 12kw Vaillant Arotherm Plus ASHP.

We've requested a call with the original installers to now discuss the next steps, as we are still unhappy with the Peak system and want to replace.

11kw Peak Energy ASHP - SunSynk 3.6kW Solar Inverter with 3.5kw panels and GSL 10kWh home battery

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johnd

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Topic starter 02/03/2026 2:00 pm

@davec That sounds like a really good idea to bring in Heat Geek. I have no updates from Peak at all or the installer. How did you get on with the installer? I have now noticed the unit cycling on and off now that the weather is a bit warmer, I suspect this 16kw unit is too big. I will speaking to installer about this too.

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DaveC

(@davec)

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02/03/2026 2:15 pm

Hi @johnd, we have the original installer coming round tomorrow to discuss what they can do next.

Since we've already claimed the BUS grant a replacement ASHP would be at full price, so hoping the installer can work with us on getting something else in.

Will update once they've been.

We've also had to drop flow temperatures down to 28 degrees due to the warmer weather and is now hitting the thermostat cut off.

The suggested replacement of the Vaillant 12kw ASHP doesn't look to modulate down very far with a minimum heat output of 5.8kw, so considering a Grant 12kw which has a much lower minimum heat output. While also working on my own heat loss calculations in Heat Punk.

11kw Peak Energy ASHP - SunSynk 3.6kW Solar Inverter with 3.5kw panels and GSL 10kWh home battery

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DaveC

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03/03/2026 7:43 pm

@johnd very quick visit today from our installer; we’ve informed them we would like to have the Peak system removed and asked what sort of credit we can get back on the equipment.

Obviously needed to go back to the office and discuss with the higher ups, but did agree that there should be something they can work out.

As they aren’t trained on fitting another manufacturer ASHP, like Vaillant or Grant, they wouldn’t be able to install a full replacement system, but could potentially supply a replacement hot water cylinder to work with one of these systems.

Should have some further details back on Thursday potentially.

11kw Peak Energy ASHP - SunSynk 3.6kW Solar Inverter with 3.5kw panels and GSL 10kWh home battery

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bobflux

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03/03/2026 8:22 pm

Posted by: @davec

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The suggested replacement of the Vaillant 12kw ASHP doesn't look to modulate down very far with a minimum heat output of 5.8kw, so considering a Grant 12kw which has a much lower minimum heat output.

ASHPs do have a minimum compressor speed. The minimum heat output at that speed depends on air and water temperature.

12kW Vaillant with 45/40°C water has a minimum power of 3.6kW at -2°C air, 5.2kW at 7°C air.

That's because Vaillant labels according to power at -7°C air, so the same machine is a "18kW heat pump at 7°C air / 35°C water" and "12kW at -7°C air / 45°C water".

No idea how Grant labels their heat pumps, some manufacturers go for the bigger number, others are more honest, but make sure you're comparing apples to apples and check the actual capacity of the heat pump at your actual air and water design temperatures.