Balancing financial efficiency and comfort using the Octopus Cosy tariff

Air Source Heat Pumps (ASHPs)

Last Post by Toodles 8 months ago

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webcmg

(@webcmg)

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Joined: 3 years ago

Posts: 107

Topic starter 29/09/2024 8:20 am

I've been fortunate to have a fixed low energy rate until recently, but this has now expired, so I've opted to move onto the Octopus Cosy tariff. I'm interested in getting views, and learning from others experience, on how to best configure my setup to maximise financial efficiency of running the heat pump whilst seeking to maintain comfort on this time of use tariff.

Previously, I've been running 100% weather compensation 24/7 (Vaillant Arotherm 12kw @ 0.4 heat curve, deactivated Neostat thermostats in all zones). This resulted in a consistent 21c house and COP of 4.6 (inc DHW) and 4.9 (exc DHW). In 2023 we used 4,218 kwh according to the Vaillant app. The house is ~300m² with UFH downstairs and oversized radiators upstairs.

The Octopus Cosy tariff offers:

Three cosy periods of super cheap rates between 04:00 - 07:00, 13:00 - 16:00 and 22:00 - 00:00 every day, 51% cheaper than the Day rate in your region.
A peak rate between 16:00 - 19:00, 45% above the Day rate in your region.

The current Octopus Cosy rates for my region are as follows:

Day rate: 21.93p / kWh (13 hours)
Cosy rate (04:00 - 07:00, 13:00 - 16:00 & 22:00 - 00:00): 10.74p / kWh (8 hours)
Peak rate (16:00 - 19:00): 31.79p / kWh (3 hours)
Standing charge: 53.94p / day

These are the options as I see them:

Option 1: Continue to run as is 24/7 and benefit from the 8 hours of super cheap rate periods, but incur additional cost during the peak rate period.

Option 2: Continue to run as is, but turn the ASHP off during the peak rate period (via Neostats), with no change to the heat curve.

Option 3: Continue to run as is, but turn the ASHP off during the peak rate period (via Neostats) and set non-Cosy rate periods to "setback" temp (on Controller), with no change to the heat curve.

Option 4: Continue to run as is, but turn the ASHP off during the peak rate period (via Neostats) and set non-Cosy rate periods to "setback" temp (on Controller), with a change to the heat curve to offset the impact on comfort.

Option 5: Only run the ASHP in the Cosy rate periods (via Neostats), but significantly increase the heat curve. This would reduce efficiency, but may result in lower electricity usage overall.

I would be interested in views, whether others have experimented with these options, and what your recommendations might be. I'm hoping to establish the best way to balance use of electricity and comfort, whilst inevitably sacrificing some efficiency.

Many thanks

This topic was modified 8 months ago by webcmg

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Anonymous 5011

(@Anonymous 5011)

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Joined: 2 years ago

Posts: 563

29/09/2024 9:55 am

So some observations

A setback or off period your heat pump will have to play catchup at the end of that period to replace the energy lost. But there way to many variables to give a black and white answer. Just looking at our garden room (well insulated) and house. The garden room would become uncomfortably cold after 3hrs on a cold day, you wouldn't notice it in the house.

Experiment and see what works for you.

A 12kW heat pump would say big house or leaky (heat) house or an oversized heat pump. Can you supply the required heat from the heat pump in 8 hrs instead of 24 - would have to be massively oversized for that, so would doubt it.

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Toodles

(@toodles)

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Joined: 3 years ago

Posts: 1955

29/09/2024 11:31 am

@johnmo Yes, I feel the same @johnmo, the 3 hours of peak might be tolerated without heat but the pump will have to play ‘catchup’ afterwards. The problem with trying to just use the 8 cheapest hours is that you are likely to lose out on comfort due to temperature modulation during the other 16 hours. Again using any setback will need compensation and this will make the heat pump have to work harder and be less efficient.

Mind you, I am an old fusspot and along with my wife, likes 22.5 degrees C. 24/7 with a smart (old Hive fitting from the gas boiler days) TRV reducing our bedroom at night. We ‘cheat’ the system by only using the 8 cosy hours and Powerwall storage and I appreciate not everyone can do this. None of us knows how long such TOU schemes will last or whether investment in storage would be worthwhile long term or not. Sorry I can’t really offer a solution webcmg. Regards, Toodles.

Toodles, he heats his home with cold draughts and cooks his food with magnets.

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