Notifications

Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

Advice needed after a successful "leftfield" experiment.....

pauli · 2026-03-11T09:00:13Z

My old (30 + years) heat only boiler died recently (gravity fed system) on a small 54m2 well insulated modern two bed end terraceI had a hot tub heat pump lying around (max input approx 800w COP of around 2 to 3)I removed the boiler and piped in the heat pump into my central heating, (was simple as the heat pump is placed directly under my existing flue hole) I then put the old grundfos CH pump onto a 13 amp plug, switched it on and then switched on the hot tumb heat pump and set it to its max temp (40 degrees)To my surprise it actually worked, i have approx 3 weeks of use now (varying outside temp) and it is heating up my house nicely (with a few caveats) at not much more cost then my old gas boiler! The advice i need is which of the following two options should i go for......1) Buy a small (non hot tub) heat pump, which i can regulate down to a max 1.5kw, add some batteries to power it (with the batteries being charged via a time of day cheap overnight tarrif)or2) I'm getting an octopus cosy quote next week, go with that, install some batteries and sign up for the cosy tarrifMy big concern is the cosy heat pump is a 6kw unit, and from my testing a 1.5kw max input heat pump would allow me to heat up my house adequately without the current caveats (even in coldest of days). It would also mean I will need less expensive battery capacity to run it daily on cheaper cosy tarrif energy ? Thanks

Page 2 / 2 Prev

Air Source Heat Pumps (ASHPs)

Last Post by ChandyKris 4 hours ago

16 Posts

5 Users

4 Reactions

140 Views

RSS

pauli

(@pauli)

Active Member Member

Joined: 2 days ago

Posts: 7

Topic starter 12/03/2026 1:20 pm

@chandykris

yup, my error, i was looking at old data

ReplyQuote

pauli

(@pauli)

Active Member Member

Joined: 2 days ago

Posts: 7

Topic starter 12/03/2026 1:26 pm

@majordennisbloodnok

just checked, didn't realise this forum has been going on for so long, must be going through a recent surge in popularity seo wise 🙂

👍

ReplyQuote

Majordennisbloodnok

(@majordennisbloodnok)

Famed Member Moderator

Joined: 4 years ago

Posts: 1687

12/03/2026 1:32 pm

No problem, @pauli. If you can find something useful, the forum has served its purpose.

105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs

"Semper in excretia; sumus solum profundum variat"

ReplyQuote

ChandyKris

(@chandykris)

Estimable Member Member

Joined: 2 years ago

Posts: 124

12/03/2026 6:36 pm

@pauli Depending on whether you can get an EV tariff, I find that Octopus Go is a better option than Cosy for me. We were on Cosy in 2025 when we initially switched to Octopus in February after our heat pump installation. Prior to that we were on British Gas EV tariff As we only had a 6.5 kWh battery, I preferred Cosy as we needed that 8 hours of off-peak energy and that too spread over the day in three slots. I was averaging 15p as I was able to avoid drawing from the grid during peak times. But, we switched to Go in April and then doubled the battery capacity to 13 kWh in May. So far this winter I am averaging 12p and now in March as there's more PV production, more like 9p.

Given that battery prices have dropped, it might work in the long run to get a bigger battery and be on Octopus Go or Intelligent Octopus Go than on Cosy. This would also reduce battery cycling and lead to longer battery life. Check out the Solar Optimiser tool on Tim and Kat's Green Walk website and model a few scenarios to help making the battery decision.

16 * 435 watts PV
13 kWh Growatt battery
1 EV - Mercedes EQB
6 kW Aira Heat Pump

ReplyQuote

Page 2 / 2 Prev