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Air Source Heat Pumps (ASHPs)
Last Post by Toodles 2 years ago
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 batalto
(@batalto)
Famed Member Member
Joined: 5 years ago
Posts: 1091
Topic starter 09/04/2021 8:47 am  

Hi all,

I am getting a new ASHP in a couple of weeks, taking advantage of the green homes grant. We have a 240m2 house, however its heavily insulated. As an FYI we used 36,000kwh of gas last year for heating and hot water - taken from gas use in ft3 converted.

We are due to get a 12kw Midea system to replace the current 32kw boiler. We have 8.4kw solar array and 7kwh of battery storage. I am looking at increasing this by another 7kwh later in the year due to the ASHP. The battery will be key for me, however it can only provide the house with 3kw of peak power. Any demand above this would come from the grid.

My plan is to swap onto a mixed price tariff, probably Octopus Agile or Go and take advantage of the cheap power at night. This way I can fill the batteries and discharge over the day with some solar top up. I will probably only do this in the winter, as the summer months its not needed. However I have struggled to find the expected average power draw from the ASHP. I assume as its a 12kw pump I should get a max draw of around 3-4kW at peak with a COP around 3 in the winter. But this is peak, what kind of draw can I expect post max?

At 3kW I would empty the battery pretty quickly and any extra power would come at normal power costs (not really an issue as the heating is the big taker).

Feedback would be most appreciated so I can start to plan my battery changes.


12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE


   
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ashp heat pump power usage
 George
(@george)
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Joined: 5 years ago
Posts: 85
09/04/2021 9:05 am  

I don't think its worth investing in more battery power if you are getting an ASHP. It would be more worthwhile to invest in a solar PV to hot water converter like an iBoost to make use of any excess solar (if you don't already have one). During the winter the ASHP will run most of the time and the reduced solar production would all go towards powering the heat pump.  


Mitsubishi Ecodan 14kw ASHP + 500l Cylinder


   
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 batalto
(@batalto)
Famed Member Member
Joined: 5 years ago
Posts: 1091
Topic starter 09/04/2021 9:20 am  

I already have a PV to immersion diverter, however with an ASHP this isn't really worth it. the ASHP will be much more efficient.

On the battery the driver is the cheap rates at night Octopus Go is 5p for a 3 hour period. With a COP at 2.8 this is 1.8p per kilowatt of heating. Being able to store this proves a massive advantage in terms of cost vs 10-14p unit rates on a normal tariff. At 14p I've estimated the heating might cost me roughly £1,500 per year, at 5p its going to be just less than £600. The trick will be eeaking this out in the winter (Dec, Jan and Feb) where the demand is highest.


12kW Midea ASHP - 8.4kw solar - 29kWh batteries
262m2 house in Hampshire
Current weather compensation: 47@-2 and 31@17
My current performance can be found - HERE
Heat pump calculator spreadsheet - HERE


   
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JulianC
 JulianC
(@julianc)
Noble Member Member
Joined: 5 years ago
Posts: 336
09/04/2021 1:18 pm  

I’ve seen a few YouTube videos about the Midea - looks a good bit of kit. 
I hadn’t thought about batteries feeding the ASHP. I was discussing batteries elsewhere on this site and thinking they weren’t for me. But as I install my ASHP..... But I’m thinking you will need massive batteries to drive the ASHP through the cold day time, and fill them at night. But it is definitely something to investigate. But I’d like to see the maths. And wonder how I will fund 😊


Daikin Altherma 3H HT 12kWh ASHP with Mixergy h/w cylinder; 4kW solar PV with Solic 200 electric diverter; Honda e and new Hyundai Ioniq 5 N electric vehicles with Myenergi Zappi mk1 charger


   
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 robl
(@robl)
Honorable Member Member
Joined: 4 years ago
Posts: 205
24/07/2023 1:03 pm  

Posted by: @batalto
↑

As an FYI we used 36,000kwh of gas last year for heating and hot water - taken from gas use in ft3 converted.

We are due to get a 12kw Midea system to replace the current 32kw boiler.

s

The rule of thumb I’ve seen has been total yearly kWh/2000 gives the peak power in kW.  It works for us- but would give 36000/2000=18kW.  Are you sure 12kW is enough?

 



   
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Toodles
 Toodles
(@toodles)
Illustrious Member Contributor
Joined: 3 years ago
Posts: 2203
24/07/2023 1:27 pm  

@robl I have a Daikin EDL series 8kW/h monobloc for heating only (DHW handled by a Sunamp Thermino ePV210) and my short exposure of winter weather from mid-Feb. onwards this year leads me to think our daily need will be for 25-45 kW/h for milder to cold conditions. Using Octopus Agile, I’ll be looking for most of this energy at the cheapest hours and decanting it into 27 kW/h of battery storage. Two periods pf charging a day might make this feasible. I realise that oodles of Tesla expenditure is not in itself an economic proposition but, I have other reasons to want the security of power when we need it as we have experienced long power outages when we could not heat or eat! Regards, Toodles.


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


   
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Toodles
 Toodles
(@toodles)
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Joined: 3 years ago
Posts: 2203
24/07/2023 1:36 pm  

If I may add to my previous posting: I am ‘sticking’ with Agile at the moment as I think the average pence per kW/h works out lower than the fixed low cost periods offered by the other tariffs in the Octopus ‘Stable’ (I like mixed metaphors!) as I am a nerd who in his retirement has time to peruse the 48 daily rates to select the cheapest and thus greenest. Regards, Toodles.


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


   
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