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British Gas versus Octopus: Two possible heat pump routes - how to evaluate them?

 
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Air Source Heat Pumps (ASHPs)
Last Post by Judith 2 years ago
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 Lucia
(@lucia)
Honorable Member Member
Joined: 2 years ago
Posts: 354
Topic starter 13/07/2024 9:21 am  

@heacol thank you Brendon - that's helpful. 

BG want a buffer tank Octopus want a volumiser.

To be fair, I didn't push back on the buffer with BG at this point because the guy who visited me was happy to run 2 heat loss assessments one with and one without cavity wall insulation which proposed a 4kw or 6kw Daikin respectively. If I go with BG I would push back on the buffer. 



   
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Jancold
 Jancold
(@jancold)
Estimable Member Member
Joined: 3 years ago
Posts: 171
13/07/2024 11:52 am  

@lucia At £209 for a buffer I'd rather have an extra emitter!


Daikin 6Kw ASHP.
MG4 EV, Solis 8KW inverter, 32KWh battery
1926 Semi with loft and wall cavity insulation, all radiator.


   
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 Derek M
(@derek-m)
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Joined: 5 years ago
Posts: 4425
13/07/2024 12:29 pm  

@heacol

Thank you Brendon, though I would much prefer the information in table format which provides a great deal more detail.



   
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 Lucia
(@lucia)
Honorable Member Member
Joined: 2 years ago
Posts: 354
Topic starter 13/07/2024 12:37 pm  

@jancold Me too! 😁



   
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 Brendon Uys
(@heacol)
Prominent Member Contributor
Joined: 5 years ago
Posts: 415
13/07/2024 2:12 pm  

@derek-m So would we all, but beggars cannot be choosers 😀



   
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 Lucia
(@lucia)
Honorable Member Member
Joined: 2 years ago
Posts: 354
Topic starter 13/07/2024 7:38 pm  

Posted by: @derek-m
↑

@lucia

With regard to the bucket analogy, think of your home as a bucket with lots of pin holes covering the whole bucket, with the bucket placed inside a larger vessel containing a certain level of water that can be raised or lowered (OAT).

If the water level inside the bucket is increased (higher IAT) then water will leak through more holes (greater heat loss), if the water level inside the larger vessel is increased then less holes will be uncovered, so less water will leak (lower heat loss). 

Having cavity wall insulation is like having less holes or smaller holes in that section of bucket, while having no cavity wall insulation is like having more holes or larger holes in that particular section. This is similar to having different windows with single, double or triple glazing.

If possible assess the area of wall that has cavity wall insulation and the area that does not. It should then be possible to approximate the heat loss in the cavity wall insulated section and the section without cavity wall insulation.

Other factors that may have an affect is if the missing cavity wall insulation is on a North, South, East or West facing wall, since solar gain may then become important.

I can explain in more detail if you wish.

This is such a good analogy - it has really helped me to see the problem. I didn't realise that higher inside temps meant more heat loss.

I don't know how extensive the problem is because it got discovered so randomly starting with builders inspecting wall ties on a south, south east wall. The borescope image last week and last winter's thermal camera image show a big problem on a north facing wall. 

This explains why, as soon as it is windy or wet I feel colder in the house quite quickly. I'm near the sea in the sw - it's been windy and wet and even though it's 'summer' I've had the heating on frequently. 

Thank you for this - it really helps me to think through tackling the assessment problem. 

 



   
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 Lucia
(@lucia)
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Joined: 2 years ago
Posts: 354
Topic starter 14/07/2024 8:12 pm  

I have just discovered that BOTH my installer's heat loss proposals for 4 kw Daikins (based on imaginary heat loss and imaginary fully insulated walls) have specced radiators with LOWER wattage than what's already there...

🙈😵‍💫🫤



   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4799
14/07/2024 8:31 pm  

What are you comparing?  Watts depends on flow temperature.  So a 1kW radiator at ,70C becomes a 400W radiator at 45C

 


4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.


   
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 Judith
(@judith)
Prominent Member Member
Joined: 2 years ago
Posts: 521
14/07/2024 8:36 pm  

Posted by: @lucia
↑

I have just discovered that BOTH my installer's heat loss proposals for 4 kw Daikins (based on imaginary heat loss and imaginary fully insulated walls) have specced radiators with LOWER wattage than what's already there...

🙈😵‍💫🫤

Oh dear!

Don't let them change radiator for lower wattage ones! There are experts on the forum who would like to see the full proposals. Can you upload them? 

 


2kW + Growatt & 4kW +Sunnyboy PV on south-facing roof Solar thermal. 9.5kWh Givenergy battery with AC3. MVHR. Vaillant 7kW ASHP (very pleased with SCOP >4) open system operating on WC


   
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 Lucia
(@lucia)
Honorable Member Member
Joined: 2 years ago
Posts: 354
Topic starter 14/07/2024 9:12 pm  

@jamespa I'm using the Delta T watts for both - proposed K22s and my existing K11s. Is that not right? 🙈

One designed for 50º flow temp the other 45º but both came up with the same 2 radiator changes. 🤔

I know there's 2 kinds of deltas - leaving/returning and I thought the other was flow temp.... am I wrong? I made a joke about learning to divide BTUs by 3.41 to one of the guys and he said yes, that's right. When I showed him the bathroom radiator I found he looked at the watts and said it was fine. 



   
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 Derek M
(@derek-m)
Illustrious Member Member
Joined: 5 years ago
Posts: 4425
14/07/2024 10:01 pm  

The standard output capacity of a radiator is specified at DT50, so if the IAT is 21C, the average water temperature would need to be 71C for the radiator to output the specified quantity of thermal energy.

If you look at the Screwfix website a 1200mm x 600mm Type 11 radiator has a specified output capacity of 1141W. The same size Type 22 has a specified output capacity of 2051W.

As James has pointed out the actual quantity of thermal energy that is emitted by a radiator is dependent upon the physical size, type, and the DT between the average water temperature and the room temperature.

I have attached a copy of the Radiator Heat Output Calculator in which you can vary the parameters highlighted in yellow to see the effect.

Radiator Heat Output Calculator.xlsx

 



   
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 Lucia
(@lucia)
Honorable Member Member
Joined: 2 years ago
Posts: 354
Topic starter 14/07/2024 10:45 pm  

@derek-m  So I can't just compare the delta T @ 30 for both rads? Oh. my. days... I think I need to go to college for this. Back to playing with Heat Punk.

Thank you for the calculator.  

 



   
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Topic Tags:  Daikin Altherma ASHP (10) , Vaillant aroTherm (11) , Octopus (9) ,

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