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Replacing knackered oil boiler with ASHP - Perthshire

 
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Air Source Heat Pumps (ASHPs)
Last Post by DowallyDrifter 2 months ago
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 DowallyDrifter
(@dowallydrifter)
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Topic starter 13/03/2025 2:01 pm  

Interested in red flags, guidance on replacing 26Kw oil-boiler with in-boiler 127l HW tank, with ASHP, 300l tank.

Timber frame with block exterior, 275sq.m, bilt 1999, UFH both floors with room stats in all but 3 bathrooms which are always open.  Original heat loss as designed was ca. 18kW.  two log burners, one in front hall one in large living room upstairs. Oil aga for cooking always on.

Two suppliers quoted: one calculated heat loss at 13.3kW, the other at 18.94kW so ASHP at 16kw and 2x19kW.

Both quote for buffer tank.


   
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bontwoody
 bontwoody
(@bontwoody)
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13/03/2025 3:30 pm  

@dowallydrifter

I think I would be tempted to do my own heat loss assessment. Using heatpunk

or you could try this simple one https://openenergymonitor.org/tools/SimpleHeatLoss.html

There is a substantial difference between the two you have. Also i would look at possible easy insulation upgrades to reduce it.

Also I would ask the installers to justify why they think a buffer is necessary. Much better not to have one if possible as it will lower your efficiency.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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 DowallyDrifter
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Topic starter 13/03/2025 4:42 pm  

@bontwoody Thanks - will have a go at the calcs as I've the data that they used.  I've asked about buffer today and waiting on a reply.


   
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bontwoody
 bontwoody
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13/03/2025 5:09 pm  

@dowallydrifter if you had oil usage figures and didn’t use the wood burners much you could use that as well to get a sanity check.

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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Mars
 Mars
(@editor)
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Posts: 3161
13/03/2025 5:42 pm  

Posted by: @dowallydrifter
↑

@bontwoody Thanks - will have a go at the calcs as I've the data that they used.  I've asked about buffer today and waiting on a reply.

You will not need a buffer if the system is correctly installed. I will connect you with two excellent installers shortly that cover your area so you can get comparative pricing. 

 

This post was modified 2 months ago by Mars

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 JamesPa
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13/03/2025 5:44 pm  

Posted by: @bontwoody
↑

@dowallydrifter if you had oil usage figures and didn’t use the wood burners much you could use that as well to get a sanity check.

Totally agree and I would be suspicious of the high loss estimates.  Even if you did use the wood burners and know how much wood you burned you could add that in.  I am sceptical about even 13kW unless it's poorly insulated.  Any sanity check you could do would be a good idea 

 

This post was modified 2 months ago by JamesPa

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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 DowallyDrifter
(@dowallydrifter)
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Topic starter 13/03/2025 6:57 pm  

@bontwoody Thanks. Oil figures available but can't isolate aga from the dead boiler usage unfortunately.


   
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 DowallyDrifter
(@dowallydrifter)
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Topic starter 13/03/2025 6:58 pm  

@editor thanks!


   
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 DowallyDrifter
(@dowallydrifter)
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Topic starter 13/03/2025 7:11 pm  

@jamespa oil varies 2500-4000 litres a year for boiler and aga, ca. 4 cu.m wood p.a. Original design for UFH showed outside walls with K=0.45 W/sq.m/K roof K=0.24 W/sq.m/K, and windows at 3.5 W/sq.m/K .  Independent engineer in 1999 when built reckoned 18Kw at -5degC.  Not trivial to add wall insulation to timber frame. Loft insulation doable to some (bats permitting!) more constrained access in rest of roof.


   
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Abernyte
 Abernyte
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13/03/2025 7:40 pm  

Posted by: @dowallydrifter
↑

Not trivial to add wall insulation to timber frame

It is pointless adding EWI to a timber framed house with external block rainscreen. The movement of air around the timber frame inside of the block outer makes adding EWI irrelevant.

Also in Perthshire with leaky timber frame in an exposed site. 


   
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 DowallyDrifter
(@dowallydrifter)
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Posts: 8
Topic starter 13/03/2025 8:12 pm  

@bontwoody

image

results from the simple calculator point toward 16.5kW heat pump plugging in U values from original docs. walls 0.45, roof 0.24, windows 3.5 - the simple calc will be ignoring the large lounge with vaulted ceiling i guess.


   
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bontwoody
 bontwoody
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13/03/2025 9:15 pm  

Posted by: @dowallydrifter
↑

@jamespa oil varies 2500-4000 litres a year for boiler and aga, ca. 4 cu.m wood p.a. Original design for UFH showed outside walls with K=0.45 W/sq.m/K roof K=0.24 W/sq.m/K, and windows at 3.5 W/sq.m/K .  Independent engineer in 1999 when built reckoned 18Kw at -5degC.  Not trivial to add wall insulation to timber frame. Loft insulation doable to some (bats permitting!) more constrained access in rest of roof.

Ive done some rough calculations on those figures after trying to add in the wood and Ive come up with a demand of about 19.5kW at -3C to achieve 21C internal temperature (using the 4000 liter figure), so if James concurs then the original estimate isn't far out. (Although this drops to 13kW using the 2500 liter figure)

We like our living room warm (22.5C) so the way I have approached insulation is to concentrate on that room. Insulated plasterboard has worked very well on both the walls and inaccessible loft areas. 

 

House-2 bed partial stone bungalow, 5kW Samsung Gen 6 ASHP (Self install)
6.9 kWp of PV
5kWh DC coupled battery
Blog: https://thegreeningofrosecottage.weebly.com/
Heatpump Stats: http://heatpumpmonitor.org/system/view?id=60


   
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