@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.

@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.

@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.

Posted by: @dowallydrifter

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@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. 

Posted by: @bontwoody

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@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 

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

@editor thanks!

@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.

Posted by: @dowallydrifter

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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. 

@bontwoody

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.

Posted by: @dowallydrifter

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@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.