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Help Right-Sizing My Grandmother's Heat Pump: Installer Says 12kW, I Think It's Oversized

 
Air Source Heat Pumps (ASHPs)
Last Post by JamesPa 1 hour ago
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 Alva
(@alva)
New Member Member
Joined: 1 day ago
Posts: 2
Topic starter 16/04/2026 6:59 pm   [#2955]
I found out local installers previously massively oversized and overpriced my grandma's air conditioner system, so I'm now helping with her new renewable heating configuration to make sure she gets the right system. 
 
Her requirements are a bit different from most setups. My grandmother lives alone in Southern Europe and only uses the full house when family comes to visit during summer vacation and christmas holidays.
 
This means that she has DHW & heating requirements usually only for herself and some occasional visitors (up to 4 people total), while around 3 months each year her house is full with family (up to 8 people total).
 
I calculated the heat loss for her primary living area (master bed, kitchen and living room) at 4,650W and for the guest bedrooms at 1,150W (outdoor temp 3c) with a total UFH area of ~140 m2 (~120L total volume). The floor will be polished concrete. It does not freeze during winter.
 
The local installer proposed:
  • 12kw Chaffoteaux heat pump
  • Separate 500L DHW tank
  • 50L Thermal flywheel / buffer tank
  • Thermostats/zones for effectively each individual room
 
Based on what I've read here it feels like this system might be a bit oversized. Plus 500L DWH heated all year while it's only fully used for a few months feels wasteful.
 
I tried to figure out as much as possible from this site, but would love input from the experts here on the below:
  1. A smaller heat pump: 10kw with 200L DHW tank (like the Mitsubishi Ecodan 10 kW 200L). Plus a separate 200L split boiler heat-pump combo (like the Ariston Nuos Split) for the guest bedrooms. The separate DHW boiler would only be turned on while the family visits.
  2. Fewer thermostats and zones: one for the entire primary living area plus one each for guest bedrooms.
  3. No thermal flywheel (I think this is their word for buffer tank) as most people here recommend against it.
 
I hope this makes sense and I didn't make any major mistakes 🤞

This topic was modified 3 hours ago 13 times by Alva
This topic was modified 2 hours ago by Mars

   
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JamesPa
 JamesPa
(@jamespa)
Illustrious Member Moderator
Joined: 3 years ago
Posts: 4640
16/04/2026 8:55 pm  

Posted by: @alva
↑
The local installer proposed:
  • 12kw Chaffoteaux heat pump
  • Separate 500L DHW tank
  • 50L Thermal flywheel / buffer tank
  • Thermostats/zones for effectively each individual room

Quite possibly all wrong.  Get other quotes or regret

Can you tell us more about the house, construction, location, floor area, climate where you are

Do you have any figures for current fuel consumption?

You are thinking along the right lines but would need more detail to be certain. 

Does the family of 8 visit at coldest time of year?

Why not 2 zones one for grandmother one for family, 2 dhw tanks independently switched, all run off perhaps an 8kW pump.  You may need a volumiser, you won't need a buffer. Something like that is probably where I would start but you need to do the actual numbers for the two scenarios separately then apply a bit of thought because of the two distinct regimes 

 


This post was modified 1 hour 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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