🔥 Heat Pump
You need a huge, three-phase machine of 14 kW to 16 kW to cover losses and hot water. Purchase cost: approximately €7,500 to €9,000.
Calculating Required Heating Capacity (BTU/kW) Before and After Insulation
The most common discussion when someone builds or renovates a house is: "What heat pump should I buy? How many BTU does my living room need?" If you ask an old plumber or HVAC technician without giving them your insulation data, they will probably use an old "rule of thumb" (e.g. so many BTU per square metre). This mistake will cost you thousands of euros on equipment, because modern insulated homes have absolutely nothing in common with 1980s houses.
Let us take our virtual "10x10 House" (100 m²) and see how insulation "shrinks" your boiler and air conditioners.
1. The Golden Rule: Machine Capacity = Maximum Losses
The rule for choosing the right machine is extremely simple: The heating system (boiler, heat pump, air conditioner) must be able to produce as much heat (in kW or BTU) as your house loses on the coldest day of the year.
Quick unit reminder: In Greece, for boilers and heat pumps we talk in kW (Kilowatts). For air conditioners we talk in BTU/h. The conversion is easy: 1 kW equals 3,412 BTU.
Let us go to our house's numbers for a day with 0°C outside and a target of 20°C indoors.
2. Scenario A: Buying Equipment for the Uninsulated House
As we calculated in previous articles, the uninsulated "10x10 House" loses energy from everywhere. If we add up the losses from the uninsulated roof, bare walls, old single-glazed windows and air leakage, the house loses about 12 kW every hour at extreme temperatures.
What equipment must you buy?
❄️ Air Conditioners (BTU)
The 12 kW translate to about 41,000 BTU. You will need an 18,000 BTU unit in the living room and three more 9,000 BTU units in the bedrooms, all running flat out non-stop!
3. Scenario B: Buying Equipment for the Insulated House
We did our renovation. We installed 8cm external insulation, 8cm on the roof and new energy-efficient windows. We sealed the shell. The maximum losses for the exact same freezing day (0°C) plummet to 3.5 kW!
Let us go shopping now:
🔥 Heat Pump
You now need a tiny, single-phase heat pump of 5 kW (or at most 7 kW). Purchase cost: approximately €3,500 to €4,500.
❄️ Air Conditioners (BTU)
The 3.5 kW translate to just 12,000 BTU. Theoretically, a single 12,000 BTU unit (or two small 9,000 BTU units) idling is enough to keep the entire 100 m² house at 20°C!
4. The "Hidden" Gain of Insulation (ROI)
Here lies the magic of renovation economics. Many people say: "I will not install external insulation because it costs €6,000; I prefer to buy a big heat pump".
But if you do install the insulation, the heat pump you need will cost €4,000 less! So the insulation "paid" half its cost before you even flicked the switch for the first time!
And of course, from the very next day, the small 5 kW pump will burn a quarter of the electricity compared to the 14 kW "beast".
💡 Final Conclusion: Insulation pays you twice. Once at the equipment shop till (because it lets you buy the smallest and cheapest equipment) and once every month on the electricity bill. Never, ever buy or size heating systems before you finish (or at least calculate) your home's insulation!
Related Articles
Uninsulated vs. Insulated House: How Much Money & kWh Do You Save? The 10x10 House: The Standard Model for Calculating Heat Losses Insulation Thickness: Are 10cm Worth It Over 5cm? (Calculation)
Thermal Physics & Calculations: From Theory to kWh and Euros
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