Sizing a heat pump for an apartment vs detached house

Heat loss per m² — apartment vs detached numbers

The sizing method is the same in both cases: calculate the design heat load to EN 12831, then choose a heat pump that can cover that load at the relevant outdoor design temperature and system flow temperature (EN 12831 — Energy performance of buildings — Method for calculation of the design heat load).

What changes in an apartment is the envelope. A modern apartment typically shares 2–4 walls with neighbouring dwellings. Those party walls are close to room temperature on both sides, so they contribute little or no transmission loss compared with an exposed external wall. In practice, that can halve the per-m² heat loss versus a detached house of the same insulation grade.

For a quick buying-stage sense check, typical design-temperature heat loss figures are:

That gap matters. An 85 m² apartment with moderate insulation in an average European climate comes out at roughly 3 kW peak heat output: 85 m² × 35 W/m² = 2.98 kW. The equivalent detached home at 60 W/m² would be about 5.1 kW.

This is why apartment heat pumps often look “small” on paper. They are not undersized if the EN 12831 result says otherwise. Oversizing is common when installers or buyers carry over detached-house assumptions into flats.

Buffer cylinder vs in-line for tight installations

Apartments usually have less plant space, shorter pipe runs and smaller water volumes. That pushes the hydraulic design question to the front: fit a buffer cylinder, or keep the unit in-line with the emitters?

A buffer can help if the system has:

• very low water volume
• multiple small zones with frequent valve closure
• minimum-flow constraints from the heat pump
• defrost or cycling concerns on small loads

But buffers cost space and add standing losses. In a tight installation, an in-line arrangement is often preferable if the emitter circuit can guarantee minimum flow and enough active water volume. That is especially true where the apartment has a single heating zone with radiators left largely open, or a compact underfloor loop with stable circulation.

The buying implication is simple: do not treat a buffer as default. Ask whether the chosen unit can operate stably with the actual apartment circuit volume and control logic. If not, a small volumiser or buffer may be justified. If yes, omitting it can save both cupboard space and parasitic losses.

For product comparison, the published space-heating efficiency and output figures under EU energy labelling are useful, but they do not replace a proper hydraulic design (EU Regulation 811/2013).

Where to put the outdoor unit when there's no garden

Detached homes usually put the outdoor unit on a slab in the garden. Apartments rarely have that option.

The common alternatives are:

• balcony floor mounting
• balcony wall or bracket mounting
• roof placement for top-floor units
• façade mounting where permitted
• shared service terraces in larger blocks

Monobloc air-water heat pumps can be installed on balconies, but that is not merely a plumbing decision. The load path into the slab or balustrade support needs structural review, and HOA or body-corporate sign-off is usually required. Access for condensate drainage, maintenance clearance and noise propagation also need checking.

The hard constraint is often not thermodynamic but administrative. A unit that fits the load may still fail on placement, vibration control or visible-façade rules. Buyers should confirm four points before fixing the equipment size:

1. permitted location 2. structural capacity 3. condensate drainage route 4. acoustic limits at the property boundary or neighbouring windows

Body-corporate and condominium consent

Apartment heat-pump projects fail more often on consent than on sizing.

External units can affect:

• common property
• façade appearance
• noise exposure to neighbours
• maintenance access
• penetrations through shared walls or roofs

For that reason, body-corporate, condominium or HOA approval should be treated as an early-stage requirement, not a final signature after equipment selection. The practical sequence is usually:

1. establish that an outdoor unit is allowed in principle 2. confirm acceptable locations and visual requirements 3. obtain structural and acoustic evidence if requested 4. select the smallest unit that covers the EN 12831 heat load

This sequencing favours apartments, because the required kW is often smaller than expected. A 3 kW-class requirement is easier to place than a detached-house-sized unit, provided the building rules allow any external plant at all.

DHW heat-pump options for apartments

Domestic hot water can be solved separately from space heating, and in apartments that is often the cleaner route.

Indoor-only DHW heat pumps — effectively heat-pump water heaters with an integrated cylinder — are a viable option where space heating remains on gas or district heating. They avoid an external refrigerant or hydronic heating retrofit and can fit apartments that only need a lower-carbon hot-water upgrade.

Where the apartment also uses an air-water heat pump for space heating, DHW can come from:

• an integrated indoor cylinder module
• a separate indirect cylinder served by the heat pump
• a stand-alone heat-pump water heater where systems are split by function

The right choice depends mostly on space, peak hot-water demand and whether the apartment already has a central heating source that will remain.

Sizing examples for typical EU apartment sizes

These are buying-stage examples using the apartment rule of thumb of 30–35 W/m² for moderate insulation. Final selection still needs an EN 12831 room-by-room calculation.

For context, the same 85 m² at detached-house assumptions of 60 W/m² would imply 5.1 kW. That is the core apartment sizing difference: same physics, much less exposed area per square metre, therefore a smaller practical kW.

The mistake to avoid is buying by dwelling type label rather than by design load. “Apartment” does not automatically mean easy retrofit, but it often does mean lower peak heating demand than the market instinct suggests.

Sources

• EN 12831 — Energy performance of buildings — Method for calculation of the design heat load
• EU Regulation 811/2013 (energy labelling)