Air-to-water heat pump in an apartment: unit, tank and underfloor heating

What changes in a flat if you drop gas and fit a heat pump?

The gas boiler is replaced by an air-to-water heat pump: the outdoor unit draws heat from the air and the indoor module heats the water. The flat needs three things: a place for the outdoor unit, a place for a hot-water tank of about 150 to 200 litres and enough electrical capacity.

The advantages are clear: no gas, no periodic gas inspection and the ability to cool. The drawbacks are too: everything hinges on the outdoor unit and the electrical supply. Those two questions are settled before buying equipment and before the electrical design, or the scheme has to be changed later.

How much space does a heat pump tank take?

A 150 to 200 litre hot-water tank for a family of three or four has a footprint of about 60 by 60 cm and stands 1.6 to 1.8 m tall. If everything is in one casing with the hydraulic module, it becomes a cabinet of the same size that goes in a technical room, utility room or built-in cupboard.

Beside the tank you leave room for servicing, for the expansion vessel and the drain, and you check the floor load: a full tank weighs about 250 kg on a small area. In an old building with a timber floor this is a serious question, and the tank is better placed by a load-bearing wall. The room with the tank should not be a bedroom: the hydraulic module hums audibly while heating.

In compact flats it is often better to move the washing machine or a wardrobe than to sacrifice a room.

Can you fit a heat pump without a terrace?

You can, but the outdoor unit still has to stand somewhere. Without a terrace the options are those for air conditioning: roof, inner courtyard, an enclosed ventilated niche. On the facade of a protected building it is not allowed, and in an ordinary one its position is processed separately.

A 6 to 10 kW outdoor unit is roughly a metre wide, half a metre deep and a metre high, with free space around it for air. In practice it goes on anti-vibration mounts with condensate drainage, and the link to the indoor module is made as a refrigerant split so water does not freeze in outdoor pipes. Line length is limited by the data sheet.

If the comunidad objects, electric heating and multisplit remain. That decision is made before buying, not after.

Heat pump and underfloor heating: are they required together?

Not required, but they work best together. A heat pump performs best when it supplies water at 30 to 35 C, exactly what underfloor heating wants. Conventional radiators need 55 to 65 C and the pump uses more electricity.

In a flat there are variants. Underfloor heating in the main rooms plus fan coils or low-temperature radiators where you do not want to touch the floor. Fan coils work at 40 to 45 C and also cool in summer. Underfloor heating adds 6 to 8 cm to the floor build-up, so in a flat with low ceilings it can decide the project. Underfloor details are in our article on radiant floors in homes.

Water temperature and pump modes are tuned to the weather, not by a button: the system must know the outside temperature.

Will the electrical capacity be enough?

A heat pump of 6 to 8 kW thermal draws roughly 2 to 3 kW of electricity in normal running, plus the tank's heating element and the anti-legionella cycle. Many homes are contracted at 4.6 to 6.9 kW, and with an induction hob, oven and washing machine at once that may not suffice.

So before ordering equipment, the load of all appliances at once is calculated and a decision made on whether to raise capacity or move to three-phase. After changing the board or the supply line, the electrical certificate (CIE) is filed, a process described in our article on it. If the building is old and the supply to the flat is limited, the increase is arranged with the distributor.

Part of the load is removed by control: the tank heats at night or in solar hours, and the hob and pump do not run flat out together.

What happens to the gas pipe and the paperwork after switching?

Gas is cancelled with the distributor and closed off by a licensed installer: a capped pipe is not simply left in the wall. This is agreed in advance so you do not end up chasing a wall for a new riser.

After the works the electrical paperwork is filed and, if you plan to sell, the energy certificate is updated: the rating usually improves and shows in the ICAEN register. The process is in our article on the energy certificate in Catalonia. If the building has a communal boiler or central heating the scheme differs and the statutes are read first.

Keep the equipment sheets, hydraulic diagrams and pump settings: in five or seven years a service engineer finds a fault much faster.

How can BCN YARD help with the switch to a heat pump?

We visit the flat, see where the unit and tank can physically go, check the electrical side and work out how the scheme fits floors and ceilings. The comunidad process can be run in Russian and Spanish. We work with our own crew and trusted subcontractors, one person is responsible for the site, and the exact price comes in the estimate after the visit and measurements.