Air Source Heat Pumps: What Edinburgh Homeowners Really Need to Know

What an air source heat pump does

An air source heat pump, or ASHP, takes low grade heat from the outside air and uses a refrigerant cycle to lift it to a temperature useful for your radiators, underfloor heating and hot water cylinder. It runs on electricity but produces around three units of heat for every unit of electricity it draws. That ratio is the coefficient of performance, and it is why a well designed ASHP can be cheaper to run than a gas boiler even at current electricity prices.

The unit itself sits outside, usually to a side or rear wall, and connects into a hot water cylinder and a redesigned heating circuit inside the house. There is no combustion, no gas flue and no visible flame anywhere in the system.

Do they suit older Edinburgh housing stock

This is the question we get most often, and the answer is a qualified yes. Tenements, Victorian and Georgian villas and interwar bungalows can all run well on a heat pump, but only after some honest assessment of the property.

Heat pumps prefer to run for long periods at a lower flow temperature than a gas boiler. Where a combi might push water round the system at 70 or 75 degrees, a heat pump is far happier at 45 to 55. That flow temperature has to be enough to warm the rooms on the coldest day, so radiators and pipework need to be able to give up the required heat at that lower temperature.

In practice, that often means:

• A room by room heat loss survey, not a rule of thumb.
• Larger radiators in some rooms, especially north facing bedrooms and older single glazed spaces.
• Insulation improvements before or alongside the install, particularly loft insulation and draught proofing.
• A hot water cylinder if you are coming from a combi, since heat pumps do not supply instant hot water the way a combi does.

Tenement flats present specific challenges. External unit placement is not always straightforward on a listed building or a communal wall, and consent from neighbours or the local authority may be needed. It is not impossible, but it requires early conversation before you commit.

Running costs against gas

A properly designed ASHP with a strong seasonal performance factor should be roughly cost neutral or cheaper to run than a modern condensing gas boiler at current tariffs, and significantly cheaper than an old non condensing boiler or electric heating. Poorly designed ones can end up more expensive, which is why the survey stage matters more than the brand of pump.

Two things drive real world running cost. The first is the flow temperature the system settles at. Every degree lower means a better efficiency figure. The second is how the controls are set up. A pump that short cycles because a timer is telling it to fire in short bursts will never hit its rated performance.

Pairing with an unvented cylinder

Almost every heat pump install we do includes an unvented cylinder. The pump heats the cylinder to a set temperature, then holds it there. A weekly Legionella cycle is scheduled automatically to lift the water above the temperature at which bacteria can survive.

Cylinder sizing matters more with a heat pump than with a gas system because you cannot instantly reheat a tank the way a fast recovery combi can. For a family of four we typically look at 250 to 300 litres, with a coil designed for heat pump duty. The service and warranty then follow the same annual G3 schedule as any other unvented cylinder.

The Home Energy Scotland Grant, in plain English

At the time of writing, Home Energy Scotland offers grant funding for owner occupiers moving to a clean heating system such as an air source heat pump, with an interest free loan available on top of the grant. Figures are subject to change, and the scheme has an uplift for rural and island properties.

The rules that tend to catch people out:

• Only heat pumps that provide 100 per cent of your heating and hot water are eligible. Hybrid systems that keep the gas boiler in place do not qualify for the grant.
• The installer must be MCS certified for the technology being fitted, and MCS certified products must be used.
• Replacement heat pumps are not eligible for the grant, though an interest free loan may still be available.
• You apply through Home Energy Scotland before the work is booked. The order matters.

For up to date grant and loan values, and to check your own eligibility, go directly to homeenergyscotland.org or call the Home Energy Scotland advice line before you commit.

What "MCS certified" actually means

The Microgeneration Certification Scheme is the quality mark for renewable heating installers in the UK. It is not the same as being Gas Safe. To fit a heat pump under the grant scheme, the installer holds MCS certification in their own right, follows the MCS installation standard, submits a design and heat loss calculation, and registers your installation on the central MCS database. You get a certificate at the end that is required for your grant claim and for future property sales.

If a quote for a grant funded heat pump does not mention MCS design paperwork, that is a red flag.

Retrofit works you should budget for

A heat pump on its own is only half the story. The typical retrofit involves:

• Removal of the old boiler and any redundant flue.
• Installation of the external unit on a wall bracket or ground stand, on antivibration mounts.
• New primary pipework run from the unit into the plant space.
• A new hot water cylinder if one is not already in place.
• Radiator upgrades in one or more rooms.
• New controls and, where appropriate, a buffer or volumiser.
• Electrical works, including a dedicated circuit and, occasionally, a supply upgrade.

None of this is exotic, but it does mean a heat pump install is not a like for like swap. Plan for a week or so on site for most properties, and longer for larger jobs or where the plant room needs reorganising.

How to move forward

Start with a home energy assessment. Home Energy Scotland offers a free advice call and can point you at a survey. From there, a good MCS installer will visit, take heat loss measurements, propose a system and give you a written design. That written design is what you should compare between quotes, not the headline price alone.