Whether you’re tackling a single-storey rear extension, building a multi-storey addition
or converting a loft, chances are you’ll have your sights set on creating a cosy, low-bills zone that complements the rest of your home.
Your choice of heating system is a crucial decision, of course – but it’s not a standalone one. Before you make the final call, you’ll need to factor in energy performance, the condition and capacity of your current heating setup, simplicity of installation, running costs and more.
A key first step is to pay plenty of attention to detail when it comes to the thermal performance of the fabric of both the new addition and the existing house. Upgrading the structure means you can minimise any extra heating load and get the best bang for your buck out of your setup.
Most homeowners will look to significantly exceed the minimum insulation and airtightness standards set out in Part L1B of the Building Regulations. After all, it stands to reason that the better the addition performs, the less extra space heating load you’ll be putting onto the existing boiler (or other heat source). Thankfully, it’s pretty easy to achieve this with modern materials and a decent builder.
Rather than looking at the new addition in isolation, though, it’s worth sparing a thought for the rest of your home. Yes, you could slap on a 20m2 rear extension replete with the best insulation money can buy in the walls, roof and floors, along with the latest triple glazing.
But will that make best use of your budget, or would it make more sense to take a holistic approach and upgrade parts of the existing house, too?
It could cost as little as a few hundred pounds to top up your loft insulation to the modern standards, for instance. And doing so could not only make the whole house cheaper to heat, but might even help enable the inclusion of desirable features in your new extension. If you want vast swathes of glazed doors and big rooflights, for instance, you may need to offset the impact of these by making efficiency improvements elsewhere.
Many of us extend to create a better connection between house and garden, and the glazing design will be a big part of the energy efficiency equation. South-facing extensions will benefit from a boost of free warmth (known as solar gain) during the day – but too much glass on this elevation could lead to overheating.
Work with your architect and window/door supplier to establish the best solutions for your project. Do you need double or triple glazing, for instance, and would solar control coatings
Learn more: Glazing and the Regulations
Perhaps the extension’s design should incorporate features such as a roof overhang or brise soleil to protect against the high summer sun, while allowing in plenty of solar heat in winter when it’s needed most.
However well-built your new extension is, and however much insulation you plan to pack into it, you can’t escape the fact that you’ll be increasing the overall amount of living space that needs to be heated. So, one of the key questions you’ll need to address is whether your existing boiler has enough capacity to cope.
As a rule of thumb, if you’ve got a reasonably modern boiler and the extension isn’t huge, then you should be ok. But many projects involve creating a considerable amount of extra floor space – particularly in England, where recent changes to permitted development rights mean (providing it meets certain criteria) you can add an 8m-deep rear extension to a detached house without having to make a formal planning application. In most cases, then, the heat demand will go up, with the possible exception of major eco retrofits (see page 68 for more on this).
One part of the equation that’s sometimes neglected at the design stage is domestic hot water (DHW). If your plans involve creating an extra bathroom, for instance, this could be the trigger that indicates a new boiler (and possibly a new hot water cylinder) is required to deliver enough hot water and the right pressure.
So, it’s always a good idea to engage a professional heating engineer to determine your requirements. They will be able to calculate the exact space heating demand across the house and in individual rooms, along with your household’s DHW requirement, and let you know whether any upgrades are required. A typical boiler replacement might cost around £2,500 including installation (on top of any new plumbing works for the extension itself).
One update that’s pretty much always worth doing is insulating any accessible heating and hot water supplies – both the existing pipework and the new runs for the extension. This is an inexpensive job and could save around £20 a year off your energy bills.
Going green is a big motivation for taking on a major renovation and extension project. If you’re completely gutting the house and giving the whole place an eco retrofit, then that will open the door for tech like air source heat pumps (ASHP). But this is way beyond the scope of most extensions – so may not represent value for money.
“We supply heat pumps as well as underfloor heating (UFH), and we do speak to homeowners who want to move away from a traditional boiler, but we always talk to them to understand why they’re keen to do that,” says Jo Snell, senior business development manager at Nu-Heat.
“An ASHP is brilliant in the right situation, which tends to be a well-insulated property off the mains gas grid where you’re comparing renewable solutions with oil or LPG, but it may not be the best option for extensions.”
Learn more: Heating Products
One issue is that heat pumps run at a lower flow temperature than boilers. This will almost certainly be too low for an existing radiator setup, so you’d need to switch to UFH or oversized radiators throughout the whole house – adding even more cost and disruption. It’s also tricky to bring certain older properties, such as listed buildings, up to the standard of insulation that an ASHP requires.
“If you want to stick to radiators and it’s difficult to insulate, then we wouldn’t recommend a heat pump,” says Jo.
It’s fair to say that, these days, water-based UFH is the number one choice for self builders and home extenders. This is largely thanks to the comfortable, even warmth it provides and the fact that it frees up precious wall space for furniture, storage and displaying artwork.
Plus, when used in tandem with a gas boiler, it’s up to 25% more efficient than radiators (which will be reflected in your energy bills). But both options still have their place.
“More and more people are asking us about underfloor heating for large extension projects, particularly open-plan kitchen-diners,” says Jo. UFH transforms the entire floor surface (tiles, engineered wood, etc) into a giant low-temperature heat emitter, producing an even warmth
that’s ideal for this kind of larger zone.
“It cuts out the hot and cold spots you experience when there’s a big distance between radiators,” explains Jo.
If you’re wondering how easy it is to run UFH in an extension and radiators in the rest of the house, it couldn’t be simpler. “All that’s needed is to split the boiler primaries, so one loop goes to the radiators and one to the underfloor heating,” says Jo. “That way the UFH can call for the heat it needs independently.”
The big no-no is combining the two systems in a single room: underfloor heating provides gentle, all-round radiant warmth and is designed to run on a long, low-temperature cycle. So it doesn’t make sense to disrupt this with quick-response convection heat.
One area worth particular attention is the join between old and new in an open-plan extension. “The addition is likely to have a concrete slab with 80mm-100mm of insulation beneath to prevent heat loss,” says Jo, “whereas the existing building might feature a suspended timber floor or the covering might be concrete over earth.”
To get the best results from UFH, you need to achieve some consistency, and not just in terms of finished floor levels. “A suspended timber floor will have a heat loss of around 45%, so you’ll definitely need to insulate between the joists,” says Jo.
“But an old solid floor will probably only have 6%-10% heat loss, so you don’t have to dig
out a sound concrete slab. In fact, there’s a clause in the Building Regs saying exactly that – because the cost of doing it would never be repaid through energy savings.”
The recent proliferation of high-quality low-profile systems, such as Uponor’s Minitec and Nu-Heat’s LoPro, means that you can run the same setup across both the new and old parts of an open-plan zone without losing valuable floor-to-ceiling height. This kind of system, which is likely to cost around £40-£65 per m2 installed, can also make sense for loft conversions with decent headroom.
Depending on the size and style of your project, and your budget, there are a few more emitter options worth considering for extensions, including:
Trench heating This solution has been around for a while but never quite captured homeowners’ imaginations. The basic idea is something of a halfway house between UFH and radiators, with special convection emitters dug into the floor and topped with a grille. One potential downside is that it doesn’t offer the flexibility of UFH in terms of furniture positioning, as it’s important not to block the vents.
Skirting heating This operates on a similar principle to trenches, but is even easier to install, with the pipework embedded into the back of the skirting boards running around the perimeter of the room. Discrete Heat is one of the originators in the market, but some UFH suppliers are now offering skirting solutions to complement underfloor heating in zones where you want a quick-response system (such as bedrooms).
Electric systems Whether UFH or radiators, electric options remain popular in specific circumstances, such as for small bathrooms (for their fast response times)
Infrared radiators An up-and-coming tech in this market. Rather than heating the air in the room (which creates cyclical draughts) these electrically-powered panels work through radiation – directly and near-instantly warming the surfaces and people in a room
All that said, there’s still plenty of benefits with radiators. There’s no doubt they’re cheaper to fit than UFH, for example, with costs as low as £175 per standard radiator (installed) depending on labour rates in your area. It’ll usually be slightly less disruptive in the main house, too,
as the additional heaters can be plumbed into the existing loop rather than requiring their own circuit.
There’s tonnes more design options, too, including modern vertical styles that can fit more easily into some extensions (between runs of glazed door, for example) than horizontal models. Some contemporary designs could even be considered works of art.
And on a practical level, you may find it useful to have a heated towel rail in a kitchen or bathroom. But radiator running costs are higher, and you won’t get the space-saving and comfort benefits of UFH.
Of course, whatever setup you choose, it will only work efficiently if it’s paired with the right controls. Even a basic programmable thermostat (which you’ll be required to fit as part of a modern system) can achieve savings of 40% compared to an uncontrolled heating setup.
The latest smart tech can eke out even better levels of efficiency, not to mention adding real convenience, especially for extensions and other zoned setups.