Expert Advice - Boilers







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Home comfort depends hugely on your heating boiler, and so does the affordability of your energy bills. But how do you make the best choice? Christian Engelke, Technical Director at Viessmann UK, answers our questions.

How do I know when to replace my boiler? 

It’s time to replace your boiler when parts eventually wear out that are uneconomical to replace, or if you do not have a modern condensing boiler by now, we’d strongly suggest a switch. The good news is that your boiler should last 10 years or more and annual servicing will extend its lifespan. And when you do have to buy a replacement, your new boiler will almost certainly be more efficient than your old one, reducing your energy bills.

How much does the average boiler cost to install? 

It will cost about £2,500 to £5,000 to fit a good quality boiler to heat an average-size family home. This includes materials and labour to install it and depends on the property. Different makes of boiler will give you different levels of performance, energy efficiency, and reliability. Going for the cheapest brands can be a false economy which ends up costing you more in running costs and repairs. It will pay back if you invest in a boiler from a quality manufacturer.

We’re proud to be one of only two gas boiler brands recommended by the Which? 2017 survey.

When should I choose a combi or conventional gas boiler? 

This is very much a personal choice of the customer. Combi (combination) boilers combine a central heating boiler and a high-efficiency water heater in a single compact unit. This has space-saving advantages, ideal for smaller properties. With a combi there is unlimited hot water on tap whenever you want it. This is delivered subject to limits defined by the output of the boiler, but there might not be a steady flow if more than one hot water tap or shower is used at the same time.

So-called conventional (or system) boilers use a storage cylinder for the hot water, usually located in an airing cupboard or sometimes the loft. A conventional boiler will be better-suited to larger properties where there is a large demand for hot water and where numerous showers or hot taps are likely to be run at the same time.

A few years ago, we developed a combi boiler that had a larger, 46-litre integral hot water storage tank for higher hot water demand. The Vitodens 111-W can fill a standard bath (140 litres) in around 8 minutes, with a quick heat recovery of less than 5 minutes.

How do I know which size of boiler to choose for my home and how does it impact performance? 

‘Size’ means heat output, which in domestic boilers ranges from about 5 to 15 kW (kilowatts) for space heating. If the boiler’s too small, it won’t be capable of heating your home properly. But be aware of the once-common practice of oversizing boilers by about 30% - with today’s technologies this is unnecessary, and if the boiler’s too big it will waste energy and increase your gas bills.

The appropriate size is determined by a heat loss calculation which takes into account a property’s floor area, insulation, and hot water requirements. A heating engineer can work this out for you, or there are a number of free online calculators available. A combi is often over-sized so it delivers a high volume of hot water e.g. for showering, but it will be proportionally less efficient.

What is the most energy efficient kind of boiler? 

Modern gas condensing boilers are the most energy efficient. Compared to previous generation non-condensing boilers, these are making fuel savings of up to 35%, which is why all gas boilers are now required to have condensing technology by law. Beyond this, there is a way to save up to 15 percent more, and that’s with Weather Compensation technology. This is available with Viessmann’s Vitodens range of condensing boilers.

The colder it is outside, the more heat our homes lose through the walls, windows and roof. Weather Compensation takes this into account by connecting the boiler to an outdoor weather sensor. This ensures the boiler burns no more than exactly the right amount of fuel to match the heat lost. This significantly improves efficiency under partial-load conditions - especially relevant in the UK climate, where for two-thirds of the year the boiler can maintain constant room temperatures while providing only a fraction of its maximum heat load.

Where is best to position the boiler in my home and is it possible to move the location of my new boiler? 

A gas condensing boiler can be located in many places in a home, but there are some essential considerations. It will need to be attached to a wall strong enough to take its weight; will need a flue which passes from the inside of the home to the outside via a wall or the roof; and will need a condensate pipe which can discharge condensed liquids from the boiler into the household drainage system.

Building Regulations specify the minimum distance required between the flue terminal and other objects such as windows, air bricks, and other buildings. Wiring Regulations stipulate where in a bathroom it is and isn’t permissible to locate a boiler. If the boiler’s placed inside a cupboard, the cupboard won’t need air vents.

What benefits will I see with my new boiler? 

Because newer boilers are more efficient than the older ones they replace, they’ll consume less gas to keep your home comfortably warm, meaning you can expect lower energy bills. With heating accounting for about 60 percent of a typical household’s annual energy bills, an efficient boiler makes a big difference. If you are replacing a really old boiler, it’s likely your home will be warmer too.

What maintenance is required for my boiler and how long should a new boiler last for? 

Boilers need servicing annually. For safety reasons, servicing is crucial because faulty boilers can cause fatal carbon monoxide poisoning. For financial reasons servicing is also essential, because a well-performing boiler will cost you less in gas bills and preventative maintenance is less expensive than fixing break downs. It will also mean that your boiler works more quietly. Most new boilers are supported by long warranties these days.

For boiler servicing and installations, you’ll need an engineer who is Gas Safe registered. At Viessmann we encourage the highest levels of engineering expertise by running a Viessmann Trained Installer recognition programme at our highly-regarded Viessmann Academy.

How long a boiler lasts will naturally depend on how well it is engineered (brands do differ in quality and reliability), how extensively it is used (boilers have to work harder and longer where it’s colder), and how regularly it is serviced. A good, and well-maintained, boiler should last for 15 years or more.

Anything else I need to consider? 

Although a gas condensing boiler is the default choice for heating a home and its water, you might want to consider whether you can heat (and cool) your home with renewable technologies or with a ‘hybrid’ combination of conventional and renewable technologies. These can further reduce your home’s energy bills and greenhouse gas emissions. You are welcome to contact Viessmann for advice about this.

Our range of renewable energy systems features solar thermal systems to heat domestic hot water and provide central heating backup and cooling, as well as special boilers for logs, woodchips and pellets. There are also hybrid appliances; heat pumps and ice store systems that exploit heat from the ground, from groundwater, or from the ambient air; and solar photovoltaic systems.

And if you’re looking for the very latest thing, why not check out a fuel cell heat and power appliance, which is essentially a boiler that, together with an integral fuel cell unit, makes electricity for free, out of your gas supply.

Published: June 3, 2020

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