What is a high efficiency gas boiler?
A high efficiency gas boiler is what’s known as a condensing boiler. Since the 1st of April 2005 the condensing boiler regulations state that all new gas-fired boiler installations must be condensing boilers, this also applies to boiler replacements as well. Before then most gas boilers installed in homes and buildings were non-condensing standard efficiency boilers.
This was a big problem because unlike condensing boilers standard efficiency boilers were not designed to extract the latent heat from the flue gases, losing energy and costing more to run. This also has a profound effect on the carbon emissions these older boilers produced to get the same amount of energy out of them.
Mains gas and LPG (liquid petroleum gas) are hydrocarbons. This means they are made up primarily of hydrogen and carbon. During the ignition and firing stage, the gas is split into the different elements and each reacts with oxygen to produce heat. The by-products of carbon reacting with oxygen are carbon dioxide and if complete combustion is not achieved, carbon monoxide.
The by-product of hydrogen reacting with oxygen is water vapour. Within this water vapour, we can extract latent heat to increase the output of the boiler and drive up the efficiency of high efficiency gas boilers/condensing boilers. By cooling the water vapour down to levels where it condenses into water, it releases previously trapped latent heat into our system that would have otherwise been sent out the flue.
See the heat geek blog for more information on condensing theory. On older standard efficiency boilers you might have seen an overall combustion efficiency of maybe 90%, probably lower, whereas when a condensing boiler operates the overall combustion efficiency can be mid to high 90's.
As you can see the advantages to a high efficiency gas boiler are far better for the homeowner in terms of saving energy but they are also less hazardous to the environment.
Are gas boilers still legal?
Whereas the government is working towards a carbon-neutral target in the future, gas boilers are still legal at the moment and offer the cheapest means of generating power currently.
Although the government's aim, in the long run, is to utilise renewable energy sources wherever possible, right now this is not completely feasible for every property throughout the UK and can incur heavy installation costs making properties ready for such technologies for those that are, especially older properties with poor insulation and high heat loss. This means that gas boilers are still viable and will continue to be used for the foreseeable future.
So what makes a high efficiency gas boiler a good boiler?
When choosing a high efficiency gas boiler brand for a new build or boiler replacement it's important to choose a manufacturer with good heat exchangers, good modulation and good controls, as well as good quality and reliability obviously. As well as choosing the right boiler and controls for your specific needs it's also important to consider the condition of any existing system as this can impact the function of the new system if not taken into account.
Why is modulation important?
Modulation is where the boiler modulates its gas input down to suit a changing environment and can be referred to as its turndown ratio. Let’s say your system is designed to an outside temperature of -2 and your room target temperatures are a consistent 21 degrees celsius, this is what is called max load conditions in the industry.
Now say in this scenario we had a heat loss of 23 kilowatts (quite high for modern buildings) but not for older poorly insulated buildings. This means under these conditions the boiler output would remain at a steady state of 23 kilowatts to reach and maintain target room temperatures, the problem is we as a country typically only see these conditions for a few days of the year though.
Now say we’re in the warmer months of the year and our heating load has decreased. This means the output from the boiler needs to be less to match the changing load and that's when the boiler begins to modulate to do so. Typically gas boilers try to maintain a set flow temperature relative to design conditions.
If the output of the boiler is too high for the current requirements the water returning to the boiler will be hotter and as it passes through the heat exchanger absorbs its energy and starts leaving the boiler hotter than required, the flow temperature sensor reads this and see that its output needs to reduced to match the load. The whole time the demands are changing the boiler either modulates up or down to maintain the correct output.
A problem within the industry is though that some gas boiler manufacturers have poor turndown ratios, this means they find it harder to match changing loads, or aren't even capable of modulating down enough to suit. This can cause a lot of boiler cycling (switching on and off) which puts a lot of wear and tear on boiler components which can lead to replacing parts over its lifetime, but also forces the boiler up in temperature and out of condensing.
Another problem common in the industry is homeowners choosing or being advised on high output combination boilers to suit their hot water desires, typically in a small flat or house that has a low heat loss. As above this can cause a lot of cycling at the boiler, increased gas usage and higher carbon dioxide emissions. This is why it's important to choose a boiler with good modulation to match/suit the changing loads. This is covered in much more depth on our other site Heat Geek.
Selecting a boiler with the right heat exchanger?
Within gas boilers you typically find heat exchangers constructed of either stainless steel or aluminium. Aluminium has a high thermal conductivity, which means it is excellent at transferring heat, however it can rust, corrode, warp or break down over time. Furthermore, if a boiler has an aluminium heat exchanger it can be hard to leave the system water quality after commissioning right if using different materials throughout as different materials will corrode at different PH levels.
Stainless steel however has a lower thermal conductivity value but is much less prone to corrosion and problems with other materials within the system as it can operate in all ranges of the PH scale without corroding, however, the thermal conductivity is simply made up for by being larger.
Choosing a boiler with a good quality stainless steel heat exchanger will increase the lifespan of the boiler and will help prevent the need for early replacement.
Another thing to consider as I mentioned earlier is the quality of an existing system's water and whether the system has been fouled over the years. Although it's a requirement for any new system to be flushed, or an older system as well during a boiler replacement it might be advisable to separate the new boiler from the old system using a suitably sized plate heat exchanger.
This keeps the boiler circuit and water separated from the old system where fouling has occurred and prevents the boiler heat exchanger from corroding or blocking, which would not only decrease the useful lifetime of the boiler but would also invalidate any manufacturer warranty.
Importance of the correct controls for a new system?
As with choosing the right boiler for your needs, selecting the right control strategy is just as important. Boiler plus is a set of standards that were introduced in April 2018 to improve the way people use energy in their homes and save energy. Typically the types of controls permitted for these standards are weather compensation, load compensation or smart controls featuring automation and optimisation functions.
Advanced weather compensation is considered proactive. This means you would have an outdoor sensor on a north-facing wall constantly reading the outside temperature. This then feeds a signal back to your boiler which interprets this reading and adjusts the boiler flow temperature from the boiler to suit the load conditions. Weather compensation can also be achieved without an outdoor sensor by using the internet. This tells the boiler the average temperature for your region and again the boiler regulates its flow temperature to suit. Weather compensation is ideal for homeowners with a lot of occupancy.
The weather compensation will keep the house at a nice steady constant temperature for those inside while using as little energy as possible. Weather compensation is best suited for homes with high occupancy, underfloor heating systems and low-temperature systems.
Load compensation or room compensation is more reactive than weather compensation. This means using room influence controls the system is reacting to any changes throughout the room and sends a signal back to the boiler to again adjust its flow temperature and output.
This type of control strategy is best used in homes with low occupancy, low thermal mass, high insulation temperatures, high-temperature systems, larger properties or unusual layouts and high airtightness buildings.
As you can see when designing the system there are lots of variables to consider and selecting the right control strategy for your particular needs is important to keep the house at the target temperature for you while using the least amount of energy.
The importance of correct system installation?
From the above you can see that a high efficiency gas boiler is now mandatory these days and selecting the right boiler and controls system for your home or building is vital for system functionality, lifespan and also reducing your energy consumption and resulting carbon emissions but finding the right installation firm is equally important.
Ideally, a customer should be getting at least three quotes from different firms and go through all their needs with them. The firm that undertakes the work should then do the proper heat loss calculations to size your boiler and then design the system to run as efficiently as possible. As with any system installation though the firm that does the work should be capable of carrying it out to industry standards.
Unfortunately though not every boiler installer out there really understands what he or she is doing and often the end result is below acceptable and future callouts are required, in fact, some systems never work effectively from the start.
This is a big problem in the industry as too many people are installing high efficiency gas boilers that are designed poorly and not executed properly, so finding the right company with the right experience and understanding is paramount for a good installation.