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Where to start with replacing a boiler

Pete Mills, technical operations manager for Bosch Commercial & Industrial details the commercial boiler replacement market.

Pete Mills, technical operations manager for Bosch Commercial & Industrial

We are all aware these days of the need for our heating systems to use less energy and ultimately to use energy that is derived from a renewable source. How we get to this stage and what the implications of this transition are, will very much depend on the circumstances of the building we are heating, its age and thermal performance. Government has for some time laid out a broad policy objective that highlights the potential routes of the electrification of heat, connection to district heating or eventual decarbonisation of the gas network. As guidance and regulation develops further, the choices we face when having to replace our heating boilers may be affected by tighter regulation and increased aspirations to meet our social obligations.

Right now in 2020, one of the most challenging years we may face in our lifetimes, having to replace aging heating boilers that are a necessity for the operation of a building, whilst wanting to reduce carbon emissions can seem a daunting challenge. 

Where to start?

We should perhaps start this process with a review of the options that are available for the specific building we need to heat and its location. Many older buildings with poor thermal insulation and high temperature heating systems will require significant investment if the electrification route of using heat pumps is to be realised, whilst buildings that are more modern may be adapted easier. This can present a significant barrier if businesses simply do not have the money to invest right now. You might be fortunate to be located near to a developing district heating scheme, where connection to the scheme may be welcomed. This can certainly be an attractive option, but will typically require some upgrades to the system to ensure the low return temperatures required by heat networks can be achieved. In the years to come, it is highly likely that regulations will tighten, meaning hard choices about how we improve and decarbonise our heating needs will have to be made.

What if your choices are limited?

For many building operators, the only option at this time may be to remain with their gas supply, optimise the control operation and reduce energy use with improvements to building insulation. In the medium to long term, it may be that a decarbonised gas supply has to come to the rescue if we are to tackle these hard to treat buildings. The developments being made in exploring the use of hydrogen in our gas supplies look very encouraging. It is now highly likely that we will see blends of hydrogen being added to natural gas, as the gas grid starts to go green. In the longer term, 100% hydrogen looks to hold the most cost effective route to decarbonisation for many buildings, as demonstrated in the recent report from Delta EE.

Many of our buildings may still have inefficient non-condensing boilers, which could see genuine improvements through installing condensing boilers and reviewing the system operating temperatures. Modern condensing boilers, like the Bosch Condens 7000 F, are particularly well adapted to the changes we may begin to see to our current gas supply. They will certainly handle any blends of hydrogen up to the proposed 20% as well as the more complicated proposal for widening of the wobbe band, which helps future proofing. Full 100% hydrogen will require a new generation of appliances of course, with the most likely route being that of a hydrogen ready gas appliance. This will largely depend on the outcomes of Government funded programmes such as Hy4Heat and HyDeploy.

There is certainly some genuine interest for large industrial boilers, which have very long service lifetimes, to see how these could be made ready for a hydrogen conversion. Having a 10% oversized boiler shell at the outset makes this a possibility and is well worth considering if you have appliances running with package burners into the MW size.

Condensing boilers need lower return temperatures to get the greatest energy savings and sadly, this is still often over looked, particularly when a new boiler is installed in a distress situation. Rebalancing systems from 82/71 to 80/60 is generally achievable for all but the oldest of heating systems. Coupling this with upgraded control systems that can offer weather compensated control means that system temperatures can be lowered for a significant number of days each year, when the demand will be less due to mild winter temperatures. Taking these extra steps over and above a simple boiler replacement, can at least begin to improve energy efficiency and will be a positive step. If for example later there is an opportunity to connect to district heating, these measures will have started the process to reduce return temperatures to that needed for these connections. 

Selecting boilers that will offer a good modulation range will help to ensure that loads are matched better by the boiler capacity and boiler cycling is reduced. This is not limited to the individual boiler turn down, but should be considered across multiple boilers within a cascade. This arrangement can give very wide turn down ratios and has a much better load matching capacity. Good control is important here too and with modern off the shelf internet connected control packages, such as the Bosch 8313 controller, the cost of sophisticated boiler sequencing and rotation has been much reduced. Individual boilers are only bought into operation when demand for heat increases. Their output is modulated to match exactly what is needed to meet the flow temperature demanded. As a boiler gets to the maximum 100% of its output, another is bought into operation at its minimum, typically 20%. At the same time the first boiler drops back to 80%, ensuring there is no sudden jump in output. Lead and lag boilers are rotated based on the number of run hours they have performed, to ensure even wear across a cascade.

Making these suggested incremental improvements to energy efficiency is certainly not the ultimate goal for decarbonising heat, but in these strange times when businesses may be forced to make hard choices, steps like this should be seen as a minimum step in the right direction.

18 November 2020


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