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Out with the old

(written for Energy in Buildings & Industry)

Refurbishing or upgrading existing heating plant has the potential to deliver very significant increases in efficiency but there are a number of factors that need to be taken into account. Kevin Stones, Technical Director with Hoval, takes a look at the key considerations.

With growing pressure on organisations to reduce their energy consumption, emissions and overheads there are many strong arguments for refurbishing existing boiler plant or upgrading it to new, more efficient plant – using, perhaps, an alternative fuel such as biomass. Indeed, if a boiler is being replaced the Building Regulations require an increase in efficiency. All of which has implications for the entire system, not just the boiler, so building operators and energy managers need to be aware of all the factors.

For example, where a non-condensing boiler is replaced with a condensing boiler the chances are the existing heat emitters will be sized to provide flow and return temperatures of around 82ºC and 71ºC respectively; a return temperature that is too high for condensing. However, this doesn’t mean that a condensing boiler shouldn’t be used, because condensing boilers will still give greater seasonal efficiencies than atmospheric boilers, even when they’re not condensing.

In addition, there are ways of squeezing as much condensing out of the system as possible, such as making the most of the cooler return temperatures when the system starts up in the morning. Also, if the system combines different kind of emitters – say underfloor heating and radiators – it may be possible to split the return flows into high and low temperature branches on the associated boiler.

Another benefit of using condensing boilers is the better turndown they offer, so that the boiler and burner will run with fewer starts and stops and NOx emissions will be reduced accordingly.

When a pressure jet oil boiler is being replaced it may be possible to add a condensing economiser to introduce condensing, and solutions such as this will become increasingly viable as low sulphur fuel oils become available. Of course, if a pressure jet boiler is being replaced with a gas condensing boiler there will be additional benefits in terms of reduced emissions, noise and electrical consumption.

This isn’t to say that condensing boilers are the ideal solution for every project. There are many higher capacity, low temperature hot water applications that will benefit significantly from other technologies. For instance, a relatively new type of heat exchanger, constructed from an inner aluminium finned surface, heat shrunk within an outer stainless steel tube will extract the maximum heat from the exhaust gases leaving the combustion chamber, with the added advantage of reducing NOx emissions. Such boilers help to fill the gap between older low temperature boilers and condensing boilers and can also be very compact, freeing up space in plant rooms for refurbishment projects.

Increasingly, building operators are opting to include biomass, either as a sole heat source or integrated with gas boilers and possible solar thermal and/or heat pumps. Here, it’s important to ensure that the site can accommodate the fuel storage and delivery requirements associated with biomass fuels. Where this is the case, biomass will make a significant contribution to the environmental performance of the building(s).

In all cases it’s just as important to consider the control of the boiler as the choice of boiler itself. In a retrofit situation, if the existing controls are quite old, they may not interface easily with the new plant – or additional sensors may be required to provide the additional information required by modern controllers. If different boiler types and other heat sources are being mixed the controls will be particularly important to get the sequencing correct.

Under pressure

New boilers may require higher gas and minimum water pressures than older boilers so this is something else to check, in case pressurisation kit for either service is required.

Pressure can also be a challenge in taller buildings where access to the plant room is difficult. For example, the building may need 10-12 bar of working pressure but it may not be feasible to get replacement equipment to match, especially where access is restricted. In such cases, it may be more practical to assemble a 6 bar cast-iron sectional boiler in situ and use a plate heat exchanger to separate the two systems. In fact, access and footprint will often be the deciding factor in retrofit projects.

Flues also need to be checked for suitability, especially when condensing boilers are introduced to the system. The same is true for valves and if these need replacing in situations where hot water services need to be maintained the project requires very careful planning.

This can also be an ideal time to install extra valves and increase zoning to make maintenance easier and less disruptive. It’s also a good time to consider replacing sacrificial anodes in calorifiers with permanent anodes if that’s appropriate.

Installing new plant will almost certainly impact on the maintenance regime and the system should certainly be cleaned out before new boilers are installed. Ongoing maintenance should also be planned to address the performance and legislative needs of the new system, rather than the old, and we would strongly recommend using maintenance operatives that have an in-depth understanding of the plant, the controls and the complexity of integrating different heat sources.

The important point about all of this is that changing or even refurbishing a boiler will inevitably affect the way it interacts with other parts of the system, so it is vital to think it through with attention to every detail. And it makes a lot of sense to call on specialist assistance to ensure you get the best solution.


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