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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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