How low can you go?

The water content of boilers can have a significant impact on performance. Simon Mitchell of Hoval asserts that low water content isn’t always the best option.

In designing heating systems that will deliver optimum performance for their clients, building services engineers need to take account of a wide range of factors. They also need to avoid being overly influenced by what might be considered to be ‘fashionable’ factors and focus on the underlying engineering principles.

A case in point is the water content of boilers and the impact this can have on the rest of the system. In recent years low water content boilers have become popular with specifiers and, in the right circumstances, they can offer worthwhile benefits. However, there are other situations where a higher water content boiler will be a more suitable option, in terms of longevity and gaining the maximum efficiency of the system. Indeed, this is to be reflected in the latest version of CIBSE Guide B1: Heating.

The principal factors at work here can be illustrated by considering a typical system providing space heating and domestic hot water (DHW) to an office block. It is reasonable to assume that the DHW requirements are relatively low and that the space heating system is made up of underfloor heating in the atrium, radiators in hallways and fan coil units in office spaces. We can also assume that the DHW and fan coils require a constant flow temperature of 80°C, while the compensated radiator and underfloor system temperatures can fluctuate during the course of the day.

If the boilers have low water content they will require a minimum flow rate, often the same as the design flow rate. The boiler sequencing control will typically be configured to maintain a constant temperature after the flows from the three boilers have mixed.

Such an arrangement is fine on a design day when the system and boilers are operating based on the design conditions (often with outside temperature lower than 0°C). For most of the year, however, heating circuits will be controlled so that they are provided with flow rates proportional to the heating load. This situation is often addressed by the introduction of a low loss header that allows the boilers to operate with the pre-set minimum design flow rates but diverts hot flow back into the boilers’ return.

As a consequence of this arrangement the return water temperature is raised, so that there is less, if any, condensing. This means that the low water content of the boilers and the resultant requirement for a minimum flow rate has a direct, and negative, impact on the overall energy efficiency of the system.

Higher energy bills

Observations in the field suggest that this reduced condensing, resulting in lower volumes of acidic condensate, enables some, typically cheaper, boilers to provide a reasonable service life when manufactured from less appropriate materials. However, the higher energy bills during this likely shorter life are hardly what the end client is looking for.

In contrast, higher water content boilers can operate with a variable flow so there is no need to divert flow water via a low loss header. This arrangement enables the system to take advantage of lower return water temperatures to maximise condensing.

Some higher water content boilers will also increase the potential for condensing by having a split return that separates the two heating systems. Returning to the example above, there would be separate returns for the underfloor heating and the fan coil system. This is because in such a system there will almost certainly be occasions when the return temperature from the fan coil system is considerably higher than that from the underfloor heating system. With a single return the two will mix and the temperature of the return from the underfloor heating will be raised accordingly, reducing the amount of condensing. With a split return the return water from the underfloor heating circuit can remain separate to maximise the condensing potential. In our experience using a split return can increase efficiency by as much as 9%.

Reduce cycling

Another downside of low water content boilers is that they use smaller heat exchangers than high water content boilers, which restricts the waterways and increases the pressure drop. A primary circulating pump is therefore incorporated into the system to supplement the heating system circulators.

Adequately sized and vertically stratified heat exchangers can provide heat rapidly whilst reducing burner cycling. Higher water content designs also do not depend on high velocities and pressure drops to achieve effective heat transfer. As a result the variable flow rate system circulators are able to manage the flows without an additional pump, helping to reduce both capital costs and energy consumption. The wider waterways are also less susceptible to blockage.

For all of these reasons I believe it is very important to give due consideration to water content when specifying boilers. I am not suggesting that higher water content boilers will be the ideal choice for every project – but neither will low water content boilers. The important thing is that all of the options are considered during the design and specification process.

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