Modulation is an essential capability of modern condensing boilers. There are of course many factors that combine to create the impressive efficiencies today’s boilers are capable of delivering, but modulation plays a crucial role in helping the boiler meet the demand placed on it at any given time as efficiently as possible.
In the most basic sense, modulation prevents the inefficiencies of on/off cycling, and by matching the output power to the thermal load of the system, a modulating burner can manage the return flow temperature, which is of course the number one influence on the efficiency of a boiler.
Commercial systems typically have to cope with a far greater spread of demand than the average domestic system. Domestic systems typically only require an ability to adjust outputs according to the external temperature and comparatively predictable demands from a handful of users. In this situation, a modulation rate of up to 5:1 is almost always adequate.
In a commercial scenario however, demand can vary dramatically and unpredictably, so a greater scope of operation can deliver significant benefits, both in terms of the efficient use of fuel, and the longevity of the system. It’s for this reason, commercial boilers are usually engineered to offer a modulation rate that affords more control over the supply of the correct amount of heat for the demand.
Another consideration in a commercial application is downtime. While no one enjoys being without heat and hot water for any period of time, in many commercial applications, downtime – whether for routine maintenance or due to mechanical breakdown – can be disruptive, expensive, and even damaging to reputation or health.
A cascade set up can therefore offer the best of both worlds. So, for example, rather than a single 500kW unit, five 100kW boilers arranged in a cascade will provide the same output and – in this scenario – an even greater modulation rate of 25:1.
This configuration allows each boiler to modulate up until the demand requires output from a further unit, and so on until the demand is met. Work is shared equally between the units in operation, meaning each boiler can operate at its optimum efficiency.
And in the unlikely event of one of the boilers failing, the system will remain in operation, and for routine maintenance, each boiler can be taken offline independently of the others, again allowing for continuation of supply.
In summary, a boiler’s modulation rate has a significant influence on its ability to operate at its most efficient point. In commercial applications, where demand can fluctuate dramatically, a higher turndown rate allows these efficiencies to be maintained. Correct specification, as ever, is therefore imperative.
This article first appeared in HVR magazine, March 2017.