Design Considerations - AquaCon™

The comprehensive range of AquaCon™ Modules is ideal for application in multiple dwelling units - such as apartment blocks - served by a central boiler plant. SAV Modules will be happy to assist in deciding on the optimum boiler requirements and in sizing the AquaCon™ Modules and other system components required for each apartment.

Here are a few general points to bear in mind.

The Boiler

A high efficiency condensing boiler with modulating burner control is recommended. For effective condensing, the return temperature must not exceed 57°C. This can be easily achieved with underfloor heating systems, which only require low flow temperatures. Where radiators are employed, units may need to be sized to give a higher than usual delta t. However, it's worth bearing in mind that benefits in fuel consumption are still possible where high temperatures are involved - in Denmark a minimum delta t of 33.0 degrees is quite the norm!

AquaCon™ Modules will work with heating water flow temperatures in the range 60-90°C. Under winter conditions, the boiler's supply temperature can be allowed to modulate within this range depending on the ambient outside air temperature. In summer, when only domestic hot water is required, a constant supply temperature of 60°C can be set.

The Buffer Tank

The buffer tank serves as an energy store, providing for short term, high load demands for hot water. Without this store, the boiler may be unable to react with sufficient speed to the increased heating load imposed by sudden hot water demands. The buffer tank also avoids the need for frequent boiler start-ups in response to hot water demand, thereby increasing the service life and efficiency of the boiler.

In principle, a buffer tank is not required if the system volume is large enough to create its own thermal store. As a rule of thumb, an installation serving more than 40 apartments is unlikely to require a buffer tank. The buffer tank should be sized to accommodate a 10 minute peak load demand for the entire building.

To minimise return water temperatures and therefore maximise boiler efficiency, the buffer tank must be arranged so as to encourage temperature stratification. Thus, primary and secondary flow pipes are located near the top of the tank, while primary and secondary return pipes are located near the bottom.

The firing of boilers should be controlled so as to maintain the required heating water flow temperature at a point two thirds of the way down the height of the buffer tank. This creates an adequate store of heating water for high demand periods, while allowing space for cooler return water at the base of the tank.

The Primary Pump

The primary pump should be a constant speed type to ensure that the boilers have sufficient water flow through them under all operating conditions, and are not at risk of over-heating. The flow rate in the primary circuit must not be less than the total flow in all secondary circuits fed from the buffer tank.

The Secondary Pump

The secondary pump can be variable speed to take advantage of pump energy savings when the heating system is operating at part load. Pump speed should be controlled such that there is always sufficient pressure available to satisfy the most remote differential pressure control valve (DPCV) in the system (located in the most remote Akva unit). If there is no significant pressure loss in the pipework between the pump and the most remote DPCV, then little may be gained by using a remote sensor. In this case controlling pump pressure constant may be a preferable option.