N°8 – One condensing boiler and one standard boiler systems (HND level)

We have seen in the previous § that if a system has 2 boilers, it is not of much use them both being condensing boilers. Let’s study some heating lay-outs with one condensing boiler and one standard boiler.

– Connection example of a single return condensing boiler coupled with a standard or low-temperature boiler

Connection of a single return condensing boiler and a standard boiler

Note:

• The condensing boiler above (and those after) is supposed to be able to operate at zero flow.
• The motorised 2 way valves installed above on the heating returns are of the “all or nothing” type and will be piloted by the boiler cascade control, subject for example to outside temperature.
• Note, above, the 2 balancing valves necessary for the sharing of the flow between the 2 boilers (with different hydraulic resistances).

Question  

On the boiler below mark the position of the balancing valves required to split the flow between the different boilers and the 2WV (all or nothing) necessary for a cascade control.

The balancing valves required to split the flow between the different boilers and the motor of the 2WV (all or nothing) necessary for a cascade control are drawn in red below.

Answer