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N°6 – Double return condensing boiler training (A to HND level)

The advantage of double return boilers (called 3 connection boilers) is the 2 separate inlets in the boiler with “hot” and “cold” returns, with the latter ensuring maximum condensation.

Imagine a system with 2 circuits of equal power, with return temperatures respectively 65 [°C] (149°F) & 45 [°C] (113°F). If we mix these returns at the boiler inlet, the average temperature obtained will be 55 [°C] (131°F).
There will therefore be no condensation in the boiler whereas a return of 45 [°C] (113°F) would permit it.

Connection of a double return boiler

Note: the condensing boiler above is supposed to be able to operate at zero through flow.
Several design methods of design of 3 connection boilers exist :


Indicate on the diagram below the numbers shown in the description:
– The system has a radiator circuit n°1 and under-floor heating n°2.
– The condensing boiler has 2 returns n°7 (high temperature) & n°8 (low temperature).
– To favour maximum condensation, the under-floor heating return is connected on the low temperature return of the boiler.
– The regulator n°11 controls the temperature at the radiator circuit outlet on sensor n°5 as well as the temperature at the under-floor heating outlet on sensor n°9.
These controls operate subject to the exterior temperature (sensor n°10) by acting respectively on the 3WV’s n°6 and n°12.
– By action on the modulating burner n°4 the regulator n°7 controls the boiler production temperature on the sensor n°3, subject to the greatest demand generated by sensors n° 5 and n°9.

If the manufacture insists on a minimal feed flow on the high temperature return but accepts a zero flow on the low temperature circuit, we can install a decoupling cylinder.