Ariston ST 50 ProTech Instructions For Installation, Servicing & Operation page 16

WORKED EXAMPLE
The example below Is for a G1/2 temperature & pressure relief valve with a
discharge pipe (D2) having 4 no. elbows and length of 7m from the tundish
to the point of discharge.
From Table 2;
Maximum resistance allowed for a straight length of 22mm copper discharge
pipe (D2) from G1/2 temperature & pressure valve is 9m. Subtract the
resistance for 4 no. 22mm elbows at 0.8m each = 3.2m. Therefore the
maximum permitted length equates to: 5.8m. As 5.8m is less than the actual
length of 7m therefore calculate the next largest size.
Maximum resistance allowed for a straight length of 28mm pipe (D2) from
G1/2 temperature & pressure valve equates to: 18m. Subtract the resistance
for 4 no. 28mm elbow at 1 .Cm each = 4m. Therefore the maximum permitted
length equates to: 14m
As the actual length is 7m, a 28mm (D2) copper pipe will be satisfactory.
WARNINGS
The outlet from the temperature & pressure relief valve must not be used for
any other purpose. This also applies to the expansion relief valve. No other
valve is to be fitted between the cold water combination valve and the
cylinder.
The temperature & pressure relief valve must not be removed in any
circumstances. Any of the above will totally invalidate the guarantee.