Rheem 491 Owner's Manual And Installation Instructions page 36

If a drain line discharges into a tundish, the drain line from the tundish must be
not less than DN20 and one size larger than drain line from the relief valve(s).
The drain line from a tundish must meet the same requirements as for a drain
line from a relief valve.
Warning: As the function of the temperature pressure relief valve on this water
heater is to discharge high temperature water under certain conditions, it is
strongly recommended the pipe work downstream of the relief valve be capable
of carrying water exceeding 93°C. Failure to observe this precaution may result
in damage to pipe work and property.
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