As of August 1, 2000, there is no longer any requirement to install a T&P Valve with the tankless water heater. The HUD letter recognizes and accepts the Underwriters Laboratories, Inc. (UL) Standard 499 endorsing the National Electrical Code (NEC) as an appropriate standard to the Tankless Water Heater Installation of valves for temperature and pressure relief as mandated by Standard 3280.609(c). Since the tankless water heater meets the requirements of UL 499 and NEC as a tankless water heater, there is no requirement for a T&P Valve when the tankless water heater is installed in a Manufactured Home.
NATIONAL ELECTRICAL CODE
422-47. Water Heater Controls
All storage or instantaneous-type water heaters other than
(a) Storage water heaters that are identified as being suitable for use with supply water temperature of 82ºC (180ºF) or above and a capacity of 60 kW or above, or
(b) Instantaneous-type water heaters that are identified as being suitable for such use, with a capacity of 1 gal (3.785 L) or less shall be equipped with a temperature-limiting means in addition to its control thermostat to disconnect all ungrounded conductors, and such means shall be (1) installed to sense maximum water temperature and (2) be either a trip-free, manually reset type or a type having a replacement element. Such water heaters shall be marked to require the Tankless Water Heater Installation of a temperature and pressure relief valve.
FPN: See Relief Valves and Automatic Gas Shutoff Devices for Hot Water Supply Systems, ANSI Z21.22-1986.
NOTE: No pressure relief valve required according to the NEC