Motel Hot Water - Weeping T&P Valve and Long Waits at the Tap

Diagnosis: the dual-check backflow preventer turned the domestic system downstream of it into a closed system. As the heater recovers, water expands and there is nowhere for the pressure to go, so system pressure rises until the T&P valve lifts. It then drips and never fully reseats. The 620 kPa supply with no pressure-reducing valve adds to the standing pressure and shortens the valve's life. Scale or a failed seat may now also be involved.

Corrections:

1. Thermal expansion control - install a listed expansion tank sized to the system volume, pre-charged to the supply pressure, on the cold side between the backflow preventer and the heater. Verify with a gauge before and after.
2. Pressure reduction - fit a pressure-reducing valve if 620 kPa exceeds the code limit for the distribution system and fixtures, and check that fixtures and the heater are rated for the resulting pressure.
3. Discharge piping - the discharge must be the full size of the valve outlet (3/4 in., not reduced to 1/2 in.), must run by gravity without traps or valves, must not be threaded or capped at the end, and must terminate with a visible air gap no more than the code maximum above a floor drain or outside. The current 1/2 in. reduction can prevent the valve from relieving at full capacity.
4. Test and document - lift the lever, confirm it reseats, and replace the valve if it does not. Record everything; a weeping T&P is a safety item, not a nuisance.

Estimate the peak-hour load first. Assume two occupants per room, so roughly 24 showers in the peak hour, each about 8 minutes at 8 L/min: about 64 L per shower, or roughly 1,500 L of hot water in the hour.

Estimate what the heater can deliver. Recovery with a 50 °C rise (10 °C to 60 °C, i.e. about 90 °F) is approximately 75,000 Btu/h ÷ (8.33 lb/gal × 90 °F) ≈ 100 US gal/h ≈ 380 L/h. A first-hour rating is roughly 70 % of storage plus recovery: about 0.7 × 284 L + 380 L ≈ 580 L. That is well short of the estimated 1,500 L peak, so the heater is undersized - a bigger tank alone just moves the shortfall slightly.

Conclusion: the lukewarm showers at peak are a capacity and recovery problem, not a thermostat setting. Options, in order of value: add storage capacity (a second heater or a large buffer tank) with adequate recovery, or replace with a correctly sized unit based on the manufacturer's first-hour rating for the actual fixture count; then balance the load by staggering dishwasher/laundry use if those are on the same supply. Turning up the thermostat does not add Btu/h and creates a scald risk to address separately.

The wait is a distribution problem. At the far end the dead leg is long enough to hold several litres of cooled water in the pipe plus the fixture branch. At 8 L/min a 30 m run of 1/2 in. pipe holds about 5 to 6 L, which produces roughly 40 to 70 seconds of wait - matching the complaint. Runs of that length are also a code concern where the fixture is too far from the source.

Fix:

• Install a recirculating return line from the far end back to the heater, with a thermostatically controlled balancing valve at the end and a check valve on the return, so the system circulates only when the line cools. Balance the return so near and far end taps reach temperature together.
• Where a full return is impractical, a point-of-use booster or a second smaller heater at the south end are alternatives; compare installed cost and energy use.
• Insulate the hot and return lines and insulate the first 2 m of the cold line at the heater.

Temperature and scald: store at 60 °C or above to control Legionella (below about 55 °C growth is favoured in the tank), then use a master thermostatic mixing valve at the outlet to deliver a safe temperature - typically a maximum of about 49 °C for a lodging bathroom, and lower where children have access. Label the storage temperature, install a tempering valve that fails to cold, and record the settings.