Nightly T&P Discharge After a Backflow Preventer Retrofit
A contractor commissions an indirect glycol system on a two-storey house. The utility had earlier installed a dual-check backflow preventer and a 60 psi pressure-reducing valve at the service entrance. The solar loop is closed and pressurized to 40 psi with a 12 L bladder expansion tank. The potable side is a 300 L stainless-steel indirect tank with an ASME temperature-and-pressure relief valve rated 99 °C / 150 psi. Six weeks after commissioning, the homeowner reports water dripping from the T&P discharge at night, and a gauge on the tank reads 130 psi after sunny afternoons. A handyman has capped the collector-loop pressure relief valve because it 'kept weeping.' The T&P discharge pipe runs 4 m horizontally with a slight rise and terminates 300 mm above the garage floor, indoors. The homeowner wants the dripping stopped before a home inspection.
1. Explain the mechanism behind the afternoon pressure rise and the nightly T&P discharge, and describe how you would confirm it.
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The backflow preventer plus the pressure-reducing valve make the potable side a closed system: water can enter from the main but cannot push back out. Heating the tank from roughly 10 °C to 60 °C expands the water. Density at 10 °C is about 999.7 kg/m³ and at 60 °C about 983.2 kg/m³, so 300 L becomes 300 × 999.7 ÷ 983.2 ≈ 305.1 L - about 5 L of expansion with nowhere to go. In a closed system that volume compresses the trapped water and drives pressure toward the relief setting; at 150 psi the T&P lifts and dumps enough water to drop the pressure. Overnight, contraction lowers the pressure and the PRV admits a little water, resetting the cycle. If the valve does not reseat cleanly, it weeps after it has lifted.
Confirm by logging pressure with the element isolated. If pressure climbs above the PRV setting on a sunny afternoon with no electric heating, it is thermal expansion, not a failed element or a failing relief valve. Also compare the gauge reading before and after a draw, since a valve that is passing will show a slow drip at the discharge.
2. Which code and standard requirements are being violated, and what corrections would you write into the work order?
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The governing references are the solar thermal system standard (ICC 900/SRCC 300) together with the plumbing code adopted locally (IRC P2803 or UPC, depending on jurisdiction). Those require an ASME-rated T&P relief on the tank, no valve or cap between the tank and the relief, discharge piped full size, by gravity, without traps, to a safe point of disposal, terminating 150–300 mm above the floor or to the exterior. A closed system must also have a means for thermal expansion.
Violations here: no potable-side expansion tank; the discharge runs 4 m horizontally with a rise, which is a trap plus a lift and is not gravity drainage; and the valve has been cycling repeatedly, which likely left debris on the seat.
Work order: install an ASME-rated potable expansion tank sized to the 300 L tank and pre-charged to the PRV setting, typically 60 psi; verify the PRV is functioning and set correctly; replace the T&P with one of the correct rating; re-pipe the discharge in full size with continuous fall to within 150 mm of the floor or through the wall to the exterior; remove any cap, plug or reducing fitting; add a vacuum relief if the local code requires it; and document the repair with a pressure log. The T&P must never be plugged to stop the drip.
3. How do you handle the capped collector-loop relief valve?
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Treat it as an immediate hazard and do not simply uncap it and walk away. A relief valve is the last line of defense against a stagnating collector loop reaching 150–200 °C and over-pressurizing. With the cap in place, the loop is a pressure vessel with no protection.
The fact that it wept tells you the loop is generating pressure it cannot absorb. Likely causes: the 12 L bladder tank has failed or is mis-charged (a 40 psi pre-charge against a 40 psi fill leaves essentially no acceptance volume), the loop is overfilled, air is trapped in the loop, or the fill valve and check valve arrangement does not allow the expanded fluid anywhere to go.
Sequence: remove the cap and isolate the loop if it cannot be repaired that day; drain the loop and verify the expansion tank pre-charge with the loop at zero pressure; size the tank to the loop volume and expected temperature swing; set cold fill 5–10 psi below the relief setting; add or restore an air separator and automatic air vent; replace the relief valve, since a capped valve may have a damaged seat; and confirm the relief discharges to a safe drain. Leave the system documented and tagged if any part is left out of service.