What summer does not do
Worth clearing up, because it gets stated the other way round.
A water heater delivers a temperature rise. In August incoming water might arrive around 80°F, so reaching 120°F requires adding roughly 40 degrees. In January the same water arrives in the 50s and needs 65-70 degrees added.
So summer is the heater's easiest season. If yours is struggling in August, that is meaningful — it means capacity has been lost, and the same unit will be badly short in January when the work nearly doubles.
That makes a summer complaint worth acting on rather than tolerating. It is a preview.
Thermal expansion, which does get worse
Water expands as it heats, and it has to go somewhere. Most homes here have a pressure regulator or check valve on the incoming main, which makes a closed system — the expansion cannot push back into the street.
Pressure therefore rises every time the heater fires. Add higher municipal pressure, which often rises overnight, and a summer of heavy irrigation demand cycling the system, and you get the conditions where a relief valve starts discharging.
A T&P valve that weeps periodically is reporting something true. Either it has failed, or system pressure is genuinely reaching its threshold. The fix is a properly sized expansion tank, not a new relief valve, and certainly not capping the discharge pipe.
The tell people miss is a white mineral crust on the discharge pipe with no water ever observed — that is the residue of intermittent discharge you have never been present for.
It is worth checking pressure with a gauge while you are at it. Above 80 psi is quietly damaging every valve and appliance connection in the house — see high water pressure.
Worth knowing
An expansion tank is not permanent equipment. Its internal bladder holds an air charge that can be lost, at which point it is just a lump of metal doing nothing. Tapping it is a rough test — it should sound hollow at the top and solid at the bottom. Waterlogged throughout means it has failed.
The garage problem
Most water heaters here live in the garage, and in August an uninsulated Boerne garage runs comfortably over 100°F for much of the day.
For a conventional tank, that is largely benign — standby losses actually fall when the surrounding air is hot. What suffers is everything else: the flexible connectors, the T&P valve, and any plastic components all age faster in sustained heat.
For a hybrid heat pump unit, hot garage air is genuinely good news — more available heat to move, and the cold air it exhausts is welcome.
For a gas unit, the thing worth checking is the combustion air intake screen at the base. Garages accumulate dust, lint and pet hair, and a starved burner produces a lazy yellow flame and incomplete combustion. Vacuuming that screen is a five-minute job with a real safety component.
Also worth knowing: heaters in garages are generally required to be elevated or to be a flammable-vapour-ignition-resistant design, because petrol vapour is heavier than air and pools at floor level. If yours sits directly on the slab and predates that, it is worth raising.
Scale forms faster in summer
Calcium comes out of solution more readily at higher temperatures, and summer brings both warmer incoming water and heavier household demand — more showers, more laundry, more cycles.
So on 15-30 grain water, the sediment bed on a tank floor builds faster through summer than through winter.
The consequence shows up in the following January, when the heater needs its full capacity and a summer's worth of sediment is insulating the burner. That is the sequence behind most “it was fine until it got cold” calls.
Which is the argument for flushing in late summer or early autumn rather than in the depths of winter.
What to actually do in August
Flush the tank, or descale a tankless unit. Losing hot water for an afternoon in August is trivial — in January it is not. This is genuinely the best time of year for it.
Test the T&P valve. Lift the lever briefly, let it snap back, confirm it discharges and reseats cleanly. Check the discharge pipe for mineral crusting while you are there.
Take a pressure reading at a hose bib, day and overnight.
Vacuum the combustion air intake screen at the base of a gas unit.
Look underneath with a torch for rust streaks or a dried white trail — the earliest physical warning a tank gives.
Check the age on the label. If it is past twelve, plan rather than react. Choosing a replacement in August with time to compare beats choosing one on the coldest night of the year with a cold house and no leverage.