Some older homes in Daly City and South San Francisco heat with hot water rather than forced air. A boiler, often in the garage, heats water that a circulator pump pushes through pipes to baseboard convectors or cast-iron radiators, and back again. The heat is quiet and even, which is why owners keep these systems for decades, and it also means the boiler, pumps and valves in such a house have often been working a long time. We service and repair hydronic boilers, baseboard and radiator systems in Daly City, South San Francisco, Colma and Brisbane.
Start with the gauge
Most boilers have a combined pressure and temperature gauge. On a typical residential system the cold pressure sits around 12 to 15 psi and rises somewhat as the water heats, staying well below the relief valve’s 30 psi. The reading tells us where to look first:
| What you see | What it usually means | What it usually takes |
|---|---|---|
| Pressure low, upstairs baseboards cold, gurgling | Water loss and air at the high points | Find the loss, refill, bleed the system |
| Pressure near 30 psi when hot, relief valve dripping | Waterlogged expansion tank, or a fill valve letting water through | Tank recharge or replacement, fill valve repair |
| Pressure normal, boiler hot, no heat anywhere | Circulator stuck after sitting idle, failed pump motor or relay | Free or replace the pump, control repair |
| One zone cold, the others fine | Zone valve motor, its thermostat or wiring, or that zone’s pump | Valve head or actuator, wiring repair |
| Boiler won’t fire, or fires and shuts off | Ignition, flame sensing, limit control, venting | Combustion-side diagnosis |
A dripping relief valve is doing its job. Never cap or plug it; fix whatever is pushing the pressure up. If a single zone is the problem, zone control and thermostat installation cover the controls side.
What an annual service includes
- Combustion analysis: carbon monoxide, oxygen and draft measured with an analyzer; burners cleaned and adjusted.
- Heat exchanger inspection for cracks, scale and soot.
- Expansion tank: its air charge checked against the system’s fill pressure, which means isolating the tank and draining its water side first.
- Relief valve tested, and replaced if it is corroded or weeping.
- Circulator pumps: amp draw and noise checked, and older pumps with oil cups lubricated.
- Zone valves: each one opened and closed by its own thermostat.
- Bleeding: air purged from the loop; automatic vents and the air separator checked.
- Piping and fittings: looked over for drips and the green or white crust that marks a slow leak.
Older hydronic systems
A few things show up again and again in systems that have run for decades. Some still have an old steel compression tank instead of a diaphragm tank; it slowly fills with water and needs draining, or replacing with a modern tank. Valves that haven’t been turned in years can start leaking once they are moved, so we work them with care. Old piping carries sludge and rust that settle in the boiler and wear out pumps, and a dirt separator or strainer helps when parts are replaced. In a cool, damp garage, the boiler’s burners, cabinet and vent connector also rust sooner than you’d expect.
When an old boiler nears the end, the radiators and baseboard can usually stay. A modern condensing boiler with outdoor reset lowers the water temperature on mild days, and with winters this mild that is most days, so it spends more of the season in its efficient range. Generously sized cast-iron radiators suit lower water temperatures especially well. If you would rather move away from gas, heat pump installation explains that path.
Radiant floors on the same boiler
A boiler can also feed radiant floor loops, usually through a mixing valve, since floors run at lower water temperatures than radiators. The boiler side is covered here; tubing, manifolds and floor controls are on the radiant floor heating page.
Safety
- If you smell gas near the boiler, leave the house first, then call the gas utility or 911 from outside.
- If a CO alarm sounds, get everyone out and call 911 from outside, and don’t restart the boiler until it has been checked.
- Heating water is hot enough to scald, and the system is under pressure, so leave valve and piping work to a technician.
What affects the price
Boiler service and repair typically runs $200 to $2,000 per service. Annual service, bleeding, a relief valve or a zone valve head sits toward the lower end. Circulator pumps, expansion tanks and controls fall in the middle; gas valves, major burner work and larger piping repairs run toward the top. Most visits take one to four hours. You get a written estimate, reviewed with you before work starts, with no hidden fees, and the work carries a warranty on parts and labor.