Most boiler calls come down to four things: the system has lost pressure and will no longer circulate, air is trapped at the high points so radiators stay cold, a circulator or zone valve has stopped moving water to part of the house, or a component on the near-boiler piping — usually the expansion tank or the automatic air vent on the air separator — has rusted through and is leaking. All four are repairable. A licensed technician can tell which it is on site, normally in a single visit.
Common boiler problems and what the symptoms mean
Hydronic heat is a closed loop of water under pressure. That is the system this page covers: hot-water boilers feeding radiators or baseboard. Steam systems are a different and much older design, and they are not something we service — if your radiators hiss and have a single pipe and an air vent on each one, that is steam, and you will need a specialist in it. Nearly everything that goes wrong with it shows up as one of these symptoms, and each points somewhere different:
- No heat anywhere in the house
- Check system pressure and the call for heat before anyone touches the burner. A loop that has bled itself below working pressure will not circulate, and a dead thermostat or a failed 240-to-24 volt control transformer means the equipment is never asked to fire.
- Some radiators hot, others stone cold
- Air trapped at the high points, a zone that is not opening, or a circulator running without moving water. Bleeding is the first step and often the whole fix.
- The gauge keeps dropping
- Water is leaving a sealed loop. Usually a weeping fitting, a tired boiler drain valve, a relief line discharging, or an expansion tank that can no longer absorb the expansion of heated water.
- Water on the floor near a radiator, or a flooded basement
- A split line. We have traced basement flooding to a copper radiator supply line that had frozen and burst — the repair was about three feet of pipe, not an appliance.
- Rust staining on the near-boiler piping
- Something above it has been weeping long enough to leave a trail. Air vents, relief valves and the tank fitting are the usual sources, and all are replaceable parts.
Why a boiler loses pressure
This is the most common reason a homeowner calls, and the appliance itself is rarely the culprit. The loop is sealed — the same water circulates. If the gauge falls, that water is going somewhere:
- A failed expansion tank. Heated water has nowhere to expand except into that tank. When it waterlogs or rusts through, pressure spikes on every cycle until the relief valve opens. On one change order, while replacing a one-inch valve on a hydronic system, our technician found the expansion tank and the automatic air vent on the air separator both leaking and rusted out. Both were replaced.
- A leaking automatic air vent. Small, high on the piping, and quiet about failing. A vent that has rusted out loses water and draws air back into a system designed to purge it.
- Tired boiler drain valves. The lowest point in the system and often decades old. Inspecting a crawl space on one diagnostic, we found a boiler drain valve dripping steadily — the slow loss that never makes a puddle but drops the gauge week after week.
- A relief valve that has lifted. Once it passes debris it may never seat again. It never gets plugged; it gets replaced, and the reason it lifted gets found.
- A split or weeping pipe in a ceiling, a wall or an unheated space. See pipe repair and leak detection.
Topping the system up over and over is not a repair. Every gallon of make-up water brings fresh oxygen into a loop that is supposed to be sealed.
Why radiators stay cold when the boiler is running
Heat that never reaches the room is a distribution fault, not a combustion fault. We work through it in order:
- Confirm pressure and temperature at the appliance. If the loop is low, nothing downstream behaves.
- Bleed the high points. A radiator hot at the bottom and cold at the top is air-bound. That is a bleed, not a part.
- Check the zone is actually being called. On a no-heat call we found a thermostat that would not respond to a reset or to fan mode and did not change after new batteries; the equipment came on by itself after about eight minutes and then would not shut off. The fix was the thermostat and nothing else — see thermostat installation.
- Check the circulator and zone valve for that loop. A circulator can hum without moving water; a zone valve can fail closed.
- Balance the loop. Older houses have been re-piped, extended into additions or had rooms closed off, so near rooms cook and far rooms never warm.
- Bleed again after any repair and verify every radiator comes up. We bleed every hydronic system we open, as standard practice.
Frozen and burst radiator lines: what the boiler repair involves
In older housing stock this is a real emergency call. Hydronic supply lines were routinely run through exterior walls, unheated crawl spaces and ceilings above garages. When a zone stops circulating in a cold snap, the water in that run stops moving — and still water freezes first.
On one call the homeowner reported a radiator leaking and flooding the basement. What the technician found was a copper radiator supply line that had frozen and split. The repair was roughly three feet of radiator piping plus partial three-quarter-inch copper, air bled out of the hydronic system afterwards, and a permit pulled for the work.
The system was then re-pressurized and a second burst appeared: a three-quarter-inch copper line running through a ceiling and down a wall had frozen as well. That meant opening the wall to reach the failed section, replacing the copper, and insulating the line in that area so it would not freeze again. Insulating the repaired run is the difference between a repair and a repeat visit. Where the failed line is domestic water rather than heating pipe, see burst and frozen pipe repair.
Expansion tanks, air separators and air vents on a boiler
The near-boiler piping — the few feet of pipe and small components clustered around the appliance — does more to determine how long the system lasts than the appliance does.
| Component | What it does | How it fails |
|---|---|---|
| Expansion tank | Absorbs the extra volume created when loop water is heated | Waterlogs or rusts through; pressure then spikes every cycle and the relief valve dumps water |
| Air separator | Strips entrained air out of the circulating water | The body is usually sound; the vent on top of it is what rots out |
| Automatic air vent | Releases separated air out of the loop | Rusts and weeps, losing pressure and admitting air instead of expelling it |
| Relief valve | Protects the system against overpressure | Lifts because of another fault, then never re-seats |
| Boiler drain valves | Fill, drain and purge points | Old packing and old seats; they drip slowly for years |
| Circulator and zone valves | Move water to each zone on a call for heat | Seized bearings, a failed motor, or a valve that no longer opens |
Replacing a leaking tank or air vent means draining the system down, cutting the new component in, refilling and bleeding every high point in the house. Methodical rather than difficult — and far cheaper than what happens when it is left.
Boiler combustion safety, flue inspection and carbon monoxide
A boiler is a combustion appliance sharing a house with people, so our heating inspection is not just a temperature check. On a fall inspection program covering 26 apartment units, the per-unit checklist covered the heating assembly, the fan and motor, the air filters, the electrical components, the thermostat, the pressure switch, the heat exchanger and a carbon monoxide check — on the heating equipment and on the water heater sharing the venting.
- The heat exchanger, inspected for cracking or corrosion that would let combustion products into the living space.
- The flue and its connection, checked for draft, corrosion and clearance. We have found flue and chimney connections left unsealed by a previous installer and corrected them.
- The gas connection, inspected along its run to the appliance.
- Carbon monoxide readings, taken rather than assumed.
- Safety controls and the pressure switch, tested for correct operation instead of jumped out to make a unit run.
If any of that fails we say so before anything else. See heating and furnace repair for the same inspection on forced-air equipment.
Boiler repair, replacement, or converting to forced air
Most boilers are worth repairing. They are simple, robust and long-lived, and the parts that fail — tanks, vents, valves, circulators, controls — are all replaceable. When one is replaced, the choice today is between a conventional cast-iron boiler and a condensing boiler, which wrings more heat from the same gas by cooling the flue gases until they condense; a combi boiler is a condensing unit that also produces the domestic hot water, doing away with the separate water heater. Which suits the house depends on the radiators it will feed, and the estimator settles that on site. But many homeowners with radiators are not really asking about heat at all. A hydronic house has no ductwork, so there is no central cooling, no filtration and nowhere to add humidification or air purification.
Converting from radiators to forced air
On one conversion the house had a boiler with radiators and baseboards and no central air conditioning. The hydronic system came out entirely — appliance, water pipe, radiators and baseboards — and in its place went an 80 percent efficiency gas furnace of 70,000 BTU, a 2.5 ton evaporator coil, a 2.5 ton 15 SEER R410A condenser, and 40 feet each of new supply and return duct.
Adding a central system where there never was one
On another baseboard-heated house the owner wanted a complete heat pump system with new ductwork, registers and a thermostat. The whole distribution had to be built from nothing: an 18x20 trunk duct about 30 feet long, ten six-inch flex runs, ten duct boots, ten floor registers, a return box, 80 feet of thermostat wire, and a condensate pump with 40 feet of quarter-inch vinyl tubing to lift drainage out of the house. The 240 volt circuit for the outdoor unit was quoted as a separate electrical scope rather than buried in the HVAC number. See ductwork repair, sealing and venting and heating and furnace installation.
Do you need a permit for boiler repair or installation in Virginia?
A service repair — bleeding a system, replacing a circulator, an expansion tank, an air vent or a control — does not normally require a permit. Replacing the appliance does, and so does work that alters piping or venting.
We pull and process permits ourselves. Work is performed according to the Virginia building code and the county inspection result goes into the job file. We have pulled permits on hydronic pipe repairs that many contractors would have done quietly, because that is how the work should be recorded. On permitted jobs we hold back the final portion of payment until the inspection passes, and permit and processing fees are written into the contract before the job starts.
What boiler repair costs, and how the price is set
We do not quote this work over the phone and we do not publish prices. Two systems that look identical from the doorway can differ by the condition of the near-boiler piping, whether the loop has been contaminated by years of make-up water, whether a wall has to be opened to reach a failed run, whether venting was left unsealed, and whether a permit applies.
An estimator comes to the property, looks at the actual system, and gives you a written price for the work in front of him — before anything begins. Sam & Sons Services has worked this way across Northern Virginia since 1999, under Virginia DPOR license #2705070610.