BoilersOil
Dunkirk Boilers EXB5100 Boiler Manual
Setup, everyday use, maintenance and troubleshooting for the 87% AFUE non-condensing oil-fired boiler, with verified specifications.
- AFUE87%
- FuelFuel oil
- DesignNon-condensing
About the Dunkirk Boilers EXB5100
This guide covers the Dunkirk Boilers EXB5100, an 87% AFUE non-condensing oil-fired boiler from the Dunkirk Excelsior line listed as ENERGY STAR certified.
With an AFUE of 87%, about 87% of the oil it burns over a heating season becomes heat delivered to your radiators, baseboard or radiant floors. It is a non-condensing design, so it vents hot flue gases through a lined chimney or a listed power-vent system and must be protected from sustained low return-water temperatures that would cause corrosive condensation inside the boiler.
Specifications
| Brand | Dunkirk Boilers |
|---|---|
| Model number | EXB5100 |
| Model name / series | Dunkirk Excelsior |
| Fuel category | Oil |
| Product series | Dunkirk Excelsior |
| AFUE (heating efficiency) | 87.0% |
| Fuel type | Fuel oil |
| Condensing design | No |
Safety information
- Install UL-listed carbon monoxide (CO) alarms on every level of the home and outside each sleeping area. Test them monthly and replace them by the date printed on the alarm.
- If you smell oil fumes or see smoke or soot, turn the boiler off at its emergency switch and call a qualified oil-heat technician.
- Never press the burner reset button more than once. Repeated resets can flood the combustion chamber with oil, which can ignite violently (a "puffback").
- The boiler must be installed by a licensed heating or hydronics contractor following the manufacturer's instructions and local codes; most jurisdictions require a permit and inspection.
- Every boiler has a pressure relief valve (residential hot-water systems commonly use a 30 psi valve). Never cap, plug or remove it, and keep its discharge pipe running to within a few inches of the floor or to a safe drain as code requires.
- If the pressure gauge reads zero or the boiler is overheating, turn it off and call a technician. Never add cold water to a hot boiler that has lost its water.
- Keep gasoline, solvents, paint and other flammables away from the boiler, and do not block combustion-air openings.
- Turn off the boiler's service switch or breaker before opening any panel. Pipes and the boiler jacket can be hot enough to burn.
Installation and first-time setup
A boiler is the heart of a hydronic system, and the piping around it determines how well the Dunkirk Boilers EXB5100 performs. Review these points with your installer.
- Size it to the heat loss
Ask for a room-by-room heat-loss calculation (such as ACCA Manual J) instead of copying the old boiler's rating — many older boilers are badly oversized. Also confirm that existing radiators or baseboard can heat each room at the water temperatures the new boiler will run.
- Near-boiler piping
Follow the manufacturer's piping diagrams exactly. Many modern boilers require primary/secondary piping or a hydraulic separator so the heat exchanger always gets the flow it needs, regardless of how many zones are calling.
- Clean the system and treat the water
Flush old systems thoroughly before connecting a new boiler; sludge and magnetite from old piping can foul a modern heat exchanger quickly. Fill with water that meets the manufacturer's limits for pH, hardness and chlorides, and use only approved inhibitors or antifreeze. A dirt or magnetic separator is cheap insurance.
- Expansion, air and fill
Size the expansion tank for the system volume, install an air separator, and connect the fill valve through a backflow preventer to protect your drinking water.
- Venting and draft
Oil boilers vent into a properly sized, lined chimney with a barometric draft regulator, or through a power venter listed for the boiler. The installer should measure draft and verify the chimney is clean and sound.
- Oil supply
The tank, supply line and filter must meet NFPA 31 and local code, with the line protected from damage.
- Boiler protection
Non-condensing boilers need their return water kept warm enough to avoid flue-gas condensation inside the heat exchanger. Low-temperature systems such as radiant floors require the mixing, bypass or boiler-protection piping shown in the manual.
- Commissioning
Ask for a written start-up report with nozzle size, pump pressure, draft, smoke and combustion-efficiency readings, system pressure, and the outdoor-reset and limit settings.
Using your boiler
Pressure and temperature gauge
Most residential hot-water systems are filled to about 12–15 psi cold and stay below about 25 psi when hot; your installer will note the correct cold-fill pressure for your building. A pressure that keeps climbing toward the 30 psi relief setting, or keeps dropping, needs attention (see Troubleshooting).
Water temperature and outdoor reset
If the boiler has outdoor reset, it lowers the water temperature in mild weather to save fuel, within the minimum the boiler requires to avoid internal condensation. Without reset, the high-limit setting controls supply temperature; do not set it lower than the manual allows.
Thermostats and zones
Hydronic systems respond more slowly than forced air, especially radiant floors. Use modest setbacks (a few degrees) on radiant zones and give each zone time to respond. Keep furniture and heavy drapes away from radiators and baseboard so heat can circulate.
Care and maintenance
| When | What to do |
|---|---|
| Every month in season | Check the pressure/temperature gauge, look for drips at the relief valve and fittings, and make sure vents and air intakes are unobstructed. |
| Start of heating season | Bleed air from radiators or baseboard with high-point vents if rooms heat unevenly or you hear gurgling. |
| Every month | Test CO and smoke alarms. |
| Every year | Professional oil-burner service: replace the nozzle and oil filter, clean the pump strainer and heat exchanger passages, check the chimney and barometric damper, and test draft, smoke and combustion efficiency. |
| Every year | Have the technician check expansion-tank pre-charge, relief-valve condition, circulator operation, low-water cutoff (if installed) and system water chemistry or inhibitor level. |
| Every 5–10 years | Replace CO alarms by the date printed on them. |
Troubleshooting
Use the table below to narrow down common problems. Anything involving gas, oil, combustion, venting or electrical components should be handled by a qualified technician.
| Problem | Possible cause | What to try |
|---|---|---|
| No heat anywhere | Thermostat setting; boiler switch or breaker off; empty oil tank or burner lockout; low system pressure. | Check thermostat, switches and tank gauge. Press the burner reset once only; if it will not stay lit, call for service. |
| Some rooms or zones cold | Air in radiators or baseboard; zone valve or zone circulator not opening; thermostat issue. | Bleed air at high points; confirm the zone thermostat calls for heat. A zone that never warms needs its valve or circulator checked. |
| Pressure rises toward 30 psi; relief valve drips | Waterlogged or failed expansion tank; fill valve letting water through; leaking domestic-water coil. | Call a technician; do not cap the relief valve. |
| Pressure keeps dropping | A leak in piping, a radiator valve or an automatic air vent; relief valve weeping. | Look for wet spots and drips. Frequent topping-up brings in fresh oxygen and minerals that damage the boiler — have leaks repaired. |
| Banging, kettling or gurgling | Air in the system; scale or sludge in the heat exchanger; low water flow. | Bleed air. Persistent kettling usually means the heat exchanger needs cleaning or flow needs correcting. |
| Boiler cycles on and off rapidly | Oversized boiler for small zones; supply temperature set too high; minimum firing rate above the load. | Ask your contractor to review the reset curve, anti-short-cycle settings and zoning; a buffer tank can help in some systems. |
| Error code on the display | The control has detected a fault or lockout condition. | Write down the code and look it up in the code table in your manufacturer's manual before resetting. |
Energy use and running costs
These examples assume a home that needs 50 million Btu of delivered heat per year — roughly a mid-size house in a cold northern climate. At 87% AFUE the Dunkirk Boilers EXB5100 would burn about 415 gallons of heating oil a year (about 138,500 Btu per gallon). A boiler at the 86% AFUE federal minimum for oil-fired hot-water boilers would need about 420 gallons — 5 gallons more.
Multiply the gallons by your local delivered heating-oil price to estimate cost. Circulator pumps and the burner motor add some electricity use, not included here. Actual consumption depends on your home's heat loss, climate and water temperatures.
Frequently asked questions
What is the AFUE rating of the Dunkirk Boilers EXB5100?
The Dunkirk Boilers EXB5100 is rated at 87% AFUE and is a non-condensing boiler.
Can the Dunkirk Boilers EXB5100 heat domestic hot water?
The ENERGY STAR listing does not flag built-in domestic hot-water capability. Most heating boilers can still supply an indirect-fired storage tank as a separate priority zone — check the Dunkirk Boilers installation manual for the approved piping and controls.
What fuel does the Dunkirk Boilers EXB5100 use?
It is an oil-fired boiler. Plan on an annual burner tune-up and a sound, lined chimney or listed power venter.
How much oil will the Dunkirk Boilers EXB5100 use?
To deliver 50 million Btu of heat in a year it would burn roughly 415 gallons of heating oil. Your use depends on your home's heat loss and climate.
When was the Dunkirk Boilers EXB5100 ENERGY STAR certified?
It appears on the ENERGY STAR certified boiler list with a certification date of November 2021.
Sources and disclaimer
Specifications on this page come from the ENERGY STAR certified boilers dataset published by the U.S. Environmental Protection Agency (record certified November 7, 2021), retrieved October 7, 2026.
The setup, operating, maintenance and troubleshooting guidance is written by ManualHarbor's editors for this specific product type and configuration. It is an independent guide — not a reproduction of the manufacturer's manual — and ManualHarbor is not affiliated with Dunkirk Boilers. Always follow the safety instructions, installation requirements and warranty terms supplied with your unit, and contact Dunkirk Boilers or an authorized service provider for warranty repairs and replacement parts.
Running-cost figures use assumed U.S. average utility prices and are estimates for comparison only. See our editorial methodology for details.
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