Combi Boiler Not Producing Hot Water? Causes and Fixes

Quick answer: A combi boiler that heats the house but gives no hot water usually has a stuck diverter valve, failed flow sensor, clogged inlet filter or scale in the heat exchanger. If you have no heat and no hot water, look at low pressure, gas supply, ignition, power or a blocked condensate line. Check the display for an error code and the gauge for pressure, then book boiler repair. If you smell gas, leave and call 911 or your gas utility from outside.

Combi boilers are compact and efficient, but one unit handles both your heating and your hot water. When something goes wrong, you can lose one or both at the same time. The pattern of the problem is a useful clue.

How a combi makes hot water

A combi doesn't store hot water. Here's the usual sequence:

1. You open a hot tap, and water starts moving through the unit.
2. A flow sensor detects it and tells the boiler to fire.
3. The boiler sends its heat to the domestic hot water side, often using a diverter valve or similar design.
4. Hot water comes out for as long as you need it, within the unit's flow limit.
5. When the tap closes, the boiler goes back to heating duty if the thermostat is asking for heat.

If any step fails, you may lose hot water even if the radiators are still warm. See the combi boiler guide.

Start with what you're seeing

Common causes of no hot water

Low system pressure

Most combis shut down on low pressure. Common reasons include a leak, a failed expansion tank or a recent bleed. See boiler pressure too high or too low. If it drops again after being topped up, there's a leak to find.

Diverter valve

This part directs heat to the hot water side when a tap opens. If it sticks in the heating position, your radiators get warm and your tap stays cold.

Flow sensor or switch

If the boiler doesn't detect water moving, it won't fire for hot water. Scale, debris or a failed sensor can all cause this.

Clogged inlet filter

Many combis have a small water inlet filter. Sediment can clog it, especially in homes on well water, or after work on the water main.

Scale

Hard water leaves minerals inside the domestic side of the heat exchanger. You may notice temperature swings, reduced flow, or a boiler that runs hot and then shuts down. Descaling is a standard fix in many cases. See descaling a tankless water heater, which covers a similar process.

Frozen or blocked condensate line

High-efficiency combis produce acidic water that drains through a line. If it clogs or freezes, the boiler shuts down, often with an error code. In a cold snap, check any section that runs outdoors. See frozen condensate lines.

Ignition or gas problems

If your stove and other gas appliances also aren't working, there may be a supply issue. Otherwise, the igniter, flame sensor or gas valve may be at fault.

Blocked vent or intake

Snow, ice, leaves or nests can block the plastic vent or intake. Clear it safely if it's reachable from the ground, and call if the problem doesn't clear.

Control board and sensors

Temperature sensors and control boards can fail. The display code often points the technician in the right direction.

Safe checks to try first

1. Note the error code and look it up in your manual.
2. Check the pressure gauge. Compare it with the normal range for your boiler.
3. Test more than one hot tap to rule out a single fixture.
4. Open a tap fully. Combis need a minimum flow to fire.
5. Check other gas appliances.
6. Check the power switch and breaker.
7. Look at outdoor vent and condensate lines for snow or ice.
8. Reset the boiler once, following the manual.

Don't take off the cover, and don't keep resetting it. Sealed combustion parts should only be handled by a technician.

Hot water that isn't hot enough

If hot water runs but seems weak, look at these reasons:

• Winter water is colder, so the same flow delivers a lower temperature
• Several taps at once may exceed the unit's capacity
• Setpoint too low on the display
• Scale reducing heat transfer
• Clogged aerators or showerheads
• Undersized unit for your home

Slowing the flow slightly often raises the outlet temperature. If you regularly run more fixtures than the unit can handle, ask about options.

When to call right away

• A gas smell: leave and call 911 or your gas utility from outside
• A carbon monoxide alarm: leave and call 911
• Water leaking from the unit, especially near electronics (see boiler leaking water)
• No heat in freezing weather (see no heat and frozen pipes)
• A repeating error code after one reset
• Pressure that keeps dropping or rising

How we fix it

1. Note the error code and fault history
2. Check gas, power, vent and condensate lines
3. Check system pressure and the expansion tank
4. Test the flow sensor, inlet filter and diverter valve
5. Check the heat exchanger for scale
6. Run combustion and carbon monoxide tests
7. Give you an upfront price before starting any repair

Repair or replace?

Most issues above are repairable. Consider replacement if the unit is old, the heat exchanger has failed outside its warranty, or repairs keep adding up. It's also worth asking whether your household has outgrown the unit. See boiler replacement in New Jersey. New systems qualify for $500 off (terms apply).

How to prevent it

• Book yearly service, including combustion testing and a check of the condensate trap (see the annual boiler checklist)
• Descale on the manufacturer's schedule in hard water areas
• Consider water treatment if your water is hard or on a well
• Insulate condensate lines that run through cold spaces
• Keep vents clear of snow, ice and debris

What to tell us when you call

A few details help us arrive with the right parts:

• The brand, model and any error code on the display
• The pressure reading on the gauge
• Whether you have heat, hot water or neither
• Whether your water comes from a well or the town supply
• When the unit was last serviced or descaled

A photo of the display and the gauge is helpful too, especially if the code disappears after a reset.

Keep your manual near the unit for quick reference.

Combi boiler repair near you

We repair and install combi boilers of every brand, including Navien and Weil-McLain, in all 153 towns across Ocean, Monmouth, Burlington and Middlesex counties. That includes Neptune, South Plainfield, Point Pleasant and Burlington Township.

No hot water? Book online or call (732) 779-7106 for same day service. We answer 24/7.