Boiler hot-water diagnosis

Boiler No Hot Water? Check Power, Pressure, and Demand

When a boiler has no hot water, first decide whether the whole boiler is down or only domestic hot water is missing. Check power, pressure, demand settings, and one safe reset before parts.

Good clues are a blank display, low pressure, a lockout light, a timer or hot-water setting that is off, or radiators heating while taps stay cold.

The main split is no heat and no hot water versus heat works but taps stay cold.

Don’t start with: Do not open combustion covers, press reset repeatedly, or replace hot-water parts by guess.

No heat and no hot water?check service switch, breaker, display, pressure, and lockout first.

Heat works but taps stay cold?focus on hot-water demand, setting, flow, and internal control response.

Do this first

Run a hot tap long enough to confirm cold versus lukewarm water.
Check whether space heat still works, if the boiler also heats the home.
Look for boiler power, display, fault light, or reset prompt.
Read the pressure gauge before another reset.
Stop if there is gas smell, water near controls, repeat lockout, or abnormal noise.

Last reviewed: 2026-06-28

No-hot-water sorter

Boiler dark or silent?

Power, service switch, breaker, and display path first.

Pressure clearly low?

Low pressure can stop operation or make hot water unreliable.

Heat works but taps are cold?

Hot-water demand or internal control path becomes likely.

Water starts warm then cools?

Watch for short firing, lockout, flow recognition, or pressure trouble.

Fault returns after reset?

Stop repeated resets and record the code or light pattern.

No-hot-water clues to check

The strongest clues are outside the boiler: display, pressure, faucet temperature, and whether the controls are asking for hot water.

Boiler display and pressure gauge checked for no hot water — Start with power, pressure, display, and service-switch clues before opening anything.

Faucet thermometer checking lukewarm domestic hot water — A simple fixture temperature check confirms whether water is cold, lukewarm, or recovering.

Wall control and boiler context for hot-water demand check — Controls and timers can stop the hot-water call even when the boiler itself is ready.

Before you buy anything

Confirm whether this is whole-boiler shutdown, low pressure, setting trouble, or a hot-water demand fault. Match the exact symptom, boiler type, pressure reading, and safe diagnosis before ordering anything.

Jump to: Result map · Tools You May Need

First split the symptom correctly

The first decision is whether the boiler is fully down or only the domestic hot water side failed. That split prevents chasing the wrong system.

If radiators or baseboards are cold too, treat it like a whole-boiler no-heat problem.
If space heat works normally but faucets stay cold, focus on hot-water demand and internal switching.
If water starts hot then cools, look for short firing, flow recognition, or lockout timing.
If the boiler is a combi unit, fixture flow and boiler response matter more than storage-tank assumptions.

What not to reset repeatedly

A single user reset can clear a nuisance interruption on some boilers. In practice, a repeat lockout is the boiler preserving the fault clue.

Write down the light pattern or display before pressing reset.
Use reset once only if there is no gas smell, leak, low-pressure concern, or abnormal ignition sound.
Do not reset after a breaker trips again.
Do not open covers to find a hidden reset or flame sensor.

No-hot-water result map

Use the boiler response to a hot tap as the result map. The faucet, boiler display, and pressure gauge together usually point to the right branch.

Run one hot tap and watch the boiler from a safe distance.
Check whether the display changes or the boiler starts a normal sequence.
Compare that with pressure and any fault light.

Pattern	Likely branch	Next move
No display, no response	Power path	Check service switch and breaker once.
Low pressure	Pressure or water-loss path	Look for leaks before filling.
Heat works, taps cold	Domestic hot-water demand path	Check settings and call service if ignored.
Brief heat then lockout	Ignition, flow, or safety path	Record timing and stop resets.

Controls and settings to check

Programmers, thermostats, seasonal modes, and hot-water settings can leave a healthy boiler waiting for a call. Good clue: the boiler display changes when a hot tap runs.

Confirm hot water is enabled and not in heating-only or vacation mode.
Check any timer window that controls domestic hot water.
If the thermostat or programmer lost power in an outage, verify the schedule did not reset.
Do not remove control covers or handle wiring to prove a demand signal.

Pressure and leak clues still matter

Low pressure can make a boiler stop firing or protect itself before it can heat water. If pressure is low, find out why before treating no hot water as an isolated control issue.

Read the gauge when the boiler is cool.
Look for recent bleeding, visible leaks, or relief discharge.
Do not add water unless the homeowner fill procedure is clear for the exact boiler.
Call service if pressure will not hold or rises too high when hot.

Boiler Losing Pressure — Use when the gauge is low or needs repeated filling.
Boiler Locked Out After Outage — Use when the problem began after a power interruption.

Tools You May Need

These tools help confirm the symptom, read the outside clues, and give a technician a clean sequence without opening the boiler.

Boiler-room flashlight

Helps when: Helps read the pressure gauge, display, valve positions, leak tracks, and switch area without opening covers.

Skip it when: Skip close inspection if the boiler is leaking near electrical parts, smells like gas, or has locked out again.

Compare flashlights on Amazon

Faucet thermometer

Helps when: Confirms whether hot water is truly cold, lukewarm, or recovering before blaming the boiler.

Skip it when: Skip temperature testing when the boiler is locked out, leaking, or showing a combustion safety warning.

Compare faucet thermometers on Amazon

Notebook and phone for recording boiler pressure, fault codes, and symptom timing

Notebook or phone notes

Helps when: Records pressure readings, fault lights, reset timing, leak timing, zones that heat, and what changed first.

Skip it when: Skip buying one if clear photos and a written symptom timeline are already ready for the technician.

Compare notebooks on Amazon

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FAQ

Why do I have heat but no hot water?

That usually points to the domestic hot-water demand side, such as settings, flow recognition, diverter/control behavior, or an internal boiler fault.

Can low pressure cause no hot water?

Yes. Many boilers will stop or behave poorly when pressure is below the normal range.

Should I reset the boiler?

Use one normal user reset only if there is no unsafe symptom. If the same fault returns, stop and record it.

Why is the water only lukewarm?

Lukewarm water can come from weak firing, short cycling, low pressure, flow recognition trouble, or a control setting that limits domestic hot water.

What should I tell the technician?

Tell them whether space heat works, the pressure reading, whether the boiler reacts to a hot tap, and any fault code or light pattern.

Can I keep running the boiler while checking this?

Only if there is no gas smell, carbon monoxide alarm, leak near wiring, relief-valve discharge, breaker trip, overheating, or repeat lockout. Stop and call for service when any safety clue appears.

What should I photograph before calling a technician?

Photograph the pressure gauge, display or fault light, the first wet point if water is involved, the thermostat call, and any zone or fixture that proves the pattern.

What makes this a service-call problem?

Recurring pressure loss, relief discharge, boiler-body leakage, repeat lockout, burner trouble, electrical symptoms, or any check that requires opening a boiler compartment belongs with a qualified boiler technician.

How this guide was built

Repair Riot reviewed this page around whole-boiler versus domestic-hot-water failure, pressure checks, reset boundaries, hot-water demand clues, and combustion safety. The source links support boiler maintenance and carbon monoxide safety context; the diagnostic sequence is original guidance.

DOE furnaces and boilers — supports boiler heat distribution, controls, maintenance, and combustion-system boundaries.
ENERGY STAR HVAC maintenance checklist — supports safe homeowner observation and professional heating-system maintenance.
CDC carbon monoxide basics — supports carbon monoxide safety around combustion appliances and emergency stop points.