Why Does My Hot Water Run Hot and Cold?

If your hot water keeps switching between hot and cold, it can quickly become frustrating — especially when you are trying to shower or wash up. In many UK homes, this is a sign that your boiler or hot water system needs attention.

At The GHS Group, we regularly diagnose hot water temperature problems across Hampshire, and the cause is often something that can be repaired before it develops into a larger fault.

Common Reasons Your Hot Water Fluctuates

1. Blocked Plate Heat Exchanger

This is one of the most common issues with combi boilers, particularly in hard-water areas.

Over time, limescale and debris build up inside the heat exchanger, restricting water flow and causing the boiler to overheat. The boiler then temporarily shuts off the burner before reheating, resulting in hot-and-cold water cycles.

Signs of a blocked heat exchanger:

• Water starts hot, then suddenly turns cold
• The boiler keeps firing on and off
• The problem is worse at hotter settings
• Reduced hot water performance

2. Low Water Flow Rate

Combi boilers need a steady mains water flow to work correctly. If the incoming flow is poor, the boiler struggles to maintain temperature.

You may notice:

• Weak pressure at taps
• The temperature changes when another tap is opened
• Inconsistent shower temperatures

Sometimes this can be caused by partially closed valves or blocked filters.

3. Faulty Boiler Sensors

Modern boilers rely on temperature sensors called thermistors. If these sensors fail or give incorrect readings, the boiler cannot properly regulate water temperature.

This often results in:

• Random temperature spikes
• Water goes extremely hot, then cold
• Boiler cycling is unusually

4. Limescale in Hard Water Areas

Homes in hard-water areas are far more likely to experience hot-water fluctuations.

Limescale can affect:

• Plate heat exchangers
• Cylinders
• Pipework
• Shower valves
• Pumps

Over time, this reduces efficiency and causes overheating in the boiler.

5. Boiler Overheating

If circulation inside the boiler is restricted, the appliance may overheat and shut itself down temporarily as a safety measure.

This creates a repeating cycle of:

1. Water heats up
2. Boiler overheats
3. Burner shuts off
4. Water turns cold
5. Boiler reignites

What You Can Check Yourself

Before calling an engineer, there are a few simple checks you can try:

• Check the boiler pressure (normally around 1–1.5 bar when cold)
• Test if all taps are affected
• Reduce the hot water temperature slightly
• See if the issue happens more during high demand
• Listen for unusual boiler noises
• Check whether the heating still works correctly

When to Call a Heating Engineer

You should arrange a professional inspection if:

• The issue is becoming more frequent
• Your boiler locks out
• The hot water is unusable
• The boiler is noisy
• Heating performance is also affected

A qualified engineer can carry out:

• Flow rate testing
• Temperature checks
• Boiler diagnostics
• Scale and circulation assessments

Need Help With Hot Water Problems?

At The GHS Group, we diagnose and repair hot water faults across Hampshire, helping homeowners restore reliable hot water quickly and safely.

Call The GHS Group today on 01329 285993 to arrange an inspection or boiler repair.