Why is My Boiler Overheating?

If you’re having issues with your boiler overheating, this guide is all you need to determine what the problem is, the easiest way to fix it and how much it’s likely to cost. Below we’ll cover: 

• A quick summary of why boilers overheat, fixes, costs and when you should replace your boiler
• A detailed guide to the most common faults that result in a boiler overheating

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Why is My Boiler Overheating?

This guide is highly detailed. Strapped for time? Bookmark this page for later, and simply read this summary instead…

There are a few faults that might be causing your boiler to overheat, including: 

• Hot spots on the exchanger
• Thermistor that’s faulty and not monitoring water temperature correctly 
• A faulty pressure release valve (PRV)
• A boiler pump that’s become clogged with heating sludge and not circulating hot water correctly

Each fault mentioned above is likely to lead to an error code being displayed on your boiler, and it locking out. It won’t work again until the fault has been fixed, the error code has been cleared and the boiler has been reset. Use a Gas Safe registered engineer to fault find and fix any problems. 

Typical costs associated with a repair of an overheating boiler sit in the £300-£500, depending on the specific fault. 

At this price point, if you have an old boiler that’s becoming increasingly unreliable, it’s worth investigating the cost of a replacement. You can do this using our clickable boiler installation cost tool here to determine the exact cost of a new boiler.

And it’s worth noting, radiators that are getting too hot are not a boiler overheat issue; this is likely to be a fault with thermostatic radiator valves (TRVs) instead.

Top 4 Reasons Causing Your Boiler To Overheat

If you're seeing a fault code on your display relating to a boiler overheating, there are 4 incredibly common reasons for the fault including: hot spots on the heat exchanger, a thermistor failure, broken pressure release valve (PRV) or a pump that’s stopped circulating.‍

1. Heat Exchanger Hot Spots

The heat exchanger in your boiler has a particularly hard life. Its role is to heat water before it’s pumped round the system. 

For central heating, water is constantly circulated and as it does it loses temperature as it’s absorbed by pipework and radiators; the heat exchanger re-heats each time water is circulated. 

When you demand hot water, your boiler works hard to get water to temperature for taps and showers. Any fault with your heat exchanger will result in an issue with heating temperature, hot water temperature, or both. 

Most common in hard water areas, limescale build-up can be an issue. Not only does limescale cause blockages in heating components, wherever limescale settles can create hot spots. 

You can check if you’re in a hard water area here. At Dwellow we will also tell our customers that they are in a hard water area when they buy a boiler from us, we will offer and advise a limescale reducer is included.

In the case of the heat exchanger, if you hear a distant whistling (kettling) noise, this is potentially a hot spot on the heat exchanger. 

Not only does this lead to your boiler overheating, it might damage the heat exchanger too. Your boiler overheating at the point of the heat exchanger could lead to an expensive bill for a replacement, with costs generally sitting in the £450-£550. 

For older units (10-years or older), it really doesn’t make sense to invest this cash in an unreliable and inefficient boiler. You can get a quote on a new unit using our boiler installation quote tool here.

A kettling heat exchanger that’s causing a boiler to overheat can be cleaned manually or with a chemical flush. However, the time taken (and therefore, labour cost) means it isn’t cost effective. The best route to take is to replace the heat exchanger, or replace the boiler. 

Remember, check your parts warranty, the most reliable boiler brands will offer 10-years or longer warranty, particularly if they’re using a stainless steel heat exchanger.‍

2. Overheat Thermostat / Thermistor Failure

A thermistor helps monitor water temperature to stop a boiler overheating. As soon as water hits a temperature that’s unsafe to operate, the boiler will lock out and display a fault code. 

The thing is, if the thermistor is faulty, it might think the boiler is overheating when in fact, it’s not. The fix here is to get a Gas Safe boiler engineer to check your boiler’s thermistor. 

So how much will a replacement overheat thermostat / thermistor cost to replace? Prices vary as each boiler’s thermistor will be a different price, and labour rates vary throughout the country. 

As an estimate, expect a bill of between £225-£350 for the diagnosis of a broken thermistor and the cost of materials / labour to have a new thermistor fitted.

3. Pressure Release Valve (PRV) Not Working

The PRV in your boiler allows excess pressure to be released as water is heated. The heated water increases the temperature of radiators, pipework and boiler components leading to increased pressure. 

A faulty PRV will mean that pressure can’t escape and could be the cause of your boiler overheating. 

Once you’ve contacted a Gas Safe engineer, they’ll see if they’re able to fix the PRV. If a new component is needed, you should expect it to cost in the region of £300 including parts and labour.

4. Pump Not Circulating

The pump in your gas boiler circulates hot water through pipework, eventually reaching radiators, taps and showers. If water isn’t getting hot at the point of taps or showers, the pump could be to blame. Or, if you have a fault code being displayed that references a circulation error, it’s likely the pump is faulty. 

Typical causes of a boiler pump that isn’t working includes sludge and limescale build-up. 

Limescale comes from minerals in water, so this is a common fault for those living in hard water areas. 

A build-up of sludge in the boiler’s pump doesn’t relate to the hardness of water; it comes from rust on the inside of pipes, radiators and boiler components. Over time, this rust breaks off and mixes with the hot water into your central heating system, creating heating sludge (dirty black water that’s full of debris). 

If the sludge issue with your central heating system isn’t severe, a chemical flush can breakdown heating sludge and it can be removed from your system. 

However, if the heating sludge is particularly thick, it might have damaged the pump meaning a replacement is needed. The cost of a replacement heating pump could be £300-£400 depending on the model of boiler you have. 

Before investing this kind of money to repair an overheating boiler, we’d suggest comparing repair costs with the cost of a new boiler. Jump over to our new boiler cost tool and you can get a fixed price on-screen within minutes; all installations come with a free chemical flush to stop problems like this occurring again.

Fixing An Overheating Boiler

If your boiler is overheating, it will lock out (stop working). 

For jobs like this, you shouldn’t attempt a DIY fix. The only people qualified to work on gas boilers are those registered with the Gas Safe register. 

If you’re not prepared to invest time and money into your old and unreliable boiler, you can get a fixed price on screen using our boiler installation tool here. Or, if you want to read up on how installations differ in price, check out our guide to new boiler costs.

As a side note, radiators that are too hot are not a sign of a boiler overheating. Typically, if you’re finding your radiators are too hot, it’s likely the fault lies with a broken thermostatic radiator valve, not the boiler.

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