5 minute read • February 14, 2024

Causes of High Boiler Flue Temperature


High Boiler Stack Temperatures

Are you curious when you see a lot of smoke coming from the chimney of a building? Do you ever wonder where it comes from, and why some buildings give off a lot, while others let out much less? 

The short answer? The boiler flue temperature. 

If the chimney temperature, which is the boiler stack, is too high, it generates a lot of smoke and can be a problem. There are a few issues that may result in high boiler stack temperatures. We’ll explore how it impacts boiler efficiency and some things you can do to fix it. 

What is Boiler Flue?

Let’s backtrack to clarify how the boiler works and where the flue comes in. As shown in the above image, the burner houses the flame, which feeds into the first pass boiler tube. This generates steam, which travels along the boiler tubes and heats up the water in the boiler. The steam then exits the tubes as exhaust via the chimney 

The boiler flue is a pipe that runs through the chimney shaft and carries exhaust gasses produced by the boiler outside the building. When the boiler flue temperature is too high, it indicates that the boiler is not running efficiently and heat is being lost.

Causes of High Boiler Flue Temperature

Some issues that can cause high flue temperature are corroded flue, dirty tubes, development of scale or soot build-up, or missing fins. If there is a leak in one of the tubes, it will cause very high temperatures due to the oxidation of the fluid. 

Another concern is when the boiler is running with too much excess air. It can cause higher stack losses however, if insufficient air is supplied, incomplete combustion occurs, and the flame temperature is reduced.  It is a challenge to ensure that the proper amount of excess air is provided for complete combustion. (Click here for more info on this).

Is a 3-Pass Boiler More Energy Efficient Than a 2-Pass Boiler?

First, let’s clarify the difference between the two types of boilers. A 2-pass boiler has only two tubes going through the boiler, bringing the steam from the flame. A 3-Pass boiler has 3 tubes and extracts more heat from the interior surface area of the boiler and is therefore more energy efficient. Less energy and fuel get wasted this way as it heats up the water better and not as much exhaust is generated.

How Does High Stack Temperature Affect Boiler Efficiency?

If the tubes are clogged, the water in the boiler receives less of the heat. The temperature of the exhaust coming from the boiler tubes to the chimney would then be higher. Instead of utilizing the energy and fuel to heat the water of the boiler, it exits through the chimney and is wasted.

When a steam boiler flue temperature is above 500°, the boiler is not running as efficiently as it can. Having a good boiler control such as the Stealth, which has a stack sensor with remote readings, enables you to detect this problem and remedy it. Entech Pro specialists monitor these issues and notify you when it registers too high.

How to Fix Boiler Flue Issues

A high stack temperature indicates that heat is being lost and fuel wasted. So what should you do if you think your stack is too high? Call your mechanic down to check it out. It is likely that the boiler tubes need to be cleaned or possibly replaced. In some cases, it is necessary to install combustion air heaters or economizers to recover the heat that would otherwise be wasted.

Armed with some knowledge and the right boiler control system, you can maximize efficiency, cut costs, and contribute to a sustainable future. Seize the opportunity now!

Related Articles