A boiler is the most important part of a home's heating and hot water system, but it is also potentially a source of danger and trouble if it is not properly maintained and also replaced when necessary. So knowing when to replace your boiler is essential knowledge for everyone who owns one.
Boilers don't age well, because they are made from metal that will become more fatigued the longer it is in use. Consider that the metal is subjected to expansion and contraction due to temperature changes and also to potential corrosion due to dissolved salts and chemicals in the water supply, which it is in constant contact with. The boiler will normally be heated either by direct flame or by electrical heating, and each of these also can fail in various ways, putting even more stress on the boiler's superstructure. The key thing to look at, then, is the age of your boiler, and you normally want to plan to replace the entire boiler at least every 15 years, or thereabouts.
Boilers may be designed in many different ways, but when fatigue begins to affect the boiler, you will usually see a few telltale signs. Rust along the seams of the boiler may indicate internal corrosion, and any evidence of leaking of either water or steam from places where this should not be occurring is something you should immediately have inspected by a trained professional.
If you start to notice anything odd about the water that is delivered through your hot water system, you should definitely consider that the boiler might be the source of the problem. The sorts of things you may notice include flakes of metal, rust, or other contaminants in the water; discoloration of the water, water pressure may suddenly change unpredictably, or the water may have an unpleasant odor. The most important sign of a problem will be if you suddenly get rapid changes of the temperature of the water that is coming out, as that can be potentially very dangerous to anyone that is using the water when it happens. If you notice these signs you should have your boiler checked and be ready to replace it.
If your boiler is not working efficiently, it will have to work harder to do its job. The result will mean higher energy costs because the boiler has to use more fuel in order to produce the same amount of heating. At that point, keeping an old inefficient boiler is definitely a false economy, not to mention a potential safety risk.
All gas appliances, including boilers, should always have a healthy, clean blue flame. If the flame is yellow or emits smoke, it is a clear sign that the boiler needs replacing. It is dangerous to continue to use a boiler that has reached this state.
Even if you don't strictly need to replace your boiler, you may still want to do so anyway. As technology continues to improve, boilers are being made to be more energy efficient, stronger, more reliable, and better for the environment. Therefore replacing your boiler early may be a good investment that will pay for itself in terms of the savings you will make and the better safety you will enjoy.