What usually causes a boiler breakdown
Most call outs come down to a short list, and knowing which one you are looking at changes what the repair is worth.
Low pressure. If the gauge sits below about one bar the boiler will lock out to protect itself. Topping up gets the heating back on, but a system that needs topping up repeatedly is losing water somewhere, and that is the actual fault.
A failed diverter valve. The classic sign is heating that works while the hot water has gone cold, or the other way round. The valve decides where the heat goes, and when it sticks it sends it to only one place.
A blocked condensate pipe. Almost always in the first hard frost. The pipe carries acidic condensate outside and freezes if it is undersized or badly routed. It usually presents as a boiler that has stopped with no obvious reason on the coldest morning of the year.
Sludge and blockages. Circulating debris settles in the heat exchanger and the pump. Symptoms are noise, radiators hot at the top and cold at the bottom, and a boiler that overheats and cuts out.
Ignition and flame sensing faults. The boiler tries to light, fails and locks out. Often an electrode or a sensor rather than anything expensive.
A fault code is a starting point, not a diagnosis. Manufacturers reuse codes across symptoms. The code narrows it down, then the readings decide it.
Repair or replace: how to judge it
Nobody should spend four figures on a boiler that owes them nothing. The honest test is a few questions.
- How old is it? Most gas boilers are designed for a service life in the region of ten to fifteen years. Past that, one repair tends to be followed by another.
- Are parts still available? Once a model is obsolete, the wait for parts gets longer and the price goes up. Some parts simply stop being made.
- Is this the second or third fault this year? A pattern of failures is a different situation from one component letting go.
- What is the repair against the replacement? There is no fixed percentage rule worth quoting, but a repair costing a large fraction of a new install on an old boiler rarely makes sense.
- Is the efficiency costing you? An old non condensing boiler wastes gas every day it runs, and that is a cost even when nothing is broken.
A repair should come with a straight answer about which of these applies. If an engineer will not tell you the boiler is finished, that is worth noticing.
What a proper repair visit includes
- Diagnosis before parts. Fault codes read, gas rate checked, combustion tested where relevant. Guessing at parts is how people end up paying twice.
- A fixed price for the repair before it starts. You should know what the job costs before anyone opens a box.
- A working boiler tested under load, not just fired up and left. Heating and hot water both proved.
- An honest verdict on what caused it. A pressure loss that is not traced will be back.
- The safety side checked. If an engineer finds something unsafe they are obliged to act on it, and that includes situations where an appliance has to be turned off.
