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Boiler Keeps Firing Up Randomly? Top 9 Causes (+ How to Fix)

Boilers aren’t too noisy, but there’s nothing worse than having your day disrupted with constant boiler sounds. The low rumble comes on randomly and seemingly without warning. Most boilers have effective settings that require periodic heating, but your boiler shouldn’t fire up at unexpected times throughout the day.

Your boiler keeps firing up randomly because of these reasons:

  1. Preset heating
  2. Temperature changes
  3. High water pressure
  4. Faulty thermostat
  5. Frost protection settings
  6. Broken valves
  7. Malfunctioning timers
  8. Wrong boiler sizes
  9. Damaged water pumps

Today, we’ll show you how to prevent your boiler from turning on randomly. We’ll also provide a handful of repair suggestions and when you should expect your boiler to turn on.

1. Preset Heating Conditions

Many boilers have preset heating cycles that keep the water warm and ready to use. Combi boilers need to have water and heat readily available whenever you might need them. Most of these models have a predetermined timer that keeps the boiler at a specific temperature to prevent it from dipping too much.

If your combi boiler turns on randomly, we suggest timing the intervals. These boilers often turn on for no longer than two to three minutes every few hours. If your boiler turns on more frequently or without a pattern, there’s likely another issue.

How To Fix

According to Duffy Heating, preheating cycles are safe and shouldn’t be altered. They keep your boiler in working condition. However, if it’s running too frequently, you can try these tips:

  • Switch to a non-combi boiler since they don’t have adjustable heat cycles.
  • Keep your thermostat at a set temperature each day to prevent it from dipping and resetting.
  • Try not to use too many sources of hot water and hot air simultaneously.

Other than that, these cycles are necessary for your boiler. They might be a bit frustrating, but there’s not much else you can do about them. If you think this isn’t the issue, read on for more potential causes.

2. Cold Weather Fluctuations

Setting your thermostat will keep your boiler working throughout the day. It’ll run every time the temperature drops below the set temperature. It’s good if your boiler runs when it needs to warm the house or the water, but it shouldn’t happen the entire time the air is circulating. Much like the previous example, cold weather fluctuations are normal and safe.

However, your boiler shouldn’t knock, tick, or click while this is happening. If you notice strange noises when the boiler runs during the winter (or whenever it’s the coldest in your location), there’s an issue with expanding pipes. It’s best to fix any noticeable leaks to prevent the boiler from running continuously to keep up with the demand.

How To Fix

If your boiler keeps firing randomly because of temperature issues, follow these suggestions:

  • Insulate the plumbing with foam rolls. For example, the UR Best Pipe Insulation provides a ⅜-inch barrier that prevents the pipes from freezing. Each roll is six feet long, so you can use it on several pipes.
  • Keep your boiler on a frost cycle. While frost cycles make it run periodically (we’ll discuss them in-depth later in this article), they don’t let temperature changes affect the boiler as severely.
  • Maintain a steady, reliable temperature. Set your thermostat to a comfortable temperature to keep it from dropping too low and firing up all of the time.

3. Excessive Water Pressure Buildup

High water pressure is a leading cause of boilers that run without a pattern. When there’s too much pressure in the system, your boiler will work on overdrive. Not only can it damage the valves, but it can also cause the thermostat to think the water is colder than it is. This process makes the boiler run more often because it’s trying to heat the water.

Another reason excessive water pressure causes the boiler to turn on randomly is because it doesn’t have enough time to heat the water. When you turn on the faucet, the water flows at high speed through the pipes. There’s not enough time for the boiler to warm it up, which means it’s always catching up to the water flow.

How To Fix

Reducing the water pressure can be quite easy. Here’s what we recommend:

  • Ensure all of the diverter valves are open all of the way to prevent the flow from narrowing. Diverter valves decide which way the water goes in the plumbing. If they’re partially closed, they’ll restrict the flow and increase the pressure.
  • Adjust the water pressure regulator going to your house. Every house has unique water pressure systems, but most of them can be fixed by twisting the knob counterclockwise.
  • Make sure the boiler’s water pump isn’t too powerful. If you get a new pump or someone replaces it, it must be within the manufacturer’s guidelines. A ½-horsepower bigger than the recommendation can drastically increase the water pressure to dangerous levels.

4. Faulty Thermostat

If your thermostat is broken, it could randomly trigger the boiler to turn on. It might tell the boiler that the house isn’t hot enough, even if it’s well over your desired temperature. This issue is rare since boiler thermostats typically stop working if they’re damaged, but it’s not impossible. If this happens, it’s time to replace the thermostat or get new batteries.

How To Fix

If you’re unsure if the thermostat is broken, replace the batteries. New batteries should do the trick; Otherwise, replace the thermostat with this process:

  1. Turn off the power going to the thermostat at the breaker box.
  2. Remove the thermostat, revealing the wires inside of the base.
  3. Label each of the wires according to the letter of the terminal they’re connected to, then remove the base screws to pull off the base.
  4. Place the new thermostat’s wire base over the old wires, then attach the old labeled wires to their new labeled terminals.
  5. Attach the thermostat over the wire base, turn on the circuit breaker, and check if the thermostat works.

5. Frost Protection Cycles

Frost cycles are designed to prevent the boiler’s plumbing from freezing. If frost builds up, the pipes could crack. Cracked pipes cause leaks, which means you’ll have a much more expensive repair to deal with. Frost protection cycles are an excellent way to keep your boiler in good condition, even if they’re a bit annoying to hear every few hours.

Keep in mind that these cycles should only occur when it’s freezing. If it’s warm outside and your boiler is running a frost protection setting, there could be an issue with the thermostat. The device warns the boiler that it’s too cold, so it triggers the cycle.

How To Fix

If your boiler has a frost protection cycle, there’s nothing you need to worry about. Some thermostats let you turn off the cycle, though it’s highly recommended to leave it on when it’s freezing outside.

If your boiler’s freezing cycle turns on when it’s warm, the thermostat likely needs to be replaced via the previously mentioned instructions.

6. Damaged Diverter Valves and Check Valves

Boilers use a handful of valves in the plumbing. Check valves are designed to prevent water from flowing backward. They use a flap that opens when water goes through the pipes, but it closes as soon as the water stops moving. On the other hand, diverter valves allow the boiler to send warm water and air to different parts of the house.

Boiler Choice explains these valves can get blocked or locked in place, preventing them from closing or opening. When this issue occurs, the water flows and tells the boiler that it needs to be heated. It’ll randomly turn on throughout the day because it thinks it’s always being used.

How To Fix

Damaged diverter valves can be a result of an electrical failure. Ensure all of the wires going to the valve are secured and in good condition.

If you have a broken diverter or check valve that won’t close properly, follow these steps:

  1. Turn off the water and power going to the boiler.
  2. Use a reciprocating saw to cut the PVC before and after the pipes.
  3. Dry the exposed PVC pipe on both sides, coat it with PVC plumbing glue, and slide a new valve where the old one was.
  4. Hold the valve in place for at least 15 seconds to let the glue solidify.

Always make sure the valve is facing the correct direction. Each valve has arrows pointing to the way the water should flow through it. If your boiler has copper plumbing, it’s best to hire an experienced plumber since it requires soldering.

7. Malfunctioning Timer

Every boiler (except some combi boilers) should have a timer. These timers work with the thermostat to adjust the air temperature whenever you want it to. If your boiler doesn’t have a timer, it’s likely because it’s a combi boiler that works on-demand or your landlord removed it to keep them in control of the utility bills.

When the timer fails, it can tell the boiler to turn on randomly. This happens without the temperature changing, which means there’s always excess heat building in the boiler. Not only can this cause random boiler fire-ups, but it can also damage the plumbing by overheating it.

How To Fix

You have two options to fix a broken timer:

  1. Replace the thermostat with our step-by-step process.
  2. Locate and replace the timer in the boiler.

If you want to replace the timer in the boiler, simply unscrew the faceplate, disconnect the wires going to the timer, unscrew the old timer, and slide a new like-for-like timer in its place. You can connect the old wires to the new timer, then turn the power back on.

Note: Always turn the power off at the circuit breaker when replacing any part of a boiler.

Review the Ultimate Handyman’s YouTube tutorial for more details:

8. Incorrect Boiler Size

If your boiler is too small, it won’t be able to meet the demands of the house. Every time you run the hot water through faucets, showerheads, or laundry machines, it’ll run on overdrive. Furthermore, a small boiler can’t keep your house warm as efficiently as an adequately-sized boiler.

Compare the Boiler Market claims oversized boilers also cause your system to fire up randomly. An oversized boiler can drastically increase the water pressure, which triggers it to run more often than it should.

How To Fix

Unfortunately, the only thing you can do to fix an incorrect boiler size is to get a new boiler. The best way to know if you have the right boiler size is to calculate its parameters.

  • How many square feet is your home?
  • How many sink faucets, showerheads, and laundry machines do you have?
  • How many radiators or heat outlets do you have?

These questions will help you determine how many kilowatts and BTUs your boiler needs to be. Anything over the recommendations is unnecessary and can cause the aforementioned issues. Use the Boiler Central Calculator to know how big your boiler should be.

9. Broken Water Pump

When the water pump is broken, your boiler can’t provide hot water. Every time you turn on an appliance that needs hot water, the pump runs dry, and the boiler can’t keep up. This issue also happens when there’s a leak in the plumbing going to or from the water pump.

If your water pump is broken, you might notice these symptoms:

  • Reduced water pressure
  • Less water coming from the faucet
  • Cold or lukewarm water when it’s supposed to be hot
  • The boiler turns on randomly
  • Water damage or puddled water near the boiler and the water pump

How To Fix

To replace the pump, try this method:

  1. Turn off the power going to the water pump.
  2. Disconnect the pump’s unions and wires, then set it aside.
  3. Slide the new pump in its place, connecting the wires and unions when it’s bolted to the ground (if your local building code requires ground bolts).

Getting an exact model replacement for your water pump will typically prevent you from having to use saws, glue, and other plumbing supplies.

Additional Boiler Resources

If you ever experience different boiler problems, some of our other boiler troubleshooting articles may be able to help:


  • Steve Rajeckas

    Steve Rajeckas is an HVAC hobbyist with an avid interest in learning innovative ways to keep rooms, buildings, and everything else at the optimal temperature. When he's not working on new posts for Temperature Master, he can be found reading books or exploring the outdoors.

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