Boiler cycling on and off?
If you’ve noticed that your boiler is turning itself off and on again every few minutes; you might be experiencing a problem called ‘boiler cycling’ or ‘boiler short-cycling.’
Whilst it’s common for a combi boiler to turn itself off every now and then; as part of it’s ‘pre-heat’ function; constant stop-starting for very short-durations isn’t normal.
If you suspect that your boiler is turning off and on too frequently; you should try and time its cycles for an hour, to get a better idea about what it’s doing.
If your boiler is turning off and on every few minutes or less, you may have a boiler cycling problem.
Boiler cycling prevents your boiler from overheating when heat can’t be dissipated quickly enough.
But it’s bad news because you’ll be paying over the odds on your inefficient system; and there’ll be unnecessary stress on your boiler, which can shorten its lifespan.
To find out everything you need to know about this pesky problem; and what to do about it, stay with us…
What is boiler cycling?
To understand what boiler cycling is, and why it happens; you need to understand a little bit about how your boiler works. And more specifically, what heat loss and boiler modulation is…
Heat Loss
At its simplest, your boiler’s job is to heat water to provide you with heating and hot water. How much power (heat) your boiler needs to output to meet your heating demands; is determined by the heat loss of your home.
Understanding heat loss is vital when it comes to understanding heating; because if it wasn’t for heat loss, we actually wouldn’t need heating!
To learn more about what heat loss is, you can read our blog on it here. But we’ll summarise here…
Why homes lose heat
Whether we like it or not, all buildings lose heat. It’s an inescapable fact of science!
The second rule of thermodynamics means that warm air always moves towards cooler air. If you think about your house; the warm air inside will want to move towards the colder air in the garden!
So, heat losses occur because the heat inside your home will be transferred through the fabric of your building. You can probably predict that this will happen more quickly if you have poor insulation; because the job of insulation is to make it more difficult for heat to escape.
But heat is also lost more quickly on a colder day, than on a warmer one. The bigger the difference in temperature between the inside and outside temperatures, the quicker heat will be lost.
To keep you warm, your boiler needs to match the heat loss of your building. This is why it’s crucial to get a heat loss calculation to choose the correct size of boiler for your needs.
Your boiler’s maximum power output should be able to match the heat loss of your home on the coldest day of the year. And ideally be able to modulate down to a lower output to match lower levels of heat loss on warmer days.
And this is where we need to understand boiler modulation…
Boiler modulation
Boiler modulation sounds very technical. But it just refers to the ability of a boiler to reduce or ‘turn down’ its output; to better match the heat demand.
If you have a 30kw boiler; you won’t always need it to be blasting out its full 30kw of power to heat your home. And you don’t want it to do that, because as you can imagine, it’s a waste of energy.
To give an example… Let’s say that you only need 15kw of heat from your 30kw boiler for the next couple of hours. If your boiler was just to output its maximum 30kw of power; the ‘excess’ heat that can’t be dissipated into your home (because it’s not needed); would cause the water returning to the boiler to become too hot. To prevent itself from overheating; your boiler would be forced to cycle, turning on for 5 minutes, and then off for 5 minutes repeatedly.
A modulating boiler could avoid this situation by simply turning down its flame by 50%; so that it was outputting 15kw of power, instead of its full 30 kw capacity. It wouldn’t need to stop-start to replicate the lower heat demand.
Ideally, your boiler will fire-up, and run continuously by modulating down as your home reaches your desired temperature.
Boiler modulation ratios
But not all boilers are created equal. How much a boiler can ‘dial down’ its output to, is stated by the manufacturer as a modulation ratio.
A boiler’s modulation ratio is its minimum output in relation to its maximum output, stated as a fraction.
To give a couple of examples:
Boiler A has a maximum output of 30kW and has a 5:1 ratio. This means that the maximum power it can output to heat your home is 30kW; and it can modulate this down to 6kW.
Boiler B has a maximum output of 30kW and an extremely impressive 17:1 ratio. This means that the maximum power it can output to your home is 30kW; and it can modulate this down to 1.7 kW, with a total of 17 different heat outputs!
Lower power output means less fuel burned; more money saved; and less boiler cycling putting unnecessary stress on your boiler.
Why is boiler cycling a problem?
A cycling boiler is a problem because it:
- Wastes fuel;
- Increases your carbon footprint;
- Results in higher heating bills;
- Increases the wear and tear on your boiler’s components;
- Leads to increased faults and problems with your boiler;
- Can shorten the lifespan of your boiler.
What causes boiler cycling?
Now that we have a better understanding of how boilers work. Let’s take a look at some of the possible causes of boiler cycling:
1. Boiler cycling caused by an oversized boiler
One of the main culprits of a cycling boiler is that it’s been oversized.
An oversized boiler means that its output is too high (or powerful) for your needs. When it comes to boilers, having too much power is a bad thing.
Whilst you (and some dodgy installers!) might be tempted to oversize your new boiler ‘just in case’; this would be a costly mistake.
And this shouldn’t come as too much of a surprise, now that we’ve seen that a boiler will cycle if it can’t modulate down low enough to meet a lower heat demand!
Suppose that your home has a 15kw heat loss, but you have an enormous 45kw boiler. The boiler would generate more power than your system can handle. It would heat up too quickly; run for a short time; and then turn off; before the cycle starts again. This is very inefficient, and would lead to excessive energy bills, as well as taking its toll on your boiler.
This is why it’s so important to have a heat loss calculation conducted to find the correct size of boiler for your needs. If an installer just wants to count your radiators, run for the hills!
Rest assured that The Heating People conduct heat loss calculations as standard as part of your free survey. We will calculate, recommend, and install the perfect boiler for your home and lifestyle.
2. Boiler cycling caused by poor system design
We’ve said it before, and we’ll say it again… Your boiler installer is the number one contributor to your heating system’s energy efficiency.
If you want to optimise the efficiency of your heating system, and avoid issues such as boiler cycling, you need an expert engineer who understands emitter sizing, pipe sizing, hydraulic layout and principles.
The Heating People are proud to be part of the Heat Geek Elite and will design and commission your heating system to get the very best out of it.
3. Boiler cycling caused by problems with the thermostat
Your thermostat’s job is to measure the temperature in your home and prompt your boiler to fire up when it drops below a certain temperature.
A faulty thermostat, or one placed in a cold location, may take inaccurate temperature readings, causing your boiler to cycle on and off more frequently.
4. Boiler cycling caused by a faulty water pump
If your boiler’s pump develops a fault, this could lead to boiler cycling if water isn’t being pumped away from the boiler effectively. An overheating boiler will cut out after firing up (short cycling).
How can I prevent boiler cycling?
As with all things, prevention is better than cure!
To avoid the problems associated with a cycling boiler, you should start by getting the correct size of boiler installed by an expert engineer you can trust.
Choosing a boiler with a good modulation ratio will help to maximise the energy efficiency of your heating system.
Final thoughts
If you think your boiler is cycling, you should talk to your heating engineer for advice.
And if you’ve decided that it’s time you replaced your old, struggling boiler, contact The Heating People to book your free survey.
