What is boiler modulation and boiler cycling?

November 1, 2018

Boiler modulation is the backbone of what allows efficiency in heating systems. But what is a modulating boiler? How much efficiency can it really add? 

Thankfully this really is not a complicated question to answer. However, what can be complicated is understanding what the benefits are and how they work.

So, what is boiler modulation, or boiler modulation ratio?

Boiler modulation is the ability of a boiler to 'turn down' its output. That is to say if you have a 20kw boiler, but you only require 10kw of heat for the next hour, rather than outputting 20kw for 5 minutes then resting 5 minutes repeatedly, (how the older non-modulating boilers used to work) the boiler will simply turn down its flame by 50%.

This means the system will run much cooler and less stops/starts for the boiler as well as many other benefits.

Here you can see, as the load drops throughout the day or year, and eventually drops below the boilers minimum output, the boiler begins to cycle to replicate a smaller output. As part of this, the boiler will overheat and under heat.

Manufacturers typically state the ability of a boiler to modulate as a modulation ratio.

A boilers modulation ratio is its minimum output in relation to its maximum output, stated as a fraction. i.e. a boiler with 30kW maximum output and a 5kW minimum output would be given the ration 1 to 6. An average modulation for this day and age. Typically it's accepted that the bigger/wider the modulation ratio the better.

However, you'll also regularly find this same figure used for the whole range of sizes available. Often the smaller boilers cannot turn down quite as low relatively, as the larger boilers. In fact you may even find some of the smaller boilers minimum output is the same as the boilers maximum output.

This is a pretty accurate way of finding out that it is exactly the same as the larger version, just factory range rated (maximum output capped in the software). 

So take modulation ratio with a pinch of salt.

What are the benefits of a boiler with good modulation?

The benefits can be as simple or as complex as you like. But this is Heat Geek, and there are lots of benefits. So without further ado. *takes deep breath*

longer run times / low cycle rate

There are 2 main reasons longer run times are helpful, boiler wear and efficiency.

Longer run times essentially means the boiler can stay on longer without overheating. As pictured above, If you have a relatively low heat requirement the boiler would simply dial down to match what was needed.

If your requirement for heat was below the minimum output of the boiler, the boiler would be forced to 'cycle'. Which essentially means to turn on and off to replicate a lower input as pictured above. The wider the gap between the load and the minimum output of the boiler, the more the boiler will 'cycle' and run times reduced.

Extreme cases of oversizing result in the boiler 'rapid cycling', although this can be also caused by a lack of flow round the system or scaling of the boiler.


Every time the boiler turns off the fan stops, the gas valve closes and pump may or may not also stop. Each component within any appliance is built to operate a minimum amount of times before failure. It's clear that operating these components more than necessary will lead to earlier required repairs, in engineering terms this is known as 'mean time before failure' (MTBF).

This stoping and starting of the boiler also leads to the boiler running hotter and cooler. As the materials expand and contract this gives thermal stress/thermal shock to the mechanical parts of the boiler and particularly where you have a joint with two dissimilar materials.

Part load efficiency

When boilers components i.e. the heat exchanger and the combustion chamber, are designed, they are sized to effectively transfer the maximum amount of heat as efficiently as possible. Both of these components are more efficient, bigger.

A larger combustion chamber gives more room for the natural gas and oxygen to evenly mix and give a more complete combustion/flame efficiency. A larger heat exchanger gives more chance for the heat to transfer into the heating system water.

When boilers modulate down these components stay the same size, meaning they effectively become oversized. This increases the heat exchanger's relative surface area and 'heat transfer coefficient'. The larger combustion chamber gives lower NOX levels and less unburned gases.

The graph above from Viessmann illustrates quite well the increase in efficiency from modulating the boiler, even from non-condensing boilers. However, you will notice there is a drop in efficiency once the output reaches less than 5% modulation or 1/20th of the modulation. We'll go further into the reasons for this at the end of the article.

The graph below also illustrates the efficiency gain when you now combine the effects of low-temperature efficiency and low load efficiency. Note the higher Efficiency for a '40/30' system. 40/30 refers to the flow and return temperatures.


There are many benefits of a low-temperature system, so much so that it deserved its own article on the benefits of low temperature heating systems.

Here's a quick breakdown anyway.

  • Slower corrosion rates within the system
  • Less Thermal Shock to the system and components
  • Better on the expansion vessels
  • Reduces cavitation at pump and fittings
  • Less noise/ creaking in the system
  • Increased comfort through reduced heat gradient in the room
  • Increased comfort through steady emitter output
  • Safer
  • Cleaner Air within the house
  • Less loss through pipes in unheated areas
  • higher comfort at lower room temperatures

Gas, oil and LPG boilers

  • Condensate can clean heat exchanger and ensure better heat transfer
  • More extracted latent heat from added condensing as shown in graph below

Heat pumps

  • Improved COP

Combining modulation and low-temperature efficiency

Heres some further graphs showing the combined effects of low modulation and low temperature together. More reading from the source here

Combination boilers and lower load properties

There's a bit of a pandemic in the UK of oversizing boilers. Even online calculators seem to hugely over exaggerate the heat load required. What exacerbates the problem is people looking at their older boiler sizes when replacing them.

Since older boilers where installed most properties have improved draft proofing, added double glasing and loft insulation, which of course reduces the amount of heat required.

What's more is that even the smallest domestic boilers are typically around 12kw and offer right up to 40kw. Most 3 bed homes are no more than around 10kW load when its -2°c outside. When its 10 degrees outside the load will be more like 5kw! (And its 10 degrees outside way more often than -2)

If you have a 30kW boiler and you don't want it to cycle most of the year you're better off having one with a half sensible modulation. If the property is a flat then half these numbers.

We are sure this offering of 12 - 40kW boilers is from a legacy of what used to be offered years ago before insulation levels were improved.

One reason you may requires a high output boiler however is for hot water. Particularly in flats where you have no room for a hot water cylinder and have to use a combination boiler, your boiler is sized solely to provide instantaneous hot water. This requires alot of energy.

Typically 26 to -30 kW boilers are used here for good flow rates. However a typical flat only has around 4kW load on a -2°c day (less than 1% of the year), a 10oc day will only have a 2kW load. And so the 30kW boiler, with minimum output of 6kW repeatedly cycles in heating.

Reduced electrical consumption

There's a law within engineering called the square rule. This essentially states that if we half the flow, we quarter the system resistance, when we quarter the system resistance you reduce power consumption to 1/8th. This is true for the pumps (you may have multiple pumps) and the fan. Electrical power consumption is not particularly high in boilers but this further illustrates the benefits.

Decreased standby losses

Every time a boiler turns off during a cycle the pump stays on in 'pump overrun mode' to help cool the heat exchanger. During this time the fan may also run in a fan 'post purge' to assist. During this time you are literally just blowing heat outside with no energy being added yo the system.

When the boiler refires it will do a quick 'post purge' to clear any products of combustion from the burner chamber before firing, as well as running the pump again wasting energy.

Even when the fan is not running and the pump is in overrun (typically 2-5 mins) the boilers heat exchanger is warm/hot, and because the 4" flue is nearly always above the heat exchanger simply leaks heat outside.

1 cycle may be a short amount of time, but the more often they happen, the more compounded the problem is.

Downsides to high boiler modulation

There are two main difficulties with trying to achieve a low output. Flame stability (keeping the flame lit) and keeping the burner cool.

How this is achieved is mainly by design of the burner. However a greater turndown beyond the limits of the burner can be achieved by introducing additional excess air. This has the effect of reducing the humidity of the combustion and inturn dramatically lowering the due point (See condensing theory).

Another issue with extreme turndown is the introduction of laminar flow. Laminar flow is where the flow of air or water is so slow that it creates an insulating boundary layer which can reduce heat exchnage.

Both of these issues will vary depending on design. For example, Viessmann developed a heat exchanger many years ago, but is still in use today in nearly all their boilers, that has 0.8mm flow paths for the combustion air. This is supposed to prevent 'core flows', which essentially means is too narrow to create an insulating boundary layer.

Similarly, some boilers have internal baffles to increase turbulence and heat exchange such as ATAG.

Don't forget to sign up to our newsletter for our latest articles!

Leave a Reply

Your email address will not be published. Required fields are marked *

Heat Geek is the one stop to find out everything from how to bleed a radiator to selecting the right boiler, we don’t have any bias and value the facts above everything else.
November 1, 2018
See all posts by author

Share this article

HeatGeek © 2020. All rights reserved.
Heat Geek is a participant in the Amazon Services LLC Associates Program, an affiliate advertising programme designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.
Vat number: 364541984
Company number: 11887015
linkedin facebook pinterest youtube rss twitter instagram facebook-blank rss-blank linkedin-blank pinterest youtube twitter instagram