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 for 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 have fewer 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 boiler minimum output, the boiler begins to cycle to replicate a smaller output. As part of this, the boiler will overheat and underheat.

heating load profile diagram

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

A boiler's 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 a ratio of 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. You may even find some of the smaller boilers' minimum output is the same as the boiler's 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 the 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 a deep breath*

Longer Run Times / Lower Cycle Rate

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

Longer run times essentially mean 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'. Boiler cycling is where the boiler turns 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 will be reduced.

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

degradation gross boiler efficiency due to boiler cycling diagram

Boiler Wear and Tear

Every time the boiler turns off the fan stops, the gas valve closes and the 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 stopping and starting of the boiler also lead 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 and 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 fewer unburned gases.

boiler part-load efficiency graph

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 combining the effects of low-temperature and load efficiency. Note the higher Efficiency for a '40/30' system. 40/30 refers to the flow and return temperatures.

standard efficiency levels for boiler designs diagram

Ability to Run the System at Lower Temperatures

There are many benefits of a low-temperature system, so much so that it deserves 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 is shown in the graph below

Heat pumps

  • Improved COP
return water temperature versus efficiency for condensing boilers diagram

Combining Modulation and Low-temperature Efficiency

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

thermal efficiency graph with return water temperature and efficiency
thermal efficiency diagram depicting boiler efficiency and inlet water temperature

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 were installed most properties have improved draft proofing, added double glazing 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 it's -2°c outside. When it's 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 require 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 a lot 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), and a 10oc day will only have a 2kW load. And so the 30kW boiler, with a 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 to 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 the 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 in turn 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 that can reduce heat exchange.

Both of these issues will vary depending on the design. For example, Viessmann developed a heat exchanger many years ago that is still in use today in nearly all their boilers, which have 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.

boiler modulation diagram

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

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    Heating Sensei

    November 1, 2018
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