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 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 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 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. 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 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 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'. 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 reduced.

Extreme cases of oversizing result in the boiler 'rapid cycling', although this can be also 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, 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 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.

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 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
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 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 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), 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 but is still in use today in nearly all their boilers, which 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.

boiler modulation diagram

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

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22 comments on “What is boiler modulation and boiler cycling?”

  1. So I expect the temperature dial on my boiler shows output not return temperature, so it is really not clear to me what may be an appropriate lower temperature to run my condenser boiler at for something like optimum performance and efficiency (allowing for assumptions about having a typical system etc)??

    1. It's always the lower the better. So just keep lowering until you have issues. Bare in mind it will need to be hotter in the depths of winter.

  2. A question about modulating boilers. Does modulating a boiler really make a difference in energy usage when comparing a condensing boiler ON-OFF vs condensing boiler modulating?
    The longevity is a different story but I am only interested in energy usage. A Condensing boilers is 97% efficient with the right return water temperature. By adding modulation how much could that increase the efficiency?

    1. Yes of course. If it cant modulate how will it maintain a return temp? It would simply continuously overheat.

  3. Great article. One point on the heat exchanger sizing: you state "A larger heat exchanger gives more chance for the heat to transfer into the heating system water." I'm afraid this is incorrect - bigger is not better for efficiency, in fact the opposite. With smaller channels you will generate higher velocities due to mass conservation which in turn leads to more turbulence. Higher turbulence is akin to higher heat transfer coefficients (due to higher Reynold's numbers). So equivalent heat transfer coefficients reduce with increase in area/size of the heat exchanger. It is a common misconception since a greater area allows for more heat transfer/duty, but it is actually less efficient - output/performance should be distinctly separated from each other as two different concepts and usually the inverse of each other. Two smaller heat exchangers in parallel would require less equivalent surface area than one bigger one to cover the same heat load for these reasons. The issue with having smaller channels is that the higher velocity also generates a higher pressure drop, and may also be less cost effective from a capex perspective.

    1. Yes, already aware of this thanks to Viessmann. There is a point (due to the viscosity of air) that allows greater modulation without loss. 0.8mm is the perfect channel (provided the channel is long is enough) for temp to stabilise into a good turbulent pattern.

  4. Hello all, we had our system totally rebuilt when we partially gutted our house. Insulation was improved but it's still nowhere as insulated as a newly built house. New heating system works well, no complaints. Much of the online content in English is UK oriented as radiant systems are less common in North America. The big difference in North America is it gets a lot colder here. Keeping return water as cool as possible keeps the condensing boiler condensing and achieving good efficiency as stated above. The best/only way to do this is by having lots of radiators in your house. These are amazingly expensive/overpriced on this side of the pond if not using slant fin rads. My experience is when it starts to get really cold, say below -10C / 14F it's almost impossible to keep return water below 130F / 55C because 170F supply water is needed to keep the house warm. But a temperature drop of 40-50F across the radiators just isn't going to happen. Of course, like I said I could add more rads but at $500 to $1000CAD a piece I'll never see the return on my investment through higher efficiently. My boiler flow rate is well matched (low lose header) to the system pump flow rate when the boiler pump is set to the lowest speed (1 of 3) and the system pump runs at 3 of 4. System runs between 3.5 to 5.5gpm. Sending about 0.5gpm to each rad. I wanted to hear other people's opinions and experiences on this situation. Is anyone able to keep return water at 120-125F when it's 5F outside? Is this because you have lots of big old cast iron rads? Further thoughts?

    1. Hi Julian

      Yes, our victorian houses in the UK, once the insulation is upgraded to basic levels sit on a Viessmann weather compensation curve of 1. This will sit at 130F flow temp when 5f outside, which will condense. When in the rest of the year will run lower and lower temp and condense evermore. If you have underfloor heating the curve is more like 0.6 which is very low.

      Importantly though... it doesn't HAVE to condense in the absolute coldest days, it can run hotter then, but as long as you have weather compensation it will condense as much as possible for the rest of the heating season.

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  9. Can the boiler still modulate reasonably effectively if it does not have the controls to rely on? For example, Hive is an on/off control does this prevent modulation or will the boiler still do its best to modulate?
    Furthermore, if your using an S Plan system boiler does that prevent modulation due to the hot water temperature requirement?
    I am asking because all the fitters I talk to just want to do a simple swap without any further changes. I would really appreciate any advice you have?

    1. Yes it will 'modulate' but it's far too late and always end up at far too higher temperatures. Something like a Hive is an absolute enemy of condensing, because it forces the boiler on and off it will rarely allow for long enough run times and because of that the boiler will be set above 70°c which prevents optimum efficiency. To the second question - Yes that can be true, unless the boiler is capable of knowing whether it is running on heating or hot water exactly that problem occurs and condensing is out the window (or rather the flue) again.

  10. We had a Biasi 35kw boiler installed and I found the 'effective output' setting in the service manual.

    Reduced the boiler output to around 18kw, looking to decrease more after further experiment and gas meter readings. The heating water takes around 10 minutes longer to hit temperature but the plume outside is now no more than a kettle just boiled very slow and not a long jet of steam hitting next door.

    Gasses stay in the heat exchanger longer and allow more heat to be exchanged and not 'blown' straight through the boiler so quick, its heat is lost.

    Boiler outbound temp is set at a point return water is 49 degrees. Advice for others, obtain a small £2 digital temperature guage and clip to the return water pipe - use this to set the outbound temp to ensure water returns in the condensing zone. Boiler no longer runs full power to get CH water up to temperature when first switched on, no cycling and modulates down to steady running.

    35kw is available for hot water at the taps and again, by running taps and especially bath water just enough to keep the boiler on and modulated itself right down is more efficient also.

  11. I now understand why most UK boiler installations are not setup correctly.
    It's too easy for the installer to fit a new boiler the same output power size as the old boiler.
    They don't see the benefits of properly investigating the house energy demands...too many variables...just bang in a new boiler and bash onto the next install... really sad situation.

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.

Heating Sensei

November 1, 2018
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