Boiler modulation (also called boiler turn down ratio) is arguably the single most important factor in a boiler or heat pump for efficient and money saving function.
Lets assume the average heat requirements of a house is approximately 7KW (this means it needs to have 7KW of heat pumped into the house to maintain a given internal temperature when it is cold outside).
Houses are designed to be heated to a given temperature inside, often 21 degrees centigrade. The amount of heat required to achieve this is predominantly guided by the outside temperature, often -2 degrees is used as a reference point to design to.
Your house loses heat dependant on the difference between the inside and outside temperatures and that changes throughout the year
This means a difference of 23 degrees between our target room temperature and the outside temperature.
7KW or 7000 watts means that for every degree rise we want inside we need to add 304 watts of heat.
Whilst we design systems for the coldest days of the year, the reality is that the average annual temperature is much warmer, approximately 10 degrees.
This makes the difference between outdoor and indoor targets much lower, 11 degrees. 11 x 304 watts means we need 3347 watts form our boiler or heat pump for most of the year, not it’s full output.
When it is 15 degrees outside then we only need 1826 watts, 17 degrees outside needs 1217 watts etc.
Viessmann are the Rolls Royce of domestic boilers
With the maths understand it should now be clear that we only want to use enough gas/electricity to provide the correct amount of heat so why would we accept a boiler that burns gas for 5, 6 or 7kw heat when we only need maybe 2kw?
Limited Options
There are only a limited few manufacturers that build boilers that are smart enough to lower the output enough to meet the real demands of a property.
Most low to mid end boilers (and a lot of expensive ones) can only lower the output to 5, 6 or even 7kw! This means that when it is not freezing outside (most of the year) and it is 5,10 or 15 degrees they still burn as much gas as they would when it is -2. CRAZY.
Low budget boilers cost a lot more to run…they often use double the gas needed. CRAZY
The likes of the Viessmann Vitodens 200 range can modulate down to approximately 2kw…almost perfect for the average sized home for most of the year. Yes, they are more costly up front but will cost far less over their life to run. Not to mention they are the best built boilers on the market so will rarely ever go wrong and need repairs.
What does this mean for you?
It means you are using a lot more gas than you need to every year and more gas means more money being spent and wasted due to most boiler manufacturers producing poorly designed or specified boilers.
Add in modulating controls like weather compensation or room sensor load compensation and you have even more efficiency.