When it comes to boilers and other heating systems, there are two main factors that all end users want to know - the price and the efficiency rating. Although, for sanity's sake we won't delve into the precarious topic of boiler prices, we will take a look at efficiency as this is often misunderstood by users and misused by marketing people. What efficiency are we really talking about? It all depends on the point at which you make the measurement.
In plain terms, if an oil combi boiler burns one litre of oil to produce 100,000 BTU of heat, we need to know what's the theoretical capacity of that fuel. Now, typical fuel oil has a capacity to provide 31,000 BTU per litre but in order to actually generate and consume that heat, we would have to invent an ideal boiler with 100% efficiency. In reality, we divide the produced heat with the potential heat:
η = Output/Input x 100
η = 22,000 / 31,000 x 100 = 71%
That's not a very efficient boiler, although for an oil boiler it's not far from the maximum you can get.
What we just calculated is called direct efficiency. Another method is to try to calculate indirect efficiency of a heating system which is based around the concept of finding individual loss happening in a boiler and then subtracting them from 100. The losses include blowdown, radiation and stack losses amongst others. The more complicated is the construction of a boiler, the more different tests you have to carry to find all heat losses involved.
Boiler manufacturers obviously use the Indirect method as it allows them to optimise the system and find out which parts need to go back to the drawing board.
If we want to delve deeper into the water-based heating systems, we have to look at the so-called boiler wire to water efficiency. Technically, this term represents the amount of energy that is transferred to water that heats the radiator divided by the energy input that the system receives. This is an important metric when you need to calculate efficiency of a boiler that has a water pump because a certain amount of loss occurs in the pump.
WWE = Em x Ep x 100 (%)
where Em is motor efficiency and Ep is pump efficiency. One of the factors that affect the heating system is water friction. The more you can reduce friction, the better boiler efficiency you can achieve, that's why so much time and money is spent researching and developing new materials and internal profiles for pipework. We could significantly increase the efficiency of a heating system by inventing a new better type of piping.
Also, if the central heating system pump is not optimised for the pipework that it serves, the wire to water efficiency of the whole system will be lower.
You might ask, if it is productive to try to increase the efficiency of boiler water pumps as on paper their metrics seem rather unimpressive. An average pump reaches just 20%.
Now if we used expensive materials and high tech, we could probably increase it by another 10% but it would mean the pump costs $100 more than a regular one. Converting these 10% into energy savings, you'd save approximately $4 per year. So you would need to keep the pump in service for 25 years just to recoup the initial cost. This is why most of the efforts go into perfecting the pipework and improving the efficiency of heat exchangers.