An Example - HHV vs LHV
As a supplier on infrared heating systems, I am frequently asked what the efficiency of a gas fired infrared heater is by people including engineering consultants, contractors and end users alike. Lacking a defined efficiency rating system for this category of product here in North America (such as AFUE thermal combustion ratings for forced air systems), I enter into a long, technical explanation of the fundamental differences between the infrared heating mechanism, and warm air heating systems in buildings. I also suggest that a new rating system based more closely on what is used in the EU, called a radiant factor, is in the works which will at least allow different suppliers of infrared heaters to measure themselves against, letting the aforementioned groups of people make educated choices where performance is concerned. Except a radiant factor will inevitably be directly compared to a thermal efficiency, which is an unfair comparison, because it is comparing apples to oranges.
Here's a good example of how these rating systems and efficiency claims can be skewed to ones advantage; namely comparing HHV to LHV, and how a higher claim can be cherry picked and used in exaggerated ways on sales literature. So what is HHV, and what is LHV, and how do they differ?
HHV is the higher heating value of the incoming gas, or fuel, and includes the sensible and latent heat of vaporization for water created by the combustion process, where water is condensed out of the products of combustion. This efficiency number is typically lower than LHV (described below), but is more representative of reality.
LHV is the lower heating value of the incoming gas or fuel, where water remains as vapour (or steam) and is not condensed into liquid form. As a result, the latent heat of vaporization of water in the fuel and the reaction products are not recovered, creating a higher thermal efficiency - in the range of 11 - 12% higher than HHV. An HHV of 78% represents an equivalent heater performance using LHV of 90%. In fact, it is common to see efficiency claims using LHV well in excess of 100%!
If you wanted to make your lower performing product look better on paper, you may be tempted to use the LHV value, because who would know the difference? Well guess what - now you do! This is also why you can see varying performance claims, expressed in percentages, appear on competitors sales literature. As with many things in this world, a closer examination of the data will usually illuminate what's really going on. No better example of this exists than the case for global warming which has been foisted on an unsuspecting public through data manipulation by people and agencies with their own agendas! But I digress - the purpose of this article is about heating equipment efficiencies and how they can be expressed to ones advantage.