Not all condensing boilers condense all of the time

It has been mandatory to fit a condensing gas boiler for all new domestic installations since 2005 – a fact that has led to the condensing boiler market in this sector experiencing a dramatic expansion – with sales more than double those of previous years. The market is projected to show further growth predicted to be 7.9 million condensing heating systems in the UK alone by 2010.
In our own commercial sector, the 2006 revisions to Part L of the building regulations are generally encouraging innovative solutions to help reduce the carbon footprint of buildings. Hence, the application of a high efficiency, condensing boiler, together with a well-insulated hot water storage tank, is a means of complying with the regulations in a great many commercial properties.
Although at first glance this seems like an appropriate solution, and certainly will keep any installation in line with the regulations, there are a number of important considerations which tend to get overlooked. In fact, such an installation is often likely to be little more efficient than one fired by a less efficient non-condensing boiler. Why? Well, the disadvantage is that when space heating is not needed, the boiler will still be required to provide heat to the hot water storage tank whose return feed, for most of the time, will be at a relatively high temperature as it re-circulates from the heating coil in the tank.
The significance of this is that at such temperatures, condensing boilers that achieve high operating efficiencies by extracting latent heat from the flue gases become physically unable to condense. The problem is that they only condense when their input water temperature is less than 57ºC (gas fired), or 47ºC (oil fired) and to achieve the claimed efficiencies of 107% or even 109%, the input water temperatures need to be as low as 30ºC. When they are maintaining hot water storage tanks at 60ºC or more, a legal requirement as it is a temperature necessary to combat Legionella etc, they don’t condense at all, with the result that the actual efficiency is some 10% lower than the claimed efficiency.
An equally important effect on the useful efficiency of storage water heaters is that caused by the standby losses. Each storage tank maintained at temperature is continuously losing valuable heat. Depending on circumstances, the standby loss can account for 5% or 10% of the annual fuel consumption.
This operating anomaly with conventional condensing boilers is probably part of the reason why the Non Domestic Heating, Cooling and Ventilation Compliance Guide (a supporting document to the Building Regulations) explicitly recognises that the gross efficiency of a direct-fired water heater is allowed to be lower than that of the boiler used by an indirect hot water heating system. The compliance guide requires a minimum gross seasonal efficiency for direct gas-fired water heaters of only 73%, compared with the minimum efficiency of a boiler being used to indirectly generate hot water of 90%.
Using condensing technology, the penalty for indirect storage heating compared to direct-fired water heaters should be considerably higher than the 7% for non-condensing appliances. Direct-fired water heaters always operate at typical incoming water temperatures of 10ºC, thus ensuring maximum condensation and maximum efficiency.  Condensing boilers, used for heating up storage tanks operate at input water temperatures up to 60ºC. You can guess how little condensation will occur and what this means for the useful efficiency.
A condensing water heater attempts to recover the heat loss of a non-condensing water appliance through the flue by deliberately inducing condensation (recovering the latent heat of the flue gas) This is achieved by adding a second or extended heat exchanger in the flue that exchanges heat with the incoming water – hence condensing systems operate most efficiently with incoming water temperatures below 57°C. Condensation will potentially release approximately another 3.5MJ per m3 natural gas. The maximum effect, however, is only achieved at input water temperatures around 30ºC. The additional performance of a condensing water heater is not only due to the recovery of latent heat, but also to the lowering of the flue gas exit temperatures. As the flue gas exit temperature falls there will also be greater amounts of condensation hence again increasing the overall efficiency. Provided that the input water temperatures are low enough, the resulting temperature of the flue gases are typically 50-60°C.
It has been estimated that the loss of usable energy for traditional condensing boilers linked in with a secondary hot water storage system (i.e. the loss associated with having to maintain cylinders of hot water at high storage temperatures however well insulated) may be 3kWh per day, and that is without considering not recovering latent heat from the flue gas resulting from a water input temperature exceeding 57°C. The same loss of efficiency will be evident with any storage water condensing heating system.
There is a simple alternative however which overcomes all these shortcomings and as a result is much more energy efficient – the continuous flow water heater. Such devices suffer none of these problems as water is heated only while it is flowing; a flow sensor measures the rate of water flow and sends a signal to the controller. The controller monitors the temperature of the water coming into the system and calculates how much air and gas is required to achieve the combustion necessary for the desired increase in water temperature.
Typically a water outlet low thermal inertia thermistor (i.e. one with a fast response time) measures the temperature of the water being produced, and the controller alters the combustion air and gas rates to ensure a stoichiometric air to gas volume ratio to maintain the desired water outlet temperature. Should the water flow rate exceed the capacity of the water heater, a control valve will reduce the throughput of water to maintain the set temperature. A variable speed fan complete with a fan rotation sensor is used in conjunction with a modulating gas valve to supply the correct mixture of air and gas.
Feedback information provides virtually instant error signals to the gas and water controller so that it is able to provide rapid responses to a change in demand for hot water or a change in temperature requirements. The system maximises system efficiency and by tightly controlling the combustion process also minimises the emissions of NOx. Safety is assured by continuous monitoring of the system components by the controller, including both a flame sensor checking for correct ignition and exhaust flow sensing.
Here though is where the continuous flow water heater system comes into its own; it is constantly fed with water from the mains supply (generally at approximately 10°C) and that means that it will be working in condensing mode at all times. Not only does this water heater maintain its maximum efficiency by condensing all of the time, but it only uses energy when hot water is required.
The application of condensing continuous flow water heaters, therefore, can provide a highly efficient and controllable year round source of hot water that will reduce a building’s operational carbon footprint considerably in line with the goals of Part L. Moreover, it will do so all of the time rather than for only part of it. It will also offset the need (and the space) for hot water storage, pipe work and associated controls, and will eliminate the energy losses associated with maintaining stored water at the correct temperature. These heaters are also readily available as external units for mounting outside a building, rather like air conditioning units and similar devices. This further improves the cost effectiveness of their use, by removing the need for a boiler room and even a flue, and in the process minimising any pipe work needed for the draining of condensate produced.