Condensing Boilers and Heat Pumps
Condensing boilers are the most efficient type of boiler on the market. Unlike conventional boilers, they can extract the latent heat from natural gas by condensing the water vapour that results from burning natural gas. This water vapour goes up the chimney in conventional boiler plants. Condensation occurs in the heat exchanger where the hot gases transfer their heat to the system water being heated. In order for condensation to occur, the system water coming into the boiler has to be much cooler than in conventional systems – under 120° F. For maximum efficiency, cooler is better. Less than 100°F is ideal. This is illustrated in the graph to the right. Non-condensing boilers increase slightly in efficiency until the inlet or return water temperature reaches about 132°F which is about the temperature at which the flue gases start to condense (the exact temperature depends on factors such as elevation and the CO2 content of the flue gases).
Conventional boilers will also condense the flue gases if the input water is cool enough. However, they are not built to take advantage of this – the condensate is simply re-evaporated, absorbing as much heat as was captured during condensation. The condensate is also acidic enough to damage the components of a conventional boiler over time. (Inadvertent condensation is a major cause of failure in conventional boilers) Condensing boilers are specially built to resist being damaged. They often have stainless steel heat exchangers or heat exchangers made of corrosion resistant alloys. They all have the capacity to drain the condensate from the boiler.
Water-source heat pump systems are an ideal application for condensing boilers. Many new condos and commercial buildings use water-source heat pumps for heating and cooling. Water source heat pumps can be thought of as reversible refrigeration units. They ‘pump’ heat from a piped water loop to a fan-coil unit for heating or reject heat from the fan-coil unit into the water loop for cooling. The water in the piped loop is usually 70° to 80°F. If the water becomes too hot, it is run through a cooling tower to cool it. If gets too cool, it is heated by a boiler plant. Usually the temperature of the water in this type of system is between 70° and 80°F. In this temperature range, condensing boilers can extract over 95% of the thermal energy contained in natural gas. By comparison, the most efficient non-condensing boilers are about 84% efficient. These are properly called mid-efficiency boilers. They accurately mix fuel and air for good combustion efficiency, but are not designed for condensation. They will have power burners or fan-assisted burners. Atmospheric boilers are less efficient, usually in the mid to high 70s when new. Atmospheric boilers burn natural gas in open air, much like a barbeque. They are less efficient because they draw in and unnecessarily heat a lot of excess air. A condensing boiler will be at about 20% more efficient that an atmospheric boiler. Yet we mostly see atmospheric boilers used in heat pump systems.
There is another reason to use condensing boilers. In order to prevent the damagingly cool water from entering conventional boilers in a heat pump system some piping magic is required. Usually there is a shunt that takes some of the hot water from the supply side of the boiler and mixes it with the return water to raise the temperature of the water returning to the boiler. This is not fool proof, however. Slugs of cool water can still get to the boiler under some conditions. Although they are energy hogs, atmospheric boilers are less susceptible to being damaged by this than mid-efficiency boilers. Condensing boilers, however, are extremely robust under these conditions. No special piping is required because they are designed for these conditions.
The table below shows the economics of the replacing an atmospheric boiler with a new condensing boiler in a recent project. This heat pump system serves 133 apartment units. The existing boiler plant consists of three 15 year-old atmospheric boilers with a combined output of 2.3 million btuh. The actual design load is about 1.5 million btuh. The plan was to replace one of the atmospheric boilers with a condensing boiler, leaving the remaining boilers as back-up.
| SAVINGS ATTRIBUTABLE FOR REPLACING AN ATMOSPHERIC BOILER WITH A CONDENSING BOILER IN A HEAT PUMP SYSTEM. | |
|---|---|
| Annual gas consumption for heating (M3) | 170,865 |
| Estimated reduction in gas consumption | 18% |
| Estimated annual savings (M3) | 30,756 |
| Estimated cost of gas per M3 | $0.26 |
| Estimated savings | $7,996 |
| Estimated cost of 2 million btuh PK Mach condensing boiler | $30,433 |
| Installation cost, including venting | $24,000 |
| Total installed cost | $54,433 |
| Simple payback in years | 6.8 |
Of course, the savings will likely be greater because of the age of the boilers being replaced and because of superior turn-down ratio of this model of condensing boiler will reduce cycling and result in more even temperature control.
Most major manufacturers are now making condensing boilers. They are also available in larger sizes than previously. They are clearly the best type of boilers for low temperature applications like water source heat pump systems and radiant floor or ceiling panel heating. With proper design, condensing boilers can also be cost effective when used for hydronic make-up air units, pool water heating, heating with panel or cast-iron radiators, and domestic hot water.
Andrew Taylor
Weinstein Taylor & Associates