High-efficiency boilers are not as common as high-efficiency furnaces. High-efficiency boilers can be twice the cost of regular boilers, while high-efficiency furnaces cost only 30 to 40 percent more than lower-efficiency systems. There are fewer manufacturers of high-efficiency boiler equipment. Let's look at some of the advantages and disadvantages of high-efficiency hot water heating.
High-efficiency equipment needs no traditional chimneys. The combustion products are vented out through the house wall, typically through a plastic or metal vent.
High-efficiency boilers are typically direct vent systems. Not only do the exhaust gases go straight through the wall, but combustion air is piped in from outside, and the combustion chamber is sealed from the house air.
The operating costs of high-efficiency boilers are considerably lower than conventional boilers. Seasonal efficiencies in the range of 85% to 95% are possible. The seasonal efficiency of conventional boilers may be 55% to 65%.
A disadvantage of high-efficiency boilers is the high installation cost.
Another disadvantage is the corrosion issue that comes up anytime we deal with condensation. In a high-efficiency boiler, just like a high-efficiency furnace, corrosion may occur because it produces an acidic condensate.
Maintenance costs for high-efficiency boilers are typically much higher than with conventional equipment. Just like high-efficiency furnaces, they are complex and full of high-tech components. So far, the reliability of high-efficiency boilers has not been great. The exhaust gas path through the heat exchanger is longer and more restricted than with conventional heat exchangers. We expect problems with clogged heat exchangers.
Another common difficulty with high-efficiency boilers is the incompatibility of the boiler with the existing distribution system. High-efficiency furnaces use the latent heat of vaporization to grab heat from the exhaust gases, to achieve their high-efficiency ratings. The combustion products of natural gas condense when the flue gas temperature drops to roughly 125F. If the flue gases are hotter than this, the boiler will not condense, and the efficiency is diminished.
Many radiator systems are designed to be supplied with water leaving the boiler at 150F to 200F. The temperature drop as the water goes through the system may be 20F to 30F. This is a typical temperature rise across a boiler, as well.
The return water temperature in many piping systems may well be higher than 125F. Obviously, it's tough to cool the exhaust gases below the dew point if the water side of the heat exchanger is hotter than the dew point. Sometimes we get condensation at startup, but none when the system heats up to steady state.
Another difficulty encountered with high-efficiency boilers is the small heat exchanger volumes. Traditional boilers held several gallons of water. In most high-efficiency boilers, the volume is much smaller. This can cause problems. The rate of water flow through the boiler is critical on high-efficiency systems. The boiler may overheat if the water flow rate is not adequate.
Typically, the water flow requirements of high-efficiency boilers are considerably higher than conventional boilers. As the water must move through the pipes faster, increasing the friction losses in the piping, the pump capacity of the new boiler may have to be considerably larger than the old pump. This needs to be sized for the existing distribution system.
It's not easy for the boiler manufacturer to determine what pump is needed for all systems. If the boiler overheats because the water flow is too slow, the boiler will short cycle. This means that the burner will go off and on several times before the thermostat is satisfied. This shortens the life of the heat exchanger and wears out the mechanical components in the system faster.
Construction and Systems
High-efficiency boilers use many of the same components that high-efficiency furnaces do. There is often a second heat exchanger. In addition, there is some form of intermittent ignition, and a low temperature venting system. Because the ignition systems are the same, the safety controls are also similar.
Boilers tend to differ from furnaces in that there are some forced-draft, high- efficiency boilers. So far, forced draft technology has not been widely used in high- efficiency furnaces.
Heat exchanger material is also different in some instances. Stainless steel is a common heat exchanger material for both boilers and furnaces, but some boilers are also made from copper-nickel alloys (e.g.
). These alloys are more corrosion-resistant than stainless steel, and have good thermal conducting properties.
Pulse combustion is used on the Hydrotherm HydroPulse or MultiPulse boiler, a high-efficiency hot water system. This boiler uses the same combustion process as the Lennox Pulse furnace. There is no burner, no pilot, no vent connector and no chimney.
This is a direct vent system which uses PVC pipe (aluminum dryer vent or galvanized steel is also used in some areas) to bring combustion air from the outside into the sealed combustion chamber. Exhaust is sidewall-vented through CPVC pipe, typically 1.5 to 3 inch diameter. The pipe size depends on the boiler capacity and length of the vent. Both intake and vent pipes should slope down toward the boiler at 0.25 inch per foot of length on the horizontal runs. Piping should be supported every 5 feet (in Canada, every 3 feet).
The HydroPulse is a condensing boiler and uses condensate drain piping.
HydroPulse boilers, like Pulse furnaces, can be noisy. Vibration damping connectors on the distribution piping are often used to minimize the noise and vibration throughout the house. Mufflers can be used on the exterior of the house to reduce the outdoor noise.
Condensing boilers have efficiency ratings of over 90%. Non-condensing or partially condensing boilers have efficiencies in the 80% to 88% range.