Electronic Ignition Gas Furnaces
The next step up from the standing pilot gas furnace is the electronically controlled furnaces. These gas furnaces are more efficient than the standing pilot furnaces but more complex in design. There are three types of electronic ignition gas furnaces: the intermittent spark pilot, the intermittent hot surface ignition pilot (Honeywell smart valve and ignition assembly) and the hot surface ignition furnaces. While the three are different they all offer higher efficiencies and are both controlled by solid-state circuit boards. Many of these furnaces also have higher efficient heat exchangers. When the intermittent pilot or hot surface ignition controls are combined with improved and more efficient heat exchangers, the result is a furnace that can be 80 percent or more efficient versus the older standing pilot gas furnaces which average only 60 to 70 percent efficient. The photo on the left shows a pool heater with electronic ignition. The pool heater was new and being inspected for proper installation.
Intermittent Spark-Ignition Pilot: Furnace Ignition
The intermittent spark-ignition pilot gas furnace, upon a call for heat (when the thermostat closes), will go through a trial for ignition. (Note that the spark generated in this process can exceed 10,000 volts). When a pilot flame is proven (normally through flame rectification) the electronic module or circuit board sends a signal to the main valve, in the gas valve, to open. The pilot lights the main burner and it burns until the thermostat is satisfied. When the thermostat is satisfied, the electronic module or circuit board stops all ignition processes including the pilot.
Intermittent Hot Surface-Ignition Pilot: Furnace Ignition
Honeywell engineers came up with a variation between the hot surface ignition and the intermittent pilot. They simply put a small hot surface igniter on a pilot light assembly along with the flame sensor. They then put all the solid state circuitry components necessary to light the pilot light and sense the flame on the pilot inside the gas valve (versus have a separate circuit board mounted in another panel away from the mechanical gas controls). Aside from a few bugs in initial production models, the Honeywell Smart Valve works very well. The sequence of operation for these smart valves is the same as the sequence of operation for the intermittent pilot light.
Hot Surface Ignition: Furnace Ignition
The Hot Surface Ignition Furnace has no pilot light. It lights the burners direct. Therefore it is also referred to at times as Direct Ignition. The igniter is made of silicon-carbon and glows red hot when power is applied to it. The igniter is positioned above the burner. Upon a call for heat, the furnace enters a sequence of operation. Normally, after the purge cycle, power is applied to the igniter. It glows red hot the gas valve opens allowing gas to flow. When the gas makes contact with the igniter, ignition occurs. Once flame is proven (usually with flame rectification), power to the igniter is discontinued. The burners continue to burn until the thermostat satisfies when the furnace goes into shutdown mode and shuts itself down until the next call for heat.
HVAC Furnace Efficiency Defined
The percentage of efficiency as described here means that if a furnace is 80 percent efficient then 80 percent of the heat it produces is usable heat beneficial to the home and the other 20 percent is vented into the atmosphere through the flue with the noxious gases that are a result of the combustion process. Thus, the higher the efficiency of an HVAC furnace the lower the temperature of the flue gases simply because much of the usable heat produced by the HVAC furnace is used in the home instead of vented into the atmosphere. This causes two separate functions that must be overcome by mechanical means. Number one, since the temperature of the vent gases is negligible and gas typically contains approximately five percent moisture content, the moisture is condensed through the process of combustion and needs a place to drain. Normally, this drain is tied into the same drain that drains the Air Conditioning condensation but could be drained in another location. Number two, because so much heat is removed and used in the home, the pressure or force needed to exhaust the flue gases is too small to force the noxious gases outside. This is the main reason why all high efficiency furnaces have induced draft motors attached to them. The inducers motor forces a draft through the entire system from the beginning of the combustion process to the end where the noxious gases are exhausted. Modern electronic HVAC furnaces are more efficient because you don’t need to keep a pilot light burning 24/7 and there has been major changes made in heat exchangers. Changing your current HVAC furnace from a standing pilot furnace to a modern electronic HVAC furnace can make a difference in your comfort and the amount you pay on your fuel bill next winter.
High Efficiency Condensing Gas Furnaces
With today’s high prices of all fuels it is not only good to conserve as much as we can for conservation’s sake but it also makes good financial sense especially for the long run. Many people who replace their HVAC furnaces sometimes do not give due consideration to efficiencies of the furnace the purchase. They often look at the HVAC salesman and balk when they see the higher price for the higher efficiency furnace. Then a few years later whether it be market forces, an act of God, or a refinery accident and the price skyrockets they often regret not getting the higher efficiency furnace. Especially after getting that big gas bill that would have been a lot less if they had purchased the higher efficiency model. Even without the volatile cost of fuels the consumer would have always been better financially purchasing the higher efficiency HVAC furnace. On a month to month basis these HVAC furnaces will pay for themselves fairly quickly.
Two Heat Exchangers
An HVAC condensing gas furnace has two heat exchangers and makes use of so much of the heat produced that it actually condenses the moisture out of the gases. Where does this moisture come from? Natural gas has approximately 5 percent moisture content. Additionally, if installed properly, the HVAC condensing gas furnace will pull all the combustion air it needs from outside of the home or business. The bonus to this is that you don’t have to worry about an exhaust fan(s) pulling a negative pressure in your home while the HVAC furnace is running. This is uncommon but not unheard of. Fire consumes air and non-condensing furnaces usually use the air from inside the dwelling for this combustion air. There are codes concerning combustion air but over time things change and people renovate. With the HVAC condensing gas furnace it has a dedicated pipe (typically PVC) which pulls air directly from the outside (using a forced or induced draft fan). A condensing gas furnace does not need a flue or chimney to vent its gases. It typically uses PVC pipe for the flue. The PVC is necessary because the combustion gases would have a highly corrosive effect on the typical metal flue (aside from stainless steel which is what the exchangers are constructed of) and caustic effects on the masonry inside of chimneys. Somewhere near this PVC vent pipe there should be a drain to drain off the condensed moisture. Usually these drains drain off into the same drain that condensation from the evaporator coils drain to. The combustion gases have so much of the heat zapped from it that it is necessary force these gases out through the PVC flue pipe. These gases simply don’t have enough heat left in them to help them rise out of the flue to the outside. The same fan that pulled the combustion air from the outside of the house is designed to push the combustion gases out through the PVC flue pipe. The HVAC condensing gas furnace is most likely your best option for heating efficiency that is currently available aside from an HVAC geothermal heat pump.
Other Heating Applications Using This Technology
Additionally, some HVAC boiler manufacturers and HVAC oil furnace manufacturers have adopted this technology for use in the designs of their equipment. You will see these HVAC furnaces and boilers advertised as 90 plus or 95 plus. That means that 90 percent plus of the heat they produce is actually delivered into the dwelling. Amazing when you compare these furnaces to furnaces from the 80′s and early 90′s which where on average 60 percent efficient.
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