Eric Crose has personally been working with gas furnaces since 1988. During that time he has seen many changes with the design and function of gas furnaces. These changes have lead to much greater efficiency.
During a maintenance service, we will do a heat exchanger integrity test, we will clean blower area, flush the condensate drainage system (one of the most crucial aspects of a new furnace. Lack of care in this area often leads to inconvenient breakdowns, often in the coldest conditions.), filter check, flame rod cleaning, and a heat rise check. We also check the error history, a feature on most new integrated control boards.
AND, please feel free to ask our knowledgable techs what we are doing and why. Education is power.
The Evolution of Gas Furnaces
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How Were Older Homes Heated?
Houses were airy, fuel was plentiful, and labour was available so shovel the coal in. An old coal furnace may have been under 50% efficient with a very high stack temperature. The temperature of the flue gases could be as high as 800 degrees!
NOTE: There are several definitions and types of efficiency, but the one we are concerned with is thermal efficiency. In layman's terms, the BTU of the fuel is the heat content. So if you were to burn a full cord of Maple, (in a very very very big fire) and it were to burn completely (almost impossible), it would produce 22.7 million BTUs. Essentially, that heat will go in two directions, one is to heat you and your house, the other is up up and away into the air through the chimney. Not really helping you or your family stay warm. The important efficiency for you is the amount of useful heat divided by the original amount that you get.
Let's talk gas furnace for realistic numbers. One cubic foot of natural gas has a heat capacity of 1000 BTUs. As one cubic foot of gas is burned in the high efficient gas furnace I just sold and installed for you. 970 BTUs goes to make you warm while only 30 btus go out the vent, because this is a 97% efficient furnace. That is a modern furnace - very efficient.
Back 30 - 100 years ago, the old furnaces using coal, wood, and oil a bit later, produced a large amount of BTUs per hour but had very poor efficiency which let a large amount of heat up the chimney. Chimneys had to be very large, made of fire brick or stone to withstand the heat. Whatever was left over heated you.
As homes became tighter and tighter, and furnaces more efficient, the amount of heat and temperature of the flue products dropped. As the heat up the chimney reduced, chimneys became smaller and had to become lined to reduce cooling.
Remember, hot exhaust goes up, just like hot air. Just as a burning bonfire in the backyard, the hot smoke goes up until the wee hours of the morning when the fire is almost out and the temperature of the exhaust falls until the smoke just hovers.
Another problem with cooling exhaust temperatures is the water vapour as part of it. One of the products of combustion in the burning of a fossil fuel (natural gas, oil, propane, wood, coal, gasoline, etc,) is water vapour. In a wood fire, you won't see it because the heat keeps it vapour, but as the exhaust cools, the vapour condenses. For example, first thing in the morning when you start the car and things are cold, water can drip from the exhaust pipe.
The problem back then with this is, as furnaces became more efficient and exhaust temperature dropped, the water vapor would condense into water in the chimneys, mixing with other molecules in the fuel the hot exhaust causing a mild acidic solution. Go out for a walk this afternoon in an older area of town to see the large brick buildings with chimneys. You might see, 3 or 4 feet from the top of the chimney the mortar and bricks will be deteriorating. This is the condensing line of the water vapor in the chimney.
How Are Modern Furnaces More Efficient?
Fast forwarding, homes became more airtight and furnaces became more efficient. Chimneys deteriorated, then had to be lined with clay tile, then lined with a metal liner, dropping the cross-sectional area of the flue for the smaller, more efficient furnaces and to protect from the condensation.
Now the efficiency has become so high, with so much of the heat going into the house and so little going outside, that the water vapor in the exhaust is condensing in the furnace itself.
New furnaces are greater than 90% efficient, require a means to get rid of the water produced, and vent out the side wall powered by an exhaust fan (no heat means no stack effect with the hot exhaust going up a chimney.
What Are Side Effects of Tight Modern Homes?
This new tighter housing structure has created a new problem though. Fossil fuel requires lots of air to burn efficiently and completely. Back in the late '80s, new housing really started making advancements in sealing and insulation technology with the first "R2000" homes and homeowners were aware of the "benefits" in sealing the home.
Our 1st Customer has an older 30-year-old furnace and water heater, maybe 70% efficient requiring lots of air to burn.
Our customer wants to drop his fuel bill, so insulates and seals the home but still has a chimney for the two pieces of equipment. Now the home does not have enough air infiltration to support the combustion of the fuel for the furnace and water heater. This can often cause a negative air pressure condition in the basement and a downdraft of the exhaust as it tries to draw air from wherever it can get it. The insufficient air means the fuel won't burn completely, producing carbon monoxide in the exhaust which is not going up and out but being drawn back. Bathroom exhaust fans and the clothes dryer exacerbate the problem.
Another less lethal, side effect of the tight home was mold and air quality. A tough time for all.
Our 2nd customer has decided to update completely and do things right with a professional. And now the modern home is equipped with a high efficient furnace, direct vent water heater and fireplace, and a heat recovery, air exchange unit (HRV). The properly installed furnace will draw its combustion air from outside, as does the fireplace and water heater. The HRV pulls the stale inside air out and replaces it with fresh preheated air from outside.
The furnaces themselves have improved in efficiency by utilizing ways to remove more heat from the flue products before exiting the building as exhaust.
They have achieved this through the addition of three new components in all high efficient furnaces - a secondary heat exchanger, an exhaust fan, and burner direct ignition to replace the standing pilot.
The secondary heat exchanger removes that much more of the heat from the exhaust products and is the place that the water vapor condenses from the exhaust. The exhaust fan is necessary to pull the flue products through the primary and secondary heat exchangers, and to push them outside. The direct ignition, either through spark or hot surface ignitor, removes the need for a standing pilot which is an enormous saving on gas.
What Maintenance Does A High Efficiency Gas Furnace Require?
The most important maintenance needed on a high-efficiency gas furnace is to keep the air filters clean and to keep the condensation flowing out of the furnace.
A clean air filter ensures that the air flow through the furnace is taking the heat away, upstairs to keep you warm. If the filters are blocked, the heat stays in the furnace overheating components and can even cause a crack in the heat exchanger.
The condensate is a byproduct of an efficient furnace and it needs to flow freely from the furnace to a drain somewhere.
Note: the condensate is slightly acidic and some local bylaws require special consideration for the condensate.
If the condensate drainage system becomes blocked, the furnace backs up and should shut down on a pressure or rollout switch, saving your life. If not, the condensate can back up into the secondary heat exchanger, blocking the exhaust and causing incomplete combustion leading to carbon monoxide or CO.
The way to have a long and happy life with your furnace is to have it checked and tuned up yearly by a qualified, experienced company like ours.