"Hey, I want to buy and install my own furnace?or I think I'd like to try?"
I first suggest you check out a very well written article in Consumer Reports (I believe 11/98) titled "How to Buy a Furnace." These guys are great! Usually, CR will secretly purchases a broad variety of products and independently tests the manufacturers claimed performance ratings. In the mean time, they try to break stuff (what a fun job!), and eventually assign comparative ratings. In this case, they instead surveyed about 500 independent HVAC contractors nationwide. In brief, the contractors shared three common beliefs:
The majority of natural gas forced-air furnaces being sold today are either a 78-80% AFUE (Annual Fuel Utilization Efficiency) forced draft furnace, or a higher efficiency, 90-92% AFUE, condensing type forced draft furnace.
There really isn't all that much that differs in quality between most brands. They all seem to perform reliably. BUT...
Overall performance of any new furnace has more to do with how well it was installed and maintained, than any other factors.
CR's article also briefly discussed some of the more advanced features available from many brands (such as variable speed blowers, and sealed combustion designs, etc.), and that the two largest manufacturers of gas fired furnaces are United Technologies (Carrier, Bryant, Payne Day 'N Nite) and Goodman Manufacturing (Janitrol, GMC, Amana).
By reading this article you should believe that no matter how much money you spend, you better hope that your contractor has lots of experience. And I mean experience as the factory would like him/her to have it, and not just because he/she's been doing lots of things improperly for a long time. I know. I have to answer the consumers calls referred by the factory, as we or one of our competitors may have sold a furnace to your "Handyman" who called himself a professional HVAC contractor. And today, he's nowhere to be found.
We've discovered that there are quite a few folks running around and installing gas furnaces who aren't very well trained or willing to read or abide by manufacturer's installation guidelines. As for periodic maintenance, mostly what your furnace will need is still an occasional re-oiling of the blower motor (about once per year, for those designed to be re-oiled), and changing your air filters every couple of months. Oh yes, please, insist on the higher efficiency pleated air filters. The glass panel filters catch only the real big rocks (that'll have to be a future discussion). So it seems there isn't all that much you would need to do to maintain a heater ONCE IT'S INSTALLED PROPERLY. But the above tasks should STILL be done by a professional, because they will be able to use the proper test instruments to monitor your furnace's output, air flow, combustion efficiency, manifold pressure, flue draft, etc. and notice a potentially hazardous problem before it becomes serious.
"But my old heater ran for nearly 20 years and hardly ever gave me any trouble. How come there's now such a fuss?"
Those days are now long gone! Because our government mandated higher minimum efficiencies several years ago, any furnace you buy today will contain 4-5 times as many parts susceptible to failure. Parts such as fan timers, combustion sequence-monitoring circuit boards or ignition modules, draft inducing motor/blowers, draft proving sail or pressure switches, dual-seated (redundant) combination gas valves, flame roll-out switches, stack limit switches, spark electrodes or igniters, high voltage ignition cables, flame proving sensor probes, secondary heat exchangers, motor running capacitors, door panel switches and, to top it off, more wiring harnesses to connect all of the aforementioned. Whew! And then there's the issue of inspecting your chimney and the proper venting these new heaters.
"OK, the government passed all these laws to make furnaces more efficient. What's in it for me, and how do I know what I've got?"
As I said before, your old furnace that ran and ran for 20 years and didn't seem to give you too much trouble is virtually a thing of the past. If your new furnace was installed properly, it should also save you money on your fuel bills. Oh, yes. It will also be unnecessary to buy anymore thermocouples or warp switch pilot assemblies. But continue to keep a thermocouple handy for your gas fired water heater.
As for the size of what you have churning away in your basement or furnace room? There should be a permanent tag indicating your furnace's BTU input and output. If some joker managed to walk off with it (and boy that sure irks me when I see them at our parts counter), here's how you can tell. There's typically 1060 BTUs* per therm (therms are what your gas utility charges you for) of natural gas (0.60 specific gravity). One therm equals 1 cubic foot. Your utility's gas meter should measure "cubic feet" of consumption, and fractions thereof. So, go ahead. Turn you furnace on by setting your thermostat to a very high setting** and clock you gas meter (with all the other appliances off, or only their pilots burning - I'm assuming you only have a couple of pilots). You can see how many cubic feet of gas is consumed in 10 minutes of continuous operation. Multiply your answer by 6, and then again by 1060. This is how many BTUs per hour your furnace is consuming, and should be pretty darned close to the nameplate rating shown on your furnace. If not, you may wish to call a contractor or your local gas supplier to find out why not. (E.g. Your furnace may have a faulty utility pressure regulator that's over/under firing your burners, or a faulty pressure regulator in your gas train, or one that's integral to your furnace's main gas valve).
If your old heater is more than 10-15 years old (and you don't live in California - or should I say your furnace is not located in California), it's most likely to be LESS efficient than what you'd buy today. This is because much more of the heat from the combustion process raced right through your furnace and straight into your furnace flue, and then into your chimney. With today's more efficient furnaces, with advanced heat exchanger designs, more of the heat from the combustion process is transferred into the living space. This results in cooler flue gases. Savings are achieved from either shorter run cycles, or your ability to select a smaller, less expensive furnace that will run for equal, or possibly longer cycles, but burn less fuel while running. If you or the previous owner has invested in new windows, weather stripping and/or has added additional insulation, you should have a new heat loss calculation performed to determine how low (in input rating) you can safely go. Even when properly sized, your furnace will always be oversized, except on the very coldest days of the year.
"So, how do I know what size I really need?
Ah, I'm glad you asked that! Do you think a true professional HVAC contractor really cares what size furnace you have limping along in your basement? Why should he? I know, Uncle Joe insists that the old heater ran fine, and that he's going to just put in another of the same size. "How about maybe even one that's a little larger - just in case. Right?" he says. Wrong! Think about this: Every guy that's ever replaced a heater in your home (we've got some really old homes here in Pennsylvania) has probably taken the same approach.
"Hey, let's take it up a notch. It really can't hurt." The last thing he wants is a call from you telling him you're cold. He also believes that you'll probably never notice the difference.
Your Uncle Joe's probably right about his last statement, because your utility bills kept going up and up, and you just kept blaming your gas company. But what about those windows, insulation and weather stripping that you (or the previous owner) invested in a couple years back. And MOST IMPORTANTLY OF ALL, when was a professional heat loss calculation last (if ever) performed on your home? How much did you say YOU spend on your gas bills each year? Now the couple of hundred dollars you'll spend on a heat loss calculation doesn't sound like such a bad investment, does it?
"OK! I'll get a heat loss done. But what's this I hear about venting concerns with these new 78-80% mid-efficiency furnaces?"
OK, let's first get a couple of things straight. First, the two primary byproducts of combustion are CO2 (carbon dioxide) and water vapor. Other gases that will almost always exist in much smaller concentrations will be carbon monoxide, sulfur dioxide, nitrous oxide & oxygen. The real problem is caused by the combination of CO2 and water vapor forming carbonic acid at these lower stack temperatures. If your old furnace was, let's say rated at 100,000 BTU/hr input, and was less efficient, the flue gas was much hotter and did a better job of warming up the interior walls of your chimney. Now, let's say you install a new heater with the same input capacity (100,000BTU/hr). You're still producing the same amount of CO2, water vapor and trace gases, but it is now entering your chimney at a much lower temperature (remember, you're saving money). All this moisture in your cold chimney will cause its interior walls will begin to look like the outside of your ice tea glass sitting on your picnic table in you back yard on a hot, humid day! All this acidic condensation that formed on the walls of your chimney gets to race back down you chimney towards your furnace. If you oversize you heater too much (and I must say that most furnaces in our market seem to be grossly oversized), you may actually have condensation raining inside your heater. This is because the heater will never run long enough (the thermostat is satisfied too quickly) to ever warm up the cold chimney. Without enough heat in the chimney, the flue gas quickly looses its buoyancy and never reaches the top of your chimney. Even if you furnace is properly sized, you should have your chimney inspected, and install an approved flexible liner or a combination of same and class B (double wall) vent pipe. Doing so effectively reduces the size of your chimney to match the reduced stack temperatures and help reduce the potential for condensation.
"WOW! I didn't realize it was so complicated"
Yep. So, you still want to install a furnace yourself? Or do you still believe Uncle Joe can do it? Will he read all the instructions and consider, among other things: Proper flue draft? Adequate combustion air supply? Make the proper vent connections? Will you need to line your chimney? Will your installation require Class B double wall vent pipe? Will Uncle Joe properly design and construct the plenums and provide adequate supply, branch and ducted returns for your required CFM. (especially important if A/C is to be installed)? Will he provide the proper delta T across the burners and prevent negative pressure in the basement or furnace room? Will he comply with local codes (including those pertaining to proper gas supply piping)? Does he have the tools and ability to monitor and test for the presence of CO, and manage the correct levels of CO2 (NOx in some states), O2 and stack temperatures in the flue gas? Even if he knows the readings he's taken, will he know how to determine if your furnace is running at peak efficiency?
Does Uncle Joe really have all the tools and experience to check these performance areas? I didn't think so. It's probably best that you contact a reputable HVAC contractor.
You may be able to buy a gas furnace a little cheaper from a home center, but who's going to make sure it's installed correctly? Oh, they say they have qualified contractors who'll install it for you? Do you think the home center selected them based on their experience, or on how cheap they were willing to do a job?
Remember: all reputable contractors will be pretty busy taking care of their existing customers during the coldest winter days. And that's probably when you will discover the flaws in your installation - if you're lucky. It may just stop running altogether. Things like this just always know when to break down, don't they? And by then Joe will probably have a disconnected phone. Remember. I know this. You end up calling us because the factory said we're their distributor. And now you want the name of Mr. Reputable HVAC Contractor. Yes, we know who they are, but what good is that going to do you now? It's freezing out. They're busy. Their customers know they're good. They'd much rather fix something they've installed. They know anything that's wrong with one of their installations will most likely be central to the furnace, E.g. a replacement part. So you beg. "How's week after next sound?" they say. They also know that they're probably going to have to take more time to look at the "big picture" before they can be sure as to what's really wrong. The short-term solution may also just be a simple replacement part. But the cause of failure may be something more difficult to solve, E.g. resizing duct work, adding returns, rebuilding and/or relining your chimney, etc.
"OK, OK! I give up! So how and where can I look to find Mr. Reputable Contractor"
Drum roll?.. Ok, I thought you'd never ask. Check out some local HVAC professional associations. Get references. The Air Conditioning Contractors of America (A.C.C.A.) has local chapters in most major markets. A.C.C.A. provides their members with professional skills training for their installation and service crews. They know how to correctly size equipment for your home's heat loss (and heat gain during Summer), properly install equipment, connect it to a properly designed duct system, and have learned about the tools required to properly service and maintain your furnace for many years of comfort in your home.
Give 'em a call, and thanks for listening.
Sincerely,
Dave Whitnack
Johnstone Supply of Philadelphia
Philadelphia, PA 19144
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* BTU (British Thermal Unit) basic definition: One BTU is required to raise one lb. of water 1 degree F.
** A high thermostat setting does NOT cause your furnace to burn more gas or your home to heat up faster. The purpose of a high setting while performing this task is to prevent the thermostat from cycling off the burners before you're finished.
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