Generic "Older Furnaces" Services You Can Benefit From

Dec 17, 2001

Popular Products in Heaters
The Bottom Line Don't toss out that perfectly good old furnace, maintain it and it could last you another 10 or 20 years.

There seems to be more interest from epinions members in how to keep the "older" furnaces running, efficiently and safely, than there is in whether or not, I own a Leatherman Wave, or whether I oppose unvented gas-fired heaters or whether I believe that CO detectors are a fraud perpetrated on the public, using scare tactics and misinformation.

I'm not offended. Fuel costs and capital expenditures that may be deferred for a few more years are things that interest anyone who owns or rents a home and pays the utility bills.

Bear in mind that I still recommend having a competent and qualified service person in to do the tricky stuff. The following gets into things that most consumers don't want to play with. However, some of the following has suggestions that anyone with a bit of common sense can do themselves.

So let's take a look at the average "older" forced air furnace, it may be burning natural gas, fuel oil, propane or even electricity (eh Cowboy?) By older, I am referring to furnaces 10 to 30 years old.

1.) What can we do, as consumers, to get the most out of that piece of equipment, we generally hide away in the basement or the utility room? What service can we do ourselves without paying through the nose for a service tech to do it?

1a.) Depending on how much dust accumulates in your furnace filter, cleaning the filter or changing it (if it's a disposable type) is the one thing everyone should do on a regular basis. Regular may mean once a month if you have dogs or cats and naturally ocurring dust bunnies.

High efficiency air filters, with fancy names like HEPA filters, are great for extracting even the smallest microns of dust from the air in your circulation system. The only drawback it that they require much more maintenance. They're so efficient, they need to be changed on a regular basis or they simply get stuffed up.

For those with allergy problems, I recommend them over the more expensive electronic air filters, which don't do nearly as good a job.

Simply stated, when the furnace filter begins to clog up, less air is drawn through the duct system, therefore, less heat is taken off of the heat exchanger and more heat escapes up the chimney (forced air electric furnaces excluded). The furnace is working harder to satisfy the thermostat, burning more fuel and stressing the steel in the heat exchanger. The circulation fan and fan motor are working harder. Extreme blockage may even cause the circulation fan to go into a stall as the load increases. As temperatures rise on the warm air side of the duct system, due to a reduction of airflow, the high temperature limit control may knock out the furnace from time to time until it cools down, even if the thermostat isn't satisfied. The home is less comfortable. Less air, may cause cold spots within the home as the volume of heated air decreases with progressive blockage by the filter.

The heat exchanger may be considered the heart of any central forced air furnace, but the circulation fan and motor are the lungs. Drawing stale used air from all over the home via the return air duct network and reheating it or cooling it in the summer time (if you have central air conditioning) and redistributing that treated air throughout the home.

If you suspect that the airflow through the furnace isn't proper, because the outlet air is too hot, double check that the return air grills and warm air registers are all clear of obstructions throughout the home (I.E. drapes, furniture, old magazines, a pair of underwear). Cold air or return air grills and registers are equally important to a balanced duct system. The return air side of the duct work is drawing off the colder air from various points around the home. Accidentally blocking them can upset the balance of the entire system.

1 b.) As I've mentioned in a previous review, most of the older forced air furnaces use a belt driven, 1725 rpm fan motor. Some use a multi speed, direct drive fan motor, which simply means the shaft of the motor is directly attached to the squirrel cage within the fan housing.

The speed of a belt driven fans can be adjusted by (FIRST TURNING OFF THE POWER TO THE FURNACE) loosening the set screw with an allen key. Turning the motor pulley clockwise increases the diameter and in turn increases the speed of the fan. These belt driven fan motors are mounted on adjustable legs, which can be lowered or raised to keep the fan belt tension from becoming too slack. While you're in the blower compartment, check the fan belt for cracks and signs of wear. If you find any cracks, pop off the fan belt and go out and pick up a new one. Better to change it now, for a couple of bucks, than to wait until it snaps on a cold winter day.

Multi-speed, direct drive motors usually have a speed selection chart on the wiring diagram, right on the motor or on the wiring diagram attached to the furnace blower door. White is neutral and the rest of the wires are color coded for a selection of speeds, I.E. high, medium, low for a three speed motor. Some even have medium high or medium low for a full range of fan speeds. These optional wire taps are usually prewired and taped off, so it's just a matter of selecting the speed you wish from the chart, matching it to the colored wire and switching it, leaving the white attached as it was. There isn't much you can do, if the motor is already set on the highest speed. (DID I MENTION TO TURN OFF THE POWER BEFORE ATTEMPTING TO DO THIS?)

"If the motor has oil service ports, a few drops of oil is a good idea each spring and fall. Most of the motors used since the 70's were sealed bearing types and don't require oiling.

For those of you who want to get technical. Go to a wholesale heating supplier and purchase an inexpensive duct thermometer. (Range 0 - 250 F) ($10 to $15) (Pick up a roll of duct tape while you're there for a couple of bucks.) On a colder day, insert the thermometer through a (1/8" to 3/16" diameter) test hole on the warm air side of the duct system, about 24" from the top of the furnace, turn the thermostat up high so that the furnace is going to be running for at least 15 minutes and take a temperature reading. Assuming the temperature reading in the cold air side is around 60 to 70 degrees F, a reading of 160 to 180 degrees F, on the warm air side, is too high. You will want to increase the fan speed. With the furnace fan still running check the return air or cold air temperature above the furnace filter. The difference between the cold air intake side of the furnace and the warm air outlet side, known as the "temperature rise" should be approximately 60 degrees F

A lower temperature rise through the furnace may generate uncomfortable drafts through the home. A much higher temperature rise is usually an indication of a lack of airflow and poor efficiency. I.E. A 180 degree F outlet temperature is running very close to the safety cut off limit control within the furnace, which is usually set for between 180 and 200 degrees F. Under this condition the furnace may cycle on and off for no apparent reason, even if the house is cold and the thermostat is turned up.

The cold air side of the duct system is under negative pressure as the fan is drawing air from the home through this network. The warm air side is under positive pressure from the fan which is forcing the air to the various rooms. Carefully check all of the joints in the duct system for major air leaks. This is where that duct tape you picked up may come in handy. Seal them up. Check all of the take offs or branch supply pipes which lead from the main ducts to various rooms in the home. A bit of tape around any loose joints can help a lot towards delivering the heat to where you need it.

If you've got an old dysfunctional humidifier hanging off the side of your furnace and you have no plans to fix it. Remove it and seal up the holes. Old leaky humidifiers have been responsible for the destruction of many otherwise healthy furnaces over the years. A leaky, corroded water valve can rot out a furnace heat exchanger in very short order. (If you have an old style non-motorized, plate style humidifier, remove it and chuck it in the garbage. They are about as useful as tits on a bull.)

Speaking of which. If you have a central air conditioning coil mounted in the furnace plenum, alwasy check the condensation drain lines are clear. A little dirt and sludge clogging this drain line up will back up the condensation from the coil in the summer and it will drip all over the heat exchanger, effectively reducing it's life expectancy. This five minute service check may just save you a few thousand dollars for a new furnace. Not to mention a hoard of problems with air quality and possible soot damage if the heat exchanger develops tiny holes and begins to leak flue gases into your home.

Okay, so you've cleaned the filter, checked the drain line from the air conditioning coil, checked the grills and registers are clear, you feel confident that the airflow through the furnace is producing an appropriate temperature rise and any major leaks in the ducts have been sealed up with your trusty duct tape. How's the house feel now?

Maybe it's time to have a competent, licensed service person take over. You want a service person with a combustion analyzer. You want an analysis of the products of combustion in your flue pipe after fifteen minutes of operation. You want the service person to relate the results of that test to you in two ways. 1.) What is the output efficiency of the appliance as compared with the output capacity shown on the manufacturer's rating plate?
2.) What were the carbon monoxide (CO) readings with the circulation fan on and with it off. If the CO readings were zero in both cases, you have no problems. If you have a few parts per million (1-25 ppms) with the fan off and a much higher reading with the fan on, you have a hole in the system somewhere and that needs attention. The circulation air from the fan should never have an impact on the combustion process.

As long as they've got the analyzer out of the box, have them run a check on the ambient air in the duct system.
Their analyzer shouldn't pick up any traces of carbon dioxide or carbon monoxide in the duct system, with the fan off or on. (Both tests again.)

No problems with efficiency, combustion or ambient air in the home? Good.

Since he or she has come all this way, have them run a few fundamental checks. Gas pressures, both supply and manifold, or oil burner pump pressure, amperage draw from the motor on a cold start, should be less than 12 amps.

Have the room thermostat checked for proper calibration and ask the service person to double check that the heat anticipator setting in the thermostat is set up correctly for the furnace it is controlling. This is a minor adjustment for most service people and one that they often forget to check. However, if the heat anticipator in your thermostat is set wrong, it can cause the furnace to short cycle. The anticipator setting is usually in the instructions with the furnace or on the rating plate. Most older gas furnaces required .2 amps, oil furnaces were usually .4 amps. However, if it has electronic ignition it may require as much as .8 to 1.2 amps.

Sorry to repeat myself, but, I highly recommend having a two speed fan motor and having it wired to run on low speed all year. The power consumption is reasonably low and the comfort levels within the home are greatly enhanced by constantly circulating the air. That applies in the summer as well as the winter. By circulating the air throughout the year you will reduce any chances of hot air stratifying, which tend to make the bedrooms too hot in the summer and the lower levels too cold in the winter.

Chimney draft should be checked with the furnace running at least 15 minutes. Older furnaces weren't equipped with spillage switches, so having the draft hood relief opening checked for signs of flue gases spilling into the home is a good thing to check. On an oil furnace the barometric damper should be checked and setup properly as I've mentioned in a previous review.

I hope this is of some help and interest, I'm not even going to proof read it, because I'm tired and I'm going to bed now.

Good night.
The Gasman

Read all comments (6)

About the Author ID:
Member: Chasing Stoichiometry
Location: Where I Need To Be
Reviews written: 66
Trusted by: 261 members
About Me: Still venting, after all these years...