No Category for "Old Oil Furnaces"... So This Goes Here I Guess

Dec 14, 2001

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The Bottom Line For SuspectTerrain,I hope I've accomodated your questions? I got a bit carried away, so I published it. *L* Hoping it helps anyone with an old oil furnace.

This was going to be a short note to Suspect Terrain about their old oil furnace but, I got a bit carried away so it became a review on how to service your old oil furnace. There should be categories for Central Forced Air Oil Furnaces, Gas Furnaces, Gas Fireplaces, Gas or Oil Fired Boilers and Gas/Oil fired Water heaters, both storage type and instantaneous types etc. WHY ISN't THERE? Look at me, here less than a month and complaining. *L*

(Suspect) Well, light green could apply to any of about a dozen manufacturers from either Canada or the U.S. 30 to 40 years ago. It seemed to be one of the most popular colors for furnaces back then. I don't know why.

There should be two name plates, one on the furnace and one on the oil burner. Most furnace manufacturers built the furnace but purchased the oil burners from one of a handful of burner manufacturers. If you're in Michigan, I'm guessing it's either a Wayne or a Beckett oil burner. Beckett would be my first guess, manufacturing oil burners out of Ohio since the 1940's and probably the largest in the U.S. Incidentally, John Beckett is still operating that company (son of the founder). It would be useful to know the specs listed on the burner plate.

The furnace manufacturer should have rated inputs and outputs, as well as the nozzle size, angle and type recommended. Quite often, over the years, service technicians will change the oil nozzle with whichever nozzle they may have on their service vehicles. Installing a nozzle with the incorrect angle or spray type can severely decrease the efficiency of the furnace. The angle and type are determined by the design of the combustion chamber and certified for optimum combustion efficiency.

The combustion chamber (oil pot) could either be made of ceramic fiber or brick. It would be a good idea to have it checked. Make sure the ceramic chamber hasn't been damaged over the years by someone who may have been cleaning the unit. If it's brick or formed concrete, make sure it's intact.

The oil burner and pouch plate will need to be pulled off to check these things. Pouch plate is the burner mounting flange and is usually just bolted to the front. You should be able to pull the burner without disconnecting too much, by simply removing the four nuts holding the burner's blast tube to the flange on the pouch plate.

Check the oil filter at the tank. It may not have been changed for awhile and probably should be.

A basic cleaning of the heat exchanger with a flexible wire brush and a good shop-vac through the cleanout ports front, sides and rear (assuming it's a reach breach) including any flue baffles would be a good start towards restoring more efficiency.

If you plan on servicing it yourself, invest in an inexpensive draft gauge. (They run about $30.00 from a heating supply wholesaler)
If you're hiring someone to do the work for you, and that's usually the best thing to do with any fuel burning equipment. At least you will know what they should be doing to the furnace for you, to bring it up to it's best possible performance for it's age.

More critical is the primary air shutter on the side of the burner. By loosening the bands that control the air intake and opening them slightly, you will introduce more primary air and of course closing the bands will produce a yellow sooty flame. There is an observation port on the pouch plate for making this adjustment. The rule of thumb is to open or close the primary air bands just until you can see the tips of the flames from the top of the fire pot. You don't want to see long yellow flames, just a few fingers of flame licking out the top.

A service technician can verify the proper setting with a smoke spot test. On an older unit like yours, a number two smoke spot is not unusual. On a properly set up oil burner, on a new furnace today, you would be looking for a zero to a trace. However, today's burners are almost all high static burners, running at 3450 rpm's and atomizing the oil much better at the flame retention head, making for a much cleaner burn than the technology allowed for 30 to 40 years ago. (Before flame retention head burners were even invented.)

With the primary air set properly and the oil nozzle as per the manufacturer's specifications on their rating plate, you will want to measure the draft over the fire pot. There's usually a test port to insert the probe built into the front of the furnace. Sometimes right on the pouch plate. You should be reading a slightly negative overfire draft of around -.01 to -.02" w.c. (inches of water column). You shouldn't see a positive overfire pressure condition, that is an indication of a severely blocked heat exchanger or a poorly adjusted barometric damper. You should be getting blow back on start up and indications of soot on the front of the furnace.

The barometric damper on the flue pipe leading to the chimney is critical to proper performance. It is balanced (rarely right) by the installers and the service people who follow them. Adjust the weights on the damper flap, with the unit operating for more than 15 minutes to establish a draft. Adjusting the weights to allow the damper to open more will reduce the overfire draft, adjusting them to close with the burner running will increase the overfire draft.

Too much overfire draft will cause flame stripping at the head of the burner and poor combustion. Too little overfire draft causes heat to build up at the burner face. The manufacturer's recommended overfire draft "should" be noted on the rating plate, if it isn't go for a reading of around -.02"w.c. and recheck the smoke in the flue before the barometric damper. (after the damper the reading is diluted)

Sometimes you may have to make minor adjustments and recheck these points a few times before you get the readings you want. It's balancing act between the damper and the burner, and all furnace types are a little different in that they have different heat exchangers and restrictions and combustion chambers, etc.

A good oil burner technician should be able to run a quick CO2 or O2 test as well. Optimum carbon dioxide levels in the products of combustion in an older oil furnace like this one, will run between 7 and 9%. Todays oil furnaces run between 11 and 12% or even higher.
A good high CO2 and a low stack temperature (low being between 350 F and 550 F) will give you an exact steady state efficiency. (Readings should be taken after 30 minutes of operation for true steady state efficiency) Some technicians measure the oxygen so it is the opposite, a low O2 reading is a good sign. Some older oil furnaces run up to 750 F in the stack and although the chimney will be healthy, it means your efficiency is pretty low.

In the good old days, (*L*) the service people would just throw in the next nozzle size up if they thought the unit could handle it. A little extra heat so to speak. The truth is, that by derating the nozzle to one or even two sizes below the manufacturers specs, you can quite often increase both the comfort level of the home and the efficiency of the furnace. If your furnace is already running for long on cycles to maintain the temperature, it is likely set up with the correct nozzle size for your home.

While they're changing the oil burner nozzle make sure they recheck the condition of the electrodes and the gap between them.

Also have them double check the oil pump is set for 100 PSI. New high static burners run at higher pressures for better atomization of the oil, but the older models need to run at 100 psi.

Big old oil furnaces contain a lot of steel in the primary and secondary heat exchangers. That heat doesn't begin to soak through between 20 and 30 minutes of operation. So your furnace has not yet reached it's optimum operating efficiency after a five or ten minute on cycle, however, the thermostat upstairs is satisfied and shuts down the burner. Decreasing the oil nozzle by 15 to 20% means the burner will run longer to satisfy the thermostat, burning less oil in the process and the home will be more comfortable throughout. Ideally, you should try to make the furnace operate for 20 to 30 minute cycles so that it reaches equilibrium in the heat exchanger. Then it's operating at peak performance. Short on/off operating cycles can also cause condensation in the chimney.

I don't usually recommend changing the oil burner for one of the new high static burners. It won't improve the efficiency of the furnace, since the baffle weren't designed to handle a high static burner.

I highly recommend a two speed fan motor. Check with a local motor rebuild shop and buy a reconditioned one for $50 - $100. Wire it in to run on high during a call for heat and to drop down to low speed during the off cycles. Continuously circulating the air throughout the home is a good way to keep the warm air from stratifying in the upper levels and maintaining even comfort zones.

While we're on the subject of fans. Check the temperature rise or the delta temp between the return air plenum and the outlet plenum. Generally, lower the temperature rise across the heat exchanger the higher the efficiency of the system. I.E. More air flow, = lower temperature rise. You want to find a place between 60 and 80 degrees F between the cold air plenum and the warm air plenum. A temperature probe about 24" above the furnace cold air may read 70 F then you want to see about 130 or 140 on the warm air side about 24" above the heat exchanger. Center the probe and it should be a shielded type that won't pick up radiant heat. You can speed up the fan by increasing the motor pulley from 3 1/2" to 4". Or slow it down going the other way. Most motor pullies are adjustable, you just need an allen key to unlock them. Worst case, you may have to run out and buy a new motor pulley. If you can't adjust the motor pulley any more, you may want to change the blower pulley. A smaller blower pulley will speed up the fan. That sometimes means having to change the fan belt to accomodate the new pulley sizes.

As usual, you should always keep that air filter clean and the fan vacuumed. My brother could grow tomatoes on his air filter every spring and after 25 years of telling him how inefficient it is, he still forgets to clean it. I even bought him a permanent, washable filter for Christmas one year as a joke. Guess who gets to wash it? *S*

Good luck with your new home, I hope this helped.

The Gasman (once known as The Oilman) *L*

P.S. When you get settled after Christmas, drop me a line if you want some specific advice. Just try to dig up as many of the specs from the name plate as you can read.

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