Down Firing Furnace

johntrhodes81

I have a house with ridiculously oversized oil furnace.  It is a ThermoPride OL16-125 with a Beckett AF burner and 1.25 gph nozzle. Short cycles like crazy as heatload on design day is less than 60,000 btus.  I am planning to swap the burner head from an F6 to F0 and swap nozzle to a 0.65 gph.  And also swap 3/4 to 1/2 hp fan motor.  

Any thoughts?
John

pecmsg

Why are you changing the fan motor?

Without a combustion analyzer I wouldn't change anything at this point.

johntrhodes81

Of course combustion analysis must be done after a burner change.  On fan motor change. Too much airflow and electricity consumption. Top 2 speeds aren't used even with current operation.
John

captainco

Do that and the oil company will just love you!!

STEVEusaPA

Bad idea all around. Without an analyzer, and without checking heat rise, static pressure, you could make it much worse...or dangerous. You'll probably get poor 'real' efficiency, and high CO production.

pecmsg

johntrhodes81 said:

Of course combustion analysis must be done after a burner change.  On fan motor change. Too much airflow and electricity consumption. Top 2 speeds aren't used even with current operation.
John

That unit has a 3/4HP motor because that's what is required to spin that Wheel and deliver the required CFM's. Reducing it 1/3 will overamp the replacement motor and burn it out.

MaxMercy

johntrhodes81 said:

I have a house with ridiculously oversized oil furnace.  It is a ThermoPride OL16-125 with a Beckett AF burner and 1.25 gph nozzle. Short cycles like crazy as heatload on design day is less than 60,000 btus.  I am planning to swap the burner head from an F6 to F0 and swap nozzle to a 0.65 gph.  And also swap 3/4 to 1/2 hp fan motor.  

Any thoughts?
John

I would talk to ThermoPride and get their input.

I made the mistake of replacing my old boiler with the same size the builder put in, and my new one is short cycling. The only time I heard it run for 20 straight minutes was on a ten degree night when both zones (two story Colonial) were calling at the same time, otherwise 3-5 minutes is typical.

And *this* is with a downsized nozzle. My new boiler was shipped with two nozzles, a 1.25 in the accessory bag and a 1.10 installed which was what I was running before. I contacted Slant/Fin and they said the lowest to fire it was 0.85 at 100lbs, which is what I have installed, yet it still builds heat in a hurry and I get a short cycle.

I know that if you downsize too much, the efficiency falls off and you risk condensation. Even at 0.85 gph, the highest stack temp I've seen is about 360F gross. I know if I downsized any more I'd approach condensing, particularly at short calls for heat.

pecmsg

With the OL16-25 being more than double what's needed is to replace the furnace.

EdTheHeaterMan

You can damage your chimney liner (if you have one) or the inside bricks and mortar by a process called condensation of flue gasses. Your heat exchanger is designed to have a 2000+ degree fire inside and drop the temperature to around 450° the vent connector. that means that over 1500° of the flame's heat goes into your home thru the heat exchanger. When you make the fire .65 GPH, the temperature at the other end can get as low as 250° going up the chimney.

Now follow this

As the bricks in the chimney start to heat up from the 250° exhaust gas, that exhaust gas will get even cooler. It will get so cool that the water vapor in the flue gas will begin to condense. The vent pipe connector (smoke pipe) will rust, the inside mortar joints will become saturated with water. During the off cycle, that water in the bricks and mortar joints can freeze and expand. That expansion will break the mortar and masonry will slowly break apart over time. Eventually the chimney will start to break apart from the inside and pieces of bricks and liner will fall to the base of the chimney and possibly block the exhaust gasses from escaping up the chimney and leak into the home.

Another thing that happens in those small droplets of condensation is chemistry. Carbon molecules and sulfur molecules mix with the water to make carbonic acid and sulfuric acid. Those acids will further break down the masonry and turn it into powder.

In a few years you will have saved $2000.00 on fuel but you will need a new $5000.00 chimney.

You can reduce the nozzle a small percentage BUT you need to have the tools to test what that does to the entire combustion process. Many folks thing all you need to change a nozzle on an oil burner are 2 wrenches. That is SOOO NOT TRUE! You also need one of these kits and you need to know how to use it.

EBEBRATT-Ed

@johntrhodes81

You can't down fire it that much without causing furnace or chimney damage. You have to do a combustion test and maintain a minimum stack temperature of around 325 degrees.

Start with a 1.00 gallon nozzle and see what you get but it must be tested. And leave the blower motor alone. If its multispeed you can change the wire positions, but you must also keep the temperature rise in the recommended range. This is done with a combination of fan speed and burner input. Both will have an effect on temp rise

captainco

Oil burns at over 3000 degrees, up to 4000 degrees with the O2 between 4% - 6%. Natural gas burns over 2800 degrees when the O2 is at 6%.

Most of the time when I have found that much debris in a flue the unit either is underfired, doesn't have a rain cap or loses all its draft when it shuts down. The draft problem at the end is a negative pressure problem that the burner can overcome when it is running but as soon as it shuts down the draft drops drastically, sometimes all the way to zero.

Dave Carpentier

captainco said:

The draft problem at the end is a negative pressure problem that the burner can overcome when it is running but as soon as it shuts down the draft drops drastically, sometimes all the way to zero.

Can the operation of a barometric damper plate be an indicator for that ?
On a relatively calm day for wind, my plate will continue to float open for a short while and then drift closed. I always thought it was the 'package' of flue heat still working its way up and drawing room air behind it.
If thats true, then a barometric set too tight could cause flue condensation ?

captainco

Your description sounds normal. If the barometric closed almost immediately after the burner motor shutdown, then you may have a negative pressure problem.