Poor Performing Oil Furnace Service Conundrums
Perhaps, a somewhat typical (31 year old Thermo Pride OL11- 112) Oil Furnace, with evaporator coil added in 2005, Conundrum
My Scan of Carol's Oil Furnace Blower Curve. A Thermo Pride engineer sent the graph to me.
The Poor Performance Data on Carol’s Oil Furnace Situation - 12/08/12 This was written for the owner, so is low tech in nature...
Look at the blower graph first & note that it only has a quarter HP belt drive blower at 700-RPM; at only .5" ESP it only delivers 400-CFM. Totally inadequate even on a good duct system; existing one not efficient. How could that extremely low airflow not be properly addressed!
A 2-Ton coil was put directly on top of the furnace resulting in too much added back-pressure, plus a long run to the bedrooms; single story home with a basement in SW WI.
The initial acute problem is extremely low blower airflow across the heat exchanger &, also through the evaporator coil during the summer cooling mode. The installer of the upflow E-Coil paid no attention to these airflow problems.
Then we have combustion air that was set excessively high causing lowered heat output. Add to that the partial plugging of the oil line filter screen & that is another drop, which will be shown in my testing data below.
When it was new, or after the first service, an .85 GPH (Gallons Per Hour) nozzle was installed; #2 diesel fuel produces around 140,000-Btu per gallon.
140,000-Btu * .85% is 119,000-Btuh Input; the furnace only gets around .75% or less efficient Output now, compared to 80% when first installed.
119,000 * .75 is 89,250-Btuh Output now, or less, if delivering its full 100-psi fuel to the burner, which my airflow test indicates its not doing, probably due to the little known & unattended inlet screen/strainer in the Suntec A2VA 7116 fuel pump.
The blower airflow test meter indicated around only 500-CFM of airflow when 900-CFM (the 2-Ton evaporator coil’s top CFM in cooling mode) results in marginally high temperatures. 89,250-Btuh Output / 900 is 99-F temp-rise over 70-room-temp is 169-F at closest supply air register, even at 900-CFM that is yet too high/hot.
*OK with a new 95% 57,500-Btuh Output Propane furnace; 57,500 / 900-CFM is 64-F; only 4-F over recommended temperature-rise; with 3-Ton blower; not a problem as we can go some above 900-CFM evaporator limit in heating mode.
At the tested 500-CFM; & that filter screen replaced; = 89,250-Btuh Output / 500-CFM is 178.5 -F; that high a temperature is not allowable!
After adjusting air to the burner, Btu output increased a lot; here are those increased temperature figures using a digital thermometer. Maximum temperature at that best airflow kitchen register was 168-F; room temperature was 63-F temperature rise was 105-F which is too high.
The formula: 105-F * 1.1 is 115.5 * 500-CFM *(or some less) is only 57,750-Btuh output; evidently due to an undetected fuel pump filter screen restriction. Therefore, 89,250 - 57,750 is a loss of 31,500-Btuh due to that filter/screen restricted fuel line flow to the burner. (57750-Btuh / by the 115.5 temp-factor is 500-CFM, ha) (search for youtube videos concerning that Suntec pump inlet strainer/screen only a $1.95)
The very low blower airflow will NOT allow anyone to get this oil furnace setup to work properly no matter what else they do to get optimal performance from the oil burner!
The new furnace would be shorter, therefore, a transition would put the coil well above the furnace for much better airflow at a lower static pressure. Just trying to illicit some comprehensive brainstorming concerning some real-life issues.
In this area many don't seem to check airflow or the temp-rise, a few just do a smoke test & get the flu pipe temp; if it starts & continues to run & it's allowed to pass muster. These situations must change; to me seems about as negligently bad as it gets!
There were NO Oil furnaces to service where I was a contractor for many decades, only natural gas. - Darrell