Heating Issues - Dometic DF Series Service Manual

Furnaces

6 HEATING ISSUES

A furnace is a vital part of any heating system, but it is only one part. A majority of furnace problems are caused
by issues outside the product itself. When diagnosing/troubleshooting a furnace problem, always check the
following items before replacing components, as the problem may stem from one of these heating issues and not
the furnace:
6.1 Identifying Heating Issues
Operational Issue
• Visible buildup of soot
• Burner ignites, then locks out
• Furnace over-fires or under-fires
• Motor vibrates or is noisy
• Furnace is limiting
• Motor runs, but no spark
• Furnace is limiting
• Motor runs, but no spark
• Fan runs too fast
Check the Gas Pressure
➤ Check that the gas pressure is set at a minimum of 11" W.C.(at the furnace), with a minimum of 50% (ideally
100%) of other gas-fired appliances operating while the furnace is in operation. The pressure should be tested
using a U-tube manometer. If using a digital-type manometer, calibrate it often with a U-tube manometer.
➤ Check the outside air temperature. As the air gets colder, it reduces the BTU capacity of liquid propane (LP).
The gas pressure is affected by a combination of the fill level of the LP tanks, the ambient temperature outside,
and the number of BTUs the furnace requires. Use the following example chart to understand the impact of
these three factors on whether there is enough gas pressure to sustain ignition in the furnace:
20 lb. Bottle (*30 lb. bottle: multiply x 1.40) for 40,000 BTU Furnace
Tank fill-level
60%
50%
40%
30%
20%
10%
65 lb. Bottle under Mtd. for 40,000 BTU Furnace
Tank fill-level
60%
50%
40%
30%
20%
10%
Example: 40,000 BTU furnace, 65 lb. LP bottle, 40% full, 0 °F outside: tank capacity is 38,500 BTUs.
EN
20 °F 0 °F
36,000 18,000
32,400 16,200
28,800 14,400
25,200 12,600
21,600 10,800
16,200 8,100
20 °F 0 °F
95,600 47,800
86,000 43,000
77,000 38,500
68,000 34,000
58,000 29,000
43,200 21,600
Potential System Issue
Gas Pressure
Ducting
Return Air
Voltage
-5 °F -10 °F
12,750 8,500
12,150 8,100
11,400 7,600
10,450 7,300
8,100 5,400
6,075 4,050
-5 °F -10 °F
36,000 23,900
32,250 21,500
29,250 19,250
25,500 17,000
21,750 14,500
16,200 10,800
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-15 °F
4,250
4,050
3,800
3,150
2,700
2,025
-15 °F
12,100
11,750
9,625
8,500
7,250
5,400
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