27
21. Equivalent Orifice Sizes at High Altitudes
(Includes 4% Input Reduction for Each 1,000 Feet)
Reference Sources: ANSI Z233.1 & NFPA 54, National Fuel Gas Code.
For Canadian high altitude (2,000 - 4,500'), reduce gas manifold pressure to 3.0” W.C. for
natural gas and 9” W.C. for LP gas.
NOTE: Do not attempt to change the size of an orifice by drilling it out. To do so will void the
manufacturer’s warranty
Table 11. High Altitude
FURNACE ORIFICE SIZE AND ALTITUDE - FEET
MODEL OUTPUT CAPACITY SEA LEVEL 2000 3000 4000 5000 6000 7000 8000 9000 10000
ORIFICE SIZE - NAT. GAS 29 293030303031313132
M1M/G 056 ORIFICE SIZE - LP 45 46 47 47 47 48 48 49 49 50
OUTPUT- MBTUH 45.4 41.8 40.1 38.5 37.0 35.5 34.1 32.7 31.4 30.2
ORIFICE SIZE - NAT. GAS 24 252627272828292930
M1M/G 070 ORIFICE SIZE - LP 42 42 43 43 43 44 44 45 46 47
OUTPUT- MBTUH 56.7 52.3 50.2 48.2 46.2 44.4 42.6 40.9 39.3 37.7
ORIFICE SIZE - NAT. GAS 21 232324252627282829
M1M 077 ORIFICE SIZE - LP 40 41 42 42 42 43 43 44 44 45
OUTPUT- MBTUH 61.7 56.9 54.6 52.4 50.3 48.3 46.4 44.5 42.7 41.0
ORIFICE SIZE - NAT. GAS 21 232324252627282829
M1G 077 ORIFICE SIZE - LP 40 41 42 42 42 43 43 44 44 45
OUTPUT- MBTUH 59.9 55.2 53.0 50.9 48.9 46.9 45.0 43.2 41.5 39.8
ORIFICE SIZE - NAT. GAS 17 181919202122232426
M1M 090 ORIFICE SIZE - LP 36 37 38 38 39 40 41 41 42 43
OUTPUT- MBTUH 71.7 66.1 63.5 60.9 58.5 56.1 53.9 51.7 49.7 47.7
ORIFICE SIZE - NAT. GAS 17 181919202122232426
M1G 090 ORIFICE SIZE - LP 36 37 38 38 39 40 41 41 42 43
OUTPUT- MBTUH 69.7 64.2 61.7 59.2 56.8 54.6 52.4 50.3 48.3 46.3
ORIFICE SIZE - NAT. GAS 26 272828282929303030
M1B 066 ORIFICE SIZE - LP 43 44 44 44 45 45 46 47 47 48
OUTPUT- MBTUH 52.6 48.5 46.6 44.7 42.9 41.2 39.6 38.0 36.5 35.0
ORIFICE SIZE - NAT. GAS 18 191920212223242627
M1B 086 ORIFICE SIZE - LP 37 38 39 39 40 41 42 42 43 43
OUTPUT- MBTUH 68.2 62.9 60.3 57.9 55.6 53.4 51.2 49.2 47.2 45.3
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