Bryant 120 Adjustments, In the U.S, Altitude Derate Multiplier for U.S.A, 312A, Fire Hazard

Table 10 – 2-Stage Furnace Setup Switch Description

SETUP	SWITCH	NORMAL	DESCRIPTION
SWITCH NO.	NAME	POSITION	OF USE
			When OFF, allows 2--- stage operation with a single stage thermostat.
SW--- 1	Adaptive Heat Mode	OFF	Turn ON when using 2 stage thermostat to allow Low Heat opera---
			tion when R to W/W1 closes and High Heat operation when R to
			W/W1 and W2 close.
SW--- 2	Blower OFF delay	ON or OFF	Control blower OFF delay time. Used in conjunction with SW--- 3.
			See Table 14.
SW--- 3	Blower OFF delay	ON or OFF	Control blower OFF delay time. Used in conjunction with SW--- 2.
			See Table 14.

312A

Step 3 — Adjustments

!WARNING

FIRE HAZARD

Failure to follow this warning could result in injury, death and/or property damage.

DO NOT bottom out gas valve regulator adjusting screw. This can result in unregulated manifold pressure and result in excess over-fire and heat exchanger failures.

!CAUTION

FURNACE DAMAGE HAZARD

Failure to follow this caution may result in reduced furnace life.

DO NOT re-drill orifices. Improper drilling (burrs, out-of round holes, etc.) can cause excessive burner noise and misdirection of burner flames. This can result in flame impingement of heat exchangers, causing failures. (See Fig. 50.)

Furnace gas input rate on rating plate is for installations at altitudes up to 2000 ft Furnace input rate must be within +/-2% of furnace rating plate input. For altitudes above 5500 ft, a field-supplied high altitude pressure switch is required.

1.Determine the correct gas input rate.

In the U.S.:

The input rating for altitudes above 2,000 ft must be re- duced by 4% for each 1,000 ft above sea level.

For installations below 2000 ft, refer to the unit rating plate.

For installations above 2000 ft, multiply the input on the rating plate by the derate multiplier in Table 11 for the cor- rect input rate.

EXAMPLE:

88,000 BTUH INPUT FURNACE INSTALLED AT 4300 FT

		Derate		Furnace Input Rate
Furnace Input Rate	X	Multiplier	=	at Installation
at Sea Level		Factor		Altitude
88,000	X	0.90	=	79,200

In Canada: The input rating must be reduced by 10% for altitudes of 2,000 ft to 4,500 ft above sea level by an authorized Gas Conversion Station or Dealer. To determine correct input rate for altitude, see example and use 0.90 as derate multiplier factor.

2.Determine the correct orifice and manifold pressure adjustment. All models in all positions except Low NOx

models in downflow and horizontal positions use Table 15 (22,000 BTUH per burner).

Table 11 – Altitude Derate Multiplier for U.S.A.

ALTITUDE	%	DERATE MULTIPLIER
(FT)	OF DERATE	FACTOR*
0–2000	0	1.00
2001–3000	8–12	0.90
3001–4000	12–16	0.86
4001–5000	16–20	0.82
5001–6000	20–24	0.78
6001–7000	24–28	0.74
7001–8000	28–32	0.70
8001–9000	32–36	0.66
9001–10,000	36–40	0.62

*Derate multiplier factors are based on midpoint altitude for altitude range.

Low NOx models in downflow or horizontal positions MUST use Table 16 (21,000 BTUH per burner). See input listed on rating plate.

a.Obtain average yearly gas heat value (at installed altitude) from local gas supplier.

b.Obtain average yearly gas specific gravity from local gas supplier.

c.Find installation altitude in Table 15 or 16.

d.Find closest natural gas heat value and specific gravity in Table 15 or 16

e.Follow heat value and specific gravity lines to point of intersection to find orifice size and low-and high-heat manifold pressure settings for proper operation.

f.Check and verify burner orifice size in furnace. NEVER ASSUME ORIFICE SIZE. ALWAYS CHECK AND VERIFY.

g.Replace orifice with correct size, if required by Table 15 or 16. Use only factory-supplied orifices. See EXAMPLE 2.

EXAMPLE 2: (0-2000 ft altitude)

For 22,000 Btuh per burner application, use Table 15. Heating value = 1000 Btuh/cu ft

Specific gravity = 0.62 Therefore: Orifice No. 43*

Manifold pressure: 3.7-in. wc for high-heat 1.6-in. wc for low-heat

*Furnace is shipped with No. 43 orifices. In this example all main burner orifices are the correct size and do not need to be changed to obtain proper input rate.

3.Adjust manifold pressure to obtain low fire input rate. (See Fig. 54.)

36