Bosch 520HN Technical Service Bulletin

Technical Service Bulletin:
Cleaning & Replacing The Pilot Assembly
Models: 520HN, 520PN, 330PN
WARNING
LP & NG are extremely flammable. Take extra
precautions when performing any work to the heater.
Removing pilot tube and orifice - 520PN / 330PN Models
1. Shut off gas to heater and remove cover per installation manual.
2. Remove wire clip holding upper end of pilot tube into burner
bracket. (Fig. 1)
Pilot wire clip
Pilot tube
Burners
Thermocouple
lead
Pilot air screen
Pilot orifice
3. Remove lower end of pilot tube from black clip on gas valve being
careful not to damage cylindrical air screen
4. Lift up pilot tube and remove from bracket
Cleaning pilot tube and orifice - 520PN / 330PN Models
1. Blow out pilot tube and air screen.
2. Remove brass hex-headed pilot orifice from gas valve with 8mm
wrench. Black clip may have to be removed with pliers to access
orifice with socket wrench
3. Clean orifice with compressed air or aerosol carburetor cleaner.
(NOTE: Do not ream or poke any object through orifice opening).
Reinstalling pilot tube & orifice - 520PN / 330PN Models
1. Screw clean or new orifice into gas valve and tighten with 8mm
wrench. Do not over tighten.
2. Place cylindrical air screen over pilot orifice. Angle top of air
screen outwards.
3. Feed bottom of pilot tube into angled cylindrical air screen. Push
bottom of pilot tube with air screen into black clip until it snaps
into place. NOTE: Air screen may become deformed during this
process. Simply mold back into cylindrical shape by hand.
4. Replace cover, open gas supply and return heater to service.
6. If pilot flame is not strong and blue, replace orifice (NG is
stamped 4C, LP is stamped 3C).
Figure 1
Removing pilot assembly and orifice - 520HN Model
1. Shut off gas to heater and remove cover per installation manual.
2. Remove 2 philips head screws holding pilot shield and pilot
assembly to front of burner. Remove pilot shield (Fig. 2).
3. Remove pilot assembly from burners (with spark electrode and
flame sensor) being careful not to damage cylindrical air screen
on bottom (Fig. 2).
Pilot shield
Pilot bracket
screws
Burners
Pilot air
screen
Pilot orifice
Cleaning pilot tube and orifice - 520HN Model
1. Blow out pilot tube and air screen.
2. Remove brass hex-headed pilot orifice from top of pilot
electrovalve with 8mm wrench.
Figure 2