Bosch 520HN Reference manual

Technical Service Bulletin:
Cleaning & Replacing The Pilot Assembly
Models: 520HN, 520PN, 330PN
LP & NG are extremely flammable. Take extra
precautions when performing any work to the heater.
WARNING
Removing pilot tube and orice - 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)
Figure 1
Pilot air screen
Pilot wire clip
Burners
Pilot tube
Thermocouple
lead
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 orice - 520PN / 330PN Models
1. Blow out pilot tube and air screen.
2. Remove brass hex-headed pilot orice from gas valve with 8mm
wrench. Black clip may have to be removed with pliers to access
orice with socket wrench
3. Clean orice with compressed air or aerosol carburetor cleaner.
(NOTE: Do not ream or poke any object through orice opening).
Reinstalling pilot tube & orice - 520PN / 330PN Models
1. Screw clean or new orice into gas valve and tighten with 8mm
wrench. Do not over tighten.
2. Place cylindrical air screen over pilot orice. 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 ame is not strong and blue, replace orice (NG is
stamped 4C, LP is stamped 3C).
Removing pilot assembly and orice - 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
ame sensor) being careful not to damage cylindrical air screen
on bottom (Fig. 2).
Figure 2
Pilot shield
Pilot bracket
screws
Burners
Pilot air
screen
Pilot orifice
Cleaning pilot tube and orice - 520HN Model
1. Blow out pilot tube and air screen.
2. Remove brass hex-headed pilot orice from top of pilot
electrovalve with 8mm wrench.

12 | Cleaning & Replacing The Pilot Assembly Technical Service Bulletin
Data subject to change without notice | Printed in the USA | BTC 746002316 A | 07.2010 Bosch Thermotechnology Corp.
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: [phone]
Fax: [phone]
www.boschpro.com
3. Clean orice with compressed air or aerosol carburetor cleaner.
(NOTE: Do not ream or poke any object through orice opening).
Reinstalling pilot assembly and orice - 520HN Model
1. Screw clean or new orice into pilot electrovalve and tighten with
8mm wrench. Do not over tighten.
2. Insert pilot assembly into burner and t cylindrical air screen onto
orice and pilot tube.
3. Insert screws through pilot shield and pilot assembly into burner
and tighten.
5. Replace cover, open gas supply and return heater to service.
6. If pilot ame is not strong and blue, replace orice (NG is
stamped 2C, LP is stamped 1C).