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: 1-866-642-3198

Fax: 1-603-584-1687

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).

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