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Bosch Therm 520HN Technical Service Bulletin page 2

Removing & cleaning the burner
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1 2 | Removing & Cleaning The Burners
Cleaning the burners (Fig. 5)
1. Use a soft wire brush across burner fins to loosen any debris
and then vacuum out.
2. Check to see that the 18 brass gas injectors in the rear of the
burner assembly are clean. To access, remove 8 Philips head
screws to the left and right of the two center brass Philips head
screws at the rear of the burner. The burners should separate
into two halves exposing injectors. Blow clean with compressed
air or carburetor cleaner and wipe dry with cloth. Do NOT poke
injectors or at tempt to clean with drill bits, wires or other
objects. Clean rear venturi passages with a pipe cleaner and
soapy water. Rinse clean and let dry before reinstalling. Follow
instructions in reverse to reconnect burner halves.
B B urner fins
Burner fins
B B urners dis i i assembled
Burners disassembled
Data subject to change without notice | Printed in the USA | BTC 746002313 A | 07.2010
Reinstalling the burners
Figure 4
1. Reinstall burners into burner area.
2. Reinstall set screws on right side of gas valve and tighten.
3. Reinstall right and left burner brackets with the four screws. The
brass screws attach to frame.
Reinstalling pilot assembly
Models: 520PN, 330PN (Fig. 1):
Reinsert thermocouple and snap into place. Thermocouple tip
should sit 1/4" above the burner bed.
Put spark electrode into place and attach with clip.
Slide upper end of pilot tube into bracket on burner assembly
and reinsert pin.
Slide base of pilot tube to the right into clip. Ensure pilot air screen
is installed between pilot tube and pilot orifice.
Models: 520HN (Fig. 2):
Slide pilot air screen over pilot tube.
Slide pilot assembly into place. Reinsert pilot shield and secure
with screws to the burner assembly.
Returning heater to service
Models 520PN. 330PN:
1. Turn on gas and light pilot. Test for possible gas leaks where
burners connect to the gas valve.
Models 520HN:
1. Turn on gas and check for leaks where the burners connect to
gas valve by applying leak detecting solution as you initiate
Figure 5
water flow to activate the heater. Allow enough time to bleed all
air in gas line. Bubbles are a sign of a leak.
All models:
If there are bubbles at either connection immediately shut off
water and gas. Loosen the burner set screws on the right side of
gas valve. Remove the burners and inspect the o-ring. Replace if
damaged. Reseat burners and tighten set screws. Check again
for leaks.
3. Replace front cover when complete.
Gas injectors
Gas injectors r r
Technical Service Bulletin
Bosch Thermotechnology Corp.
50 Wentworth Avenue
Londonderry, NH 03053
Tel: 1-866-642-3198
Fax: 1-603-584-1687
Bosch Thermotechnology Corp.



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This manual is also suitable for:

Therm 520pnTherm 330pn