Troubleshoot Therm 520 HN

No spark at pilot with water flow

• Refer to service bulletin CT-09
• Ensure that the On/Off button on control box is pushed to the "On" position;
• Ensure that cold water supply is attached to rear of water valve and hot water outlet is on the left; 3. Measure voltage at hydrogenerator. Disconnect the wire connector at hydrogenerator and measure voltage on hydrogenerator side of connector. – a. If the wires from the hydrogenerator are black and red, the voltage should be at least 1.3VDC. – b. If the wires from the hydrogenerator are blue and brown, the voltage should be at least 2.8VAC. If the voltage is inadequate, refer to bulletin CT-29 to clean the hydrogenerator;
• Check for crossover: – a. Turn off cold water supply to heater only. If no shut off valve is present, install before continuing. – b. Turn on each hot water fixture to hot only. – c. Wait 5 minutes. – d. If any water is flowing, even a trickle, there is a crossover or the scald protection in a shower valve is adjusted incorrectly. Consult plumber or faucet valve manufacturer for repair or adjustment;
• Rotate flow control (right knob) to full clockwise position for the lowest activation rate of about .6 gallon per minute. As the knob is turned counter-clockwise, the activation rate will rise to about 1.1 gallons per minute;
• Check for obstructions to water flow. Clean inlet filter screen in water valve, whole house filters, showerheads and faucet aerators. Obstructions such as dirt, debris or mineral deposits will decrease the water flow through the heater below the activation rate.
• Check for tight and clean wireconnections in heater: – a. wire from ignition unit to spark electrode – b. wires to flue gas sensor on draft diverter – c. wires to overheat sensor on outlet pipe – d. wires to overheat sensor on heat exchanger – e. wires to microswitch – f. ground wire connection between ignition unit and fixing screw
• If a powervent and spill switch are installed, check the reset button on the spill switch (installed on the draft diverter). Consult powervent manual for more details. – a. If the button clicks when pushed, the switch was tripped. Check venting for restrictions and exhaust hood damper for correct operation.
• Check microswitch on bottom of gas valve. Disconnect wire connector at microswitch and jump connector on wiring cable. Run water through heater. – a. If heater sparks, check continuity through microswitch connector with water flow. If circuit does not close one of the following may be the cause; microswitch is faulty, water valve needs maintenance or flow is inadequate.
• If heater still does not spark, contact Bosch technical support.

Spark appears when hot water is turned on, but pilot and burners will not ignite

• Refer to service bulletin CT-10
• If there is a spark from the spark electrode with water flow, but the pilot does not ignite: • With water flowing and heater sparking, apply flame
• from a match or lighter to the rear of the pilot burner. Avoid applying flame to flame sensor. If main burners only light, repeat procedure and watch for brief flame from pilot burner: 1. If pilot does not light: – a. Measure voltage between pilot electrovalve wire
• connection and ground when heater is sparking (Do not disconnect wire); Voltage must be at least 1.1VDC. If less, contact Bosch Tech Support. – b. Clean or replace pilot orifice. See Bulletin CT- 22. – c. If heater has never fired, check for correct gas type on rating label on right side of cover. If incorrect, exchange heater for correct gas type. It is not practical to convert a heater to a different gas
• type. – d. If heater has never fired or gas lines have recently been serviced, there may be air in the gas line. Turn water on, wait until sparking has stopped, turn water off and back on again. Repeat until pilot lights or have gas technician bleed the gas line. – e. Verify that gas shut off valves are open. Gas must be supplied to heater. – f. Measure gas pressure at inlet tap with manometer. See service bulletin CT-04. – g. If no gas is present, Maxitrol regulator supplied with heater may be upside down or locked. Reinstall correctly or unlock regulator (see Bulletin CT-14). Measure gas pressure on gas line before Maxitrol. If over 14", Maxitrol should lock up to prevent high gas pressure from damaging heater. – h. Check for tight and clean wire connections to pilot and burner electrovalves. Remove connector and clean with pencil eraser if dirty or corroded. – i. Check pilot electrovalve function: Turn off gas supply to heater, remove wire connection from electrovalve, connect positive terminal from a single cell battery (1.2-1.5VDC) to electrovalve terminal and a jumper wire from negative terminal of battery to ground. When connection is complete, there should be an audible "click" from the electrovalve. If no click is heard, electrovalve may be faulty.
• If pilot and burners light: – a. Check for drafts around pilot burner. See Bulletin CT-03 but apply smoke to pilot burner without heater operating. If smoke is blown away from pilot burner, find cause of draft and correct. – b. If an AQ4 powervent is installed: – I. Check for presence of time delay in powervent. If there is no time delay, replace with newer style AQ4 powervent with time delay. – II. If powervent has time delay, check for correct setting (2 seconds). – III. Check for adequate minimum vent length. Refer to AQ4 Installation Manual.

Pilot lights, but burners will not come on

• Verify that gas type indicated in the rating sticker located on the cover's right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane (See chapter 2.2).
• 2. Pilot flame should be blue in color and completely engulfing the flame sensor. If not, the pilot orifice may be dirty or clogged. See chapter 5.3 on how to clean the pilot and orifice.
• 3. If sparking does not stop when pilot is lit, the flame sensor may not be recognizing pilot flame. Verify that wire connection clip on wire from the bottom of the flame sensor is secure. If flame sensor is fully engulfed by the pilot flame, it may be dirty. Clean entire surface area of flame sensor with a pencil eraser or fine steel wool.
• 4. Confirm that wire connection to the burner electrovalve is secure (see components diagram for location of burner electrovalve). Inspect terminals for corrosion. To clean terminals, remove the spade connectors and clean with a pencil eraser.
• 5. Ignition unit, burner electrovalve or flame sensor maybe faulty. Measure voltage from the spade connection of the burner electrovalve to ground with wire connected (see components diagram for location of burner electrovalve). After the pilot lights, voltage should read at least 1 VDC. If voltage is proper, the burner electrovalve may be faulty. If voltage is not proper, ignition unit may be faulty.

Main burner goes out during hot water use

• Hot water flow rate is dropping below required activation flow rate. Turn flow control knob all the way clockwise. Fully open a hot water faucet and fill a quart container. If the container fills in 36 seconds or less, the flow rate (0.5 gallon per minute) is sufficient to activate the water heater. If flow rate is not sufficient, the water heater's inlet filter should be inspected and cleaned.
• See chapter 5.2.
  2. Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open every hot water tap supplied by the heater. Wait 5 minutes and check all taps. Any water running, even a trickle, is a sign of a plumbing crossover. Consult local plumber or service person for help correcting a plumbing crossover.
• 3. The minimum water pressure required is 30psi. For installation on a well system with use of a pressure tank, the lowest pressure range setting recommended is 30- 50 psi (2.07-3.45 bar).
• 4. Hot water is very hot out of the tap, requiring a lot of cold water to be added to it in order to attain a useable hot water temperature. The addition of too much cold water will slow the flow within the tankless heater, decreasing it below activation point, which shuts off the burners. The end result is nothing but cold water coming out of the outlet. Reduce the amount of cold water to be mixed by turning the gas control knob clockwise for lower hot water temperatures.
  See chapter 4.4 for details in lowering temperature.
• 5. A temperature limiter (ECO) is tripped. The water heater does not have a thermostat. If the inlet water is preheated, the unit will overheat, stopping the flow of gas. Supply heater inlet with cold water only.
• 6. Flue gas sensor is tripped. Lack of adequate combustion air or venting improperly by reducing pipe diameter, improper use of elbows or exceeding maximum vent length will trip the flue gas sensor. Confirm that venting and combustion air meet requirements in this manual. Refer to service bulletin CT-03.
• 7. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to overheat and trip the temperature limiter (ECO). To descale the heat exchanger, consult section 5.6 on page 18.
• 8. The water valve assembly, which allows gas flow to the burners, requires periodic maintenance (every 2-5 years depending on water quality and use). See section 5.2.

Hot water temperature fluctuates at tap

• Close installer supplied cold water shut off valve (if none installed, install before proceeding). Open every hot water tap supplied by the heater. Wait 5 minutes and check all taps. Any water running, even a trickle, is a sign of a plumbing crossover. Consult local plumber or service person for help correcting a plumbing crossover.
• 2. Check for a clogged inlet water filter screen. See chapter 5.2.
• 3. Hot water is very hot out of the tap, requiring a lot of cold water to be added to it in order to attain a useable hot water temperature. The addition of too much cold water will slow the flow within the tankless heater, decreasing it below activation point, which shuts off the burners. The end result is nothing but cold water coming out of the outlet. Reduce the amount of cold water to be mixed by turning the gas control knob clockwise for lower hot water temperatures. See chapter 4.5. If the problem persists, go to www.boschpro.com for service bulletin CT-07.
• 4. The water heater is designed to modulate gas flow to the burners when water flow varies. If the water pressure in the home is erratic and the water flow is not consistent while a tap is opened, then the temperature of the hot water will fluctuate. The minimum water pressure for the home must be 30psi or greater. For installations on a private well system with the use of a pressure tank, the lowest pressure range setting recommended is 30-50psi (2.07-3.45 bar). The use of a pressure reducing/regulating valve before the water heater (directly after the pressure tank on well systems) may be an effective way to maintain constant water pressure to the water heater.
• 5. If the inlet water temperature is not consistent then the hot water temperature from the water heater will fluctuate. The water heater modulates its burners when flow rate is changed. It does not have a thermostat and will not adjust to changes in inlet water temperature.
• 6. If the inlet water temperature to the water heater is above 70 F, the heater may produce outlet temperatures that are too hot for most uses. A temperature balance shower valve can automatically mix in cold water to reduce very hot water temperatures. This can slow the flow within the tankless water heater, decreasing it below activation point, which shuts off the burners. In the event of any temperature instability with the use of a temperature balance shower valve, refer to shower valve manufacturer instructions for internal adjustment to minimize the amount of cold water the valve is adding.
• 7. The gas pressure also needs to be stable and adequate. Ensure that gas pressure meets specifications in chapter 3.8. A gas pressure reading is needed to proceed further. Contact your original installer or a local gas technician to obtain this reading.

Water is too hot

• Verify that gas type indicated in the rating sticker located on the cover's right hand side coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane (See chapter 2.2).
• 2. Adjust the power adjustment knob clockwise for cooler temperatures. See section 4.4.
• 3. Inlet water is preheated. This model does not have a thermostat and will not modulate flame height in order to prevent overheating. It is designed for cold water supply only. Supply the heater with cold water to the inlet.
• 4. Increase flow rate. Restrictions in the water path can slow the flow of water through the heater, resulting in very hot outlet temperatures. Restrictive showerheads and faucet aerators should be cleaned or upgraded with less restrictive ones. In addition, the inlet filter on the water heater should be cleaned and inspected. See chapter 5.2.
• 5. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to boil and produce extremely hot temperatures. To descale the heat exchanger, section 5.6 on page 18.

Water is not hot enough

• the water heater should be cleaned and inspected. See chapter 5.2.
• 5. If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to boil and produce extremely hot temperatures. To descale the heat exchanger, section 5.6 on page 18.

Low hot water pressure

• Confirm adequate flow rate through the unit. With cold supply shut off to the unit, disconnect hot outlet side to the unit. Place a gallon jug under hot water outlet to catch water. Open cold water supply and measure flow rate out of the unit. With temperature adjustment knob set all the way clockwise, the flow rate should be close to 2GPM (filling the jug in 30 seconds or less). If the flow rate is not adequate, inspect and clean inlet filter screen. See chapter 5.2.
• 2. Confirm that the incoming water pressure to the unit is above the minimum required pressure of 30psi.
• 3. Inspect the water path outside the heater for obstructions. Make sure all showerheads, faucet aerators and whole house filters are clear of debris.
• 4. Be sure to run only one major fixture at a time with this water heater. Opening too many taps at one time can disperse water flow resulting in diminished flow or pressure at all outlets.

Noise when heater is running

• If the water has a high mineral content, the heat exchanger may be scaled internally. This restricts the water path, causing the water to boil resulting in noise and overheating. To descale the heat exchanger, consult section 5.6 on page 18.

Burners do not burn cleanly, yellow flames when operating

• Verify that gas type indicated in the rating sticker located on the heater's right hand side, coincides with the gas type you are using. NG is a natural gas unit and LP is for liquid propane (see chapter 2.2).
• 2. For proper combustion at high elevations the inlet gas pressure must be set at installation to the values in table 3 on page 11 (while operating at maximum BTU). A gas pressure reading is needed to proceed further. Contact your original installer or a local gas technician to obtain this reading.
• 3. Combustion air is inadequate. Ensure that adequate combustion air is being supplied to the unit in accordance with requirements in the installation manual. See chapter 3.3.
• 4. Verify that the water heater is vented properly. Ensure that the venting meets requirements in chapter 3.6.
• 5. If the burner flames burn yellow or orange the burners must be removed from the unit and cleaned. Once the burners are removed, the fins in the top of the heat exchanger can be accessed from below and brushed or vacuumed clean. Consult service bulletin CT-13 on www.boschpro.com for removing and cleaning the burners.