Troubleshoot Therm 520 HN
Introduction
Many of the questions customers ask regarding operation of this unit can be answered by following the troubleshooting steps as outlined below. Visit our service bulletins. For best results, perform each step before proceeding to the next. The suggested solutions may require that the cover be taken off. (See chapter 3.5 of the manual).
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.
WARNING!
Warning: If you are unable to perform the tasks listed below, or need additional assistance please contact your original installer or a licensed gas technician.