I have a BOSCH 425N tanlkess hot water heater. In order to get hot water to flow to the sink taps, I need to turn more than one tap on (eg. if I want hot water at kitchen sink I must also turn a bathroom tap on). If I don't do this, the unit sparks, the pilot comes on, but fails to fire the burners. The bath and shower taps seem to be ok for the most part.

Do you live in a climate where the hot water line could have frozen? If your heater did freeze up, that could be very bad. This is because water actually expands when it freezes and can do a lot of damage to pipes and to the water heater itself. My other thought is possibly a clogged in line water filter. But typically a clogged water filter would make it run with reduced pressure rather than a complete shutdown all water flow.

take the cover off the 10 h unit light up the burner by turning on the tap turn the water flow tap on the front of unit (white knob) clock wise slowly feel hot water pipe coming out off unit when temp increases put cover back on check temp at sink if too hot turn knob anti clock wise

1. COLD MIX, HEATER DEACTIVATES
   If inlet water temperature is over 70°F due to geographic location, water maybe very hot out of the tap. This requires a lot of cold water to be added with it in order to attain a useable hot water temperature. The addition of too much cold water will overpower hot water demand from the tankless water heater. This slows the flow within the tankless water heater, decreasing it below activation point, which shuts off the heater. The end result is nothing but cold water coming out of the outlet. See: TWH E2-04 Overcoming fluctuating temperatures [PDF].
2. FLUCTUATING WATER PRESSURE
   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 hot water will fluctuate. The minimum water pressure for the home should be 30psi or greater. For people on well systems the minimum pressure range is 30-50psi. The use of a pressure reducing/regulating valve is an effective way to maintain constant water pressure to the water heater. Watts brand 25AUB- ¾" or N35B-¾" pressure reducing/regulating valves or equivalent is suggested.
3. CHANGING FLOW RATE
   Major changes in flow rate can adversely affect the output water temperature of the heater. Increases from one major fixture running to multiple fixtures running at one time can cause the temperature to fluctuate. Stay within the heater's specifications. See: TWH E2-03 Outlet temperature vs. flow rate [PDF].

From the Bosch website.

Good Luck,
Gary

The problem with all that type of water heater is that once the hot tap is turned on sufficient flow must be maintained through the water heater to keep the main gas burner alight. If the heat adjuster is turned up to high, then the person using the hot water will turn it down to a low rate which means the main burner will turn off and result in cold water. Try setting the heat control to the lowest setting that still gives hot enough water. If you have a thermometer adjust control for a temperature of around 50 to 55 degrees Celcius. You may have to fiddle with fine adjustments to get the temperature and flow rates to maintain a constant output of hot water. You may still have intermittent problems. If the heater only supplies baths or showers you could set the temperatuer even lower (45 to 50) to ensure that the hot tap is used more than the cold which will maintain flow and therefore heat.

It is likely the inlet filter is plugged. this reduces the flow and then the heater adjusts the burn rate down to protect itself. the filter screens are small and plug easily. It would be a good idea to install a larger pre filter on these heaters.

Bosch tech support will ask you to turn off the valve to the water heater and open a hot water tap. If you have water flowing it is likely from a shower valve. My experience is Price Pfister shower valves are usually the culprit. Let me know how this turns out.

Have you tried turning off the valves to the water heater and making sure you have no water flow out of the hot water tap? When this happens, often it is because of a problem with a shower cartridge allowing the cold water to cross over into the hot water side. It makes the unit think that the flow has reduced and the heater will turn down or shut off. Then things bounce back and forth Hot to Cold then back to Hot.

The water temperature may be set too high, or try turning the hot water off and opening a hot water tap. If the water is still flowing out the hot water tap, there is a problem with a faucet (likely a shower valve). It could also be water pressure below 30 psi.

Sounds like a flow sensor may be bad. It may be registering flow when there is none and it sending signal to ignitor and gas solenoid that there is.

basically the principle of the heater is water flows into a internal tank and is heated by an element. You open a hot tap and the mains cold water forces the hot out and replaces it inside the heater.As you have a flow problem i can only suggest you may have some kind of restriction or blockage in the cold water supply at the water heater or from the hot supply from the heater.If you live in an area where lime content is high then this can calcify when heated.If you can, isolate the water supply and electrical supply and check the pipework for blockage.Also investigate your hot tap as this may be the fault. Good Luck