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Preasure varies in shower

We recently had a Bosch Tankless Gas Water Heater GWH635ES installed. We now have trouble with some of our faucets as follows:
1. After water has heated up, the temperature of the shower (shower in tub configuration) fluctuates with no adjustment to the faucet. First the shower turns cold, then very hot and then comfortable. This happens 2 to 3 times in the course of a shower.
2. The fluctuation is more pronounced when using the hand held shower head attachment.
3. No other faucets or appliances are in use at the same time.
4. When the faucet in the tub is running and turned to hot, the preassure from the faucet drops.
5. Alternatly, in the kitchen faucet, the preassure drops when going from hot to cold.
6. The tub faucet is a Moen 9245 single knob unit.
7. The kitchen faucet is a Moen 7841 Single Handle eight inch center faucet.
8. Tha shower head is a Speakman Anystream.
9. The hand held showerhead attachment is from Wal-Mart and attaches to the pipe going into the showerhead.
Any advice would be greatly appreciated,

Judy in NJ

Re: Preasure varies in shower

i would suggest going to this web site for some self education. I wouldn't expect you to know how to install the water line and hook up the gas lines and all the technical points, but sometimes it is good to undertstand the basics.


Do you have an expansion tank? Did they use a mixing valve on the line? Or is there a check valve? Are the hot water lines and the cold water lines switched? Is the flow of the water going the right direction? I have serviced brand new tankless with all these problems. There should be a expansion tank and you do not need a mixing valve. The cold water supply was going into the hot water supply. All these things just take a little patience to figure out.

It also sounds like so of your water lines are undersized. If the water is cutting off when you open the faucet this is the main cause. I think all of your fixtures are ok, even the one from Wal Mart. If you have cold water pressure, the same should apply to the hot water side as well, since cold water will be entering the Bosch. It sounds like undersized pipes. Good luck. Process of elimination.

Re: Preasure varies in shower

According to the data sheet on that WH, it restricts water flow, more restrictions at higher temp, so you are bound to see a difference between it's output and the unrestricted cold water.
Your water flow going up and down. Are you on a well or city water? On a well the pressure pump pressure is set by the pressure switch and generally has a 20 Lb variance between on and off. As an example the pump runs the pressure up to 40 Lbs. and shuts off, it won't kick on again until pressure drops to 20 Lbs. The WH may be having trouble adjusting to the pressure difference.
The shower head may be a water saver head and the hand held unrestricted so you would run water through the hand held faster than the shower head.

Re: Preasure varies in shower

We have city water.

Re: Preasure varies in shower

The main thing with tankless water heaters is proper installation.
They require adequate gas pressure to reach expected temperatures and flow rates. To compensate for inadequate gas supply or lack of gas pressure the system will effectively reach the selected output temperature, but only by reducing the hot water flow rate.
Its motorized water valve makes this adjustment. Proper gas pressure will need to be delivered before the motorized water valve will allow greater flow rates through it.
Lowering the set temperature will also allow increased water flow rate through the motorized water valve.

Another important factor is the temperature of the incoming water supply which affects the output of the water heater.
For example …. In order to maintain an output temperature of 120 degrees the following is the flow rate ….

40 degrees = approx. 3.75 gpm
50 degrees = approx. 4.1 gpm
60 degrees = approx. 4.75 gpm
70 degrees = approx. 5.75 gpm

They also require a ¾ inch cold water supply to provide the required flow. Many situations homes are plumbed in a trunk and branch ( T&B ) configuration with only a ½ inch supply coming from the water meter. Then a transition from ½ to ¾ is made at the connection to the water heater …. This may not be enough to maintain the flow and maintain minimum pressure of 30 psi.
It’s also required to have a ¾ inch hot water outlet from the heater to maintain the flow from the unit.


[COLOR=#000000]No other faucets or appliances are in use at the same time.

Well sort of … when you are using the shower you are mixing cold water to regulate the temperature …. Similar to opening another tap. When this is done some of that cold water is being diverted directly to the shower and being removed from the path to the water heater … which reduces the input flow to the heater.

The water heater requires a minimum of 0.8 gpm to operate and when using a shower head rated at 2.0 gpm would depend on how much of that is cold water.

The burner is activated by a water flow valve. Numerous water related problems can cause this
water valve to malfunction such as:

Insufficient water flow volume to activate the burners at its minimum flow requirement;

Dirt in the water flow valve causing it to malfunction;

Sediment build-up in faucet aerators, orshower heads;

Uneven pressures between cold and hot
(with single lever mix valves);

Plumbing crossovers.

These water flow related problems can cause the
heater to deliver less than its full output, or to fail to
ignite or to shut down completely.

Here are some recommendations from the manufacturer.

Cold water inlet filter screen is dirty
Remove water inlet filter screen and clean. The screen is located at the inlet connection to the water heater, disconnecting the main cold supply line at the heater is required. Check and clean faucet aerator screens too.

Crossover in plumbing
The heater activates when there is sufficient water flow through its water valve, a minimum of 0.8 gallon/minute (3 l/m) is required. If there is a crossover in the plumbing, the necessary hot water flow rate through the unit may not be reached, even though the flow at the hot water tap is sufficient.
A plumbing crossover can be caused by a failed washer at a single lever faucet, incorrect plumbing or a faulty mixing valve in the piping. The crossover will create a back pressure
on the water heater and prevent an adequate flow of water through it.

To confirm there is no crossover in the plumbing, shut off the cold water supply feed to
the water heater.

Individually open each of the hot water taps. There should be no water flowing under
a constant pressure.

If there is a constant flow of water then the plumbing crossover must be
corrected before the heater can operate properly.

Unbalanced pressure in water lines
Any restriction caused by the water heater in the hot water system can result in uneven pressures
between the cold and the hot.
In such cases when mixing cold water at the tap (manually or automatically through a mix-type shower valve) the lower hot water pressure may be overpowered by the higher cold water pressure in the building and cause the hot water flow rate to fall below the minimum rate required. A pressure balance valve will help to resolve this symptom.

Temperature balance valves
If the outlet water temperature is set too high, the heater can produce temperatures that are too hot. A temperature balance shower valve can automatically mix in cold water to reduce such hot water temperature.
In the event of any temperature instability with the use of a temperature balance shower valve, refer to shower valve manufacturer's instructions for internal adjustment setting.

Check the unit itself to see if it’s displaying any error codes which may also help in determining the problems.

Hope this helps.:)

Re: Preasure varies in shower

We also have a Bosch GWH-635-ES and the water pressure does the same thing (we have a well) and a friend has the same one and same problem. I was told by Bosch that it wasn't installed properly. We have lived with it for about a year after installing. We now have a error code A2 that Bosch said was a blown fuse and to call a technician. We live 30 miles from a large city and their service tech said that we would have to pay $350 just for him to come to the house to find the problem. Then, the part would be $100 or more and that it would not be worth the money to have it repaired. I called several more than does not have a service contract with Bosch and was told that they have a lot of problems with them, it was probably a circuit board ($350 or more) and they do not sell them because of the problem. One service company had installed 2 and the owners both had the same problem and it was the circuit board. I am very unhappy that it has been a little over a year and Bosch is not being helpful to try to resolve the problem. We are thinking about tearing it out and replacing with a different brand.

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