View Full Version : Grohe Thermostat Mixer Problems

02-18-2009, 07:57 PM
I am having problems with my Grohe 34 122 thermostat mixer and just can't seem to figure out what the problem is. The parts are all brand new. The problem is that the temperature cannot be adjusted or calibrated to the 100 degrees as required in the manual. The manual says that an adjusting nut will control the temperature if you turn it right or left. This doesn't seem to be the case with mine. No matter what direction you turn the adjusting knob, the temperature is over 110 degrees F. Is the thermostat bad, or is there someting else I'm missing.

I had a general contractor do all the plumbing, but this is the first thermostat mixer he has installed. He is baffled as well. It just doesn't seem to work as documented.

We are using both outputs to drive two shower heads (top and bottom). Each output has a volume control. The pressure seems fine since both shower heads have good flow at the same time (but temps greater than 110F). We ran 3/4" pipes to hot and cold with a reducer to 1/2" at the very end. This is an 1/2" mixer.

We also have a tankless water heater that is set to 120 degrees F, so I get an endless supply of hot water. My thermometer only goes to 110, so it may always be at 120. I have removed the flow restrictor on one of the heads. It doesn't seem to make a difference.

All hardware is Grohe. Any suggestions? I have a call into Grohe, but haven't heard back yet.

Thanks, Chris

02-19-2009, 09:19 AM
Are you sure the hot and cold aren't reversed?

02-19-2009, 11:10 AM
Yes we are sure the hot and cold are not reversed. Just solved the problem however. It turns out the cold water was shut off under the house to the new bathroom. Dohhhh. It now works exactly as advertised. We were able to dial in 100f exactly. And now we have even more water pressure than before. We had sufficient pressure/volume for two heads with just the hot water before.

Thanks, Chris

03-04-2009, 07:32 PM
Hi Chris,

I am well versed with thermostatic mixers. Here are a few things to consider. First, the thermostat must be fed correctly or it will give only all hot or all cold. By fed, I mean the hot pipe goes in the hot inlet and the cold pipe goes in the cold inlet. If the valve was installed upside down or the hot pipe is on the right instead of the cold pipe, the valve is being fed backward. The reason this causes a problem is because the valve is calling for cold water to mix with the hot water it is putting out, but if it is fed backwards it only adds more hot because it is coming through the cold side of the valve.

OK, check that first.

If that isn't the problem, then you need to check for proper flow. If your valve is equipped with service stops, turn them off and remove the thermostatic cartridge. Then open the cold (right) service stop and see if the cold water comes into the opening for the thermostatic element with any gusto. If not, clean any filters or check valves on the cold side of the valve and try it again. If you cannot get good flow the problem could be the service stop or debris in the piping. If this is a remodel, ask your plumber if he installed shut off valves in the piping to this bathroom. Perhaps the cold valve is partially closed.

If you have good flow from hot and cold to the cartridge and they are correctly entering the valve, then you have a bad thermo element. Thermo elements work by sampling the outgoing water temperature with a device that expands when it is hot and contracts when it is cold. The most common is the paraffin piston type. If the piston didn't hold it's charge then it cannot expand and move the shuttle that opens and closes the hot and cold inlet ports. Opposite the paraffin piston is a spring pushing against it. If the paraffin piston cannot expand due to a failure, the spring pushes the hot port open and the cold port closed, and there is no chance of it working properly. If this is the case you need to have Grohe send you a new cartridge.

Good luck,


03-05-2009, 08:20 AM
Thanks for your detailed response. I should have updated this thread, but the solution was a bit embarrasing. For this project, we had installed shutoff valves under the house. The cold water was shut off (doh!). We never really had any problem once we figured that out. But I will say that Grohe technical support was excellent. They helped us troubleshoot the problem. The shower and tile are now complete and I have to say I really love the thermostatic valve. And the look and finish of the Grohe hardware is just superb.

Thanks again, Chris

Edit: Oh I see I did update this thread on the solution previously. I guess EuroPlumber didn't see that. Anyway, I hope this discussion might help someone else.

03-05-2009, 06:18 PM
Hi Chris,

Because I was so long winded in my response your last post just beat mine. I'm glad you have it figured out now.