Grohe 34 900 thermostatic mixer - water won't get hot

[LisaBrooklyn]

I've looked through many previous posts and am not sure of the answer to this particular problem. I have a Grohetemp 34900 thermostatic mixer with 3 taps - tub, handheld head and shower head. A little while ago the shower head stopped getting beyond mildly warm, even if we pushed the dial to the hottest setting. But we discovered if we opened one of the other taps a little, then the shower will get hot.

I haven't tried cleaning the valves and cartridge or replacing them yet, which I will do (we only just figured out how to get the handle off and open it), but from everything else I've read this doesn't sound like it will address this temperature problem. Maybe it will, I'll see, but any thoughts on why it gets hot again when another tap is opened and if it's something else that needs to be addressed?

FYI we live in a 6-story apartment building and don't have control over the heater and pressure, but we'd hear from others if there was a building-wide issue like this. Thank you!

 

[TJRAD]

as an experiment : try removing the shower head so just the shower arm remains. does the water come to temperature with the head removed? if yes then the shower head is most likely plugged and is causing your problem. do this before taking your grohe valve apart.

 

[Sponsor]

 

[LisaBrooklyn]

Good suggestion, but I wonder how if the shower head is plugged it's limiting only hot water, and why it then gets hot enough when the other tap is opened.

 

[TJRAD]

I need to ask questions to come to a solution: did you remove the shower head? what happened with the temperature? what Im thinking is your apartment building has a tankless water heater and that tankless needs a certain amount of flow to trigger the heater on. If your shower head is on the plugged side then the flow coming through your Grohe shower is not enough to trigger the tankless heater to come on. But when you open the bathroom faucet the flow is now high enough to trigger the tankless to fire and produce hot water.