I have a Gröhe thermostatic valve which came with the house. It's somehow broken such that you can't get the water as hot as advertised unless the dishwasher is running (?). And it's installed behind tile, so we can't just get a plumber to come in and replace it; we have to also line up someone to re-tile the shower immediately afterward.
We also have a pair of Gröhe sinks. We had to replace all four expensive, proprietary faucet hoses one-by-one as they ruptured. And then a couple more. It turns out they're short enough that it's hard to install them without kinking, and that eventually causes them to rupture.
I'm not a Gröhe fan.
(It also takes several minutes for the water to come to temperature in that particular shower, though it's only about 10 feet away from the water heater. I think the sink fixtures are affected too which suggests it's not all the shower valve's fault. It might be that we have pipes that are indirect and/or way too high-capacity, such that the total volume of water to drain between the water heater and the bathroom is excessive, and this is particularly obvious when using a low-flow shower-head.)
All thermostatic valves have cartridges that eventually wear off and need to be replaced. Cartridge is accessible from outside by removing the handle held by the side screw (handle can be stuck and hard to remove) and then unscrewing the cap. Remove the cartridge, look at the part number and buy the replacements at Amazon. Obviously cut off water to the shower first.
Also handle has a small button on the bottom which blocks it from going into super-hot range. If you need hotter than normal water, press and hold this button while rotating the handle. (Handle is supposed to be calibrated when installed so digits on the dial match water temperature, but nobody does that)
Some even have integrated shutoff valves, so you can replace the cartridge without shutting off the water anywhere else! What a marvelous world we live in.
Wow, I wish the last one I encountered had that. Plumber hooked up the shower valve fixture backward at my dad's new place. Also didn't even properly tighten the screws. Thermostatic valves aren't reversible, so it just didn't work right at all (basically got to choose full-hot or full-cold). They neglected to provide a shutoff for the shower, and he wasn't sure about draining whole system (it's an over-engineered water system with well-draw, sump pump, water softener, multiple redundant filters, gas water heater, array of in-floor heating, hydronic air furnace, and who knows what else). Access around/behind the fixture plate was non-existant.
In retrospect probably should have just clamped the soft lines upstream to effect the repair.
Sounds like an issue with your heater. Not sure why your dishwasher changes anything, they normally run off cold water and use their own heaters. Sounds more like you run your kitchen tap while filling the dishwasher, and flushes the cold water out of your pipes, and gives your heater enough time to get to temp.
Have you tried see if you can set the output temperature of your heater higher? Could be the output temp of the heater is very close to “shower” temperature, and a lot of heat is lost heating up pipes etc, something that could be sped up by increasing the heaters output temp, or improving the insulation on your hot water pipes.
> Not sure why your dishwasher changes anything, they normally run off cold water and use their own heaters.
For North America at least, this is not true. Dishwashers are hooked up to the hot water supply[1] (and it's a good idea to run your sink tap hot to "prime" the hot water for the dishwasher).
They _also_ use built in heating elements to keep the water hot, and to dry the dishes, though.
The water's hot enough everywhere else (this bathroom's sinks after the aforementioned delay, other bathroom, kitchen). The heater's set to ~130 deg F, which iirc is the upper limit of what's recommended and considerably above the ideal shower temp.
It really seems to be the dishwasher running that makes the difference. I rarely use the shower in question, but my wife does every evening, and it apparently doesn't help if I've done all the rinsing/hand-washing and loading of the dishwasher but not actually hit start. I don't understand the mechanism, but I believe her. I'm not sure if the dishwasher draws from the hot or cold tap.
We also have a pair of Gröhe sinks. We had to replace all four expensive, proprietary faucet hoses one-by-one as they ruptured. And then a couple more. It turns out they're short enough that it's hard to install them without kinking, and that eventually causes them to rupture.
I'm not a Gröhe fan.
(It also takes several minutes for the water to come to temperature in that particular shower, though it's only about 10 feet away from the water heater. I think the sink fixtures are affected too which suggests it's not all the shower valve's fault. It might be that we have pipes that are indirect and/or way too high-capacity, such that the total volume of water to drain between the water heater and the bathroom is excessive, and this is particularly obvious when using a low-flow shower-head.)