We have an amazing power shower in our new house in New Zealand. Only problem is the hot water is very hot for approx. 30 seconds and then gets colder and colder. Start with the mixer handle in the mid hot position and it's very hot but then gets colder, moving the mixer handle toward hot gradually, keeps the water hot but still can only keep a very hot shower for about 1.5 minutes. Can turn off the mixer, leave for a minute or 2 and it seems to reset itself, but for shorter periods of time. I think the mixer might be at fault – shall I get a new one or do I need to investigate pipes? Thanks in advance for anyone's ideas or help. Liz
Shower starts v hot and powerful but gets colder quickly even though lots of hot water available
shower
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Official term is "water velocity" but for domestic plumbing this isn't really a thing you need to concern yourself with stick to pressure and flow rates. To answer some of your questions all in one:
The flow rate can vary between showers due to the plumbing that supply's them also the pressure can vary for the same reason, they can also have water restrictors that slow the flow rate down.
In Australia this is due to regs stipulating maximum flow rates for water conservation reasons, ( I will always remove these as I hate a weak shower).
As for the flow rate you can use a flow cup to check the flow rate on a unrestricted tap and a pressure gauge to check pressure available.
You can attach a booster pump to your water supply but if this is legal in your location I do not know, the use of a booster pump will also depend on the supply pipe you have to connect to and what the pressure is already if it is near the output of the pump there is no point installing the pump.
Background:
All new valves sold in the USA must meet a Federal anti-scald standard meant to prevent sudden surges of hot water. A typical inexpensive 'cycling' anti-scald cartridge works on pressure only, and does not sense temperature. For example you're showering and the sprinklers go off, the cold water pressure drops, and the valve will reduce the hot flow to match. The higher pressure source is throttled prior to the mixing valve.
In addition the valves have an adjustable limit on the mixing valve: usually adjustable with a plastic ring easily accessible after installation.
A great background video on this topic, showing a Delta brand single handle faucet, is at http://www.youtube.com/watch?v=mI2NZMadb1A . The video shows the 'cycling' pressure balance mechanism in action.
A cutaway of a typical valve is at http://www.popularmechanics.com/home/improvement/electrical-plumbing/whats-behind-your-shower-walls (Popular Mechanics).
These 'cycling' valves have lots of small parts: periodic cleaning may be required.
For your situation:
- Try and rule out a strong pressure difference between hot and cold as it arrives at this fixture.
- If you can install a two handle faucet, that obviously would solve the problem.
- If your base rough in unit is a 'Grohe Flex', you can insert different single handle models in place and try them (without re-plumbing).
A more expensive 'thermostatic' valve would not have the problem you describe: it would keep the hot line fully open until a certain temperature is reached, regardless of the cold line pressure. Grohe sells such models.
Best Answer
You probably just need to replace the cartridge inside the mixer valve.
Thermostatic mixer valves work by expanding and contracting from heat and cold, automatically adjusting the mixture to maintain the set temperature.
It sounds like yours is worn out, and as it heats up it is adding too much cold water, or not enough hot. Turning it off for awhile lets it cool off a bit and start working normally again until it heats up.
Replacing the cartridge is a pretty simple job - turn off the water, unscrew the screw that holds the handle in place, unscrew the cartridge with a wrench. The take the cartridge with you to the store to find the exact right one. It should come with instructions for aligning the valve and handle properly, so be sure to read them.