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I have a single-handle (rotates 270 degrees) faucet in my tub/shower. I've noticed that it is always mixing in cold water, even when turned all the way to hot. I can prove this by holding the cold water supply copper with my hand and feeling it get colder as the water runs on the hottest setting.

According to this article: http://www.selfhelpandmore.com/plumbing/how-to-adjust-a-single-lever-shower-faucet.php, the 270 degree position should be hot water only.

I have read the instructions for my valve and the hot water stop is set such that it is not restricting the rotation of the handle in any way. There is no documented way to get hotter water out of the valve.

Also worth noting is that this is a brand new Grohe valve, replacing an old valve which had the exact same behavior as this one. Seems odd two valves would have the same problem if it were indeed a problem. Maybe it's just how pressure balancing cartridges behave?

The reason this is bothering me is because if not for this issue, I would be able to set my water heater's temperature lower. To get adequate heat out of the bath faucet, my kitchen sink is unpleasantly hot (kitchen sink always temps 10 degrees hotter than the bath faucet - it must not mix cold water in).

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Which model Grohe? What rough in base is used? And what jurisdiction are you in for building code purposes? –  Bryce Feb 3 at 22:58
    
Do you have a pressure reducing valve in your house? Bell shaped device usually located near the shutoff. –  Steven Feb 4 at 1:13
    
See also diy.stackexchange.com/questions/30271/… –  Bryce Feb 5 at 8:59
    
For what it's worth, in the UK (which is a much less litigous culture), mixing valves to have an anti-scald stop -- but also have a button to override it. Interesting compromise. Would never survive in a US court. –  keshlam Sep 21 at 20:36

3 Answers 3

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.

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This is very helpful, thank you. One question remains unanswered, though, which is whether what I'm experiencing is normal for this type of valve. –  Greg Smalter Feb 5 at 13:57
    
I would also love to rule out a pressure difference but I'm really not sure how to do that. –  Greg Smalter Feb 5 at 13:58
    
The behavior is not universal behavior, nor is it unheard of. You can get an idea of pressure by watching flow of hot & cold from a nearby fixture or the same fixture at different handle angles. I think it worth your while to call Grohe and then even better if you report back here on what they say. Thermostatic values are considered superior in this way and others. –  Bryce Feb 5 at 23:28
up vote 3 down vote accepted

According to Grohe, the manufacturer of the valve I am using, this is normal behavior for their valves. The max ratio they provide is 95% hot/5% cold. They also said this is true for their thermostatic valves.

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If you install a pressure reducer on the cold, you'll get more heat (the 5% figure is at assumption of equal pressure). You may have low hot pressure which is why you notice this. –  Bryce Feb 19 at 21:28
    
@Bryce I am open to the idea that I may be getting more than 5% cold, but it's pretty difficult to have different hot/cold pressure, and tests from my other sinks with independent shutoffs indicate roughly equivalent pressure, and I also don't really know how to fix the problem if there actually is one. In any event, wouldn't my problem be the opposite of what you said - low cold water pressure? With low cold water pressure, the valve would compensate by providing less hot water and more cold water, which is what's happening. –  Greg Smalter Feb 24 at 14:19
    
Balance valves are built to fail safe, therefore there will always be cold water present to cut the water temperature. It's part of the design. –  Fiasco Labs Sep 21 at 18:53

Does the valve have an anti-scald feature? This may be what it is.

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Yes (like all new valves, I think), which is why I mentioned the pressure balancing cartridge (which I think is the same thing). –  Greg Smalter Jan 18 at 14:01
2  
So what are we saying? This is just the way it is with anti-scale pressure balancing values, even when the pressure is equal on both sides? –  Greg Smalter Jan 25 at 14:20
    
If it is the anti-scald feature, if one lowered the hot water temperature enough, shouldn't it eventually no longer mix in cold water? In other words, the hottest water coming out would not change even if the hot water temperature was varied within a certain range. While you may not like the hottest temperature, it shouldn't drop if the hot water temperature were lowered, but still as hot or hotter than the outlet temperature. –  bcworkz Jan 30 at 20:34
    
@bcworkz No, because the anti-scald is based on pressure, not temperature. If the pressure of the cold water drops, it will reduce the hot water flow so you don't get burned. It doesn't know what temperature the water is. –  Greg Smalter Jan 31 at 13:38

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