I have a puzzle:

The water heater in a small building was too hot -- scalding. Turning the thermostat down to warm or even vacation didn't seem to make a measurable difference. We were about to have someone out to replace the thermostat.

The gas water heater is only one year old. This building is small. Four bath sinks, one urinal, 3 toilets, one kitchen sink.

We turned off the hot water valves in the bathrooms sinks for safety. We then observed that two sinks in ONE bathroom had just a trickle of cold water (each with their own shutoff valves). Hot water (when it was enabled) seemed to have full flow. Cold water in ALL other locations in the building seemed to have full flow.

I scratched my head and didn't put the two things together - asked someone else for their help and he noticed a hose bib outside had only a trickle and that it was part of the shut off valve for the building. So he completely opened the shut off valve.

We regained cold pressure in the bathrooms. We also saw a reduction in the hot water temperature. (I also verified that when turning the thermostat that the thermostat clicked over at the low temperature as I passed it -- which is what one wants.)

At first I assumed there must be two water entrances to the building, which would explain the full flow in some locations but not others (need to confirm there isn't a 2nd entrance. It seems so odd I'm having doubts about that theory.)

Note, this shutoff valve is also right next to the bathroom sinks in question.

I'm trying to come up with a model in my head how all of these things could be related.

Pressure built up in the water heater which pressurized part of the system? Why only part? Air in the water heater? But I never get any popping at the taps. The pressure release valve works fine.

Is there a coherent explanation of this behavior?

  • 1
    Your water heater is supposed to be scalding hot. The issue is legionella. Yes, this is relatively new. If you are concerned with scalding, get a modern mixing valve with scald protection built in. If you don't want to change your spigots, get tankless; they don't store hot water so they are not breeding grounds for leigonella. Aug 20, 2017 at 21:20
  • @harper The sinks do have some kind of mixing valve (single pull lever). Looks very similar (or is) this model: moen.com/products/Adler/…
    – rrauenza
    Aug 20, 2017 at 22:46
  • The government body licensing the school wants it 120 degrees at the tap so, yeah, we'll need to figure out something to cause a 20+ degree drop.
    – rrauenza
    Aug 20, 2017 at 22:52
  • That explains everything, then. Due to the cold valve being near-closed, the anti-scald valves were doing their college best but were unable to add enough cold water to control the hot temp. Aug 20, 2017 at 22:55
  • @harper, learned something new today. Excellent question too rrauenza. Can we upvote @harper?
    – noybman
    Aug 21, 2017 at 3:46

3 Answers 3


The final piece of the puzzle reveals what was going on.

What I thought was a 2nd or 1st building shutoff was a shutoff outside the building that only shuts off those two specific bathroom sinks' cold water supplies.

Why!? Who knows! It's an old building.

But that explains why those two taps were too hot and why none of the other fixtures in the building were affected.

Another factor was that I was using the kitchen sink faucet to test the water temperature, but those are probably not anti scald.


The legionella game

In 1976, people who attended an American Legion convention started dying for no apparent reason. The Center for Disease Control went on red alert, and years of exhaustive research discovered that a bacteria was responsible. It was later discovered this breeds in water of a certain temperature, such as that found in the hotel's air conditioning coolers, municipal water systems, and especially water heaters.

The cure was only developed in the early 2000s and many people are still unaware of it. It is to store hot water at least 55C (131F), preferably 60C (140F). Except this will scald. So additional steps are required: Either

  1. tankless heaters that output safe temperatures. They don't breed legionella because they don't store water at breeding temperatures. They do allow continued use of 2-valve (hot and cold) faucets.

  2. "smart water valves" which automatically mix cold and hot (or throttle hot) to assure output temperature is in safe limits. These must, by necessity, be mixing valves with one faucet. This is why every facility has these now.

What happened here

Someone closed (or nearly closed) a cold water supply valve downstream of the hot water supply -- cold flow was restricted, but not hot. This seems like a very unusual condition; in most houses that would be impossible unless someone turned off the cold valve under the sink.

There are two ways to build an anti-scald valve: #1 Reduce "hot" flow until output is within safe temperatures, turning it off if necessary, and #2 increase "cold" flow until output is within safe temperatures. Your valves use method #2. The problem is, this method is vulnerable to the cold water being shut off. The valve was doing its level best to blend in additional cold water; there just wasn't enough cold supply. In all fairness to the valve manufacturer, that is an unusual condition.

Your facility staff would have had to turn off a very particular valve for this to happen, and honestly that valve should probably be eliminated for these here reasons. Perhaps it was installed before legionella** when you could just store hot water at 120F.

Honestly, if you have the AC power for it, I'd install electric tankless heaters directly at point-of-use, and not look back. They're cheap, and instant. Trying to adapt a boiler system for legionella/anti-scald is too much work and hazard, and still gives slow hot water that runs out.

** there isn't really a "before legionella", it's probably been killing people since there was hot water. Just the 1976 convention made us notice.

  • Any explanation why I had full cold water and hot water flow even though the service entrance was mostly turned off?
    – rrauenza
    Aug 22, 2017 at 4:18
  • @rrauenza The odd positioning of the odd valve, surely. That valve lacks any reason to exist, I don't see where its location will make any more sense. Aug 22, 2017 at 4:21

I'll go for the obvious: The thermostat is set correctly, right?

You can find the thermostat on the side of the tank about one-quarter the way up from the floor. It should be set at 125 degrees F.


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