Cold getting into Hot?

[danxp]

hi...

tried searching but couldn't find anything conclusive... i'm trying to figure out if my plumbing system has a problem... while doing maintenance on my tankless water heater i did the following...
- turned off the supply into the water heater
- turned off the hot water going out of the water heater
- turned on all hot water fixtures to let drain (except for laundry, dishwasher)

all the fixtures didn't shut off completely but still had a low flow to it... my single lever faucets had even more of a flow (naturally, i guess) and my first floor shower had a full blast (of cold) while my two other showers simply had the low flow...

- shouldn't there be zero flow coming out the dedicated hots since the output to the water heater is shut off?
- is cold water somehow getting into the hot water supply before it reaches the fixtures?
- could my first floor shower be the culprit somehow allowing the cold to feed back into the hot?

other notes
- my water heater is set to 135 (my plumber had to set it that high to reach satisfactory temps at the 2nd flr faucets)
- water heater is in basement
- copper piping uninsulated throughout

thanks in advance!

 

[Reach4]

No experience... just thinking about how to troubleshoot. It seems to me that if
1. Incoming cold water is colder than the inside air temperature
2. You run the "hot" at a trickle on all of the suspect faucets

then the coldest trickle would be nearest the source of the leak. The difference will probably be small, so you would need a quick thermometer to distinguish the temperature differences. An electronic thermometer with a probe sounds good. The theory is that the cold water spilling into the hot water pipe would warm more the farther it is from the place of the leak.

 

[Sponsor]

 

[hj]

Since there is not "zero flow" with the water supply to the heater turned off, (and it has nothing to do with valves on the outlet, which are a convenience SOME times, but seldom necessary), obviously one or more single handle faucets are "mixing" the cold into the hot, which COULD cause a water temperature problem. But which one, or ones, takes some diagnostic and testing, which we would have to be there to diagnose.

 

[danxp]

thanks hj (and reach4)...

that is what i suspected and i think the culprit may be my 1st floor shower (single lever)... with the water heater output valve shut off and that shower faucet turned to hot all the way it was still coming out at a pretty good clip...

typically when something like this happens, is it normally a problem with the mixing valve or a faulty install? without opening it up myself i'm guessing mixing valve but i'm pretty clueless here...

thanks again.

 

[hj]

Depending on the make and model, some are notorious for "mixing". Some can be repaired to eliminate the problem, but with others is systemic and cannot be corrected.

 

[Reach4]

The I think problem could also be a tempering valve, if you have one or more of those.

 

[danxp]

all my faucets are delta... would it be a good idea to put a check valve before a suspected mixing valve or faucet?