Which heat trap?


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Old 11-23-09, 08:02 PM
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Which heat trap?

I have a 40 gal. elec DHWH. Cold feeder line runs up one side of the house, across the building between first and second floors, then drops down into the top of the tank. There are cold water run-outs all along the main feed that go to sinks, fridge, etc.

Problem: I am 99% that hot water is thermosyphoning up from the hot water tank into the cold water lines. The cold water taps closest to the tank run warm when they are first turned on. I'm not happy getting drinking water coming from the hot water tank.

I need a heat trap. Can someone make a simple call for me? Do I do an S-bend in the plumbing, put in one of the ball types or a rubber gate? Or does someone have a better idea?

Thanks
 
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Old 11-24-09, 04:15 AM
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You can do a heat trap with a 12" or so drop of pipework then back up.
You can put a check valve on the cold line, but then you must put a potable grade expansion tank on the line between the tank and the check or after the the tank on the hot side.

I would try a heat trap on the hot and cold side before doing a check valve.
 
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Old 11-24-09, 04:48 AM
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Here is a drawing of what to do. You can do it on both sides to stop the flow. I normally do not find it on the cold unless there is a cold water faucet dripping to create a pressure drop. The cold will either be piped into he bottom of the tank or into the top with a dip tube to the bottom. The bottom of the tank is very cool compared to the top of the tank when the unit is idle.
http://www.comfort-calc.net/Indirect_Piping.html
 
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Old 11-24-09, 09:47 AM
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I have found that with indirect tank that are stored hot and piped with 3/4" or larger, they can in fact slip stream up the cold pipe.
Hot will go up one side and cold will come down the other.
Never seen it with 1/2" pipe of course, but for sure with 3/4".

Many plumbers these days just put a check valve in to save the trouble of a heat trap, but they often "forget" about the expansion tank... Imagine.
 
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Old 11-24-09, 03:07 PM
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I have noticed that homes with private well water will sometimes send 'warm' water up the cold supply. This is because of the wider variation in system pressure. These systems might see 30-50 or 40-60 PSI low and high. If you open a cold tap when the pressure is high, as the entire system de-pressurizes before the pump kicks on, you will get some backflow out of the water heater into the cold supply.

Answer is the check valve and potable expansion tank on the cold supply as TO suggested.
 
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Old 12-03-09, 11:17 AM
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Pressure differential. Didnt occur to me but I guess that is also a factor. In any case, the series would be DHW -> cold water inlet -> expansion tank -> check valve... right? Like this?

| <---- cold water in
V
|
X <------ check valve
| _____
|____|____| <----- exp. tank
|
V
___|____
| |
| H2O |
| TANK |
|_______|

Also, no "heat traps" needed it doesnt seem like (U-bends in the pipe).

Thanks everyone!
 
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Old 12-03-09, 03:34 PM
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What you typed is different than what you drew?

Cold water supply --> check valve --> expansion tank tee --> water heater.

You still should have a heat trap on the hot outlet, because hot water will float up the pipe.
 
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Old 12-04-09, 03:46 AM
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OK. Heat trap on hot line. Also, you are right - the art doesnt match what I wrote... all makes sense now.

THanks again!
 
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Old 12-04-09, 02:31 PM
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I just noticed that you said 'U bend' in a post...

That heat trap doesn't need to look like the traps that are found in drain/waste/vent systems. You can and should make that up with regular elbows and such. As long as the hot comes off the top, down about a foot, and then back up again toward the home.
 
 

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