Strange Issue with Hot Water Flow (Heat Trap)

[57]

My neighbour has a strange problem. After he uses a lot of cold water - say for his in-ground lawn sprinkler system, even after the system has been shut off and there is no cold water flow, he can then get very little hot water flow inside his home. Cold water flow is just fine - this only affects the hot water.

The flow is quite slow from any tap and even from the drain at the bottom of the hot water tank (gas). Once the hot water has been flowing for a while, there is a "bang" like a water hammer and the hot water flows fine - from all the taps in the home and from the drain at the bottom of the hot water tank.

Is there a back-flow preventer valve or something inside the tank that could cause the water to flow slowly and then "bang open" with normal flow?

I'm thinking that after the cold water is used for his sprinkler system, it's "sucking" some flow from the tank causing a back flow preventer valve to "stick" closed. Only after the hot water flows for a while do you hear the "bang" and the hot water flow is normal again. If there is no back flow preventer valve, perhaps there is some sort of "vapour lock" that is created inside the tank?

Thanks for any feedback.

The main inlet for the home is right next to the tank and the runs to the tank are very short with the inlet and outlet valves in those lines right next to the tank. I believe all lines are 3/4" copper near the tank. The obstruction must be inside the tank since the drain valve at the bottom of the tank also flows slowly at first but then flows fully after the "bang". This only happens every few days or after he's used his lawn sprinkler...

[hugh]

MY guess.

I didn't think there was any type of valve on the hot water output.

I'm wondering if the slowness is because the input valve (where the cold water comes in) is stuck so cold water is not coming in as he tries to draw hot water. Perhaps once a bit of a vacuum is created inside the tank it opens the stuck input valve with a bang.

[57]

Input valve appears to be a ball valve with a 3-4" lever attached. I don't think that's the issue. Thanks though. There is a similar valve on the output line, but that can't be the problem since the flow from the drain valve at the bottom of the tank is slow until the "bang".

Forgot to mention - hot water tank is relatively new - only two years old. Water from the drain valve is perfectly clear.

So, the only items between the slow flow from the drain valve on the bottom of the tank and the main inlet line to the home are:

- about 3' of 3/4" copper line
- the ball valve
- the hot water tank and anything inside it. (that's where I'm unsure what's inside the tank)

[dirtroad]

By chance does your neighbor have a pump that drives his sprinkler system? It sounds like his water service into the home can't keep up to the sprinkler system, if that is the case.

[57]

That doesn't apply since the sprinkler is turned off when the hot water problem arises. There is no cold water flow to "keep up" with, and as mentioned in post 1, cold water flow is fine and hot water flow is fine after the "bang", which occurs a minute or so after you turn on any hot water tap in the home.

Something is causing the hot water flow to be slow after using the sprinkler and something is causing the "bang", after which you have full hot water flow again. The "bang" occurs while hot water is flowing from any tap in the home, at which time you have full flow again until the problem resurfaces several days later, usually after using the in-ground sprinkler.

[dirtroad]

Code usually require backflow valves when connecting anything that could inject anything into the potable water and sometimes hot watertanks. For instance the sprinkler pump, which could by creating a low pressure (vacuum) close the backflow preventers (valve), and it sticks until the differential in pressure allows it to bang open.

I think your original diagnosis was correct, I just thought it may have been cause by the pump and a sticky valve.

[57]

The backflow preventer valve would have to be inside the tank though since the cold water flow in the rest of the home is fine.

As mentioned in post 3 there are only 3 "things" between the main inlet line and the tank drain valve. (copper pipe, shutoff valve, Hot water tank)

I honestly don't know if there is a sprinkler pump - I though the sprinkler system just ran off the main water feed line.

[57]

The following article seems to indicate that (some/most?) hot water tanks have backflow preventers:

http://www.ehow.com/info_12169767_ba...er-heater.html

A backflow preventer operates as one of two check valve devices located at the bottom of a hot water heater.

The inlet pipe for my neighbour's tank goes in the top of the tank though, unlike the bottom as described in the article and unlike my own tank. I would assume that the actual "end" of the inlet line is near the bottom of the tank though... I assume there are many different designs. His tank is a GSW, but I didn't make note of the model.

[hugh]

The inlet pipe for my neighbour's tank goes in the top of the tank though

Mine too. It connects to the dip tube that extends to near the inside base of the tank. From my understanding, it's the pressure relief valve that keeps water pressure from building up inside the tank and flowing back out.

I'm confused though by your link. When I investigated installing sprinkler systems, I was told a backflow preventer was something that was required when you used a pump to increase internal water pressure and had nothing to do with hot water tanks. It essentially prevented water under higher pressure from being forced back into the municipal water supply.

[TKG26]

Some not all gas tanks have what is called "heat traps" Basic check valves. They are not there for back flow prevention but rather to trap the hot water in the tank on the off cycle. To reduce some of that hot water moving back up the cold line. Supposed effeciency boost to the tank .I suppose every bit helps..

The heat traps are located in the hot and cold inlet fittings (3/4" threaded inlets). One side uses a glass marble and the other a plastic marble that floats.

Its totally possible that one of the heat traps is getting hung up. And infact i have come across it a few times.. Low flow call and it turns out the heat trap was not freely moving. Caused usually at installation if the fittings are over heated when soldering the copper lines on.. I usually just cut them out. We also get noise complaint calls on these as well. In some homes the make a racket when running the hot water.

I would have them removed.


*** noticed you said 3/4" copper for sure more heat is used to solder these, and if the tank top fitting is brass vs copper even more heat is needed. They take more effort to heat up and get the solder flowing. Adding to the possibility of melted heat traps

Regarding sprinkler systems(i am not plumber or sprinkler guy) any of the ones i have seen have a large brass check valve. I think they are a good idea to prevent any back flow of lawn water or chemicals used on the grass back into the water supply.

LEt us know how this turns out

[57]

TKG26: Thanks for the feedback. I'll advise my neighbour, but it may take some time to get back to you since this may not get fixed right away. It's an intermittent issue that is resolved by running the hot water for a short while.

[rpr]

I have the exact same problem...

[TKG26]

Below is the official info regarding the heat traps i was mentioning.



As part of the redesign of our water heaters to meet the 2004 Natural Resources Canada energy
efficiency standard, GSW has added heat traps to the cold water inlet and the hot water outlet on
most of our products. These thermoplastic devices are designed to reduce stand-by heat loss from
water heaters. GSW heat traps have a tadpole design.

The tadpole in the cold water inlet heat trap (blue) is lighter than water. As cold water is drawn into
the tank, it is pushed down, it then floats up to reseat when water flow stops. It is furnished as a dip
tube assembly.

A heavier-than-water tadpole in the heat trap for the hot water outlet (pink) is pushed up when hot
water is drawn from the tank, and then sinks to reseat in the absence of water flow.

Each heat trap fitting features safety relief ports to prevent accidental closure of the waterway. In
addition, these plastic lined galvanized steel nipples create a dielectric waterway that prevents
galvanic action between dissimilar metals.

[57]

Originally Posted by TKG26

Some not all gas tanks have what is called "heat traps" ...One side uses a glass marble and the other a plastic marble that floats... Its totally possible that one of the heat traps is getting hung up...

You were bang on (as was my assumption regarding a "check valve"). My neighbour had the "trap" removed. He also called GSW who knew instantly what the problem was... Poor execution of a good energy saving idea I guess.

Neighbour had previously paid a plumber to look at the problem, but I guess that plumber wasn't the best...

[TKG26]

Thats great news...

Funny story about the heat traps and plumbers. Years ago when they first came out i was called in by a new home builder to look at the water heater for some mystery noise... They where not familiar with the new "feature" and i knew right away what the noise was. I removed it to eliminate the noise completely... They site foreman was supper happy and looked almost relieved. He then showed me the trail of ripped up drywall in the home where the plumber had tried to track down this sound! LOL He had ripped open about 10 holes in the home before he gave up and called us in to look at the tank.