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Old 12-07-2008, 03:38 PM   #1
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Gravity Hot water recirculation system.


Hi

I am new to this forum. It appears to be very helpful. I have installed two other gravity hot water recirculations before and I am running into problems with this last one. I read a detailed article on the ASKTHEBUILDER website to no avail. Here is my problem: After I installed the system. I get warm water at the furthest site and then it turns cold the again hot. Ask the builder said that I need to install a checkvalve just prior to the return line entering the drain valve because cold water was flowing back through the line from the water heater. Well I installed the check valve and it was not helpful. I put a 1/8 inch hole in the flapper valve as described and later increased it to just less then 1/4 and it was still of no help. Can anyone help me with this problem???? Thanks so much. I don't want to install a pump if I don't have to. Let nature do its thing.
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Old 12-07-2008, 06:20 PM   #2
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Gravity Hot water recirculation system.


Can you describe the system in a bit more detail?

Here's my experience; I installed a loop of 1" black iron pipe inside my woodstove, bushed it down to 1/2" where it came out of the top, and installed a 50 gallon electric water heater in a closet directly above the woodstove. This serves as a pre-heater for my regular water heater.

This water heater is not connected to the electrical system, it is nothing more than a storage tank. One side of the loop goes to the top hot water out fitting (and has a T&P valve installed), and the other side of the loop goes to a tee installed in the drain valve fitting.

All worked as planned for about 10 years. Whenever I had a fire in the woodstove, one of the 1/2" pipes would be warmer than the other, and with minimal hot water use, the cooler pipe would get lukewarm after about a day or so.

The temperature difference of the pipes was usually noticeable, but not huge. Over time, the temperature difference became more pronounced. I figured that sediment was building up in the 1" part of the loop. One summer I took it apart, and sure enough, it was mostly blocked off with sand-like sediment. The water around here is very soft, but has alot of calcium in it.

I installed a circulating pump in the loop, and run it for a few hours every week or so, and haven't had any problems since. I run a bunch of water out the drain every year, and see quite a bit of sediment.

If your system is similar, you might have the same problem I did.

Rob
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Old 12-08-2008, 03:52 AM   #3
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Gravity Hot water recirculation system.


Thanks Rob for the detailed response. Sounds like you have a pretty elaborate system. My system is in a brand new house so it wouldn't be a sediment problem. It has something to do with the loop back process not working correctly. I installed the loop at the furtherest faucet from the hot water tank and then ran it back to the drain outlet valve in the water heater and now I get warm water at the faucet and then it turns cold then back to warm/hot. I am told that I am getting a back flow from the bottom of the water tank. ( water running in the wrong direction). I installed a check valve in the drain outlet valve and that hasn't worked. Any ideas??? Thanks.
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Old 12-08-2008, 09:50 PM   #4
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Gravity Hot water recirculation system.


I think I'm beginning to understand your system a bit better. Is it a normal, typical plumbing installation with the addition of a pipe (loop) from the farthest faucet back to the bottom of the water heater?

If so, you do indeed need a check valve at the bottom of the water heater. Here's why; Inside a water heater is a 'dip tube' on the cold side. Its purpose is to send the cold water to the bottom of the tank.

If we follow a molecule of water, it enters the tank at the top, and goes down to the bottom via the dip tube. If flow continues, it will slowly rise to the top of the tank, and eventually exit the tank through the hot water outlet. Before this molecule of water can get to the top, all the existing hot water must be pushed out.

Same situation, except no dip tube. Our molecule enters the top through the inlet, and simply moves over a bit to the outlet, leaving most of the hot water in the tank.

If a pipe is connected to the bottom of the tank, and no check valve or pump is installed, when a tap is turned on, water will flow from both the hot water outlet, and the bottom fitting. The bottom of the tank is cold water from the dip tube. Make sure the check valve is installed to allow water to enter the bottom of the tank, but not exit.

Gravity loop systems like this don't work very well if the pipe is not graded continuously. If there are dips in either pipe, it'll 'trap' the heat, not completely, but it'll sure cut back on its effectiveness.

Rob
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Old 12-09-2008, 03:33 AM   #5
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Gravity Hot water recirculation system.


Thanks Rob,

I will install a check valve today. You are quite knowledgeable.
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Old 12-09-2008, 09:12 AM   #6
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Gravity Hot water recirculation system.


Quote:
Originally Posted by micromind View Post
Can you describe the system in a bit more detail?

Here's my experience; I installed a loop of 1" black iron pipe inside my woodstove, bushed it down to 1/2" where it came out of the top, and installed a 50 gallon electric water heater in a closet directly above the woodstove. This serves as a pre-heater for my regular water heater.

This water heater is not connected to the electrical system, it is nothing more than a storage tank. One side of the loop goes to the top hot water out fitting (and has a T&P valve installed), and the other side of the loop goes to a tee installed in the drain valve fitting.

All worked as planned for about 10 years. Whenever I had a fire in the woodstove, one of the 1/2" pipes would be warmer than the other, and with minimal hot water use, the cooler pipe would get lukewarm after about a day or so.

The temperature difference of the pipes was usually noticeable, but not huge. Over time, the temperature difference became more pronounced. I figured that sediment was building up in the 1" part of the loop. One summer I took it apart, and sure enough, it was mostly blocked off with sand-like sediment. The water around here is very soft, but has alot of calcium in it.

I installed a circulating pump in the loop, and run it for a few hours every week or so, and haven't had any problems since. I run a bunch of water out the drain every year, and see quite a bit of sediment.

If your system is similar, you might have the same problem I did.

Rob
Really cool idea and set up. I love hearing about projects like this.

One thing. Black pipe is not rated for domestic water (or any water for that matter) and will rust and eventually clog up. You should convert to copper or galvanized.
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Old 12-09-2008, 09:28 AM   #7
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Gravity Hot water recirculation system.


Thermosyphon systems are based on differential temperatures. They have to have a hot side and a cold side. There has to be check valves to work efficiently or at the very least a large differential in pipe diameter. Design can make a big difference also but I won't get into that.

The basic principle is that the water has to have a clear direction of travel which is decided by valves or design. It is not as easy as just slapping a few pipes together and thinking it will work.

The higher the differential temperature the faster it will flow. Minimal temperatures can cause flows within the pipe and the colder water will settle to the bottom of horizontal pipe and warmer water will go to the top. Even with vertical pipe there can be water flow problems especially with minimal differences in temperature. As the water cools going up the pipe the flow will stop and eventually start to drop slowing rise of warmer water.

There are limits on thermosyphon systems.
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Old 12-09-2008, 10:49 PM   #8
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Gravity Hot water recirculation system.


I looked back and read my post, and sure enough, I did state black iron. I don't know what I was thinking, it is actually galv.

Obviously, I can't handle more than one thought at a time!

Rob
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Old 12-09-2008, 11:00 PM   #9
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Gravity Hot water recirculation system.


Quote:
Originally Posted by micromind View Post
I looked back and read my post, and sure enough, I did state black iron. I don't know what I was thinking, it is actually galv.

Obviously, I can't handle more than one thought at a time!

Rob
Sure you can. You were just distracted.

I was thinking that your sediment was due to the black pipe rust.
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Old 12-10-2008, 08:43 AM   #10
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Gravity Hot water recirculation system.


This system is more than 12 years old, had it been black iron it'd likely be rusted out by now.

There's a lot of calcium in the water around here, what came out of the hose certainly looked like calcium.

Rob
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