HEATED WATER RETURN IN UNPRESSURIZED TANK

[Fred61]

I'm in the process of plumbing my unpressurized storage and looking for input on where is the best place to inject the return water after it has gone through the flat plate HX to minimize mixing, thus perserving stratification. At the moment the return pipe is roughed in about 2 inches above the proposed hot high water line.

 

[sparke]

Are you saying the return h20 line is coming into the top of the tank? If so - you should change it so the return comes into the bottom of the tank. However, since you are using a Hx, I wonder if it changes the return location. What is the temp difference from supply to return after it goes through the Hx?

 

[Fred61]

Are you saying the return h20 line is coming into the top of the tank? If so - you should change it so the return comes into the bottom of the tank. However, since you are using a Hx, I wonder if it changes the return location. What is the temp difference from supply to return after it goes through the Hx?

Since I'm just now doing the plumbing and have not completed the project I can't tell you the temperatures. My max. boiler temp is 180* with only about a 10 foot run and of course, like everyone else I'm hoping for 100% HX efficiency (LOL). My supply on the tank side exits the bottom. My logic is that the colder water will absorb the most btu's. I had felt that the best place to return the heated water was to the top but I wanted to distrubute it so that it minimized the mixing. I could be wrong but that made sense to me. That's why I'm asking. I could tee it off and distribute it with lower pressure and let it "splash" on to the surface or I could immerse the outlet a couple inches or if someone has data that says it should be distributed elsewhere in the tank, I can do that.

 

[sparke]

This is a link to one of Eric Johnson's old threads. I am not sure if he found a better way to do this. I am sure he will chime in at some point

https://www.hearth.com/econtent/index.php?ACT=24&fid=21&aid=4777_FW7GhK9jp2uqvqRkhugi&board_id=1

 

[sdrobertson]

You want the return as far a possible from the hot pipe out. I have 500 gallon tanks standing upright and I thought since I would run the tanks to 190, and my return would be in the 160's I would pipe the return to the middle so I would not mix the stratification up to bad. What actually happened was I was mixing the whole tanks and never getting any stratification. I moved my return to the bottoms and installed a controlled circ so that my water always returns from the heat zone at 127 degrees. I can't believe the difference. Right now I'm recharging my tanks but I started at 187 top, 145 middle and 125 at the bottom. I still have allot of btu's available to use as the cold water had just started to get to the middle of the tanks. I'll usually run by tanks up to 190 top, 187 middle and try and get the bottom to 170 before letting the boiler run out of fuel. Watching the stratification move slowly up is amazing.

**EDIT** I read over your original post and realized it was for unpressurized...mine in pressurized but the principles are still the same. You still want the coldest at the bottom and no mixing of the different temps.

 

[sdrobertson]

When you put the water into the tanks at the top and its "slow" enough not to mix with allot of the water in the tank, the hottest water will remain at the top as hot water is less dense. As you charge the tank his hot water will slowly move down when you take the same amount of water out of the bottom of the tank. If you can stop any mixing when you use the heat to your zones, the hot water will remain at the top and the cold will replace it at the bottom. The secret is to figure out how not to mix any of the water.

 

[pybyr]

What I am aiming to do is to have the water go from a tube to the center of a plate, stacked moderately close to another plate, with the plates placed horizontally (and in the case of the lower connection, I'll use the bottom of the tank as the second plate) so that water is pushed or pulled in a 360 degree horizontal plane. My hope is that such an approach will minimize the stirring effect that any uni-directional flow would inevitably have.

 

[Fred61]

You want the return as far a possible from the hot pipe out. I have 500 gallon tanks standing upright and I thought since I would run the tanks to 190, and my return would be in the 160's I would pipe the return to the middle so I would not mix the stratification up to bad. What actually happened was I was mixing the whole tanks and never getting any stratification. I moved my return to the bottoms and installed a controlled circ so that my water always returns from the heat zone at 127 degrees. I can't believe the difference. Right now I'm recharging my tanks but I started at 187 top, 145 middle and 125 at the bottom. I still have allot of btu's available to use as the cold water had just started to get to the middle of the tanks. I'll usually run by tanks up to 190 top, 187 middle and try and get the bottom to 170 before letting the boiler run out of fuel. Watching the stratification move slowly up is amazing.

**EDIT** I read over your original post and realized it was for unpressurized...mine in pressurized but the principles are still the same. You still want the coldest at the bottom and no mixing of the different temps.

I have a unique situation in that the pex in my radiant is not oxygen barrier. It was set up to be heated by a hot water heater. When I decided to use wood is when I discovered this. I can only charge my storage tank presently under construction and it will be heated with a flat plate. The heat for system will be from immersed copper coil. I got a little confused with your explanation. Are you saying I should take the water from the bottom and return to the top or vice versa? The return water from my radiant obviously goes to the bottom of the immersed coil but where the tank water enters and exits in my question.

 

[sdrobertson]

I'm also kinda screwed on the pex also..I first put in a home built OWB and used the wrong pex. It was 3/4 inch to additionally add to my problems. I'll replace all my pex as I'm losing some heat to the ground as its not insulated very well either. For the adding and removing of heat from the storage...the hottest water will be at the top of your tank, so this is where you put the heat into the storage from the boiler and the top is where you will take the heat from to send to your heat load. The bottom will be the coldest so this is where you will return the water after it has heated the house and you will take the water from the bottom and put it into the boiler as its the coldest and it will transfer heat better into cold water than hot from the top. If your using tubing in the tank, both fittings can be out the top but run the water in whatever direction you need to to get the hot on top, and cold in bottom.

 

[Fred61]

I'm also kinda screwed on the pex also..I first put in a home built OWB and used the wrong pex. It was 3/4 inch to additionally add to my problems. I'll replace all my pex as I'm losing some heat to the ground as its not insulated very well either. For the adding and removing of heat from the storage...the hottest water will be at the top of your tank, so this is where you put the heat into the storage from the boiler and the top is where you will take the heat from to send to your heat load. The bottom will be the coldest so this is where you will return the water after it has heated the house and you will take the water from the bottom and put it into the boiler as its the coldest and it will transfer heat better into cold water than hot from the top. If your using tubing in the tank, both fittings can be out the top but run the water in whatever direction you need to to get the hot on top, and cold in bottom.

Thanx, That's exactly what my original thought was and what I thought you were saying in your post above. Czarcar had some concern about the colder water returning to the boiler could be a concern but my Danfoss should handle that. I also intend to start the circulator at a high temperature. Maybe 170* or so.