Calling all boiler experts, advice on adding a buffering tank for my DHW...

[brenndatomu]

I'm in the planning/parts buying stage of putting a buffering tank ahead of my electric DHW tank to preheat the water. The plan is to mount a tank next to my Yukon Husky furnace, run lines to a coil in the furnace, circulate water through the coil by gravity. But I'm not sure if I want/need to mount the buffer tank vertical or horizontal. I'm sure there are probably some "rules of thumb" about getting water to circulate by gravity as far as, length of pipes, angles, (rise/run). Does the "hot" side have to have more "rise/run than the cold return side? Can the cold return go uphill from the tank to the coil or does it have to run downhill?
BTW, my buffer tank is an old galvanized pressure tank 5' x 12".
I would tie into the DHW tank directly, but it is 20' away and would be problematic to move.
I am more than willing to check out some old threads or links if you can point me in the right direction. I didn't find anything quite like this (other than some good ole boy on U tube that hung a DHW tank on its side above the furnace) when I searched for info. Anybody have any advice or suggestions? Thanks in advance!

 

[Clarkbug]

So I think Im following you, but to be clear, are you intending to mount the tank above the coil, correct?

 

[brenndatomu]

Yeah, if I mount it horizontal, then the whole tank would be above the coil. If I go vertical, then the top 1/3 would be above the coil.

 

[ewdudley]

Horizontal would get all the cool water as high as possible, which is what you want.

The driving force is the difference in density between the hot side and the cold side and the height of the tank. The densities depend on how hot the water gets and the temperature the water starts out at, which you probably can't affect much. The more elevation the better up to a point, I would speculate you would need at least 30" elevation to be effective and 60" might be more than enough.

The resistance to flow depends on pipe diameter, pipe length, and pipe material smoothness.

Pipe diameter makes the biggest difference by far. For instance, even if the ports on the tank are 3/4" and the heating coil is 1/2", it would pay to make the pipes to and from the tank 1" or 1.25" diameter.

Pipe length depends on how close the tank is to being directly above the boiler and the total number of elbows. It's the total length and the total number of elbows that makes the difference, so if hot side is more direct than cold side or vice versa it's a wash. (But only if we ignore the differential viscosity of cool vs. hot water. Technically it makes sense to make the cold run the shortest and most direct.).

Smoother is better but no big deal. Hopefully you're planning on using copper or PEX, as opposed to galvanized, and they are about as smooth as you're going to get anyways.

 

[brenndatomu]

Yup, I was planning 3/4" copper pipe run from coil to tank and back. Tank to coil is gonna be about a 5-6' run of pipe. I was planning on using 45* elbows instead of 90s.
Sounds like the horizontal tank mount that I was leaning toward is the way to go. It's more work to mount that way, but oh well, nothing's ever easy

 

[STIHLY DAN]

Brenndatomu, Here is a couple pics of mine, works great. The buffering tank is an old 80 gal water heater.

 

[brenndatomu]

Thanks Dan (& everybody) that is pretty much what I had in mind, except why not just tie it direct to your water heater? More capacity?

 

[STIHLY DAN]

By having the tempering tank, you will have a greater delta T for better transfer of heat. The tempering tank constantly gets heat from your furnace leaving your hot water tank at temp. When you use the hot h2o, the cold h2o enters the bottom of tempering tank taking the hot water, or warm from the top into the cold water supply of your water heater. So even if the tempering tank is only at 80* your hot water heater has a 30* head start on reheat.

 

[brenndatomu]

OK, so now I'm trying to figure out how best to connect my lines to the buffering tank. The tank will be mounted horizontally, from the ceiling, keep in mind that it is just a regular water tank (does not have a cold water down tube or any baffles) So I'm thinking that there are several possible combinations, some obviously will work better than others. I'm just trying to come up with one that will best take advantage of the natural hot/cold currents both when no water is being drawn, and when it is. This is a drawing of my first thoughts on how to do it. The numbers in the tank represent available port holes on the tank. Suggestions...comments?

 

[ewdudley]

Suggestions...comments?

Moving hot inlet to 5 or 6 would promote better stratification, but at the expense of adding more elbows, depending on your situation. It's hard to say without a proper flow model based on temperature delta, elevation, pipe diameters, and fittings.

And moving cold inlet to the lowest port of HX coil would be needed to get the system to start up in the intended direction. As it is depicted I think flow would start up going from the right side of the tank to the left and would intensify in that direction as the left side pipe heated up.

And finally, need to use good sized pipe to and from HX to tank. Going from 3/4 inch to 1 inch should double the flow rate.

 

[iceguy4]

Moving hot inlet to 5 or 6 would promote better stratification

I'd be tempted to T it into 6... But short of that, wouldn't #1 be next best?

 

[iceguy4]

OK, so now I'm trying to figure out how best to connect my lines to the buffering tank. The tank will be mounted horizontally, from the ceiling, keep in mind that it is just a regular water tank (does not have a cold water down tube or any baffles) So I'm thinking that there are several possible combinations, some obviously will work better than others. I'm just trying to come up with one that will best take advantage of the natural hot/cold currents both when no water is being drawn, and when it is. This is a drawing of my first thoughts on how to do it. The numbers in the tank represent available port holes on the tank. Suggestions...comments? View attachment 119669

Another thought...# 2 port will be the old "Cold in" with its standpipe intact...most are flexible enough to lat on bottom side of horizontal tank...helping further with the stratification. I'd be willing to bet the standpipe would flex enough to go from port #1 to the bottom...easy to verify...

 

[ewdudley]

I'd be tempted to T it into 6... But short of that, wouldn't #1 be next best?

Yes indeed, horizontal and closer to the top.

 

[brenndatomu]

Good stuff guys! I'm ears/eyes wide open here. So, hot in the top, cold out the bottom...I can do that! If I tee in the hot from my coil to the return line back to the DHW tank, won't it try to pull cold water through the coil when I'm drawing hot water?

And moving cold inlet to the lowest port of HX coil would be needed to get the system to start up in the intended direction. As it is depicted I think flow would start up going from the right side of the tank to the left and would intensify in that direction as the left side pipe heated up.

Good point ew, my drawing is a little deceiving, in that the coil was actually going to be mounted flat, but I understand what you are saying, I will now be mounting on an angle or even vertical.
Thanks guys, advice much appreciated!

 

[ewdudley]

If I tee in the hot from my coil to the return line back to the DHW tank, won't it try to pull cold water through the coil when I'm drawing hot water?

Good point, it would likely be best to force DHW to draw from the the tank alone and not create a parallel path through the preheat coil. So 1, 5, or 4.