Top of storage tanks

NCFord said:

I have 2-250 gallon propane tanks that I am going to sit vertically and am wondering if I need
to have an air vent on the very top "tip" of the tank. It seems to me that there would be no other way for air to get out. My supply ports are located on the vertical surface about 6-8 inches from the top. I still have the tanks at the welding shop and can have them just add one more 3/4 inch port if need be.

Click to expand...

Yes, do that. Why not have supply right in the top?

I'd put the ports on the top & raise the shorter tank up so they are both the same height at the top. I would also though make sure there was some way to bleed air from the top - either with a T in the supply line or a dedicated port. Likely would add a small (1/2"?) extra port anyway. And maybe one on the bottom for drain.

with different tanks I think I would plumb them in series

You might get more benefit plumbing the tanks in series. Your two red lines would connect to the top of one of the tanks, and the blue line and the boiler return line connect to the bottom of the other tank. The two tanks are cross-connected with a line from the bottom of the first to top of the second.

What this does is effectively give you a tall 500 gallon tank. Hooking them up in parallel gives you a short 500 gallon tank. The goal is to maximize the amount of thermal stratification in your storage tank - stratification means that more of the thermal energy stored in the water is available at a temperature that's usable to your heating system. Taller tanks develop a better stratified system than shorter tanks. Tanks in series kind of act like tall tanks, especially if you can mount one higher than the other.

Here's a nicely written design guide from the UK that covers the topic - buffer tanks start on pg 13, and they have an example of two smaller tanks on pg 18:
http://www.biomassenergycentre.org....ICAL/BEST PRACTICE/37821_FOR_BIOMASS_2_LR.PDF

Don't worry about uneven height. I can't see any significant disadvantage plumbing parallel side-by-side with tops of tanks even, bottoms of tanks even, or anywhere in between. There's really very little that could go wrong stratification-wise.

Also, if using ports on sides of vertical tanks, any volume above the top ports or below the lower ports will be lost for storage purposes, unless internal riser tubes/dip tubes are used. If anyone doesn't believe it just put all ports at the middle of the tanks and prove me wrong.

I have three 250 gallons tanks vertical in my basement. I did not have the room to raise the tanks up and get the return at the bottom (wish I could have). I did the top and I glad I did, plus i put a boiler drain up there via tee of the supply which gives a nice collection point for air. When I charge the tanks the bottom 4-6 never heats ups. Use old truck rims for stands. Consider adding ports for thermowells too.

Make sure the pex you use is the kind that has an oxygen barrier in it. The standard stuff for regular house plumbing allows oxygen to diffuse through the walls which will corrode out your boiler.

I can say that I wish I had an air vent on the very top of my tanks, otherwise you have to move the air that collects to somewhere where there is an air vent.

A domed tank top makes an ideal air vent. Velocity of the fluid is slow or stopped which helps, and air rises to the highest point normally.

Heating water also causes the air to rise up.

Either add a 3/4 port at the very top, or a side nipple bent up into that air space with a ball valve for manual air removal.