Wrapping Outside of Thermal Storage (LP Tank) with Pex for Domestic Water?

[JJSHELLEY]

Looking for feedback and suggestions for domestic hot water.
I have 2 500 gallon LP tanks side by side horizontally in a super insulated box. I’m thinking of heat taping Pex to the top of the LP tanks and connecting to a 40 gallon hot water tank. I would set the hot water tank above the LP tanks so it would thermosiphon. No circulator.
I would connect into the bottom of the hot water tank then run pex above LP tanks and then T back into top of Hot Water tank.

I have a on demand tankless water heater so this system would at least pre heat my water for the cost of an old water tank and pex tubing before it goes to tankless.


Plan A

• ¾ pex tubing ontop of storage
• 100 feet long (Thermosiphon)
• Connected to 40 gallon water tank

Plan B

• ½ pex tubing ontop of storage
• 300 feet long (Thermosiphon)
• Connected to 40 gallon water tank

Do you see a possibility of over heating the hot water tank?

 

[hobbyheater]

Use 3/4 inch copper on top of your tanks; 90 to 120 feet. This can be your feed directly to your tankless demand water heater. No other tank required. Depending on how hot you are going to run your propane storage tanks, you may have to install a mixing valve after your tankless water heater.
I run 120 feet of 1/2 inch inside the top of a horizontal 1,200 gallon storage tank. The water comes out at whatever the storage tank temperature is.

 

[maple1]

I stuffed 2 100' rolls of 3/4" pex on top of my tanks before I boxed them in. Just laid the rolls in & spread them out like. It's just plumbed in series with my electric water heat supply. I can bypass it if needed. So it preheats. But I suspect handles most of the DHW heating when I'm burning wood - although, once all that hot water in the pex is replaced with cold water, then the pex soaks up that cold, it takes a little while for it to get hot again due to the relatively poor heat transfer of pex. Doing it like that though will reduce your hot water flow at the taps vs. cold tap water flow. I'm not sure how good it would thermospihon. Might do OK? I wouldn't bank on it - would likely come down to trying it. In my experience, thermosiphoning is best 'helped along' when the water that is being heated (i.e. your pex on storage) is as low as it can be vs. the height of the tank you want it to run into. Doesn't sound the case here? The heated water leaving the pex will be what drives the flow, and if that isn't up, it might stall.

Having said that, it wouldn't take much to put a small circulator in line if needed. I have a little B&G E3 on the DHW side of my flat plate. Adjustable speed, uses next to no electricity. A 20 plate HX works really well for heating DHW.

 

[JJSHELLEY]

Use 3/4 inch copper on top of your tanks; 90 to 120 feet. This can be your feed directly to your tankless demand water heater. No other tank required. Depending on how hot you are going to run your propane storage tanks, you may have to install a mixing valve after your tankless water heater.
I run 120 feet of 1/2 inch inside the top of a horizontal 1,200 gallon storage tank. The water comes out at whatever the storage tank temperature is.

I like the idea and with out a holding tank!

Does it matter if I use M Copper pipe vs L?