Hot water tank for a indirect wet back system

[Colaman]

Hi all,

Well, the 30+ year old copper hot water tank for our combustion stove finally sprung a leak or five in the last week. Checked it out and there's lots of solder patches all over it already.... and a lot of green weep marks as well.

So! Time for a new tank. Don't know about where you are, but standard domestic hot water systems here are pretty much all mains-pressure, unvented types. I want a vented tank that's compatible with my stove and gravity fed system and has a heat-exchanger coil in it to run a pressurised hot water service for a shower. Trickier (and a lot more expensive) than it sounds to actually get one. It's also a tight spot in the cupboard behind the stove, and there's nowhere else to put it. There's about 6 or 7 foot of head when the main tank's full, hence the need to pressurise the shower to try and save water over the baths that we're having now.

I've decided to talk to a sheet metal maker about constructing one instead, either out of thin(ish) aluminium or galvanised sheet. Here's what I've got so far:

400x400x900mm tank, with a removable top sealed with 8 bolts and a gasket, giving approx 140 litres storage. It will be insulated with about 75mm of "pink batt" insulation all round,but more on the top half. For connections , it has:

• 1/2" BSP socket at top of tank for air vent connection.
  
• 3/4" BSP socket at bottom of tank for drain/ main water tank connection.
  
• 3/4" BSP socket near top of tank for stove hot water connection, probably 100mm down from the top.
  
• 3/4" BSP socket near bottom of tank for stove cold water connection, probably 100mm up from the bottom.
  
• 2 x 1/2" BSP sockets welded through the tank for the internal heat exchanger loop, one 50mm from the top of tank, one 200mm from top of tank.This, with the removable lid, gives the the opportunity to access and replace the copper loop when needed.The sockets are high up, so I can reach the internal fittings with the lid off.
  
• Copper heat exchanger loop, 3 meters or so of 1/2" copper, wound on a 300mm dia former, going from the bottom 1/2" socket down to the same level as the cold water connection to the stove (100mm up from bottom),then coiled back up to the top 1/2" socket.
  
  

Questions I have:

- Anyone got an idea of the "ideal" position of the stove's inlets and outlets? Eg. Do you have the top one lower down to give more "room" for warmer water above it? I presume the water in the wet back won't rise until it's warmer than the water at the tank, so if it's outlet is right at the top of the tank, will this mean circulation will slow once the water there gets hot? Should I lower the return a little to 3/4 of the way down the tank to give some room for the heated water to rise up out of the way?

Original tank takes all day to get the water up to enough temp for two quick baths at present, and I have to wonder if the thermosiphon is set up correctly. The wet back takes up about 300x150mm in the side wall of the firebox in the combustion stove. I would really like to improve on heating times, so I don't have to burn 1/2 ton of wood to have a shower. It was definitely an improvement after I insulated the bare metal tank(!!) a week after I arrived.... so the more insulation, the better I guess.

- Anyone see any fatal flaws?

Cheers

Dave

[ghurd]

Being a cheap SOB, I see a couple flaws.

Copper is crazy expensive.  I would look into stainless.  Might be cheaper.

The coils may need to be longer and / or larger dia to conduct the heat.

Only need to build it once from SS.

I don't know much about it, but this thing works great.

Standard gas heater connected behind the wall.

Hot from the stove goes into a T at the cold input (top).

Cold goes to the stove from a fitting near the bottom.

No pump. Gravity fed.

Only takes a few minutes for 120~140F water.  Coal fired.

The potable water flows through both (no coils).

http://www.otherpower.com/images/scimages/2050/watrheatr.jpg

G-

[Colaman]

Already got the copper... about 10 metres or so. I've seen similar mains-pressure systems about.... but it's hard to judge the length of the loop. It's water-water across 1mm wall thickness copper at about 9L/min or so... I'll generally be trying to cram as much in as (neatly) possible, anyway. There'll be a gas booster after the stove anyway if it's lacking.

Talked to a sheet metal shop today, they can fabricate it out of 3mm aluminum for about $400 or so. For a comparative pricing, a standard 50 litre unvented mains pressure hot water system goes for $500 here, and prices just keep going up from there (eg 180L approx $700 or so). But from memory, Aluminum and copper don't play nicely electrochemically, so stainless might be the go for the loop anyway.

That's an interesting pic... I take it that it's a mains pressure (not gravity-fed) system? And the stove's just looped in? My concern with a pressurised system/tank is that the stove can superheat the water above 100 degrees without the user being aware of it - once you turn a tap on and drop the pressure, it can flash to steam with much nastiness/spattering/percolation.

I can get the current hot water tank hot enough on occasion that I can hear the water go "glooop, gloop-gloop" in the wet back every 30 seconds or so. It's also one of the reasons I suspect the wet back isn't circulating properly..... I would have thought it could have thermosiphoned out of the wet back before it began to boil. This is with inlet temp of the wet back of around 50-60 degrees C.

Cheers

Dave

[ghurd]

The thing has a thermostat controlling the air flow to the fire.

The pressure relief would sometimes blow before it was adjusted right.

The system is pressurized.

Yes, the stove is in parallel with the storage tank.

I don't recall it making those noises. Makes me wonder if yours is not circulating properly too.  Maybe the circulating pipes are not large enough dia?  Just a quick thought, no idea if that's right.

G-

[Colaman]

Thermostats are always handy :-)

Bit hard with a wood stove.... can't turn the heat down if you're cooking on top.

I suspect there's not enough rise from the wet back to the hot water tank in the current one - it rises possibly two or three inches over about 16 inches horizontally. The cold water side goes out that same distance horizontally then vertically about 18 inches or so to the bottom of the wet back. I wonder if there's not enough rise on the hot side and too much on the cold....

[BigBreaker]

Consider plumbing a vertical riser from the bottom of the hot water loop, where the colder water comes out of the tank to a level a few feet above the highest point in the loop.  Steam bubbles will never go up that riser since they are formed above it but the water level in the riser can "rise" to offset any steam production in the loop.  It's a very simple, last forever form of pressure relief.  Make sure that any water coming out of the tube lands in a safe place.  That's also where you can top off the water in the loop.  Alternatively you can plumb the riser into the tank itself if a tiny amount of mingling of that water is ok with you - it should be.