Do's and Don'ts of Unpressurized Storage

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CANUCK GLAZIER

New Member
Jul 10, 2008
34
south eastern Ontario
Hi All,

I was just taking part in the wood consumption survey and noticed how many people that have unpressurized storage. I have my tank built and liner ready to go but I was wondering about flat plate HX or coils in the tank. Any thoughts either way? Pros and cons?

Mostly for anybody who has been using their storage for awhile. How's it going? Anything you might do differant?

Any advice welcome.

Thanks

Rob
 
I'm approaching 1 yr. The tank has 2 - 3/4" x 120' coils of copper in parallel for heating, and 1- 3/4" x 160'. These are as per Tarm's model for unpressurized storage. I have no flate plate to compare. I am happy with this rig. We never run out of hot water. The tank gets up to temp. (175-185) in what I think is a reasonable amount of time. The stratificatin is 20 °F . In reverse the coils deliver plenty of BTUs to run 3 zones, keeping the house at 70 °F or better.(whatever the Boss sets the TT to).

I have no holes in the EPDM; I went in through the top and am very glad about that. 180 degree water finds any weakness very quickly.

We just have to hope the EPDM holds up well.
 
Don't put holes in the liner. We have used liners with penetrations and they can work, but they are certainly not worth the risk. And definitely not worth doing
as a first time DIY deal, unless you are patient and have a good floor drain.

You can get more info at our website and the other folks who make tanks for a living.

Most of the info on the threads here on Hearth are pretty useful.

Coils are simple and work. Plates can be less expensive, but require an extra pump and hardware.
I will leave plate hx to others to voice their experience. We have used both with good results.

Happy New Year.
 
I don't have my tank quite online with my boiler yet (project always multiply and morph faster than available time), but one of the things I did was to put my DHW coil up around the perimeter near the top of the tank, suspended by brackets bolted on above the water level. My thinking is that, unlike loading the tank, where you may want heat going in/out/countercurrent flow at different vertical strata, with DHW, you really just want max heat to the DHW coil. In my case I am going to be using a 5x12x70 plate HX between the boiler and the unpressurized tank; when I was buying my supplies (a while back now) copper tubing was insanely priced, so that the plate HX was more affordable- that may be different now.
 
I was originally planning on doing a flat plate HX for my American Solartechnics tank. There were 2 reasons that I was leaning that way. I liked the fact that the tank would be charged without any stratification and the rate at which the HX removed heat from the boiler could be varied with a variable speed pump. The boiler water could always flow through the HX and when the tank side pump was not running, it would not draw any heat. In the end, I decided on an in-tank HX due to the difficulty of controlling the flat plate setup along with the added cost of a bronze circulator needed on the tank side of the Hx. Also there is a potential to occasionally having to de-scale the HX which is easy enough if you add in a bunch of shut-offs and boiler drains at the right places.

I ended up making my own in-tank space heating heat exchanger for this tank. I decided to go with 300 feet of 5/8" copper refrigeration tubing divided up into 6 parallel loops each 50 feet long. I found a Soux Chief manifold that is 1 1/4" pipe and has 12 ports at 3/4" each. I cut this in half and capped off the cut ends. I used one piece as the inlet and the other as the outlet manifold. Total cost for the tubing, manifold and adapters was around $500.

I added a small Taco 006 bronze circulator to stir the tank water when the tank is being charged. This allows the entire tank to be heated but still allows desirable stratification when heat is drawn out. This circ was originally designed as a DHW re-circulation pump. I gutted the control and just run it whenever the solar controller senses that the boiler is hotter than the tank. The circulator draws water from the top of the tank at the center of the space heat exchanger and pushes it out at the bottom of the tank in the far corner. It works well which is surprising for such a small pump. On draw-down I see 10-15 degrees of stratification, on charging there is none.

The un-pressurized tank can never get as hot as pressurized storage. The HX can get it to within 10-15 degrees, but no closer. But on the plus side, you don't need a large expansion tank and these American Solartechnics tanks are super well insulated and very cost effective. Thanks Tom!

Regards,
Scott B.
 
I have a home made EDPM 60 mil 1500 Gal with a 5x12 70 plate. It works very well. I would and should have added the disconnects to remove the HX if desired for cleaning.

I would also like to have a better "Hatch" on top to be able to enter and repair/adjust the plumbing. The size creates a big load for the EKO 60 to heat on one fill but it is GREAT to not have to burn all the time.

Unit is a simple box fold no seams all pluming from the top

My system uses 3 pumps. One on boiler circuit, one on tank charge, one on furnace HX's ( I have two ).

I think a 5x12 80 would be a better fit for the tank size, or larger -- but what I have works pretty well. I like the charging layout I have , the output from the HX is Tee'ed to house and tank top. If house is running hot water is drawn from HX Directly - if boiler is on, Else it siphons hot water from the same feed line off the top of tank. No waiting for the tank when it is cold, no valves pretty simple really.
 
I have a 1000 gallon fiberglass tank (ebay) with 240 feet of 3/4" copper tubing as a heat exchanger from my Tarm Solo Innova 30. I wound the copper tubing in a coil, evenly from the top to bottom of the tank. The problem is, during tank heating, the tank becomes extreemly stratified. As the tank heats from top to bottom, the coils near the top lose there ability to transfer heat, as they are the nearly the same temp as the boiler water. Near the end of the burn, most of the heat transfer area has been lost - and the bottom of the tank remains relatively cool. If you use the tube method, I would recommend the heat input coil be on the very bottom of the tank. I would also recommend another 120' of copper tube. The more heat transfer area, the lower the delta temperature during heating. This will allow you to push the temp in the storage tank higher - similar to what the pressurized storage people enjoy!
 
I have an EPDM liner in my 900 gal. tank. I installed 320' of 1/2" copper tubing. In gereral I am very happy with its performance. I do find that 175* is about the max temp that I can transfer to the tank. I have cast iron rads so I still have useable temps down to about 130* so this system works fine. If I were starting from scratch and had the room, I would really look into pressurized storage as you can have higher storage temps.
 
NHFarmer said:
I have an EPDM liner in my 900 gal. tank. I installed 320' of 1/2" copper tubing. In gereral I am very happy with its performance. I do find that 175* is about the max temp that I can transfer to the tank. I have cast iron rads so I still have useable temps down to about 130* so this system works fine. If I were starting from scratch and had the room, I would really look into pressurized storage as you can have higher storage temps.

How many coils of what lenght do you have and do you use reverse flow?
 
I use 2 150' coils of 3/4 tubing, and heat from the top to the bottom, reverse that when drawing heat for the house. Standard TARM setup figure E.
That way your water in the coils is always hotter than the surrounding water all the way to the bottom. I can get 172 at the top and 157 at the bottom. That takes some nice dry wood though, lately with the wood I've been getting I'm looking at 168 max at the top.
 
I have 4 1/2"coils 80' each ,I used 1'' fittings in a manifold and yes I use them as counterflow. I used the Tarm design for non-pressurized storage.Works for me.
 
NHFarmer said:
I have 4 1/2"coils 80' each ,I used 1'' fittings in a manifold and yes I use them as counterflow. I used the Tarm design for non-pressurized storage.Works for me.

Thanks, I have enough copper for 4 1/2" coils of 100 ft each and setting it up for counter flow aswell. I also have some roll ends of 1/2" pex left that I may use for on the bottom of each coil to squeeze just a little more Btu's out of each gal hot water. :lol:

Henk.
 
NHFarmer said:
I have 4 1/2"coils 80' each ,I used 1'' fittings in a manifold and yes I use them as counterflow. I used the Tarm design for non-pressurized storage.Works for me.

Where could I find the Tarm design for the coils? Is it available on their website or is it in a manual somewhere?

Thanks again for chiming in on this topic I could really us the help.

Rob
 
The Tarm design should be on their website. Their layout is for nonpressurized storage with coils. The coils themselves and their design is my own. I did my share of research and it seemed the best for my needs. It does get a little tough to transfer heat at the upper end of temps but that is to be expected as your delta-t gets smaller.
 
My brother in law replaced a copper coil (with several patches) with 3/4" copper pipe making several loops around the tank (square, home made) and it seems to work fine. I don't remember the size of the tank or length of coils / piping.

I was always curious about using baseboard piping with the aluminum fins. Should increase heat transfer and reduce the length required.

Does anyone have any simple formulas to calculate the heat transfer of tubing etc in a tank so one could get a reasonably accurate estimate on the amount of tubing required?
 
Derek Campbell said:
My brother in law replaced a copper coil (with several patches) with 3/4" copper pipe making several loops around the tank (square, home made) and it seems to work fine. I don't remember the size of the tank or length of coils / piping.

I was always curious about using baseboard piping with the aluminum fins. Should increase heat transfer and reduce the length required.

Does anyone have any simple formulas to calculate the heat transfer of tubing etc in a tank so one could get a reasonably accurate estimate on the amount of tubing required?

You do NOT want to put copper and aluminum in the same tank!!! That kind of finned tubing is a quick route to major issues of dissimilar metal corrosion...

According to Tom from Maine, it doesn't really do much to put fins or other such things like corrugated tubing in a tank - the fin spacing for air cooled stuff is to tight for water to pass through easily, and the improvement you get using the stuff designed for water cooling is small enough that it isn't worth the extra cost and hassles of working with it...

Gooserider
 
Gooserider said:
[
According to Tom from Maine, it doesn't really do much to put fins or other such things like corrugated tubing in a tank - the fin spacing for air cooled stuff is to tight for water to pass through easily, and the improvement you get using the stuff designed for water cooling is small enough that it isn't worth the extra cost and hassles of working with it...
Gooserider

Additional note-- not only does Tom from Maine build heat storage tanks for a living, he's also been involved with research efforts on them.
 
Derek Campbell said:
My brother in law replaced a copper coil (with several patches) with 3/4" copper pipe making several loops around the tank (square, home made) and it seems to work fine. I don't remember the size of the tank or length of coils / piping.

I was always curious about using baseboard piping with the aluminum fins. Should increase heat transfer and reduce the length required.

Does anyone have any simple formulas to calculate the heat transfer of tubing etc in a tank so one could get a reasonably accurate estimate on the amount of tubing required?

Goose said it right, aluminum fin tube will not work very well and the fins would eventually corrode and plug anyway.

There are enough folks here who have done fine jobs with smooth copper and PEX hx. You do need to pay for the surface area of either material to get a decent hx, but this is the most economical way to go.

Plate hx are certainly a different way to go, with many people getting excellent performance. Of course, you trade off an extra pump against a slightly more passive design.

One final way to look at this is to consider using the tank water directly into a heat load. This works fine with radiant floors on a house that is on a single floor.
I am heating my home via a tank, with only a DHW hx. We pump tank water via a bronze 007 to the radiant floor and walls. I did replace a steel radiant panel
with a double layer of fin tube baseboard that I built into a custom tall case. The taller the case, the better it works with low temp water.

The house stays at 70-72F with 105F water. Yes, it is superinsulated, but that only lets me get away with lower temp water.
The unpressurized system would work with most systems if you remove all the iron/steel or thoroughly treat the water.
We have done both. And we are heating two levels.

We do have a 25 year old system at the University of Maine that operates this way with iron pumps and corrosion inhibited water.
Only the DHW hx and we pump tank water to two levels.
 
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