Another Storage tank option?

[SteveJ]

Has anyone used a Unitech Solar storage tank?

There is a 528 gallon listed on eBay (more than the DIY plywood but less than the STSS Tarm):

http://cgi.ebay.com/528-GAL-Solar-DHW-Hot-Water-Tank-Copper-Coil-Heat-Exch_W0QQitemZ330206008978QQihZ014QQcategoryZ3240QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

 

[Bob Rohr]

Not a bad looking tank. I'd be a little concerened about the amount of coil and the size. It doesn't get into a lot of detail but a 1/2" coil of that length may not be a good match for a wood burner BTU capacity. It may be a little constipated in that area? It may be a headered loop style so pressure drop may be ok, but it's a surface area game when transfering energy into water.

These folks http://www.stsscoinc.com have quite a bit of experience and data to show the performance of their heat exchangers. I know they are more $$. But when it comes to heat transfer exposed copper coils are where it all happens. And copper is very expensive these days.

hr

 

[Eric Johnson]

Do you think there's a future in stainless steel hxs, hot rod?

What about cast iron radiators? Is there enough oxygen in a well-sealed, pressurized tank to cause corrosion problems?

 

[Bob Rohr]

Stainless may be an overkill if you plan on a closed loop system. Once all the air is purged and the system heats and drives off all the microbubbles there should be no corrosion going on. You could always add an inhibitor, either a hydronic specific chemical, or just an O2 scavenger like the open system OWF people sell.

The bigger challenge comes with the HX. If you want a tank with internal heat exchangers the price goes up quickly. Most, even steel ones, are designed for DHW use and are glass or stone lined. But for just a buffer for boiler fluid a plain steel tank should be fine.

If money wasn't an object I would own one of the tanks in this pic. They are commonly used in Europe. This tank has three internal coils for solar, wood, supplemental inputs or take off and it contains the DHW. It really simplifies the piping and need for three way divertor valves. I've heard around 4 grand for this 1000L tank. But that's not a lot of buffer or storage capacity.

I currently have a 500 gallon LP tank matched to my 40KW EKO. I'd like a bit more capacity for longer off cycles. Even with a small design load of 32,000 BTU/ hr for my shop and office, 8- 12 hours of time is all it buys me.

Like buying a truck, tool, or any equipment.. You need to decide how nice you want and what you are willing to spend to get there.

When I do find an additional LP tank to add to mine, I'm going to try wrapping some copper coils around the outside for heat exchangers. The Rheem/ Rudd Solaraide tank is built that way. You don't get quite the exchange as an internal coil, but it would be easy to field build and repair or replace.

I bought a bunch of copper solar panel absorbers from www.thermafin.com to use as my external HX to maximize the surface area.

So much to do so little time.

My advice when designing you system is to do a lot of math first. It's cheap and easy and the formulas are readily available.


First insulate your buildings as best you can. By far the best place to spend money. Lower the load to the smallest possible number.

Next a room by room heat load calc.

Then select heat emitters that can operate at the lowest possible temperature. Often radiant floors are the winners. Unless you plan on a lot of carpeting

By designing around the lowest possible supply temperature you can leverage thermal solar. A solar panel, like a boiler will deliver the highest efficiency with the lower return temperature. Use and leverage the delta T. If you can charge a storage to 140, 160, or even 180, and drawn down to 90- 100F you have a huge delta t to work with.

If money allows weather responsive controls are nice. They allow the supply temperatures to change as the load does. Never overshooting, always trying to match the heat supplied exactly to the heat loss of the building, and with the lowest supply temperatures. Plenty of outdoor reset options available these days.

There are a bunch or really nice mixing assemblies showing up from Europe. PAW has some variable speed pump modules to both charge and unload your buffer. Solar pump modules that can watch east and west facing panels, and charge one or two tanks on priority, to let's say handle you DHW load first, then charge the heating side. All from one digital control. These plug and play modules take a lot of the guess work and experimenting out of your design. Sometimes money well spent.

A bit long winded but it may help you in the planning stage.



hr

 

[Eric Johnson]

I misspoke in my post. I meant to say using a cast iron radiator for a heat exchanger in an "unpressurized tank."

 

[Bob Rohr]

Interesting idea a cast radiator. Cheap and available, I suppose.

The two main things about HX are surface area, which is why finned coils are often seen in HX design. The other being turbulence. When you see a cutaway of quality HX materials they often have a means to tumble the flow. Some as simple as a rifling inside the tube.

Copper tube hydronic boilers have cork screw turbulator inserted inside those finned tubes. Just as you would use a turbulator inside the boiler passage ways.

Also two pumped, or moving flows transfer heat better. That the concept in a plate style HX with counter flow piping.

Far from ideal, your radiator idea, but I'm sure it would warm some water.

My first buffer tank was a concrete septic tank bottom. 500 gallons with an EPDM liner and a bunch of copper coils. It worked fairly well until summer rolled around. Some sort of bacteria or algae got started in the tank and created a terrible smell in the shop. I suppose off season temperatures presented an ideal cultural?

Perhaps a better sealed lid may have helped, or occasionally firing the boiler to put some higher temperature into the tank. Maybe a bactericide chemical like they use in ponds around here.?

All things considered I feel a pressurized tank on a closed loop has been a better choice for me. At least in my shop.

As a contractor I need to be concerned with codes and safety when pressurized vessels are used. Lp or other manufactured tanks have an ASME label welded on to assure I am covered. I'm sure there are plenty of home brewed open and pressurized tanks out there working just fine. Be sure to insulate well to keep that thermal energy inside the tank until you need it.

hr

 

[Willman]

HR, On the subject of septic tanks for buffers. Do they need a liner for water leakage or is it for an oxygen barrier ? Was your septic tank pressurized ? I talked to a tank supplier and was told the septic tanks double as pump stations and therefore can be pressurized. Also does a bactericide have any negative effects on any of the components in a hydronic system such as pex or pexalpex, copper iron and such ?
If the tank is totally sealed from outside air I am wondering how the smell gets into occupied space. I would like to use propane tanks but everyone I have called including scrap yards, won't sell them due to "liability issues". A 1000 gal. septic tank weighs in @ 8400 lbs or so and insulated on the outside with foam board would present quite a large thermal mass, unless I am missing something.

Will

 

[Willman]

Hi, Any feedback on using a septic tank (new of course) for storage ? I would like to bury a 1000 gal tank encased in foam board directly below my future boiler room.

Will

 

[Bob Rohr]

HR, On the subject of septic tanks for buffers. Do they need a liner for water leakage or is it for an oxygen barrier ? Was your septic tank pressurized ? I talked to a tank supplier and was told the septic tanks double as pump stations and therefore can be pressurized. Also does a bactericide have any negative effects on any of the components in a hydronic system such as pex or pexalpex, copper iron and such ?
If the tank is totally sealed from outside air I am wondering how the smell gets into occupied space. I would like to use propane tanks but everyone I have called including scrap yards, won't sell them due to "liability issues". A 1000 gal. septic tank weighs in @ 8400 lbs or so and insulated on the outside with foam board would present quite a large thermal mass, unless I am missing something.

Will

The liner is to assure waterproofing. All concrete runs the risk of cracking, you want to keep the water INSIDE the tank. Mine was unpressurized. I had a hard time getting a good seal on the lid. Mine was the bottom 1/2 of a 1000 gallon tank. It had a lap joint that was intended to be glued to the top. It was really to rough to get a foam lid to seal onn that rough, un-even surface.

I have not heard of smell or bacteria issues with any of the Tarm systems I installed. Maybe due to the fact they run them year 'round for DHW? Or a much better sealing lid. Or both?

Mine was no doubt an operator/ builder error. It took up way to much room inn the shop anyways. You really shouldn't need any water conditioner, in my mind.

Yes insulation on the outside would allow the concrete mass to be a better part of the storage. Remember insulation only slow the heat transfer. Even 4" of foam will allow some loss.

hr

 

[eekster]

Here is my thought for myself, I have no room avaliable for storage except for a four and a half foot crawl space. I was thinking about 4 or 6 50 gal. hot water heaters run in tandem. They would be pressurized of course, but it would help on those 40 to 50 degree days. Just a thought.
Keith P.S. Didn"t mean to butt in, but I finally have people to talk about these really neat topics.

 

[Willman]

Thanks for your input hr. I enjoy reading your most informative posts, knowing full well you speak from well versed real world experience.

Will

 

[Bob Rohr]

Here is my thought for myself, I have no room avaliable for storage except for a four and a half foot crawl space. I was thinking about 4 or 6 50 gal. hot water heaters run in tandem. They would be pressurized of course, but it would help on those 40 to 50 degree days. Just a thought.
Keith P.S. Didn"t mean to butt in, but I finally have people to talk about these really neat topics.

Sure, the newer tanks are foam insulated for less heatloss. Use electric tanks, or blank insulated tanks, not gas fired ones as they have a hole, aka chimney through the center

There are plenty of ways to skin that cat, use what is readily available in your area and price range.

Befriend some folks at the plumbing supply stores and box stores. Scratch and dent water heaters are usually heavily discounted.

hr