pressurized vs. nonpressurized storage

[Jim in CT]

Firstly, thanks to you guys for helping us newbies out.
I've been reading about heat storage options, and I don't see why you'd want to use pressurized storage. Corrosion issues are higher, the possibilities of leaks on some of these used tanks, the need for very large expansion tanks...and getting these vessels into the basement... Is it the savings on heat exchange coils? I must be missing something here, I see very little upside to going pressurized. Wouldn't you have to tap into the tank for a dhw coil anyway?
My understanding is the storage is just a large mass of thermally dynamic stuff, easy to add or extract heat from. Why add that storage to your boiler system?

BTW, I'm going with a Tarm 2000 wood/oil (only 1 flue) and a homemade storge tank.. or I might just bite the bullet and get their 600 gal tank.
Thanks
Jim

 

[jebatty]

...pressurized storage. Corrosion issues are higher, the possibilities of leaks on some of these used tanks,...

I think you've got it backwards. Corrosion issues are much less with pressurized storage, and with regard to leaks, your Tarm and most wood boilers (other than OWB's), and all gas and oil boilers, are pressurized; as is your domestic water system. While anything is possible, probability is about 0.

Pressurized storage is pretty much "heat it and forget it."

 

[MrEd]

The only downside to pressurized tanks I can see is the need for a large (expensive) pressurized tank. A possible benefit of a non-pressurized tank is the ability to get a DIY tank into a small area where it would be impossible to move a propane tank - so that is site specific.

As far as cost, there is no comparison however - a used pressurized tank can be had for about $1000, probably less, and add a $500 expansion tank - in my case, my tank was free so the cost is $500 for the expansion tank and maybe another $500 in pipes, pumps and controls and I am in business for $1000 - a purchased non-pressure system from Tarm to match my Solo 60 was about $7000. I thought the STSS setups were pretty slick, and would have gone with it if the price was $3000 or less, but with the price of copper, that seems unlikely anytime soon.

 

[Nofossil]

I'm assuming that your boiler is pressurized either way, and the debate is about pressurized storage. Jim is dead on about having the boiler pressurized.

Five advantages of pressurized storage:

1) No need for primary heat exchanger. At today's copper prices, not a small issue.

2) With appropriate baffles, you can get better stratification

3) Since there's no primary HX, you can get significantly higher tank temperatures.

4) No evaporation worries - easier to enclose and insulate

5) Tank can be ordinary non-stainless steel without a liner.

Three disadvantages of pressurized:

1) Large tank(s), hard to handle, hard to get into basement

2) Hard to add secondary HX. Probably sidearm or external flat plate are best options.

3) Requires large expansion tank

Bottom line - pressurized is higher performance and 'cleaner' installation, but not possible or even ideal in all situations. I have three HX coils (primary, DHW preheat, and solar) in an unpressurized stainless tank.

 

[muncybob]

Can you elaborate on advantage #2 regarding baffles?

 

[Bob Rohr]

Every time you pass through a HX you will lose some efficiency. So connecting pressurized storage directly to the boiler is a bit more efficient. Both from a heat transfer stand point the the electrical energy required to run an additional HX circulator.

Once water is heated and all the air, and O2 removed they will be no corrosion possible. Steel and cast iron boilers have been used for many many years with plain tap water and no corrosion issues. Some call this "dead water" when it is O2 free.

Be sure the vessel you chose for pressurized system use is designed and listed for pressurized use! Not all tanks are meant to be pressurized. And check the maximum pressure allowed and add a safety relief valve with adequate BTU rating to protect it. and the boiler of course.

Former LP tanks are a common option, as they are thick steel with ASME pressure vessel ratings. safe and typically long lasting. In a closed loop application, of course

hr

 

[Nofossil]

Can you elaborate on advantage #2 regarding baffles?

If you look at tanks designed to attain and maintain maximum startification, they typically have a horizontal baffle just below the top port and just above the bottom to spread and slow the incoming water. You don't want a jet of water shooting in from the top and going all the way to the bottom, mixing up everything as it goes. The higher the fluid velocity in the pipe, the worse this effect is. You want to spread the incoming stream out so that it's moving horizontally and very slowly before it encounters the water in the tank.

There are real limitations on how much of this you can do when using an existing tank such as a propane tank. Still, I'd look at using the very largest pipe diameter that I could for the last few feet (after the last elbow in particular) and I'd think very hard about trying to rig at least a minimal baffle if possible. If the port is on the top of the tank, imagine a section of 2" pipe with a plate welded on the end and many large holes drilled in the sides, protruding downwards a few inches from the port into the top of the tank. That would at least slow the flow and divert it sideways.

Hope this explains what I'm talking about.

 

[jebatty]

Once water is heated and all the air, and O2 removed they will be no corrosion possible.

This is true in a closed system, which is why usually in a residential closed system, no water treatment is necessary, as the little bit of O2, which causes rust, is quickly consumed, and no more corrosion from O2 results. Not quite true in an open system or a system to which water is added from time to time. Both of these continue to allow air into the water and some additional corrosion. In many cases it may be minimal and of no consequence.

But in addition to O2, you need to be aware of the pH of your water. If pH is less than 7, acidic water, can cause quite a bit of corrosion. You want to treat your water to make sure it is above 7, and probably between 8 and 9. Once done in a closed system, should be no further issue unless water is added. In an open system, pH needs to be monitored from time to time, as water is added, to make sure pH stays where it is supposed to be.

I was surprised when I checked my water and found pH at 6.5. In my old open storage tank system, I got lots of corrosion, and I presume it mostly was from the acidic water. With the LP tank, I increased pH with sodium hydroxide (also added sodium sufite as O2 scavenger), and there just was a bit of initial rust, and for months now the water is crystal clear.

On stratification, on my inlet to the LP tank I constructed an elbow about 4" into the tank and inlet water directed horizontally in the opposite direction from the outlet, which is at the bottom and at the end of the tank. Not perfect, but does help to keep the hot water at the top and increase stratification.

 

[Dave T]

Thanks for the explanation of the baffel system NOFOSSIL are zone valves favored over pumps for this mixing reason,and is this the reason people choose to go to a certified welder and have their tank inputs modified to fit the big 2" pipe?
DAVE

 

[Nofossil]

Thanks for the explanation of the baffel system NOFOSSIL are zone valves favored over pumps for this mixing reason,and is this the reason people choose to go to a certified welder and have their tank inputs modified to fit the big 2" pipe?
DAVE

I don't know of anyone who has used zone valves as a strategy for increasing stratification. Zone valve do have the potential to reduce flow rates somewhat, but you have to size your circulator(s) to move the necessary flow rate through your entire system based on supply / return temperatures and required BTU output. If you look at it that way, you might find that you need a bigger circulator to move enough water through your zone valve.

In my system, I'm dealing with relatively small circulators and flow rates - a Taco 007 at 7-8 GPM is more than enough. Some of the guys with bigger boilers need 30 GPM or more. At those rates, you need larger diameter pipes and zone valves are probably out of the question. The need for really high flow rates might be the reason some people have welded 2" fittings to their tanks. At this point, there's a certain amount of improvisation in all of these installations. I will suggest that at the very least, if you have a tank with a 2" or even 2 1/2" fitting, don't reduce it at the tank.

Maybe some day we'll start seeing tank manufacturers who will make a pressure tank with modifications that make it work better for storage. Lighter gauge than is used for propane, with top and bottom baffles.

Keep in mind that I have no direct experience with pressurized storage for wood boilers. Such knowledge and insight as I might have comes from general experience in the world of engineering and thermodynamics. The rest I make up as I go along ;-)

 

[Jim in CT]

Well thanks for setting me straight, I was compleatly turned around. And the info on ph control is something I hadn't considered, thanks alot. I've got to go with unpressurized, the only way I could go pressurized would be something like 2 275 gal oil tanks connected- they would fit through the door.
For HX, is copper tubing enough or do you need rolled copper pipe? ...size?

 

[Willman]

2 275 gal oil tanks connected-

Oil tanks are NOT pressure vessels !

Will

 

[nhjohn]

FYI - I am well into the construction of a roughly 1700+ gallon tank - I'll post pictures soon. I am looking at spending about $1200 for all the materials - liners, insulation, wood etc. The one thing no one ever mentions when purchasing an old LP tank is the amount of money to insulate it. That must be a big number (north of $400 at least) and add a lot of complexity given the shape of the tanks. The insulation number is usually built into the figure for open storage tanks.

 

[WoodNotOil]

nhjohn - You are right that the insulation is one of the major costs in a project like this. I have 3" of polyiso in my open tank all around. Cost just over $300 for a 1000 gallon tank.

Mixing is also a problem in an open storage situation using a flat plate. However, since you can open the tank up it is easier to solve. I will use a long piece of copper with holes drilled in it to spread the water the length of the tank horizontally to both side.

 

[jebatty]

Insulation of my 1000 gal LP tank was not a big issue, mostly because it is installed indoors and in an area that is heated. I used 6" fiberglass batts, like that to insulate house walls, and simply wrapped the tank. Any heat that escapes just heats space I would heat anyway.

 

[Bob Rohr]

I built a square foam box around my 500 gallon LP tank. Double layer of 2" blue foam board. Real easy to install and remove if needed, and waterproof for those rouge, errant leaks

I also stack my kindling wood on top of it for pre-het and drying.

It is also the prefered sleeping "shelf" for the shop cat.

hr

 

[rickh1001]

I wanted to follow up on a question raised earlier, regarding proper chemical treatment of the boiler water (I'm a chemist, so this stuff fascinates me). I've been reading up a bit on various boiler water treatments, and apparently it is a huge field, with some very deep technology. Preventing corrosion and controlling bacterial growth, etc. in large heating systems is not trivial it seems.

Anyway, I have been researching how to treat the boiler fill water once I get our EKO 60 system all together. Oddly, I can't find too much in the way of ready made products or tech support for our residential systems. The only one so far has been the one that Cozy Heat promotes (Wood Solutions I think it was called). They have a line of products to reduce residual oxygen, and raise the pH to 8-9 to prevent corrosion. Legionaire's bacteria can be a problem, but apparently are not an issue as long as the water temp is over 120 deg, although it might become an issue in stagnant systems turned off during the summer.

I am going to continue my research, but what have the veterans on the list been doing in terms of boiler water chemical treatment, and what sources have they found for products? I also intend to build in some type of valving arrangement to allow for periodic water sampling, and the introduction of chemical additives if necessary, as has been discussed elsewhere in the forum.

 

[Nofossil]

Put me down as very interested in this as well. Probably should be another sticky:

'Boiler water test and treatment best practices'

Want to include general theory as well as references to specific products, tools, and techniques.

Could somebody who knows what they're talking about start a new thread with the goal of making it a sticky?

 

[Adios Pantalones]

I'm a chemist as well and was involved with a division of our company looking at cooling tower water treatment. The issue in cooling towers is that warm water is a great breeding ground for Legionaire's disease. It would form in the bio-scum on walls until this material reached a critical point at which it sloughed off. The free-floating material would then get caught up in the aspiration for cooling and be turned into a nice fine mist to spread out and be carried into residential areas. Not nice. The problem is a lot different in a closed system (pressurized), or a system that remains hot nearly continuously to the point of killing disease.

Corrosion, etc- also different. Industrial systems are often continuously monitored so the issue is different. Just wanted to scare folks with the disease story

 

[jebatty]

It took me awhile, and I should have saved the sites, but Google boiler treatment and variations and I think you will come up with enough to answer your questions.

I have opted for sodium hydroxide (lye) to raise boiler pH. I bought 100% sodium hydroxide at the local hardware store. Inexpensive, and enough to last forever. Handle carefully and research how to mix to prevent splatter and burns. Sodium sulfite (O2 scavenger) also is readily available and not expensive. In a closed system may not be as important as in an open system. I bought mine from Bi-Chem Co, 30189 690th Ave, Roosevelt, MN 56673-3416. Phone: (218) 442-6391. Ideally should monitor ppm but I never have. The acceptable range is quite broad.

Easy testing is with a swimming pool test kit to check pH and total sulfites. Litmus paper test strips for pH also available from health food stores.

Once proper treatment in a closed system should be done with this, unless much water added. Open system need to monitor as water is added from time to time.

 

[jebatty]

P.S. - total sulfite in a water boiler system, if I remember correctly, is up to 700 ppm. A swimming pool test kit will say sulfite too high when it is above about 200 (I think).

 

[Adios Pantalones]

40-50C is optimum temperature for breeding it (what's that- like 104-120F or so?), but it can multiply at room temp also. In cooling towers, water recirculators, etc, there are normally wide ranging temperature gradients so that there will be a spot SOMEWHERE that the bio scum can accumulate and be a good region for legionella.

Temps over 60C kill it, IIRC. Even in a summer home, or an unused tap it can grow. That garden hose that hasn't been used in a while? Think about flushing water through it and draining it. You generally get it from breathing aspirated droplets, so the shower in the summer home? maybe run the water a whilebefore using it.

People most susceptible are the typical crew- over 45, weak immune, smokers (it's a lung disease), rodeo clowns, minotaurs.

 

[rickh1001]

Fascinating stuff about the Legionaire's microbes.

Looks like we have an interesting problem here. I certainly don't know the details of the chemistry of boiler treatment, but I agree it would be great if anyone on the forum did. I have started to do a little more reading and research, and have found a few places that sell boiler treatment chemicals to individuals. All the books I have found so far seem to either be about large boilers in big buildings or ships, and most are quite expensive. I will keep looking. If only I was still in academics, I would assign this as a fascinating term paper subject for the class! I suppose people make good money servicing boilers in industry, and don't have much incentive to discuss the fundamentals of boiler water treatment for novices. In any event, it is definitely necessary to keep the pH fairly high, and sulfite as an oxygen scavenger appears to be a routine additive. Other than that, I haven't found many specifics yet.

If I find any good information sources, I will post them.

 

[Willman]

i’d read that a water heater should be raised to 140*? ever so often to sanitize against legionaire’s

Keep the temp up to point of killing or not allowing Legionella to breed. Use a mixing valve for water use temperature.

Will