Hi all ....I have been searching all over this site over the last day or so to get a handle on heat storage setups for my possible boiler install. I confess I am lost ....can anyone point me to "Heat storage 101" where I can learn the basics of putting heat storage in with a boiler system. Being that I intend to use a used propane tank unless I find a better solution that is cost efficient ...I find the various sources for purchasing a boiler lacking in information here. Thx, RH
Heat storage 101
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jebatty
Minister of Fire
Try searching this forum for "pressurized storage" and "open storage." I think you'll find a lot. Then come back with the fine-tuning questions.
I recently got up and running 1000 gal of pressurized storage, a used LP tank. Wish this had been available last fall rather than this spring.
I recently got up and running 1000 gal of pressurized storage, a used LP tank. Wish this had been available last fall rather than this spring.
Hi ...Thanks for the response. I have searched all over the forum and have read pretty much all the posts ....but I still find myslef in a quandary. Since you just put in a 1000 gallon LP tank ...do you have a diagram of all the ingredients and pointers on installation. I have to admit ...I really have a lot going on and really would like just a simple how-to on this ....Thx, RH
Eric Johnson
Mod Emeritus
There are two kinds of storage, as Jim points out: pressurized and non-pressurized. Each has its advantages and disadvantages. I can't say which is better. That really depends on your situation, although the high price of copper lately, which is used for making heat exchangers (hx), tips the balance in favor of pressurized.
Nonpressurized storage consists of a tank or other insulated vessel full of water, containing a heat exchanger which allows pressurized water from the boiler to heat up the water in the tank. You might have separate heat exchangers for putting heat into the tank and taking it out again, or you might use one for both jobs. In any event, you need an efficient way to charge up the tank and an equally efficient means of getting the heat back out of it. If you think about the heat distribution in your unpressurized tank, you have to realize that it's a lot hotter at the top than at the bottom. Ideally, you put the hx (usually a copper coil, though not always) into the bottom of the tank to heat the water in the tank, because that's where the coolest water is. Heat transfer is most efficient with a high delta T (greatest differential between the two temps), which is why you want to introduce the hottest water to the bottom of the tank.
At the top of the tank, you want the hx that is going to draw heat from the tank and distribute it into your heating zones. Again, the hottest water is at the top of the tank, and the hx supplying hot water to the zones contains relatively cool return water from the house. It's cool because it's already dissipated much of its heat into the house. So again, you've got a situation with a relatively high delta T. The heat differential within the tank itself is called "stratification"--i.e., hot water at the top of the tank and cooler water at the bottom, with a gradient of temps in between. You get stratification because there's no movement of water in and out of the tank, so it's free to migrate to its appropriate location in the tank. As my friend nofossil likes to point out, "stratification is your friend" for reasons that should be obvious in light of what I've described so far.
The main reason for having a nonpressurized tank boils down to the fact that finding a vessel capable of holding heating system pressure in the 1,000-gallon range can be difficult. Most steel tanks that big are not rated for pressure that high. The bigger the tank, the less capable it is of holding pressure safely. And since most hydronic heating systems are pressurized, with a nonpressurized tank you need the heat exchanger(s) to transfer the heat from the boiler into the tank and back out into the heating system.
The alternative is pressurized storage, which typically these days involves a 500- or 1000-gallon propane tank. These tanks are designed to hold pressure, but they are heavy and hard to get into most basements. You don't need a heat exchanger with pressurized storage because it's the same pressure as the rest of the system, so water can flow in and out of it. But you do need a pretty big expansion tank, and that can run up the bill and present some technical challenges, aside from the logistics of getting a big steel tank into position. One tradeoff is that the flow of water through the tank diminishes your opportunities for stratification. And it would be difficult to take advantage of it even if you did, though some people do use dip tubes to exploit what little there is. (correct me if I'm wrong here, fellas).
Used propane tanks can be had for around $1.20 per gallon these days. You can buy a nonpressurized tank and/or hx from commercial suppliers (for big $), or figure out how to build your own. Many of us have wood or concrete or steel tanks that we insulate, line with EPDM rubber roofing, fill with water and seal up really well.
So that's two ways to skin the same cat. The result in both cases, if you engineer them properly, is that you can run your boiler (typically a gasification boiler) full out and stash the heat for use later. Think battery. Gasifiers run most efficiently at maximum output, so you save wood, produce little or no smoke during operation and extend the life of your boiler if you use storage. Plus, you can do neat things like fire your boiler up every other day when it's relatively mild out, or fire it up once a week in the summer to provide all your domestic hot water. You fire the boiler to maintain the temperature in the tank--not the temp in your house. That's a big difference that gives you a lot more flexibility in firing, since the tank can perform over a much wider temperature range than your house can. The tank heats your house, in other words, and all you have to do is keep the temperature of the tank above, say, 140. That's why some people call it a heat buffer.
Insulation is critical in either case--pressurized or unpressurized--as you want to keep standby losses to a minimum. Some people who have propane tanks don't bother with insulation, but that's because the tank is located in a shop or other space that they want to heat. But if you want to do the summer DHW thing, then you need insulation for obvious reasons.
That's it for Storage 101.
Nonpressurized storage consists of a tank or other insulated vessel full of water, containing a heat exchanger which allows pressurized water from the boiler to heat up the water in the tank. You might have separate heat exchangers for putting heat into the tank and taking it out again, or you might use one for both jobs. In any event, you need an efficient way to charge up the tank and an equally efficient means of getting the heat back out of it. If you think about the heat distribution in your unpressurized tank, you have to realize that it's a lot hotter at the top than at the bottom. Ideally, you put the hx (usually a copper coil, though not always) into the bottom of the tank to heat the water in the tank, because that's where the coolest water is. Heat transfer is most efficient with a high delta T (greatest differential between the two temps), which is why you want to introduce the hottest water to the bottom of the tank.
At the top of the tank, you want the hx that is going to draw heat from the tank and distribute it into your heating zones. Again, the hottest water is at the top of the tank, and the hx supplying hot water to the zones contains relatively cool return water from the house. It's cool because it's already dissipated much of its heat into the house. So again, you've got a situation with a relatively high delta T. The heat differential within the tank itself is called "stratification"--i.e., hot water at the top of the tank and cooler water at the bottom, with a gradient of temps in between. You get stratification because there's no movement of water in and out of the tank, so it's free to migrate to its appropriate location in the tank. As my friend nofossil likes to point out, "stratification is your friend" for reasons that should be obvious in light of what I've described so far.
The main reason for having a nonpressurized tank boils down to the fact that finding a vessel capable of holding heating system pressure in the 1,000-gallon range can be difficult. Most steel tanks that big are not rated for pressure that high. The bigger the tank, the less capable it is of holding pressure safely. And since most hydronic heating systems are pressurized, with a nonpressurized tank you need the heat exchanger(s) to transfer the heat from the boiler into the tank and back out into the heating system.
The alternative is pressurized storage, which typically these days involves a 500- or 1000-gallon propane tank. These tanks are designed to hold pressure, but they are heavy and hard to get into most basements. You don't need a heat exchanger with pressurized storage because it's the same pressure as the rest of the system, so water can flow in and out of it. But you do need a pretty big expansion tank, and that can run up the bill and present some technical challenges, aside from the logistics of getting a big steel tank into position. One tradeoff is that the flow of water through the tank diminishes your opportunities for stratification. And it would be difficult to take advantage of it even if you did, though some people do use dip tubes to exploit what little there is. (correct me if I'm wrong here, fellas).
Used propane tanks can be had for around $1.20 per gallon these days. You can buy a nonpressurized tank and/or hx from commercial suppliers (for big $), or figure out how to build your own. Many of us have wood or concrete or steel tanks that we insulate, line with EPDM rubber roofing, fill with water and seal up really well.
So that's two ways to skin the same cat. The result in both cases, if you engineer them properly, is that you can run your boiler (typically a gasification boiler) full out and stash the heat for use later. Think battery. Gasifiers run most efficiently at maximum output, so you save wood, produce little or no smoke during operation and extend the life of your boiler if you use storage. Plus, you can do neat things like fire your boiler up every other day when it's relatively mild out, or fire it up once a week in the summer to provide all your domestic hot water. You fire the boiler to maintain the temperature in the tank--not the temp in your house. That's a big difference that gives you a lot more flexibility in firing, since the tank can perform over a much wider temperature range than your house can. The tank heats your house, in other words, and all you have to do is keep the temperature of the tank above, say, 140. That's why some people call it a heat buffer.
Insulation is critical in either case--pressurized or unpressurized--as you want to keep standby losses to a minimum. Some people who have propane tanks don't bother with insulation, but that's because the tank is located in a shop or other space that they want to heat. But if you want to do the summer DHW thing, then you need insulation for obvious reasons.
That's it for Storage 101.
Eric
Very good explanation of the two types of storage. Definitely worth the read. Now all I have to do is find a tank. Local scrap yard was recently bought out by a national company and will not sell used propane tanks due to "liability" reasons. Still searching though.
Will
Very good explanation of the two types of storage. Definitely worth the read. Now all I have to do is find a tank. Local scrap yard was recently bought out by a national company and will not sell used propane tanks due to "liability" reasons. Still searching though.
Will
Thanks that helps me understand a lot more ....I could not get the idea of the need for a heat exchanger before. I am under the impression that I am going to be able to get a used propane tank for pretty cheap ...down here in Tennessee I may be one of the first putting in one of these systems. Hence, propane tanks are pretty much scrap metal down here and since I have been spending hard earned dollars with my propane company for the last 8 years ....it is a good possibility to get a 500 gallon tank for a lot less ($100). Also, if I do this ...getting the tank in is not a problem because I will literally build the shed around the whole boiler operation.
What is a problem is that there is little to no help down here on this kind of system ...at best the big thing down here are the outside wood boilers. So before I do anything I want to feel very comfrotable that I know exactly what I need to do ....and the heat storage need seems to be the least outlined part of a system from sources for gasification units ...unless you are going to buy one of their expensive storage tank solutions ...of course this is understandable because that is what they want to sell.
So now ....it looks like with a propane tank I can do non-pressurized and I need to understand that better. I have looked at a lot of the discussions on this forum and the diagrams that go with them ....but candidly everyone here is far more knowledgeable than I am ...or may be able to be become. Which is why I still feel the need for some easy tio understand instructions and drawings to put in a non-pressurized system with heat storage.
Any more guidance would be more than appreciated.
RH
What is a problem is that there is little to no help down here on this kind of system ...at best the big thing down here are the outside wood boilers. So before I do anything I want to feel very comfrotable that I know exactly what I need to do ....and the heat storage need seems to be the least outlined part of a system from sources for gasification units ...unless you are going to buy one of their expensive storage tank solutions ...of course this is understandable because that is what they want to sell.
So now ....it looks like with a propane tank I can do non-pressurized and I need to understand that better. I have looked at a lot of the discussions on this forum and the diagrams that go with them ....but candidly everyone here is far more knowledgeable than I am ...or may be able to be become. Which is why I still feel the need for some easy tio understand instructions and drawings to put in a non-pressurized system with heat storage.
Any more guidance would be more than appreciated.
RH
Eric Johnson
Mod Emeritus
Thx again ....I have looked at most all of those posts but find that I am still at a loss to design my system. Doesn't anyone have a simple to understand diagram of using a propane tank for storage with a boiler? Thx again ....RH
I am a newby at this and have been reading this site almost every night for the last month. Thanks to everyone but especially to Eric; that explanation about storage (above) has nailed the whole storage thing for me.
Eric Johnson
Mod Emeritus
headrc said:
It's pretty simple with a pressurized tank: Supply from the wood boiler going into the bottom of the tank and hot water to the house zones coming out of the top. You'd want to pipe in a bypass so that you could heat the house directly with the wood-fired boiler if you wanted to. The control on that could be as simple as a couple of ball valves, or a more complicated 3-way zone valve if you wanted to automate it.
Sketch it out and you'll see what I'm talking about.
jebatty
Minister of Fire
headrc said:
Start with this post on my system: https://www.hearth.com/econtent/index.php/forums/viewthread/17371/
For some good sample drawings, looks at this. Just page forward and take a look:
http://www.thermavolt.com/page19.html
Also look at this thread: “Boiler piping with pressurized storage.”
Now the pointers:
1) Both a gasification boiler and a pressurized storage tank are heavy and not easy to move or relocate, especially the tank. My Tarm weighs about 1000 lbs., and the 1000 gal LP tank weight more than 2000 lbs. You need space + access which allows you to move the boiler and tank into the space you have.
2) The Tarm is pressurized, typically 12+/- psi. Note Eric's comments on using pressurized or non-pressurized tank.
3) Although I am using a pressurized tank, I still am isolating the tank from the Tarm through a plate hx, probably mainly because I started with non-pressurized storage (aka open system) and isolation was necessary. But I also am finding that with this large of storage and the signifcant water expansion, my tank is running up a little higher pressure than my Tarm, so the isolation is serving a purpose. The jury is still out on whether I will continue to isolate the boiler and tank.
4) Some wood storage in your boiler room is an advantage for some owners. I have a separate wood shed about 100 feet from my shop, and I bring in wheel barrow loads of wood as needed. My wood is well-seasoned and dry.
5) I'm going to strongly advise using a plate hx vs a coil hx of any kind for most if not all heat transfer applications. Plate hx's are small, very efficient, and relatively inexpensive compared to coil hx's. Also, a plate hx is nearly essential if you are going to use an LP tank, as there really is no access for a coil hx, unless you cut a hole and weld it shut, something I would not want to do with a pressure vessel.
6) In my experience I think a 5" x 12" x 30 plate hx, with 1" inlet/outlet, is sufficient for my Tarm 40. You might be able to go smaller, and larger would not hurt, but cost is a factor. I started with (approx) a 2.75" x 8" x 20 plate, and that definitely was not large enough. When I say large enough I mean that (keep stratification of storage tank in mind) water in to hx from tank at 100 (from bottom of tank) and water our to tank (to top of tank) at 160, and in this scenario my Tarm is running full-out, no idling. Idling will start to occur as water in gets into the 120-130 range, as water out remains at approx 160-170. Heat transfer is more efficient at large delta T's. Following a boiler firing, my tank typically will be 155-165 at top and 130 at bottom. I have not had a need to bring the whole tank closer to, say, 160.
7) Regardless of type of storage, you need to condition your water to alkaline, oxygen scavenger, and probably an acid buffer chemical. Low pH water (below 7.5) and oxygen in the water are enemies of boilers, as they cause corrosion. From what I've read pH can go up to 9, and maybe higher, without difficulty. If you have open storage, you also will need to check pH from time to time as you will be adding water to the system as some evaporates off, and you will need to add O2 scavenger, as oxygen will dissolve into the water from the atmosphere. With a closed system, once the water is stabilized, there should be little else to do.
8) Use black iron, copper, pex or other plumbing but not galvanized. Use isolation valves around circ pumps, heat hx's and other devices that might have to be removed for replacement, servicing or maintenance. Use an air separator (need one at a high point on each side of hx if pressurized), and use air vents at other high points for air purging as necessary. If pressurized, once air is out of the system, further venting should be minimal. Adequately size expansion tank(s) - two needed if pressurized, one for pressurized boiler side and one for pressurized tank size. The big ones are pricey.
9) A power failure over-heat loop is essential for safety. Mine is triggered by any over-heat condition (boiler out temp greater than 210F) plus any power failure.
10) A boiler mixing valve (Termovar, for example) or other return water temperature control is essential to boiler efficiency and long life. This insures that boiler return water temp usually does not fall below about 140.
11) Pay attention to proper chimney install.
The first biggest pointer of all is that if you like to figure out how things work, you have an engineering and inventive spirit, you are fairly mechanical in skill, and you find a great reward in accepting a challenge to do something you haven't done before, then self-installing a gasification boiler/storage system will be a great enjoy and result in tremendous satisfaction when you are done. Most of us like to continue to tinker with our system, just because we enjoy it, so the adventure can be ongoing.
The second biggest pointer of all is this Forum should be able to come to your rescue as you face problems or things are not operating as you think they should. This Forum has saved me considerable angst, and I cannot thank the contributors enough for the benefit of their experience, as well as their patience.
The third biggest point of all is that a wood gasification boiler system is amazingly efficient, about as non-polluting as wood burning can get, and can save you lots of money over other energy sources if you have an adequate wood supply at a reasonable price. My conservative estimate is that the wood stove and now gasification boiler have saved my wife and me a minimum of at least $20-30,000 over the past 18 years, far more than the cost of the systems, and our annual heating bill for other heating energy is less than $100.
Good luck on your decision and forthcoming project.
Excellent ...Thx guys ...this novice is now starting to understand the storage tank. A couple of questions for Jim however ...in your posts you do not mention an expnasion tank with the install ...I have been led to believe that this is a necessity with a presasurized system. Then I get the impression that a hx is necessary with your pressurized system ....is this just because of your application? Please help me understand this. Lastly ....it appears that all the ins and outs of your storage tank are at the top of it .....whereas previous recommendations have been to have water from the boiler go into the bottom of the storage tank and water out at the top. Are you using a dipstick method like Eric described? Thx again, RH
jebatty
Minister of Fire
I have the Amtrol SX-160V expansion tank. You can see it in the last picture on the left. I draw water from the tank bottom through the drain fitting. I return hot water to the tank at the top. A hx, one or more, serve a variety of purposes.
1) You can isolate the tank from the boiler through use of a hx. I have done this for now, as I am seeing a higher pressure at the tank than is recommended for the Tarm. If they are operating at the same pressure, which is certainly possible, then no hx is needed unless you want to isolate the two for some other reason.
2) If you have a non-pressure boiler, and want pressurized storage, you must use a hx.
3) You might use a hx for DHW supply: heat potable water for house use.
Others can chime in on additional hx applications.
1) You can isolate the tank from the boiler through use of a hx. I have done this for now, as I am seeing a higher pressure at the tank than is recommended for the Tarm. If they are operating at the same pressure, which is certainly possible, then no hx is needed unless you want to isolate the two for some other reason.
2) If you have a non-pressure boiler, and want pressurized storage, you must use a hx.
3) You might use a hx for DHW supply: heat potable water for house use.
Others can chime in on additional hx applications.
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