why is temp stratification better in layers and not the whole tank hot as possible one would think if tank was all 180 vers top 180 middle 170 and bottom 155 it would hold more btu's I don't know? help
storage question
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The goal when trying to store max btu is to start with tank at your choice of temp top to bottom. I usually go 185. As your heat load uses btus the return water to the bottom of tank will naturally lower the bottom tank temp. Depending on type of heat emitters most people like to keep the hottest possible water at the top for supply. To do that your tank needs to stratify and not mix to the same lower temp.
That being said, I sometimes don't heat the tank all the way top to bottom the same temp. If it is mild outside it really does not matter all that much. I just charge and put some hot water back on the top to supply my heat load.
gg
That being said, I sometimes don't heat the tank all the way top to bottom the same temp. If it is mild outside it really does not matter all that much. I just charge and put some hot water back on the top to supply my heat load.
gg
dzook
Member
The goal most times when batch burning a wood boiler with storage is to charge the tank top to bottom at what point the boiler is out of wood and goes out at what point you are then coasting on the tank for the next number of hours maybe a day or more depending on the size of the tank and the heat load. If the tank is not over pumped during the pull off ie. just pump what is needed for the load and return as cold as possible the tank still remains quite hot at the top and the colder statification line moves up thru the tank. If the tank remains well stratified the hot water at the top is more "usable" than if the tank was mixed and ended up with a lukewarm mixture. This can be most helpful for someone using a baseboard fintube heaters that typically transfer best with hotter water or if you are charging an indirect domestic hot water tank. Following is a link to a 5 minute power point video showing a typical burn cycle for someone batch burning a boiler with a loading unit pump and utilizing a storage tank. It is a visual that demonstrates how stratification works.
sorry guys I have an old system from 2003 its a non pres system called sst usind 2 coils in a rubber bladder I send picts today sorry mark
dzook
Member
The tank that is hot top to bottom will hold more BTU's than a partially charged tank. I wish my tank stayed hot top to bottom all the time but I batch burn with storage. For a batch burn method where the boiler is only fired when the tank falls below a usable level then charged back up the tank will usually start loosing the BTU's right away after the firing is over. The only way to keep the tank hot top to bottom all the time is to run the boiler all the time, but then it will probably idle a lot and would probably defeat the purpose of having a tank. Assuming you only run a load of wood thru as needed to charge the storage tank, a well stratified tank during the de-charge will allow you to achieve a potentially higher output supply system temperature when pulling out the heat opposed to if the tank was just an average temp. The indirect tank volume storage you have is charged and de-charge with coils and should naturally stratify well because you are not stirring up the tank volume. To help understand how a stratified tank can be better than one that is blended see the following hypothetical.
If a 1000g tank was 150 deg to to bottom and assuming you could use the water till 120 deg you have the potential of about 250K btu. If water is pumped thru the coils the hottest water you potentially could get out wouldn't be higher than 150deg. Now looking at the same tank if the top half was 170deg and the bottom half was 130 deg the tank has the same potential energy assuming you can pull it down to 120deg but your output temperature has the potential to be hotter. Some heat emitters will work better with this hotter water. If you were only heating radiant were you only needed 120 deg water output the stratification naturally occurring in the indirect tank would not necessarily be as much of a benefit, but it shouldn't hurt the cause either.
If a 1000g tank was 150 deg to to bottom and assuming you could use the water till 120 deg you have the potential of about 250K btu. If water is pumped thru the coils the hottest water you potentially could get out wouldn't be higher than 150deg. Now looking at the same tank if the top half was 170deg and the bottom half was 130 deg the tank has the same potential energy assuming you can pull it down to 120deg but your output temperature has the potential to be hotter. Some heat emitters will work better with this hotter water. If you were only heating radiant were you only needed 120 deg water output the stratification naturally occurring in the indirect tank would not necessarily be as much of a benefit, but it shouldn't hurt the cause either.
hobbyheater
Minister of Fire
Our storage is 1046 imp gallons with the tank laying horizontal . So from bottom to top its height is 54 " . On charging ,the bottom temperature can achieve a maximum temperature of 185 F but more typical would be 180 F , with the top temperature maximum of 195 F. As the storage cools it stratifies at consistent differential of 20 degrees , between the top and the bottom .
In this system the boiler and storage share the same water ,but only with static pressure ( open to atmosphere expansion tank on the floor above ). There are two copper coils inside the storage one for dhw and the second for heat ,these coils are 8" from the top of the tank extending the storage's total 110" of length for good heat transfer. The charging pipe from the boiler enters the tank at the top from one end, 36" from the charging pipe there is a baffle that the coils rest on this baffle extends down into the storage 16" . If the storage temperature is low when you start to recharge this 36" space plus the remaining top 8" heat very quickly giving you a readily available heat or dhw . This setup eliminates the need to have the boilers output switched from storage to the house if the storage temperature is low , and secondly no water has to be circulated an outside the storage for dhw or house heat loads . Though storage is horizontal this set up allows for good stratification .
For the short time I've been on "Hearth " this system is definitely not typical ! But has worked with no problems for over 30 years .
In this system the boiler and storage share the same water ,but only with static pressure ( open to atmosphere expansion tank on the floor above ). There are two copper coils inside the storage one for dhw and the second for heat ,these coils are 8" from the top of the tank extending the storage's total 110" of length for good heat transfer. The charging pipe from the boiler enters the tank at the top from one end, 36" from the charging pipe there is a baffle that the coils rest on this baffle extends down into the storage 16" . If the storage temperature is low when you start to recharge this 36" space plus the remaining top 8" heat very quickly giving you a readily available heat or dhw . This setup eliminates the need to have the boilers output switched from storage to the house if the storage temperature is low , and secondly no water has to be circulated an outside the storage for dhw or house heat loads . Though storage is horizontal this set up allows for good stratification .
For the short time I've been on "Hearth " this system is definitely not typical ! But has worked with no problems for over 30 years .
thanks a lot for the infro I must think on it a wile my srorage is no psi with 2 coils for heating up the tank and 1 for taking out the heat I think it called the sst type
Nofossil
Moderator Emeritus
Storage does two things:
In order to accomplish mission number 1 above, the storage system MUST provide return water that's cool enough to keep the boiler loaded. If the boiler is set for a maximum outlet of 180
and provides a 20 rise, then return water must be below 160 to prevent idling. For storage to be effective at absorbing heat, it must return relatively cool water as long as possible. If proper stratification can be maintained, it's possible to push hot water into the top above a layer of cool water that gets gradually pushed down. This allows the boiler to run at peak output until storage is completely charged.
For mission #2 above, it's desirable for storage to provide water that's as hot as possible for as long as possible. Ideally, water would be drawn from the top of storage at a high temp - say 180, and returned at a very low temp - say 100. If stratification is maintained, you can be drawing very hot water right to the point where the tank is completely discharged. To get a huge delta T like this requires a lot of thought (and likely a radiant slab), but it's by far the most effective way to use storage.
Think of it this way: a tank that has 500 gallons of water in the top half at 180 and 500 gallons in the bottom half at 120 is much more useful for either absorbing heat or providing heat than a tank that's 150 top-to-bottom, even though they both have the same amount of BTUs.
- Absorb excess heat while the boiler is operating. This allows the boiler to run at peak efficiency without idling.
- Provide heat to the heated space, DHW, and/or other destinations when the boiler is not operating.
In order to accomplish mission number 1 above, the storage system MUST provide return water that's cool enough to keep the boiler loaded. If the boiler is set for a maximum outlet of 180
and provides a 20 rise, then return water must be below 160 to prevent idling. For storage to be effective at absorbing heat, it must return relatively cool water as long as possible. If proper stratification can be maintained, it's possible to push hot water into the top above a layer of cool water that gets gradually pushed down. This allows the boiler to run at peak output until storage is completely charged.For mission #2 above, it's desirable for storage to provide water that's as hot as possible for as long as possible. Ideally, water would be drawn from the top of storage at a high temp - say 180
, and returned at a very low temp - say 100. If stratification is maintained, you can be drawing very hot water right to the point where the tank is completely discharged. To get a huge delta T like this requires a lot of thought (and likely a radiant slab), but it's by far the most effective way to use storage.Think of it this way: a tank that has 500 gallons of water in the top half at 180
and 500 gallons in the bottom half at 120 is much more useful for either absorbing heat or providing heat than a tank that's 150 top-to-bottom, even though they both have the same amount of BTUs.
maple1
Minister of Fire
Swapping out ones 'conventional' circulator for a new Alpha or Bumblebee can alleviate a lot of that thought burden. 'Bigger Delta T For Dummies'. Yours truly included.
Nofossil
Moderator Emeritus
True, but in many cases slowing down the rate of circulation creates hotter and cooler areas in the structure. Not good for a long baseboard loop, for instance. I've toyed with the idea of introducing a charge of hot water from storage into a baseboard system, then recirculating it at relatively high speed until it's cooled down, then returning it to the bottom of storage. That would only work if you had plenty of excess baseboard output capacity, but would maximize stratification.Swapping out ones 'conventional' circulator for a new Alpha or Bumblebee can alleviate a lot of that thought burden. 'Bigger Delta T For Dummies'. Yours truly included.
The VS circ works really well for other types of heat loads, though.
maple1
Minister of Fire
Yes, quite agree.
I haven't quite hit heating season with my new Alpha, but I'm sure looking forward to it. I have some of those long baseboard runs. I will play some with slowing some flow via ball valves, and also play with the shutters on the baseboards - closing some up early in the run while leaving the later ones open. Not expecting perfection by any means, but should see some improvements in my storage stratification.
I haven't quite hit heating season with my new Alpha, but I'm sure looking forward to it. I have some of those long baseboard runs. I will play some with slowing some flow via ball valves, and also play with the shutters on the baseboards - closing some up early in the run while leaving the later ones open. Not expecting perfection by any means, but should see some improvements in my storage stratification.
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