A Question I've Never Been Able To Answer

[hobbyheater]

The Jetstream was a pretty remarkable Boiler with a 1,000 lbs refractory burn chamber base. With 30 plus years firing experience, I could get the 1045 imperial gallon storage to 195 F with every batch burn. With top of storage at 195 F and the bottom at around 170 F with all the wood burned, I would let the circulator run for another hour and half and it would raise the bottom of storage to 185/ 190 F with the top remaining at 195F! The question is why did only the bottom temperature rise from the heat in the boiler's base but not the top ?

 

[brenndatomu]

Man that's a very interesting question! There are a lot of things with hydronic heating that I find "non intuitive"
My best guess is that the heat being added just seeks the largest temp differential

 

[EbS-P]

Did you ever see a temp above 195?

 

[hobbyheater]

Did you ever see a temp above 195?

The top temperature never increased once the wood was burnt up!

 

[hobbyheater]

Man that's a very interesting question! There are a lot of things with hydronic heating that I find "non intuitive"
My best guess is that the heat being added just seeks the largest temp differential

Good answer!

 

[OffGridICF]

In your case, the circ is not turned off for 1.5 hours so the residual heat in the refractory is transferred to storage just as if the boiler was still firing. The heat is moved at the same rate so the boiler output probably never exceeds your 195F setpoint. The thermocline is pushed down further causing the heat at the bottom of storage to increase, just like in your regular charge cycle. If you turned the circ off and allowed the boiler temp to increase above 195F then turned it back on, the top of storage would go above 195F and the bottom of storage would probably not change much until the new higher temp propagated all the way through storage to the bottom.

I think the temp at the top of storage will move toward the bottom as water at the same or lower temp is added. If the incoming water is hotter than storage then that 'new' increased temperature must further charge storage before it increases the temp at the bottom of storage. The mass of water in storage will absorb the new hotter water (store the heat) before the bottom temp will move much. If the boiler output temp is the same temp as storage then no more heat needs to / can be stored before the bottom of storage temp will start to increase.

The Froling has a boiler output target setpoint of 185F. It will adjust the primary circ speed and burn rate to maintain this target. While it is running, the 185F water will push the thermocline down in the storage tank. Once the wood is consumed, the primary circ will turn off. If the boiler temp goes above 190F due to residual heat, the circ will turn back on until the boiler temp drops again. As long as there is more 'space' for the heat in storage, the top of storage will stay around 185F (maybe up to 190F) and the hotter water will continue to move toward the bottom of storage.

This plot shows two firings of the boiler. In the first, the storage is not fully charged. There are 4 sensors on the storage tanks (top, 2/3, 1/3 and bottom). The boiler output sensor is on the output pipe, not in the boiler itself so once the circ stops it is no longer accurate. When the pump stops the actual boiler temp will increase above 185F on its display. Once it hits 190F the circ turns back on which you can see in the plot at about 25 hours and again at just before 26 hours. You can see that the top and 2/3 sensors do not show a change but the 1/3 sensor does jump up a little as the hot water is pushed further down in storage. This would be more clear except the radiant floor is pulling heat from storage the whole time. The second charge cycle runs to 100% and at the end the boiler cycles the circ to move residual heat to storage as needed. There is still room for a little more heat in storage since the bottom sensor has not hit 185F yet. The boiler delta-T is on the right axis.

 

[hobbyheater]

In your case, the circ is not turned off for 1.5 hours so the residual heat in the refractory is transferred to storage just as if the boiler was still firing. The heat is moved at the same rate so the boiler output probably never exceeds your 195F setpoint. The thermocline is pushed down further causing the heat at the bottom of storage to increase, just like in your regular charge cycle. If you turned the circ off and allowed the boiler temp to increase above 195F then turned it back on, the top of storage would go above 195F and the bottom of storage would probably not change much until the new higher temp propagated all the way through storage to the bottom.

I think the temp at the top of storage will move toward the bottom as water at the same or lower temp is added. If the incoming water is hotter than storage then that 'new' increased temperature must further charge storage before it increases the temp at the bottom of storage. The mass of water in storage will absorb the new hotter water (store the heat) before the bottom temp will move much. If the boiler output temp is the same temp as storage then no more heat needs to / can be stored before the bottom of storage temp will start to increase.

The Froling has a boiler output target setpoint of 185F. It will adjust the primary circ speed and burn rate to maintain this target. While it is running, the 185F water will push the thermocline down in the storage tank. Once the wood is consumed, the primary circ will turn off. If the boiler temp goes above 190F due to residual heat, the circ will turn back on until the boiler temp drops again. As long as there is more 'space' for the heat in storage, the top of storage will stay around 185F (maybe up to 190F) and the hotter water will continue to move toward the bottom of storage.

This plot shows two firings of the boiler. In the first, the storage is not fully charged. There are 4 sensors on the storage tanks (top, 2/3, 1/3 and bottom). The boiler output sensor is on the output pipe, not in the boiler itself so once the circ stops it is no longer accurate. When the pump stops the actual boiler temp will increase above 185F on its display. Once it hits 190F the circ turns back on which you can see in the plot at about 25 hours and again at just before 26 hours. You can see that the top and 2/3 sensors do not show a change but the 1/3 sensor does jump up a little as the hot water is pushed further down in storage. This would be more clear except the radiant floor is pulling heat from storage the whole time. The second charge cycle runs to 100% and at the end the boiler cycles the circ to move residual heat to storage as needed. There is still room for a little more heat in storage since the bottom sensor has not hit 185F yet. The boiler delta-T is on the right axis.
View attachment 338488

Sounds like a very good explanation ,never thought of turning the circulator off and back on to see if it would bump the top .