I think what was going on was my zones had all gotten open at the same time, and since they also have a 15-58 pump on low speed, all the flow was going to the zones & none to storage. So I was getting hotter water returning to the boiler than if it was going through storage - the zones by themselves couldn't get rid of the heat as fast as the boiler was making it. So now I'm thinking I should maybe bump my loading pump up to second gear, so it will pump more than the zone pump & always get some to storage.
Bumping up your loading circ will help and might, or might not, solve the problem.
I think this may be a design issue with a boiler circ (15-58, for example) or a loading unit on the boiler, circs on zones, and storage where system return water is relatively hot. Simple schematic follows. Keep in mind that water leaving the boiler must also return to the boiler, and water leaving storage must return to storage. Scenarios:
1) Boiler firing and boiler circ "on", no circ zones active. All boiler output (hot) will flow to top of storage, all return (cold) from bottom of storage: if boiler/storage loop has adequate flow based on delta-T, everything works fine.
2) Boiler not firing and boiler circ "off", circ zones active. No flow through the boiler. All output will flow from top (hot) of storage, all return will flow to bottom of storage: everything works fine.
3) Boiler firing and boiler circ "on", circ zones active, and system return water relatively "hot" (the quoted example): all hot boiler output to zones + possible additional hot water flow from top of storage, relatively "hot" system return to boiler + possible additional hot water return to bottom of storage. Boiler "sees" a low delta-T due to "hot" system return, the delta-T is much lower than if return was from bottom of storage, boiler can't get rid of heat as fast as it is making heat, and boiler goes to idle even though plenty of storage availability in the tank.
Possible solutions:
4) Increase flow from boiler in high demand situations (bump up the loading circ). This may work, more or less. It will work if the flow rate based on delta-T is sufficient to move boiler heat to prevent idling. Keep in mind that what goes out of the boiler must return to the boiler, so still all system "hot" return will go to the boiler. There may be some flow to storage if by reason of increase in flow the head to system is greater than head to storage, and if so, then there will be some boiler output flow to storage and some "cold" return from storage to the boiler. This may then result in solving the problem due to increase in delta-T. My thought on one way to do this would be to install a bypass circ on the boiler return line to bypass the return water protection valve/loading unit. This circ would activate when return water was greater than 160F (or a higher setpoint), for example.
5) Change the plumbing so that the boiler only supplies top of storage with return from bottom of storage, and separate plumbing so that circ zones draw from top of storage and return to bottom of storage. The storage would function as hydraulic separator, and boiler would see return solely from bottom, which in most cases should be less than system return. The result is higher delta-T and improved function of the system.
6) Solutions others propose: ???
There is another problem, especially with a high output boiler, 160,000 btu for example, and use of a loading unit. Based on the specs of the loading unit maximum flow may be less than boiler output, even at relatively "high" delta-T. Assume output of 160,000 btu, assume maximum loading unit flow rate of 12 gpm (actual spec on one loading unit is a little less than 12 gpm), assume pump head will allow full 12 gpm with the loading unit: If delta-T=30, this will work: 160,000 / 500 / 30 = 10.7 gpm. But if delta-T = 20, then 160,000 / 500 / 20 = 16 gpm required flow rate, which is greater than loading unit maximum flow. The result is that boiler output exceeds capacity of the loading unit to move the output, and boiler will idle. An appropriate design with a bypass circ to increase flow may solve this problem. Of course, even a 160,000 btu boiler does not output at this rate over the full burn, but at high burn output likely will equal or somewhat exceed 160,000 btu. And it is at high burn that it is particularly important to move boiler output to prevent idling.
In a design where boiler only supplies/return to/from storage, and with somewhat careful loading/firing of the boiler so that high output does not occur as tank delta-T starts to close and boiler output is falling, then even a loading unit with maximum flow of 12 gpm should work fine. But with a design like the schematic, flow always may be insufficient at high output and the problem remains.
![[Hearth.com] Flow Rate From Boiler to Storage](https://www.hearth.com/talk/data/attachments/94/94234-49f281fc179e4dd5197be87be7d0c6ee.jpg?hash=k0Kvux7tsb "[Hearth.com] Flow Rate From Boiler to Storage")
