I would like to use parallel injection mixing circulators per a J. Siegenthaler article:
It’s also possible to use multiple (and identical) injection circulators in parallel, as shown in Figure 5.
When multiple and identical injection pumps are configured as shown in Figure 5, only one needs to be speed-controlled. The others can be controlled as on/off. This array of circulators would operate like a multiple boiler system having one fully modulating boiler in combination with additional on/off boilers. The variable-speed circulator would ramp up as the load begins to increase.
Once the load increases beyond what can be supplied through the variable-speed injection pump at full speed, one of the on/off injection pumps would turn on and the variable-speed circulator would drop back to a low speed.
Assuming the load continues to increase, the variable-speed circulator ramps up again until two of the on/off pumps can be on and the variable-speed circulator drops back down to low speed. In theory, this control action could be used with any number of identical circulators, only one of which is speed controlled. This technique allows smaller wet-rotor circulators to be used to meet the design-load injection flow rates that are beyond the range of a single small circulator. It also may reduce cost relative to a single larger injection pump with a VFD control. It also can provide injection flow rates lower than what a single larger circulator may be able to achieve based on a typical VFD that only reduces motor speed into the range of 20-30% of full speed."
The reason being I will be using a wood gasification boiler plumbed to storage. When the water in the tanks is at 180F, I only need a small gpm to meet my in floor loads. As the temp in the tank drops, I will need more and more gpm from the tanks. I would like to be able to use the tanks down to perhaps 100F supply temp. On a design day, that would mean 12gpm. When the water in the tanks is at 180F, I might only need 1 gpm or less. By using two pumps, I can use a small pump and smaller supply line (allowing me to operate within a 2ft/sec~4ft/sec window) for low flows, then use the larger or both pumps when the temps in the tank drop and injection flows reach 12gpm (again, keeping flow velocities within the recommended range. If I use one pump and size for the max flow, I will not meet the recommended flow velocities at low flow rates....
My question is, how would I control these two pumps (with outdoor reset) so that the smaller pump runs at a variable speed until it is maxed out, then the larger pump turns on until maxed out, then the smaller pump turns on, and with both running, I can reach the max flow required? Mr. Siegenthaler doesn't spec the control mechanism to accomplish this, he just describes how it works...Any suggestions appreciated....