Lead LAG LL Master General Operation | Bryan Boilers & 300, 250, 200

There are parameters that are available to set the features for Lead Lag.

Many of the descriptions used are internal functions or tables. The names help define the functions but are not controlled or selectable outside Sola, unless noted as a parameter.

4.1GENERAL DESCRIPTION OF THE LEAD LAG APPLICATION

Sola devices contain the ability to be a stand alone control, operate as a Lead Lag Master control which also uses the Sola control function as one of the slaves or to operate solely as a slave to the lead lag system. Conceptually it is not a part of that specific control, but is an entity that is "above" all of the individual Sola controls (including the one that hosts it). The master sees each slave (including the one that hosts it) as a set of Modbus devices, each having certain registers, and in this regard it is entirely a communications bus device, talking to the slave Sola controls via Modbus.

Sola devices utilize two ‘ModBus™’ ports (MB1 and MB2) for communications. One port will be designated to support a system S7999B display and the other port will support communications from the LL Master with its slaves. The diagram on page 4 shows a simplified wiring diagram connecting the system display with a 4 system Lead Lag arrangement.

The Lead Lag master is a software service that is hosted by a Sola control.

The LL master uses a few of the host Sola's sensors (header temperature and outdoor temperature) and also the STAT electrical inputs in a configurable way, to provide control information.

4.2LEAD LAG (LL) MASTER GENERAL OPERATION

The LL master coordinates the firing of its slave Solas. To do this it must add stages and drop them to meet changes in load, and it sends firing rate commands to those that are firing.

Date: 8-4-2010

Revision: 0

Form: 2396

The LL master turns the first stage on and eventually turns the last stage off using the same criteria as for any modulation control loop. When the operating point reaches the Setpoint minus the On hysteresis, then the first Sola is turned on. When the operating point reaches the Setpoint plus the Off hysteresis then the last slave Sola (or all slave Solas) are turned off.

The LL master PID operates using a percent rate that is, 0% is a request for no heat at all, and 100% means firing at the maximum modulation rate.

This firing rate sent to the slaves as a percentage, but this is apportioned to the slave Solas according to the rate allocation algorithm selected by the Rate allocation method parameter.

For some algorithms this rate might be common to all slave Solas that are firing. For others it might represent the total system capacity and be allocated proportionally.

For example, if there are 4 slaves and the LL master's percent rate is 30%, then it might satisfy this by firing all four slaves at 30%,

by operating the first slave at 80% (20% of the system’s capacity) and a second slave at 40% (10% of the system’s capacity).

The LL master may be aware of slave Sola’s minimum firing rate and use this information for some of its algorithms, but when apportioning rate it may also assign rates that are less than this. In fact the add-stage and drop-stage algorithms may assume this and be defined in terms of theoretical rates that are possibly lower than the actual minimum rate of the Sola control. In any case a Sola that is firing and is being commanded to fire at less than its minimum modulation rate will operate at its minimum rate: this is a standard behavior for a Sola control in stand-alone (non-slave) mode.

If any slave under LL Master control is in a Run- Limited condition, then for some algorithms the

LLmaster can apportion to that stage the rate that it is actually firing at.

Additionally when a slave imposes its own Run- limited rate this may trigger the LL

Master to add a stage, if it needs more capacity,