These issues may be relative to how many stages are practical for any given system, and the methods of transferring water flow from the boilers to the system, to ensure that the boilers will not encounter substantially cooler system return water temperatures.

BOILER STAGING: HOW MANY IS ENOUGH?

The proper number of stages for any particular application can be difficult to determine. The things that will help determine this include the installation budget, type of boiler system, workable floor space, and, most importantly, the built-in redundancy required of the new system.

Redundancy will vary with each locality or state, but it is generally there. It may be there for the possibility of future building growth, or for the ability to carry the load during a possible equipment outage. The redundancy factor may vary from 135% in one state to 165% in another.

The thing we need to bear in mind is that the larger the redundancy, the more stages we should have available. Tests based on a given design temperature have proven that heating systems are generally only operating at 40% to 60% of the demand most of the year. (See Figure 1.)

With a larger redundancy factor and only two boilers with on-off firing, one of these boilers will carry the load almost all of the time; if no boiler rotation is incorporated into the system, the second boiler will probably never run.

A better configuration for this system would be two boilers with individual low-high firing or three to four on-off firing boilers. This would more closely match the individual system requirements to available boiler outputs during the lower demand or warmer climate conditions. This, of course, is once again dictated by budgets and available floor space within the boiler room.

In comparing equipment costs with multiple stages, the decisions are generally based on whether the boilers may be atmospherically vented or pressure fired. Depending on the available workable floor area, multiple atmospherically vented boilers may fit into the same budget as two pressure-fired boilers with low-high firing.

The overall thing to keep in mind is that the closer the boiler output is matched to the demand of the system at any climate condition, the more efficient the system will be.

TEMPERATURE RESET

As I already stated, today’s temperature reset systems have done a remarkable job of matching the output of the heating system more closely to the demands at any given outdoor temperature condition.

These controls may be as simple as an add-on device or as complex as a network-based computer management system. In most cases, however, they are only managing system temperatures and not boiler return temperatures.

The majority of cast iron boiler manufacturers would prefer to keep the temperature rise through the boiler within their individual standards. This may be a standard of 20 to 40 degrees F, depending on the piping to and from the boiler and system size. In a boiler that would be trying to maintain an output of 180 degrees to the system, this would mean return water no colder than 135 to 140 degrees coming back to the boiler.

The intent is to reduce the likelihood of condensation and thermal stresses from occurring. We need to incorporate a way to reduce the amount of flow of cooler system return water to the boiler, or to keep it warmer.

Most reset systems have a way to maintain a minimum system temperature. If this were set to a higher temperature to maintain warmer water on the return side of the system, it would accomplish what we need but it would defeat most of the control system’s intended purpose. You would still have the stage-firing aspect working for you, but the temperature modulation part would be defeated somewhat.

BOILER CONSIDERATION METHODS

Systems from years past may have been less sensitive to cooler return water conditions due to the larger masses and water volumes that the older boilers used to have. Increased efficiency standards have dictated that today’s boilers have lesser mass and smaller water contents.

There are numerous methods of controlling the likelihood of cool system water returning to the boiler(s). Boiler bypasses are a proven method of controlling return water flow through the boiler, but they can’t necessarily provide proper protection on a system that may have a variable return or system temperature. They are generally sized and set based on one set of temperatures to and from a system.

Variable-speed injection into a boiler primary loop or main is an excellent way to properly control the amount of heat that a boiler is handling, but once again, most of these systems operate off of a system supply sensor. It would be more desirable to incorporate a boiler return sensor to control this type of system.

Variable-speed injection also would require the use of a wet rotor design pump that can operate at reduced speeds. However, based on system capacities and the number of modules or stages, a wet rotor design pump may not be large enough to handle the capacity necessary for the system. Larger injection pumps using a variable-frequency drive (VFD) are effective on larger boiler systems.

Other methods include modulating three- or four-way mixing valves between the boiler and system loops. Dual-modulating two-way valves are also an excellent choice that may fit into the budget better than three- or four-way valves.

BOILER TEMPERATURE CONTROL

Other methods of ensuring that the system return temperature does not drop below minimum standards would involve primary-secondary piping using two or more tiers or temperature levels for injection.

Allow the boiler(s) to inject into a primary loop that will always maintain at least 140 degree return-side water temperatures. Inject again from the primary loop into a secondary system loop that can be modulated by the temperature reset control.

Once again, the injection from the primary to the secondary loop can be easily accomplished by using a variable-speed pump control or, if that is not feasible, a three- or four-way mixing device. This would guarantee, by system design, that the boiler(s) would never see return water cooler than would be desired by the boiler manufacturer.

The building owner, design contractor, and installation contractor can have maximum efficiency within the system by taking advantage of full temperature reset, multiple-stage firing — and all with the ability of keeping the return temperature to the boiler where it should be.

Stanton is manager of training for Burnham Corp., Lancaster, PA; 717-397-4701.

Publication date: 09/23/2002