TORONTO, CANADA — Contractors should team up multiple smaller boilers to handle a large load, hydronic engineer John Siegenthaler told contractors attending the Great Lakes Hydronic Conference here in late April.

Multiple modulating boilers are even more efficient, Siegenthaler said.

Multiple boilers are two, three or more boilers in a large custom residential or commercial application. Each boiler, or stage, is independently controlled, and the boilers divide the heating load among them with the use of an intelligent controller.

The reasoning for this type of installation, Siegenthaler said, is that boiler efficiency decreases slowly as a unit's duty cycle decreases, but efficiency plummets when the duty cycle drops below 35%. A multiple boiler installation can minimize running time at less than a 35% duty cycle.

Multiple boilers can provide heat even if one boiler fails, he noted. Smaller and lighter boilers are easier to retrofit than a knockdown cast iron boiler. Repair parts are easy to obtain at local wholesalers, Siegenthaler continued.

Multiple boilers can shift to high output for domestic hot water or snow melting while retaining high efficiency for low spacing heating loads.

The space-heating load can be tracked even more closely with four boilers than with two, Siegenthaler said, although that more-is-better logic stops when an installation reaches eight boilers. After that there is so much boiler jacket surface area that jacket losses drop the net system efficiency.

Turndown ratios to 20%
The newest concept is multiple modulating boilers, of which there are at least a half dozen on the market, Siegenthaler said, with turn-down ratios that go as low as 20% before problems develop with ignition, emissions and flame stability. Seigenthaler believes that boiler manufacturers will be able to get the turndown ratio lower in years to come.

If a boiler can modulate from 150,000 Btuh down to 30,000 Btuh, four such boilers could fire from 600,000 Btuh to 30,000 Btuh, a turndown-ratio of 20:1. The object is to run the heat exchangers at low combustion surface temperatures to produce sustained flue gas condensing in the stainless steel or cast aluminum heat exchangers. Efficiencies while condensing are in the 95% range with seasonal efficiencies in the high 80s. Most such boilers are sealed combustion, venting through PVC or CPVC pipe.

The flow in such an installation must be independently controlled though each boiler — water should never be pumped through an unfired boiler. Reverse return piping will ensure that each boiler sees the same return water temperature. The boilers should be run at the lowest possible water temperatures to encourage condensing.

Unlike cast iron boilers, circulators on condensing boilers should pump into the heat exchanger to decrease chances of cavitation or steam flashing. Because such boilers may contain only 1/2-gal. of water, it's easy to make steam. For the same reason, water must be moving through the boilers constantly, even if only one zone valve out of six is calling for heat. Siegenthaler often uses closely spaced tees between the supply and return sides to allow for continuous circulation. Some manufacturers are producing "low loss headers," which are essentially a wide spot in the pipe to mix boiler water with distribution system water while still achieving flow separation between the boiler side and distribution side. The devices, also called hydraulic separators, are common in Europe, Siegenthaler said, and one model has an air vent on top and a sediment drain in the bottom.

No mixing valves
The low system temperatures on both the boiler and distribution side eliminate the need for mixing valves.

Siegenthaler positions his supply temperature sensor on the supply side of the distribution system, either before or after the system pump. That sensor, along with the outdoor sensor, controls the boilers' firing.

Conventional bang-bang controls measure the differential between the target temperature and the actual water temperature to turn boilers on and off. Modulating boiler controllers try to maximize operating time at the low output range of the boiler, so the control will typically run two boilers at low levels rather than one boiler full blast. The object is to spread smaller flames over maximum heat exchanger area in order to encourage condensing.

Different manufacturers have different control logic, Siegenthaler said. Some will fire one boiler to 80%, then fire a second and modulate both boilers back down to 40%. Some will fire three boilers at low levels and ramp all of them up at the same rate.

The size of multiple modulating boiler systems is remarkable when compared with the paradigm 25 years ago, Siegenthaler said. One manufacturer has put together a heating plant of eight boilers firing from 3.2 million Btuh to 100,000 Btuh, a turndown ratio of 32:1 or 3% of load, that occupies only 50 sq. ft. of mechanical room space. Siegenthaler has seen boiler rooms with multiple modulating boilers on racks, taking up even less floor space.