Lochinvar CP-5M-4/08, 45 Low Temperature Bypass Requirements, Three Way Valves, Boiler Flow Rates

LOW TEMPERATURE BYPASS

REQUIREMENTS

This piping is like a primary/secondary boiler installation with a bypass in the secondary boiler piping. Inlet water temperatures below 140°F (60°C) can excessively cool the products of combustion resulting in condensation on the heat exchanger and in the flue. Condensation can cause operational problems, bad combustion, sooting, flue gas spillage and reduced service life of the vent system and related components. The bypass allows part of the boiler discharge water to be mixed with the cooler boiler return water to increase the boiler inlet temperature above 140°F (60°C). This should prevent the products of combustion from condensing in most installations. The bypass should be fully sized with a balancing valve to allow for proper adjustment. A valve must also be provided on the boiler discharge, after the bypass. Closing this discharge valve forces water through the bypass. Start boiler adjustment with the bypass valve in the full open position and the boiler discharge valve half open. A small amount of the higher temperature boiler discharge water is mixed with the system water to maintain the desired lower system temperature. A remote low tem- perature range operator is recommended to control the boiler operation based on the lower system temperature. This remote operator should be wired across the R and W terminals (see Room Thermostat Connection and Terminal Strip Instructions).

THREE WAY VALVES:

SECONDARY

BOILER PUMP

EXPANSION		FROM
TANK	PRV	SYSTEM

MAKE-UP

WATER

	LWCO	THREE-WAY VALVE
	(OPTIONAL)
		SLOW
		OPENING/SLOW
TO SYSTEM		CLOSING
	AIR SEPARATOR
	SYSTEM PUMP

FIG. 45 Boiler Piping with a 3-Way Valve

The installation of a three way valve on this boiler is not generally recommended because most piping methods allow the three way valve to vary flow to the boiler. This boiler is a low mass, high efficiency unit which requires a constant water flow rate for proper operation.

Low flow rates can result in overheating of the boiler water which can cause short burner on cycles, system noise and in extreme cases, a knocking flash to steam. These conditions can cause operational problems and non-warrantable failures of the boiler. If a three way valve must be installed, please pipe in a primary/secondary system as shown in Figure 45. Based on boiler sizing and system flow requirements, this piping may still result in boiler short cycling.

BOILER FLOW RATES

￿CAUTION

The maximum flow rate through the boiler with a copper heat exchanger must not exceed 30 GPM.

MAKE-UP WATER

PRV

EXPANSION TANK

TO SYSTEM

LWCO (OPTIONAL)

SYSTEM PUMP

AIR SEPARATOR

BYPASS VALVE

FROM SYSTEM

FIG. 46 Boiler Bypass Piping

The heat exchanger is generally capable of operating within the flow rates of a residential heating system. Should the flow rate exceed the maximum allowable flow rate through the boiler (30 GPM) an external bypass must be installed. The bypass should be fully sized with a balancing valve to allow for proper adjustment of flow. Flow rate can be determined by measuring the temperature rise through the boiler.

The basic guide for minimum flow in this boiler is based on a 40°F temperature rise in most installations. Lower flow and a higher temperature rise is acceptable if the boiler system is specifically designed for the characteristics of a higher temperature rise. A system not specifically designed for a higher temperature rise may experience overheating of the boiler water. This can cause short burner on cycles, system noise and in extreme cases, a knocking flash to steam. These conditions can lead to operational problems and non-warrantable failures of the boiler. High temperature rise systems must be designed by a qualified engineer.

35