Copper Finned Tube Boilers and Primary/Secondary, Why?

"Hey Jim, I need to bring something to your attention. This copper finned tube boiler isn’t piped primary/secondary. Unfortunately we won’t be able to complete our start-up until the boiler piping is corrected".

"Dave, what do you mean you can’t complete start-up, we have to be on line tomorrow for the grand opening!"

"I’m sorry Jim; this boiler should be piped primary/secondary to insure trouble free long lasting operation. If it’s not piped primary/secondary, you may get a low flow condition or condensation may form. Both will cause tube bundle failure and it will not be covered by warranty"

"But Dave, this is the way it’s shown on the plans…"


This is one of the most uncomfortable situations I’ve been faced with as a manufacture’s representative and, unfortunately, one that occurs all too often. The use of copper finned tube boilers has grown tremendously in the last ten years through plan/spec, design build and replacement projects. The recent improvements of copper finned tube products, their low cost and ease of use have increased their use. In the past, the consulting engineer or contractor has designed/installed copper finned tube boilers on the job as they always have using a steel tube boiler with primary pumping. What they don’t realize is that copper finned tube boilers differ from other boiler types and special consideration must be taken when piping them, especially in comfort heating applications.

The biggest difference between the two technologies is the amount of water they hold. Steel tube boilers are high mass units, typically holding many gallons of water. They also take longer to reach set point and are slower to respond to system demands. Due to the amount of mass they hold, they can handle variations in flow well as variable speed drives and 2-way control valves respond to system demands.

Conversely, copper finned tube boilers are low mass units and hold a small amount of water. The water they hold is based on the amount of water the heat exchanger tubes can hold, which depending on the size of the boiler may only be a few gallons. They basically heat the water via convection and gas radiation by passing the products of combustion through a series of baffled finned copper tubes, thus heat transfer is very effective and warm up time is fast, almost too fast. In fact, copper finned tube boilers are often referred to as "quick-start" boilers because they are able to heat the water inside the tubes up to 9 times faster than cast iron or steel tube boilers.

Copper finned tube boilers are designed to handle high "heat flux" or heat transfer, and rely upon higher tube bundle velocities to keep the tubes free from scale build-up and to "unload" all of the heat they can transfer. Therefore, a basic element of design for all copper finned tube boilers is the flow rate that the heat exchangers require. Piping these boilers using direct primary pumping can pose a myriad of problems and in most installations will shorten the life of the boiler drastically. Problems that may occur include:

• Over pumping: if the pump isn’t matched to the boiler properly, excessive velocities may occur in the tube bundle leading to tube erosion and premature failure. When higher flow rates are required, such as in domestic water heating applications with high hardness levels, cupro-nickel finned tubes that have greater resistance to erosion may be ordered instead of the standard copper finned tubing. I must stress that even cupro-nickel finned tubes have limits on internal velocities and direct primary pumping may still pose a problem.

• Low/no flow over heating: in a 2-way valve variable volume and/or variable speed system, the flow constantly varies to match the system’s load. As the valves begin to modulate closed or the pump speed is reduced, the flow through the boiler may not be enough to take away all of heat the boiler is producing, causing excessive heat flux, liming inside the tubes, and ultimately, tube bundle failure. Even if the boiler turns off, there still may be enough residual heat in the combustion chamber to cause damage. Thus the pump must continue operating until the heat is dissipated.

• Condensation: In today’s world, equipment manufacturer’s have been forced to design equipment to operate at the highest possible efficiency. For boilers, this may or may not pose additional design considerations. As boiler efficiency increases, the temperature of the products of combustion will decrease. If the temperature of the products of combustion falls below the dew point, condensation will form and fall back into the boiler. It turns out that this condensation is mildly acidic, about like Coca-Cola and prolonged exposure over time will cause oxidation of the copper fins, increasing the potential for soot build-up which will obstruct hot flue gasses from rising, as well as serious corrosion to all ferrous materials it comes in contact with. Premature tube bundle failure is sure to follow. (Boy, I guess I better stop drinking so much Coca-Cola!)

One way to prevent condensation from forming is to keep the return water temperature at some minimum temperature, typically 110F–130F, depending on the boiler of course. This is especially important in low temperature return systems such as water source heat pump or swimming pool applications.

Primary/secondary piping provides a good method of preventing all three scenarios I have mentioned above. I always advocate to the customer to specify/purchase a boiler with a factory mounted pump to serve as the primary pump. In some cases the factory furnished pump is not large enough, thus an external field mounted pump would be required.

Figure 1 shows typical primary/secondary piping for a single boiler comfort heating system. The boiler is piped directly to the secondary loop by placing two tees 6-12" apart. A pump dedicated to the boiler and two tees, it’s that simple.

Figure 2 shows a multiple boiler system where the primary loop is piped in reverse return fashion, assuring each boiler’s inlet temperature is the same.

Additional attention must be given to low temperature return systems because they typically operate below 100F. Figure 3 shows an additional blending loop with a balancing valve within in the primary loop to assure some of the leaving water is blended back to the boiler, raising the inlet temperature to the minimum required by the boiler.

The boiler line that Dawson represents, Laars Heating Systems, has designed a header for the Laars Pennant that includes a factory mounted 3-way mixing valve specifically designed for low temperature return system (see below). The factory mounted three way valve also comes in handy in large systems where initial system warm-up is slow, thus condensation may still be a factor.

Primary/secondary piping is not the solution to every installation, but more times than not it will keep the manufacture’s representative and contractor out of trouble and the engineer and building owner happy. Primary/secondary is a simple low cost method that works well and I strongly recommend that it be considered as the starting point of every copper finned tube installation.