FAQs
Copper Finned Heat Exchanger Design
If you were designing a heat exchanger for a gas fired appliance, what material would you choose? We at Weben-Jarco believe the best material for a heat exchanger is solid copper. Excellent conductivity helps increase efficiency while maintaining small footprints. Low thermal mass provides rapid heat output and minimal standby heat loss. The ductility of copper makes it immune to thermal shock.
All our heat exchangers have extruded fins. The fins cannot separate from the tube and there are no thermal boundaries to inhibit heat transfer.
We use the largest inside diameter (1-1/4") available in our heat exchangers. The large diameter makes our heat exchangers more resistant to the most common water heating problems; scale and improper water flow. Weben-Jarco's single tube design makes failure of the heat exchanger due to scaling virtually impossible.
Scaling
As water is heated, dissolved minerals settle out as solid particles. As the particles build-up on heating surfaces they inhibit heat transfer and increase heating costs. In extreme cases, water heaters can be destroyed due to hot spots in the heat exchanger or restriction of water flow.
Underfired, storage water heaters are hurt the most by scaling. Almost all of the particles that form out of the water settle on the bottom of the tank. The heat transfer surface on the bottom of the tank is coated by layers of the insulating scale and there is no method for removing the build-up.
Forced circulation water heaters and boilers (such as the Weben-Jarco A-2000™, E-finiti™, MVP™ and Vision 3000™) have most of the particles flushed away from the heat exchanger by water flow. Even so, scale can build-up inside the heat exchanger tubes.
If scale does build-up on the walls of a forced circulation heat exchanger, it will restrict water flow through the appliance. Since Weben-Jarco uses the largest diameter tube in the industry, it can handle more build-up of scale without becoming completely blocked.
Weben-Jarco's appliances have two more advantages regarding scale due to the single tube design. Competitors use manifolded heat exchangers that have multiple water passageways. If a single tube in a heat exchanger manifold becomes blocked, water will continue to flow through the unblocked tubes while the blocked tube will melt. If Weben-Jarco's single tube design is blocked, the water flow switch will prevent boiler operation and subsequent heat exchanger damage.
The second advantage is the ease of scale removal. If the single tube requires cleaning, a chemical that dissolves scale can be circulated through the heat exchanger without any disassembly. Most manifolded heat exchangers will not allow chemical deliming, since any tubes with less scale may be damaged by the solvent.
Condensation
When a hydrocarbon is burned, the primary products of combustion are CO2 and H2O. Typically, all the products of combustion go out the exhaust vent as gasses. However, if the flue gasses are cooled down enough, the water vapor will begin to condense on surfaces and possibly rain out of the air. Higher efficiency appliances and cooler water temperatures increase the potential for condensation.
Condensation can be prevented in most appliances by allowing no water under 120°F to enter the heat exchanger. Appliances up to 85% efficient will stop condensing between 130 and 140°F return water temperature. 90%+ efficient appliances will stop condensing between 140 and 170°F. Consult the operation instructions for each appliance for the manufacturer's requirements.
The bad side of condensation: corrosion. The water that condenses out of flue gasses is acidic and will corrode most unprotected metals and other construction materials. Condensation can damage heat exchangers, cause oxides to plug air passages and eat holes in combustion chambers and vent stacks. Condensation in appliances that are not designed for condensation will damage the appliance and may result in injury or loss of life.
The good side of condensation: efficiency. Most high efficiency boilers and water heaters are categorized as "condensing appliances." Not all "condensing appliances" are designed to condense. Non-condensing boilers loose out on the added efficiency of steam condensation. When the steam produced by combustion is condensed on the heat exchanger, an additional 970 BTUs is gained for every pound of steam condensed. Weben-Jarco's MVP™ and Vision 3000™ appliances are designed to capitalize on the efficiency of condensation due to their "no minimum inlet water temperature" design.
Stand-by heat loss
Heat exchangers never stop working. When a boiler or heater is firing, heat is transferred into the water. When the appliance is off, heat is transferred out of the water. This in a nutshell is standby heat loss. There are two common ways to reduce or prevent standby heat loss: never turn the appliance off or minimize the thermal mass in the heat exchanger.
The next section, Modulation vs. On-Off Firing, will cover the "never turn the appliance off" strategy. This section is focused on thermal mass.
Cast iron, steel tube, fire tube and storage type water heaters all have large thermal mass in their heat exchangers. Copper finned tube heat exchangers are commonly referred to as low mass heat exchangers because they have relatively small metal mass and very little water mass in them. The key to preventing standby heat loss in a low mass appliance is to stop the water flow through the heat exchanger when the appliance is off.
Weben-Jarco offers a "pump delay switch" as an option with all water heaters and boilers. The switch turns on the circulating pump when the call for heat begins and keeps the pump on for 90 to 120 seconds after the appliance is turned off. The delay allows all the heat to be drained from the metal in the heat exchanger and then stops the water flow. With the flow off, only a few gallons of water remain in the heat exchanger to lose standby heat. With a low mass heat exchanger and a pump delay switch, standby heat loss is virtually eliminated.
Modulation vs. On-Off Firing
Modulation, proportional firing and step firing are all ways of matching the heat output to the heat demand. Rather than turning a boiler on and off, the firing is throttled, like the accelerator on a car. Modulation allows more stable outlet water temperatures, can increase the efficiency at low firing rates and reduce standby heat loss. On-Off firing is just what it sounds like, fully on or fully off.
One of the selling factors for modulation is the standby heat loss issue. As stated in the previous section, standby heat loss for properly installed low mass appliances is negligible. Efficiency studies that show fuel savings by converting on-off boilers to modulating boilers are based on high mass boilers and while valid in that context, do not apply to low mass appliances.
Another efficiency related modulation issue is the improved efficiency at low fire. As the firing rate is decreased there is more heat transfer surface per BTU of heat input, and the efficiency increases. It is human nature to look at the best efficiency and presume that is what you will get. Consider a boiler that runs at 25% of full firing rate 80% of the time and at 100% of its firing rate only 20% of the time. This boiler will burn 1/4 of the fuel for 4 times as long as it will at full fire. In other words it will burn equal amount of fuel at full fire and low fire, even though it is only on high fire 20% of the time. Half of the fuel will be burned at the worst efficiency, and the fuel prices will probably be adjusted higher when demand is at its peak.
On-Off boilers are simple. The controls are typically non-proprietary and less expensive. Set-up is easier and adjustments and maintenance are less frequent. With more and more multiple boiler installations, on-off boilers can be staged on to provide the same stable output temperature that modulating boilers offer. Finally, control systems for on-off appliances are simpler and less expensive.
Common Problems
All on-off, low mass appliances must be controlled on inlet water temperature. Attempts to control these appliances on outlet water temperature will result in short cycling, premature parts failure and potential sooting. Similarly, there must be a minimum difference between the settings of the operating aquastat and the high temperature limit switch to prevent short cycling and lockouts. Weben-Jarco recommends the high limit set point be no less than 40°F above the operating set point.
Night-time temperature setback or turning off water heaters over week-ends may cause condensation. Repeatedly feeding low temperature water to an appliance that is not designed for condensation will cause maintenance and safety problems. Refer to "Condensation" section above. Frequent shutdowns must be avoided and night-time temperature setback must not be set below the minimum allowable inlet water temperature.
If you have other engineering questions, please contact your local representative or e-mail questions directly to Weben-Jarco.
