Engineers & Manufacturers of the Heatsponge Economizer


HeatSponge Frequently Asked Questions and

Boiler Economizer Specification Compliance Information


Do you have an existing specification for a boiler economizer and would like to know how a HeatSponge unit compares to it ? Many common specification items are listed below along with comments. If you have a question that is not listed below feel free to contact us. Please note not all information on this page applies to the Sidekick model hot water boiler economizer.



Question: The specification requires individually replaceable tube elements that connect to headers by compression type fittings. Does your unit meet this requirement ?

Answer: Absolutely ! The ability to replace individual tube elements without welding is a key feature for all HeatSponge models including the industry leading SHORTY helical coiled economizer.


Question: The specification requires a coiled unit.

Answer: All HeatSponge models except the SHORTY are rectangular units. The HeatSponge SHORTY model coiled-tube economizer is typically offered for boilers under 300 HP and unlike any competive coiled designs offers the same replacable tube features as larger rectangular HeatSponges.


Question: The specification requires a hinged access door.

Answer: Hinged panels cannot be cleared out of the way for easy of entry. The HeatSponge unit includes handles to remove the access door and place in an area that does not interfere with access to the unit.


Question: The specification requires side access doors.

Answer: The HeatSponge rear access door was made large enough to provide access to the inside of the economizer for maintenance and inspection. Our rear door is large enough as to not require removable side doors as well so are not included.


Question: The specification states gas flow must be unrestricted over the entire tube bank.

Answer: That is standard for a HeatSponge. The only part of the heat exchanger kept out of the gas flow is the headers and compression fittings.


Question: The specification requires a water side bypass.

Answer:  Water side isolation and bypass valves are not included in the base HeatSponge scope of supply. The installing contractor should supply these components.


Question: The specification requires the unit be designed to ASME Section I. We see your standard is Section VIII.

Answer: The industry standard for these types of units is Section VIII. We can supply a unit to Section I with no problem. There may be an additional cost so contact your local representative or us directly.




Question: The specification states a requirement for a damper.

Answer: The HeatSponge does not require a damper for proper operation therefore it is not supplied. The HeatSponge is a rectangular economizer utilizing a true square pitch design. This is the configuration used almost exclusively with large industrial boilers. We modeled our design on industrial standards. Most units that have a damper require it because their tube arrangement does not allow the gas to evenly flow over the tube bank. The damper is little more than a diverter that forces flue gases over the tube banks. These units do not provide for a physical isolation of the gases from the tube bundle. Dampers introduce moving parts that over time will be expected to bind, stick, and need servicing. Worst of all, when you are bypassing flue gas you are letting energy blow right through the unit at go out the stack. What is the point of installing an economizer if your design has to allow flue gas to blow through the unit unrecovered ? Our answer is the correct answer: design a better unit that does not require the hassle and problems associated with a damper.


Question:  The specification states the damper must be of a modulating type.

Answer: Great, another controller and actuator to fail and another damper to bind and stick. More complexity, more maintenance, more expense. We'll take exception to that as well. Economizers by their nature are straightforward devises, why take something simple and make it more complex than it needs to be?


Question: We were told we need a damper for occasional #2 oil firing.

Answer: A HeatSponge economizer with 5 fins per inch is designed to accommodate periodic firing of #2 fuel oil. If the economizer will see extended periods of #2 oil service then the removable rear door can be opened and the unit pressure washed to avoid fouling. If the unit will see prolonged #2 oil firing the economizer should be selected with 4 fins per inch to reduce fouling concerns.


Tubes and Fins:


Question: The specification requires a specific grade of stainless steel.

Answer: The standard HeatSponge uses either 304 or 316 stainless steel depending on bulk availability although we can supply to other grades at additional cost. Contact us for more information.


Question: The specification calls out for welded finned tubes.

Answer: The HeatSponge uses a brazed metallurgical bond to accomplish fin to tube welding as do most of the companies that manufacturer competitive units.


Question: The specification calls out an ALFUSE attachment, what is it ?

Answer: ALFUSE is a metallurgical bonding process used primarily by a competitive manufacturer. The HeatSponge unit will utilized a brazed metallurgical bonding process. We are unable to comment on a competitive tube bond method.


Question: The specification requires stainless steel tubes but the application is feedwater from a deaerator so your selected carbon steel tubes. What should we do?

Answer: If the customer has specified stainless steel tubes for an application that carbon steel tubes would suffice the choice is up to the customer to either save money and use carbon steel tubes or input a low water temperature just to get a price for a stainless steel tube unit. The performance of either a stainless steel tube unit or carbon steel tube unit will be close although a carbon steel tube unit will perform slightly better. Upon placement of order we will confirm all performance.


Question: The specification calls out a triangular or staggered fin pitch.

Answer: All HeatSponge's except the Sidekick utilize a true square pitch tube arrangement. From behind it would resemble a checkerboard or grid. Staggered pitch and triangular tube pitch arrangements do not line up perfectly square. For this reason staggered units are not 100% drainable by gravity in a horizontal installation and triangular pitch units are more subject to fouling and are more difficult to clean. A staggered pitch in a horizontal installation is a suitable arrangement but a triangular pitch should never be utilized.




Question: The specification states the unit shall be selected as not to go into condensing.

Answer: Stainless steel HeatSponge units are design to accommodate condensing. The recovery of a cold water HeatSponge is significantly increased when the unit is allowed to go into condensing. The correct answer is the unit should be design to accommodate condensing should it occur. Carbon steel HeatSponge units supplied with feedwater over 228 deg F will rarely if ever go into condensing however should it occur the lower transition is design to capture and drain the water before it can enter the boiler. Applications such as cold process water heating applications can utilize a fully stainless steel HeatSponge to take advantage of condensing operation.


Question: We will be heating a cold process water that will resulting in condensing inside of the economizer. Does the performance reflect this ?

Answer: No. At this time our software does not calculate the additional heat recovery from condensing. All heat recovery is based on a sensible exchange only. We are currently working on new software that will allow the additional latent heat recovery to be accounted for.


Question: We have a specification that calls out the BTU duty to be based on 80% efficiency. What does this mean ?

Answer: Our program calculates the BTU duty of the unit as the true heat transferred. Some manufacturers will express the heat recovery in terms of energy input that is offset by the inclusion of a new economizer. For example if the btu duty of a new economizer is 100,000 btu/hr and the existing boiler is 80% efficient while the true heat duty may be 100,000 btu/hr the amount of energy you would be offsetting with a higher efficiency system would be 120,000 btu/hr. It is a good way to understand true heat recovery and competitive manufacturers are good to point this fact out however make sure you never confuse the true heat recovery of the unit, the BTU duty, with this offset concept.


Question: Can we heat a glycol solution instead of water ?

Answer: Yes, our unit can be utilized to heat fluids other than water such as glycol. To assist in preliminary selection we have placed the ability to change the specific heat of the water on the performance page. this is the most important physical property to be changed. You will need to change it to the proper specific heat for your application. If a project reaches an order stage then we will review the application to insure the performance is valid by looking at all of the unique physical properties of the fluid.


Question: Can a HeatSponge be used for domestic or potable water heating ?

Answer: Yes assuming a 304 stainless tube and header is acceptable for use in the facility for the intended service. A HeatSponge can be used to heat domestic or potable water. Stainless steel tubes and headers will be required as well as having a certified company perform the required cleaning of the tube bundle after installation and prior to use. This expense is not included in the HeatSponge pricing as is by others. Since most domestic water systems use very cold water stainless steel inner casing upgrade may also be required if the unit goes into condensing.




Question: How accurate can the performance be considered and how is performance calculated ?

Answer: The AutoPick software is based on the industry-standard Escoa approach to heat transfer. An industry standard, this proven and publicly available series of heat transfer equations can be relied on for providing accurate and consistent heat transfer ratings. All units that are awarded are further reviewed on a third-party software to insure accuracy with the preliminary online performance.




Question: Do I need a safety relief valve ?

Answer: Yes, every economizer will require a safety relief valve. We include one in our base price for every unit. Be sure to remember this when comparing pricing, many competitors make purchasing a safety relief valve an optional adder.


Question: Are inlet and outlet transition pieces included ?

Answer: Every HeatSponge features a fully insulated one piece construction that includes the inlet and outlet pieces integral to the unit. Imagine a BASE HeatSponge unit is the shape of a dishwasher and a HIGH unit is the shape of a refrigerator.


Question: What about mating flanges.

Answer: Our base price does not include the mating flanges that are required to connect to our unit. A typical firetube boiler already has the proper flange and the popular modular stack assemblies likewise also have the flange. If a mating flange is required we will provide them at additional cost.


Question: Does a HeatSponge carry any stack load.

Answer: By default HeatSponge's only carry a limited static loads for the simple reason every installation is unique and it is paramont that no details be missed therefore we keep our published load carring limits very low on purpose. Different HeatSponge units have different capacities, check Bruce's proposal for information on your specific unit. HeatSponge's can carry greater loads however the factory must be consulted.


Question: How is a HeatSponge supported ?

Answer: A HeatSponge is designed to either be suspended or to be carried by a suitable external structure. If you intend to set it on top of a firetube boiler you must verify the boiler can carry the additional weight or provide other provisions for the load.


Question: Is a constant water flow required ?

Answer: Yes. The water inside of a HeatSponge cannot be allowed to stagnate. Stopping water flow will lead to steaming inside of the economizer. For on-off feedwater systems that cycle very frequently you may be able to avoid the need for upgrading to a modulating feedwater system. Your boiler engineer should make the final determination as to the ideal way to avoid economizer steaming.


Competitive Units:


Question: What should I look for when comparing competitive units ?

Answer: The key design parameter for any economizer is the flue gas pressure loss. This is the amount of backpressure the forced draft fan will have to create to overcome the restriction in the stack caused by the new economizer. Efforts to make a cheaper economizer usually mean a smaller unit therefore a higher pressure drop. COMPARE THIS CLOSELY ! once you have insured the gas side pressure drop is equip look at the btu recovery of the unit. Make sure the units are recovering the correct amount of energy.


Question: Our procurement process requires us to solicit multiple bids. Do other companies make units with these same features ?

Answer: Yes. While our unit is unique there is nothing proprietary or patented on a HeatSponge. There are other companies that build economizers, some with similar features to ours. We hope you would understand why we wouldn't want provide any advertising for our competitors so we would decline from listing them. If you do require names please contact your local representative or us directly and we will see what we can do to assist your needs.


Question: It te HeatSponge a duplicate of another unit ?

Answer: No. The HeatSponge is a all new design that started as a clean-sheet-of-paper approach to designing an economizer to meet the demands of today's market. While there are some features that are similar to units manufactured by competitive manufacturers the HeatSponge itself is unique and in our opinion superior to any other unit. For information on how we developed the HeatSponge click this link.



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Boilerroom Equipment, Inc.

11 Rich Hill Road

Cheswick, PA 15024