Introduction
Minnesota has adopted a modified version of the Universal Plumbing Code (UPC) as the new code for the state. The UPC allows for the use of single wall heat exchangers for boilers with domestic water heating appliances. The Minnesota Plumbing Board had originally planned to amend the MN UPC code to require RPZs be installed anywhere a single wall exchanger was used. That extra requirement would have made the use of single wall exchangers economically unfeasible. Jon wrote and presented the white paper below to the Plumbing Board to help convince them that adding the extra RPZ requirement was overkill and detrimental to the industry and its customers. As a result, the Plumbing Board removed the RPZ requirement, and single wall heat exchangers can now be installed legally throughout Minnesota.
Defense of the UPC Single Wall Heat Exchanger Language
The proposed UPC single wall specification revision requires that a Reduced Pressure Zone Backflow Preventer (“RPZ”) be added to systems incorporating double wall heat exchangers. This offers a more technically achievable provision for single wall heat exchangers than the current Minnesota code. However, the added expense of the RPZ installation, annual testing, and the inconvenience to homeowners, offsets a majority of the energy and cost savings opportunity, making the installation of single wall exchanger solutions impractical and economically unfeasible for most applications. The result will be far less indirect water heaters, combi heating units, and solar thermal installations in the state than there could be; thus depriving homeowners of some of the most innovative and efficient water heating alternatives, and the State of Minnesota of an opportunity to significantly lower our carbon emissions.
I will discuss below why single wall exchanger devices are important, and quantify the energy savings potential and corresponding environmental benefits that these devices can provide. I will also take a closer look at the elements of the UPC single wall code as it is currently written and illustrate how the unamended code is very effective at protecting the health of the citizens to whom it applies. The UPC code is a safe, practical balance that provides an opportunity for energy savings and environmental benefits, and application advantages that single wall equipment provides.
The existing UPC single wall code is effective
The UPC single wall code effectively addresses the proper and safe use of SW exchangers. A team of very capable industry professionals put a lot of thought, concern and hard work into creating the UPC single wall code language. Questions and concerns have been thoroughly researched and thoughtfully addressed. The current language of the UPC single wall code was adopted around 2007 as a rational balance of safety and practicality.
603.5.4 Heat Exchangers. Heat exchangers used for heat transfer, heat recovery, or solar heating shall protect the potable water system from being contaminated by the heat-transfer medium.
603.5.4.1 Single-Wall Heat Exchanger. Installation of a single-wall heat exchanger shall meet all of the following requirements:
Pressure Differential
(1) Connected to a low-pressure hot water boiler limited to a maximum of 30 pounds-force per square inch gauge (psig) (207 kPa) by an approved safety or relief valve.
A typical residential boiler’s pressures range between 10 and 20 psi. Domestic water systems have normal pressures much greater than 30psi. The requirement that the maximum of a 30lb relief valve be installed on the boiler side ensures that if there is a heat exchanger leak, under normal circumstances, water from the higher pressure domestic side will be forced in to the boiler system side. This results in the relief valve for the boiler blowing fluid onto the floor of the boiler room, providing a dramatic visual and audible alarm for the occupants. In unlikely event that the domestic pressure drops below 30psi in the domestic side, while there is an unnoticed leak on the SW exchanger, the provision that the boiler be filled with nontoxic fluid and labeled as such ensures that the integrity of the domestic water supply is maintained.
Non-toxic Heat Transfer Fluid
(2) Heater transfer medium is either potable water or contains fluids having a toxicity rating or Class of 1.
The statement in the code that “The heat-transfer medium shall be water or other nontoxic fluid having a toxic rating or Class of 1 as listed in Clinical Toxicology of Commercial Products, 5th edition” could not be more definitive.
Labeling
(b) The pressure of the heat-transfer medium shall be limited to a maximum of 30 psig (207 kPa) by an approved safety or relief valve.
The word “Caution” and the statements in letters shall have an uppercase height of not less than 0.120 of an inch (3.048 mm). The vertical spacing between lines of type shall be not less than 0.046 of an inch (1.168 mm). Lowercase letters shall be compatible with the uppercase letter size specification.
The UPC clearly calls out the verbiage for the label that has become a standard on all indirect water heaters sold in the US. The UPC language even goes so far as to specify the size of the text and spacing of the lines. The reference is listed for the Class 1 material list. Anyone who does not care to take the initiative to research the Class 1 alternatives can simply default to potable water.
To fail beyond the safeguards of the current UPC code would require that a statically implausible sequence of events occur simultaneously:
- As discussed earlier, a leak in a SW exchanger between a boiler and DHW system will result in the boiler’s 30lb relief valve blowing a steady volume of water onto the boiler room. For the heat transfer fluid to move into the domestic side, a leak in would have to develop in the heat exchanger simultaneous to a freakish pressure drop in the domestic system.
- For the backflow to be a health issue, someone at that same address would have had to ignore the prominent label on the tank, industry norms, and common sense and fill the system with a toxic fluid. One could argue that the sort of person who would do that has already demonstrated they aren’t concerned about following codes and will install whatever they see fit.
- Additionally, unnoticed exchanger leak simultaneous with the freakish pressure differential would have to exist long enough to pull a sufficient amount of heat transfer fluid into the water supply to be a toxic concentration.
It is logical to argue that the probability of all of these events happening simultaneously is very small.
Below is a reasonable attempt at quantifying that probability. Each of the assumptions could be debated in either direction. Doubling, tripling, or even increasing or decreasing any of them by a factor of 10, however, yields the same conclusion. That is that the UPC single wall exchanger code as written does a very good job of protecting the public.
Probability of Harmful Heat Exchanger Failure Incident
Table 1. Probability of Harmful Heat Exchanger Failure Incident
| Failure rate of heat exchangers – units/100 | 3 | 3% |
| Domestic pressure < Boiler – Hrs/Yr | 4 | 0.0457% |
| Toxic Fluid in boiler with indirect despite warning label – Installations/1000 | 5 | 0.5% |
| % of time a negative pressure leak results in enough cross contamination to be harmful | 25 | 25% |
| Probability of an Incident that causes harm | 0.0000000171 | |
| Percent of time the UPC constraints have the intended result | 99.9999983% |
Why single wall heat exchanger devices are important.
Indirect water heaters, combination “combi” heating units, and solar thermal systems save energy when compared to conventional tank style and tankless water heaters. They have lower operating costs, conserve energy, and are a greener solution for the environment. They also provide application benefits of abundant domestic hot water, simplified installations, and space savings.
Indirect Water Heaters
Today’s indirect water heaters are very durable and dependable pieces of equipment with industry standard lifetime warranties. Tanks and exchangers are generally made from robust, corrosion resistant materials such as SST, Cupronickel and engineered polymers. Tanks that meet the UPC code requirements are tested to working pressures of 300psi and manufactured to ASME or IS 191 standards.
Water heating requires the second largest demand for energy in a home after heating and cooling. As such, it offers one of the best opportunities for energy savings. Adding an indirect water heater to an existing boiler is an effective way to reduce the energy required for DHW production. Indirect water heaters are paired with boilers typically ranging from 80% AFUE to 98% AFUE, yielding combined annual efficiency rates (EF) of .86 plus. This compares to a typical combustion efficiency of a conventional gas water heater with burner in the 70% – 80% range with large stand by losses up the flue, yielding an EF of .60. Pairing high efficient condensing boilers with well insulated indirect tanks results in superior combustion efficiencies and stand by losses as low as 1oF per hour. Typical annual savings from a boiler/indirect relative to a conventional gas water heater are in the $200 range. Savings over electric water heaters are closer to the $400 annual range. Installation cost of a residential single wall indirect water heater with an existing boiler is around $2000 . This provides a 5 – 10 year payback on the investment; a reasonable return on a piece of equipment with a lifetime warranty. In addition to energy savings, single wall indirect heaters can produce hot water 2-5 times faster than a comparably sized conventional gas or electric heater.
An estimated 120,000 homes in MN currently have gas boilers installed. Based on industry data, approximately 3300 boilers are replaced in MN annually and another 1500 are installed in new installations. Assuming 80% of those homes with boilers currently have conventional water heaters, there are 96,000 homes with boilers that could add indirect water heaters. The typical installed mix is 60% gas, 40% elect, therefore, we can use a conservative weighted average energy savings of $280 per year per home. The total annual energy saving opportunity becomes 96,000 homes x $280 = $26,880,000 annually.
Burning that much less gas and coal (for electrical generation) means that we could reduce statewide CO2 emissions by 250,000 tons. One ton of CO2 reduction is the equivalent of planting 140 trees. This reduction would be the equivalent of 70,000,000 trees. This sort of environmental benefit opportunity is consistent with direction from the governor to look for ways that our industry can be greener.
The industry manufacturers track the total number of indirect water heaters sold into different markets in the US. Last year there were 77,000 sold throughout the US. 302 were sold in MN. By contrast 1700 were sold in WI. That is hard data showing the current code requiring double wall exchangers makes the products less viable in MN. The extra costs associated with an RPZ installation will ensure that Minnesotans continue to miss out on energy savings and application benefits that the products provide.
Combination (Combi) Heating and Hot Water Devices
There has been a tremendous amount of innovation in the area of hydronic heating and DHW generation devices. Manufacturers are finding creative ways to combine the functionality of gas and electric boilers, hydro air, indirect DHW, tankless water heaters, solar thermal, air source heat pumps, geothermal, ventilation, and biomass boilers. The results of this innovation include efficiency improvements, enhanced comfort, space savings, cost savings, and environmental benefits. The new products are evolving from multiple directions, but one common aspect is they incorporate multiple types of heat exchangers. These exchangers often function to trade energy between space heating and DHW heating devices.
One example of such a device is the Matrix from NTI. It combines hydronic heat, DHW, hydro air heat, cooling and an HRV all in a single piece of equipment. The Matrix uses a brazed plate exchanger for its DHW generation. Another example is the Versa-Hydro Solar from HTP. The Versa-Hydro is a tank style, modulating, condensing water heater with a brazed plate exchanger for supplying radiant heat, auxiliary ports for open loop hydro air handler, and a heat exchanger coil at the bottom of the tank for charging the tank with energy from solar thermal panels or a wood boiler. The common denominator with these and nearly 75 other combi devices on the market from over a dozen manufacturers is that they are only available with single wall heat exchangers.
Solar Thermal Systems
Solar thermal options offer the potential for a near zero energy cost and a corresponding reduced carbon foot print. However, the high cost of the system, even with 30% federal tax credit incentives, makes it difficult to justify economically. Solar thermal heating is most often applied to DHW systems because they have a year round demand. The extraneous extra cost of DW heat exchangers or RPZs, makes Solar that much more difficult to justify financially.
Effectiveness of Single Wall vs. Double Wall Heat Exchangers
SW heat exchangers are, in general, much more effective at transferring heat than DW exchangers. Heat transfer takes place across a single layer of material rather than across two layers with an insulating air gap in between. In some cases, manufacturers oversize the DW heat exchangers to increase surface area and compensate for the reduced efficiency in an effort to improve the performance. Extending the length of the coil results in that need for larger circulators and more pumping energy to overcome the additional head loss and still achieve the required flow. In either case, DW heat exchangers are more costly to install and operate.
As an example, a 45 gal single wall heat exchanger from one manufacturer will provide up to 141 gal of 140F water in the first hour. The 45 gal DW version yields only 70 gal at 140F in the first hour. The consumer cost of the DW is $600 (50%) more. To achieve similar performance with the DW, the cost would be $1900 higher
There are several reasons that DW heat exchanger HVAC devices tend to be significantly more expensive than similar SW devices.
- DW exchangers are more expensive to manufacture because they require more complex processes and more material.
- Manufacturers must make the DW heat exchangers larger with more surface area to compensate for the lower heat transfer rates across the two layers and insulating air gap.
- The DW market is very small relative to the SW market. According to equipment manufacturers, Minnesota has the only additional state wide restriction on the use of single wall heat exchangers. As a result, in the rare case when a manufacturer does decide to offer a device in a DW version, the production volume is comparatively much lower, resulting in higher product costs.
The bottom line is that the extra cost and reduced performance of the DW exchangers makes them difficult to justify economically from an application perspective. Additionally, there is a strong argument that in reality, double wall exchangers provide little or no additional safe guards relative to singe wall. I’ve had conversations with several plumbers who have stated that, in their experience, leaks in DW systems often show themselves the same way as they would in a SW system. More often than not, if a leak develops in a DW exchanger, it starts small and ends up calcifying up the air gap and plugging the weep hole between the heat exchanger walls. It isn’t recognized until the inner wall of the exchanger fails as well, which results in the relief valve blowing on the boiler. To that point, the most popular DW indirect on the market has a combined exchanger wall thickness less than most of the SW exchangers.
Why an additional RPZ requirement is detrimental
The cost to install an RPZ can range from several hundred to several thousand dollars. A typical charge for the required annual testing is $120 plus the city permitting fee, which ranges from 0 – $80 in the metro area; thus adding an annual maintenance expense for a homeowner of $120 to $200. The extra installation and maintenance expenses combined with the inconvenience of scheduling and hosting the testing service is a significant disincentive for someone interested in investing in equipment. In most cases, the average annual maintenance cost offsets the majority of the energy savings making it impossible to justify the installation financially. Restricting or adding additional expense to the installation of SW device installations deprives Minnesotans of the opportunity to conserve $26 million dollars of energy, and eliminating 250 thousand tons of CO2 into the environment of our state.
The fact that there are no know instances of a single person in this country or the rest of the developed world ever being harmed by a single wall system that UPC compliant, is the best evidence of all that the UPC single wall code serves its purpose as written. To add the additional RPZ requirement to single wall heat exchanger usage, adds unnecessary constraints and is a disservice to Minnesota’s citizens, environment, and economy.
References
UPC CURRENT CODE LANGUAGE WITH PROPOSED AMENDMENT
4715.0603 Cross-connection control
Subp. 1. Section 603.2. UPC section 603.2 is amended to read as follows:
603.2 Approval of Devices or Assemblies. Before a device or an assembly is installed for the prevention of backflow, it shall have first been approved by the Authority Having Jurisdiction. Devices or assemblies shall be tested in accordance with recognized standards or other standards acceptable to the Authority Having Jurisdiction. Backflow prevention devices and assemblies shall comply with Table 603.2, except for specific applications and provisions as stated in Section 603.5.1 through Section 603.5.21.
Devices or assemblies installed in a potable water supply system for protection against backflow shall be maintained in good working condition by the person or persons having control of such devices or assemblies. Such devices or assemblies shall be tested at the time of installation, repair, or relocation and not less than on an annual schedule thereafter, or more often where required by the Authority Having Jurisdiction. Were found to be defective or inoperative, the device or assembly shall be repaired or replaced. No device or assembly shall be removed from use or relocated or other device or assembly substituted, without the approval of the Authority Having Jurisdiction.
Testing shall be performed by a certified backflow assembly tester in accordance with ASSE Series 5000.
Subp. 2. Section 603.5.4. UPC section 603.5.4 is amended to read as follows:
603.5.4 Heat Exchangers. Heat exchangers used for heat transfer, heat recovery, or solar heating shall protect the potable water system from being contaminated by the heat-transfer medium.
603.5.4.1 Single-Wall Heat Exchanger. Installation of a single-wall heat exchanger shall meet all of the following requirements:
(1) Connected to a low-pressure hot water boiler limited to a maximum of 30 pounds-force per square inch gauge (psig) (207 kPa) by an approved safety or relief valve.
(2) Heater transfer medium is either potable water or contains fluids having a toxicity rating or Class of 1.
(3) Bear a label with the word “Caution,” followed by the following statements:
(a) The heat-transfer medium shall be water or other nontoxic fluid having a toxic rating or Class of 1 as listed in Clinical Toxicology of Commercial Products, 5th edition.
(b) The pressure of the heat-transfer medium shall be limited to a maximum of 30 psig (207 kPa) by an approved safety or relief valve.
The word “Caution” and the statements in letters shall have an uppercase height of not less than 0.120 of an inch (3.048 mm). The vertical spacing between lines of type shall be not less than 0.046 of an inch (1.168 mm). Lowercase letters shall be compatible with the uppercase letter size specification.
(4)A reduced-pressure principle backflow prevention assembly shall be installed on the building supply before the first branch
603.5.4.2 Double-Wall Heat Exchanger. Double-wall heat exchangers shall separate the potable water from the heat-transfer medium by providing a space between the two walls that are vented to the atmosphere.