The risks of sizing plumbing systems using Hunter’s curve
By Tony Furst and Kazi Nasir
As plumbing professionals know, Hunter’s Curve is a common method of estimating the water demand of a building using statistical probabilities. For more than 70 years, the Hunter’s Curve method has been used to size plumbing systems, including booster packages, supply and drainage piping, and storage tanks.
When Roy B. Hunter first created this basic method of measuring a plumbing fixture’s water flow in duration of flow and the probability that it would be in use, it was a very useful guideline (see Figure 1). Although its simplicity is still seductive, we now consider it to be a very conservative approach that significantly oversizes plumbing systems and can lead to substantial increases in total cost of ownership. More importantly, it can also cause some potentially serious health concerns.
The Limitations of Hunter’s Curve
Hunter’s original sizing concept was based on a large population for estimating demand, but the accuracy of the sizing curve diminishes as real-world populations are examined. When compared to other methods of sizing plumbing systems, such as IPC, UPC, IAMPO Water Demand Calculator, and so on, there can be huge discrepancies in estimated and actual results. Hunter’s and the other sizing methods are almost always much higher than the actual measured results.
Impact of using Hunter’s Curve: Biofilm
Unfortunately, there is an even greater problem with using Hunter’s Curve and that is the potential danger of harmful bacteria growing, and for contaminants to leach from the pipes into the water. Because this method often overestimates water demand, designers sometimes install oversized pipes. This can cause water to remain in the plumbing system for longer than intended and/or circulate in the piping at lower velocities, allowing harmful bacteria to grow.
Regardless of the type of water treatment the municipality and the building might use, there will be some degree of bacteria growth in your system. Domestic water systems that have low or no flow can develop bacteria blooms due to lack of chlorine and lack of fluid movement can result in biofilm coating the pipes. Not only does this biofilm create health risks, it also constrains the flow in the piping; increasing pressure drop and therefore energy costs.
There can also be the issue of “dead legs,” which are defined as any area of the distribution piping system where water does not flow. These are prime areas for bacteria to blossom.
Increased costs as a result of oversizing
There are, of course, a number of other problems with oversizing plumbing systems, including the tendency to oversize the various system components. Oversizing booster pumps, for example, can add significant costs and require much more power to operate. Providing more power over a period of time can also dramatically increase electricity costs.
Another costly consequence of oversizing is the larger footprint required to fit the larger equipment. In new buildings we have seen footprint reductions of more than 60 per cent, which save the building owners significant money. In retrofit situations it frees up valuable space to use for other purposes.
Alternative sizing methods
Now that we have a better understanding of some of the drawbacks of using Hunter’s Curve the obvious question is “what is the best alternative?” It should be noted that the IAPMO Water Demand Calculator is currently under review and we are advocating for the adoption of this new method. It’s currently only for residential, but we suggest using it as a guideline in combination with engineering judgement for use in other building types.
However, none of the methods mentioned here are 100 per cent accurate and most of them still significantly oversize the system. In a recent survey conducted in conjunction with a webinar on this topic, we found that 43 per cent of participants still used the Hunter’s Curve or a derivation thereof. Another 35 per cent used IPC or UPC, 10 per cent used the IAPMO Water Demand Calculator and 12 per cent were self-developed based on experience.
There are a variety of consulting engineering firms and manufacturers who can help you determine current or anticipated needs, measure existing flow (where applicable), and create a sizing system that can save you time, money and energy. However, there are two indispensable steps in the process that need to be followed:
1) The first step in the process is to have a clear understanding of pipe sizing and your flow velocity. As plumbing professionals are aware, there are two types of flow – laminar flow (slow flow) and turbulent flow (fast flow).
We also know that pressure loss in piping is related to the velocity of the flow and the friction loss along the pipe wall reduces the available pressure. The two most common equations for friction loss are Darcy-Weisbach and Hazen-Williams. We have found that the Darcy-Weisbach method provides the most accurate results.
2) The second step is factor in the following information:
- Type of piping material
- Supply pressure (inside the building)
- Demand pressure
- Flow rate (quantity)
- Velocity limitation
Once you are armed with this information, you are now ready for booster and drawdown tank sizing. Booster pumps are arguably the single most important piece of equipment because they are responsible for increasing pressure and delivering fresh water to tenants and equipment alike throughout the entire building. Although more contentious, drawdown tanks can also be important in certain circumstances, especially in the following installation types:
- Large buildings that have a large variance between peak flow, minimum flow and average flow.
- Tall buildings where the pressure drop resulting from water demand will take some time to propagate down to the booster.
- Older buildings or buildings more susceptible to leaks, which can erroneously be treated as if it was actual demand.
Sizing your specific plumbing system
You are now ready to size your system and determine design flow. Of course, your needs and demands can vary greatly depending on the type of building. For example, a hotel or office building will obviously have different water demands and usage patterns than a residential or university building.
But regardless of the specific installation, there are certain factors to consider. Look for engineering partners who can offer proven experience in measuring water usage and designing customized solutions for buildings that are 100 per cent (or close to) occupied and don’t have variable occupancy levels; booster systems with advanced software solutions with advanced controls and analytics, including energy and water consumption data and profiling; and small mechanical footprints plus ease of installation and commissioning.
By following these steps and working with the right partner you should be able to more accurately design your plumbing sizing system while saving valuable time and energy.
Tony Furst is RSEC manager, U.S. Armstrong Fluid Technology. He is a mechanical engineer with over 30 years of success overseeing all phases of multimillion-dollar construction, infrastructure and energy projects worldwide. Kazi Nasir is offering manager- plumbing, Armstrong Fluid Technology. Prior to assuming this position, Nasir was Armstrong’s global product experience specialist – plumbing, heat transfer and expansion.