Properly Sizing a UV Light

A UV Secret Revealed

Here at HKF we use and sell only Pentair (formerly Emperor )UV lights. There are a few reasons for this. The first reason is the size of the housing on all of their units is such that you truly get the proper water dwell time through the unit.

Reason #1

Dwell time is the amount of time that the water is actually in the unit getting zapped by the UV bulb. This action is what actually kills pathogens and green water algae cells. Pentair/Emperor units are sized in diameter and length to best match the wattage required to do this. The longer the unit and the larger the diameter of the housing the better the dwell time will be.  This is one reason they are guaranteed to clear green water in 3-5 days if properly sized and installed!

Reason #2

The second reason we use Pentair/ Emperor is for the output wattages of their bulbs. You see, most UV’s are advertised by INPUT WATTAGE like 40, 50, 120 and so on, and this includes Pentair/Emperor. However, the input wattage is irrelevant in determining what wattage is actually zapping the water. It is the OUTPUT WATTAGE that is doing this job! In short an input wattage might be 40 watts but the output wattage may be as low as 5 watts for that 40 watt bulb of some brands, but a Pentair/ Emperor 40 watt has an output of 14 watts! Because Pentair/ Emperor has the highest output wattages of anyone, this means you can purchase a smaller wattage unit as compared to other manufacturers. This will save you tons of money. Pentair/Emperor is the only manufacturer to list their output wattages, and others do not. The fact that these others don’t list that important stat is a clue that they do not want you to know their low output wattage.

Anyway these are just two of many reasons that we only use and sell Emperor. Whether you understand what I wrote above or not is not very relevant, so don’t be intimidated if you don’t.

Pentair/Emperor Aquatics UV lights are some of the best made for the money! They have the best OUTPUT wattages of any manufacturer as previously discussed. With this fact  you can have a smaller wattage UV as compared to other brands and this alone will save you big!

Pentair/Emperor UV makes two distant series of UV lights;

The Sterilizer Series come in input wattages of 18, 25,40 and 65 watts. These are all single bulb units with 1 1/2″ union PVC slip connections.

Sterilizer Series UV’s are designed to achieve optimum germicidal disinfection, protecting your valuable Koi against harmful waterborne pathogens.The design of the UV Sterilizer Series is the result of a calculated balance between equipment size, cost and performance; making  UV Sterilizer Series the best option for Koi Pond disinfection for smaller ponds and flow rates. The Sterilizer Series are also GUARANTEED to clear green water in 3-5 days if sized and installed correctly!

The High Output Sterilizer Series come in  input wattages of 50, 80,120,150 watts. These are all single bulb units with 2″ union PVC slip fittings.

High-Output  Sterilizer Series deliver optimum protection against harmful pond pathogens and destroy green water algae. They feature our high-output T6 style UV lamps, offering approximately twice the UV lamp output when compared to standard low-pressure UV lamps of the same arc length. With the increased UV output, The HO UV Sterilizers offer increased performance in a unit that requires less space. Each  HO UV Sterilizer model is designed to operate safely in wet environments and they are UL listed. HO UV Sterilizers are designed for Koi ponds and light commercial applications, filling the gap between our standard and commercial models, and are ideal for larger Koi ponds from 4500 gallons to 15000 gallons.

The Real Reason You Need a UV Light

Most UV manufacturers including Pentair/Emperor advertise their UV lights for clearing green water algae and killing harmful pathogens that make your fish sick. This is a little misleading, especially when we are talking about their use in outdoor Koi and Goldfish ponds. I am here to tell you that you need a UV light for one primary reason, and that reason is to clear stained or green water. As for the pathogen killing, well this is where there is some missing info in their ads. Most fish pathogens that hurt the fish live on the fish. They are not free-swimming through your water in most cases, and most times. Most of them stay on the fish at all times and it is only a fraction of the time some may be free-swimming. So as you can imagine the UV may kill a percentage of the free-swimmers, it is not killing the ones that live on the fish. These are the ones that hurt the fish. So pathogen elimination is not a reason to have a UV in my opinion.

As for clearing green water there is nothing better than a properly sized and installed UV light. If you have an outside pond you will at some point be dealing with stained or green water and a UV light is a 100% cure for this. So basically put everyone with an outside pond will need  a UV light, and this is the real reason to have one.

Properly Sizing a UV Light

Many manufacturers advertise their UV lights as being good for a certain size pond. Even Pentair/ Emperor is guilty of this. In reality a UV is sized based on the flow rate or speed of the water that is going through it. This is usually expressed in GPM or GPH. So the flow rate of your water is what is critical in choosing the proper size unit, and more specifically the actual flow rate where the UV is installed. This spot in reality will be less of a flow than the actual pump GPH. This is due to head loss and friction loss. In general though it is fine to size the UV based solely on the pump’s labeled GPH.

As you may know a UV light works by zapping the water with ultraviolet light. Given this fact the water has to be inside the unit long enough for the given wattage to do this. This is the “dwell time” as discussed previously. In combination with the speed or flow rate of the water and the dwell time, the wattage of the bulb has to be sufficient to accomplish the proper killing of the algae cells. This is why there are different wattage units, of varied diameters and lengths. If the water passes too fast through a given wattage it may not kill the algae cells. The faster the flow rate or GPH the higher the wattage required. So as you can see there are a number of factors to properly sizing the unit. The good news is that most of this calculating is done for you by the manufacturer and the only thing that you need to be concerned with is what your flow rate is of the water passing through the unit.

With most makers of UV units they list the suggested and the maximum flow rate that each wattage is good for in their specs. They usually list this in two groups. One Min and max flow for just killing algae and bacteria, and another set for killing parasites. It takes much more wattage to kill parasites than algae. We really only care about the flow rates needed to kill green water algae cells for reasons previously mentioned, and we want to use the suggested column and not the maximum column in most cases.

For example if you look at the spec sheet below you will see that an Emperor 40 watt Sterilizer Series is rated for a suggested  flow rate of 943 gallons per hour (GPH) to kill algae and bacteria. This means for the unit to be able to kill the algae that the water cannot exceed the flow rate of 943 GPH going through it. If you did exceed the this the unit will not work properly. So again as you see it is the flow rate and not the size of the pond that determine the size of the UV.

You will also see that next to that they list the suggested and maximum flow rates for killing protozoan parasites. Again in most ponds using the suggested flow rates for algae control is all we need and all that is truly practical.


You can see that I highlighted the column to use below.

Hanover Koi Farms Emperor Aquatics Smart Sterilizer Series UV light flow chart
You can also notice that the chart above lists the maximum pond size each wattage is good for. This is the part of the specs that are simply not good to use to pick a UV size. Do not size a UV based on pond size alone  How big your pond may be in volume is irrelevant to properly sizing the UV without taking other things into consideration.

For example someone may have a 4700 gallon pond and have a single pump that is rated at 4500 gallons per hour (GPH). If you used the chart based on maximum pond volume you would pick the 40 watt unit which is rated at 4725 gallons maximum pond size. However, the flow rate going through the UV is 4500 GPH. In this example you would be picking a UV that is way too small in wattage because a 40 watt unit is only rated at a maximum flow rate of 943 GPH! It simply would not do the job intended as you can see from the chart.

So as you can see you need to size the unit based on flow rate and not pond size! With that said you need to know the size of your pump in GPH in order to do this. Now you must also understand that not all brands of UV lights are created equal. Even though two brands may offer 40 watt units, these units will have varied flow rate capabilities. This is primarily due to the OUTPUT wattages from brand to brand. With some brands their 40 watt light may only have an output wattage as low as 6 watts! A little deceiving to say the least! Remember it is the output wattage that is actually doing the work to kill the targeted algae or parasites. Most brands don’t even list their output wattages and I am sure you can imagine why. Emperor has some of the highest output wattages on the market and therefore you get the most power for least money, and thus you are able to buy a smaller wattage with Emperor than other brands.

Full Flow Method

There are two ways to size a UV depending on the brand you get. Most brands only have one way to size them. This is the full flow method. With this method you have to size the UV based on the flow rate of the pump.  If you have a 3000 GPH pump you need a UV rated for 3000 GPH to kill algae.Then all of the water being pumped must go through the unit to do the job properly. So as you can imagine this will usually mean you are forced to buy large wattage, and expensive unit.

As well you can only have one pump associated with your pond/system. Some folks do have multiple pumps in their ponds for various reasons. In this case you may most likely need a UV for each pump! This is because all the water in the system has to eventually go through the UV for the UV to work. With multiple pumps this may not be the case depending on the exact situation and design.  In some instances if you had two pumps and one pumps was substantially larger than the other in GPH, you may be able to get away with just putting a UV on the larger of the pumps. However there is no surety or engineering that can guarantee this will work to clear the green water.

The Secret

Bypass Method

With this method you install the UV in such a way as to be able to only allow part of the water to go through the UV. If you purchase your UV from HKF we will guide you through the installation process. Again this method is only assured to work in single pump systems/ponds and if the UV is properly installed and in the correct location. If you have two or more pumps associated with you pond you would most likely need two UV’s. A s well this method will not work with most Uv brands. It does however work with Pentair/ Emperor due to their excellent output wattages. Do not attempt the bypass method with any other brand of Uv.

The bypass method is the most efficient method of sizing and installing a Uv. With this method you can size the UV based on the volume of the pond, but again assuming a one pump system, and a properly matched pump to the system. A properly matched pump is not only critical for Uv systems but also for the overall functionality of the filtration as well. So we will now discuss what this means exactly.

A Properly Sized Pump

With any system or pond outdoors for Koi, the pump should be sized to accomplish a once per hour turnover rate through the filtration. What this means is that you want to pump the entire volume of your pond through the filtration in an hour. As an example if you have a 3000 gallon pond you want to size the pump at least to be 3000 GPH. If you have a 5000 gallon pond you would want at least a 5000 GPH pump so on and so forth. I discuss turnover rate in more detail in the article The Science of Water, and I would suggest you read it now. Some folks have a larger pump than they should and are pumping a faster turnover rate than once per hour. I do not recommend this as there is not enough dwell time of the water in the filter itself for the bacteria to do their job efficiently.

Sizing the UV

So finally if your pond or system has only one pump, and that pumps GPH equals the volume of the pond (5000 GPH pump and 5000 gallon pond) you can use the bypass method of sizing and installing the UV. You can also use the bypass method if your pump is smaller in GPH than the overall volume of the pond. However you do not want your turnover rate to be less than the complete volume going thru the filter once per every three hours. Preferably I would never have less than once every two hours .

You may have a slightly larger pump than the pond volume and get away with it in some cases however, as long as you are within 10% difference between the two. For example if you had a 5000 gallon pond but had a pump rated at 5500 GPH you would still be OK to use the bypass method. (Remember, your pump should be sized as such to give you a once an hours turnover rate.)

With all the above in mind you would simply take 1/3 the volume of your pond and use that figure to size the UV. Then the UV is installed in such a way as to only have that 1/3 in GPH going through the Uv.

How the UV is installed is critical to the bypass method working properly.

So as an example if you have a 5000 gallon pond, and a single pump of 5000 GPH or less you simply take 1/3 of the 5000 gallon pond volume which is 1666 gallons, and pick the proper wattage Uv under the Suggested column that is good for 1666 GPH to kill algae! In the case of the chart above you would need the Pentair/Emperor 130 watt Uv.

One more time….If you had a 3000 gallon pond  and a single pump let’s say rated for 2500 GPH or less (or possibly up to 3000 GPH ) you would simply divide the pond volume by 3. This gives you 1000 gallons. Then you simply use the chart above and find what wattage Uv is rated for 1000 GPH. In this case it would be the Pentair/Emperor 40 watt unit.

So to size the UV take 1/3 of your pond volume

That is all there is to it! However you must be 100% sure of the true volume of the pond. If you measured the pond and used a formula to figure your pond volume, this would NOT be accurate, and you would most like be from 25% to 50% off! I discuss how to accurately calculate the true volume of your pond using salt in the article Koi Treatments. I highly recommend you use this method unless you metered the water into the pond with a flow meter and are sure of the volume. Otherwise you WILL size the Uv incorrectly and it will not work.

Now if you have a pump with a faster GPH than the actual pond volume, than you should use 1/3 of the pump GPH as opposed to the pond volume. For example if you have a 2000 gallon pond but you have a single pump at 3000 GPH, than you should use 1/3 of the 3000 GPH which would be a UV rated for 1000 GPH. The other option here would be to use a valve at the pump discharge to slow the GPH of the pump down to the required 2000 GPH for a 2000 gallon pond.

Remember the bypass method will not work with any other brand Uv and you would be forced to by a much larger more expensive unit other than Pentair/ Emperor.

If you do all that I say here, then Pentair/Emperor guarantees clear water within 3-5 days in most cases.

Written by John Fornaro, Hanover Koi Farms. ALL RIGHTS RESERVED BY HANOVER KOI FARMS, COPYRIGHT © 2017

Green Pond Water? You Need a UV Light

Shop the best quality UVs for Koi Ponds- These lights come with personal support and installation advice from the experts at HKF- the same experts that wrote this article!

Back to Top
Previous Page