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Feature Story

Know the basic rules when checking a domestic hot water system

By George Lanthier


In this article we are going to take a look at basic rules for troubleshooting domestic hot water heaters. Most of this information is from the book The Hot Water Handbook by Bob Suffredini and myself and many thanks for the orders; that always seems to happen after we print an excerpt from one of our books.

Most of what I’m going to describe is interchangeable between a tankless heater, indirects and water heaters, so when in doubt, give it a try. After all, what have you got to lose? There are five things you must do while troubleshooting around domestic water heaters and we will discuss them in depth as we go along.

The first is patience. Most people can’t see the forest for the trees when it comes to domestic hot water troubleshooting. Keep in mind that it is the customer that has the problem, not you, so be understanding and patient. Remember, people have a right to demand good hot water, and it is part of the health laws.


Figure 1

The second and third things are a bucket and a timer. The bucket should be used only for DHW troubleshooting and should have been calibrated for that purpose. It’s really easy to calibrate a bucket (See Figure 1). Take a known quantity container like a gallon milk bottle, fill it to capacity, pour it into the bucket and mark your bucket so you know exactly where a gallon of water is in the bucket. The picture shows a six-gallon bucket and as you will see, in this case ‘bigger is better.” The fourth thing is a pressure gauge and we’ll discuss this later.

Finally, you need a good thermometer. Not only will it keep you from getting into trouble with a scalding issue, but you’ll also know if those adjustments are doing anything. When you were taught at some point in your career the basics of combustion, you were probably shown a ‘mind jogger” or memory tool called the ‘Circle of Combustion.” It is the basis of all combustion troubleshooting for solid, liquid and gaseous fuel sources. As you probably remember, the three parts of the wheel are fuel, air and ignition, and if any part of the wheel is broken or incomplete good, reliable, clean and efficient combustion of any fuel cannot occur.


Figure 2

In a variation on this theme and concept, in Figure 2 we show the ‘Hot Water Wheel.” Again, as in the case of combustion, all of the elements must be present and in equal relationship to one another. If any of the components are missing or incomplete, a good supply of domestic hot water will not be created or maintained. Instead of fuel, air and ignition to establish and maintain combustion, now we must be concerned with heat, flow and circulation. Let’s look at how this relates to the tankless heater.

Tankless Heaters
‘Heat” is described as the temperature maintained within the boiler or heater. ‘Flow” is the amount of water in gallons per minute that will pass through the inner surface of the coil or heat exchanger as found in the typical tankless. ‘Circulation” is the amount of water or steam that will pass around the outer surface of the coil. So, to have a properly designed system, three things must be known and verified.

The first thing to always do is verify that enough Btus are available to provide that heat source. This may require checking that the proper controls are in use, the piping is of the proper size and that the boiler is fired and capable of delivering enough heat to begin with. After you have determined that you have enough Btus to do the job, the next step would be to verify the flow and the circulation.

To verify flow, use a one-gallon pail or some larger device that will allow you to draw a fixed amount of water while timing the draw. If it takes 30 seconds to draw one gallon, your flow rate is 2 gpm, right? But what if it only takes 15 seconds to fill that pail? Well, your flow rate is 4 gpm and that’s why you’re having problems with that OEM 2.5-gpm coil. Don’t forget to try this test with the cold water outlet too, it might give you some more information. Remember that there is a pressure drop across that coil. Also, keep in mind to always perform any flow test with an open pipe outlet for each source, if possible. That means no aerators on the faucets while testing. Let’s look at another example, OK?

Let’s say that after measuring a tub, whirlpool or spa you find you have a vessel that will hold 100 gallons. How long will it take to fill that tub with a 5-gpm flow rate? It would take 20 minutes, right? But, can you supply hot water for a continuous draw of 20 minutes? That’s the question, isn’t it?

OK, circulation is next. This is an area that sometimes seems too simple to many and yet many common mistakes are made. As an example, let’s begin with the first test for circulation, the old ‘touchy-feely” test. This test consists of carefully testing the flow in and out of the heater by quickly touching the near boiler piping. Like we said, be careful, the piping may be dangerously hot to the touch, especially steam systems. If the supply pipe is hot to the tankless and the return pipe is cold, there is no circulation. If there is also no noticeable drop in temperature, very hot on both sides, it could indicate that the potable waterside of the coil is contaminated. In either case it says that the water is not traveling through and absorbing the heat from the boiler. No heat transfer is taking place and no energy is being expended into the coil.

Be careful of the venting of an external tankless. On hot water systems you want an automatic or ‘float” vent to make sure that the tankless shell and piping are free from circulation blocking air pockets. On steam systems, automatic vents are a no-no and all that is needed is a good quality coin or key vent. As we have said so many times already, the system piping is critical for good circulation. Sticking a circulator on a system that is already bad or plugged with debris and muck is not going to fix anything but your wallet, and you will still have an unhappy customer. Keep all of the piping as free of obstructions and avoid turns, making the water flow as easily as possible.

If all of the piping is very hot and the coil does not seem to be transferring heat, the outside or the inside of the coil could be plugged. Another thing to check, however, is that if the circulation is being created by a circulator, that the circulator is not too large for the application. Contrary to public belief, bigger is not always better in everything, and if the pump is moving the water too fast by the coil, inadequate or no heat transfer may be taking place.

Another thing to check is the use of boiler sealers, silicone products, pipe dopes and other agents that may have been added to the system for various reasons. Be especially careful of this when diagnosing problems on systems installed by others and where all of the major components are brand new. Before that new steam boiler and tankless went in, how many and what kinds of chemicals were thrown into that old leaking clunker?

Internal Contamination
To test a tankless for internal contamination, first get the water coming back from the tankless as hot as possible, or at least as hot as the boiler. With steam boilers get it hot enough to make some steam. Then, begin to draw hot water from the tankless through a non-aerated faucet or hose and within your view. As the water is drawn, bang on the piping near tankless valves with a rubber or wooden mallet. Be careful not to bang hard enough to break the pipes. If the water leaves the faucet or hose with discoloration as you bang on the piping, you probably have a plugged or contaminated coil. Your options are now to shock the coil, acid treat it or replace it.

To shock the coil, get the boiler as hot as possible while shutting off the cold water supply to the tankless. Make sure that the drain valve on the tankless is open and allow the water in the coil to boil out. After a few minutes and when no water is seen leaving the coil any longer, close the drain. Then open the test faucet or hose again, and open the cold water inlet. Be careful that you don’t burn yourself or anyone else when opening the test outlet; it’s going to be hot. It’s also a good idea to pull the mixing valve element, if possible.

To acid treat the coil, a good quality chemical designed for the purpose should be used. One of the brands available throughout the industry is Everhot Internal Coil Cleaning Solution. It will remove encrusted lime sediment, as well as algae, silt, silicates, sulfates, aluminates or iron. Lime is very common since it is formed from calcium and magnesium carbonates that are found in many water supplies and central piping systems.

With the addition of a motorized chemical pump, acid treating can be very easy and effective. Good safety practices, common sense and all plastic components should always be used around these powerful acids. The acid should be discarded in a proper manner following the manufacturer’s recommendations and EPA guidelines, not thrown on someone’s lawn.

In addition, make sure that the system is isolated on both sides with shut-off valves during the cleaning process. After the coil cleaning is completed according to the chemical solution manufacturer, flush the system thoroughly while tapping on the near-tankless piping with a wood or rubber mallet.

To troubleshoot a mixing valve for internal contamination, the same basic tests as for a tankless should be done. A ‘touchy-feely” test or a simple banging with a mallet should tell you something. If these tests fail, try replacing the thermal element. The condition and appearance of the element and valve interior will probably give you a good indication of what the inside of the coil looks like.

See ya.

George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax at (781) 641-7099 and his e-mail is FiredragonEnt@comcast.net.

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