Boiler DHW Coil Troubleshooting
When using an instantaneous hot water coil installed in a boiler, you can encounter some problems and we will address these problems. The three major problems with domestic hot water coil (DHW coil) are water not hot enough, too hot or is hot enough or too hot but runs cold shortly after use starts. The following information will assist in troubleshooting these problems.
Before we address problems, let's look at how the DHW coils should be piped when installed. I also would like to address the fact that, in my opinion, they should not be offered or used in boilers anymore.
The following diagram is the proper piping of a DHW coil. It is important to control the way the control senses the temperature change to have the cold water on the inlet side and the hot water on the outlet side of the DHW coil. With lower water volume in the boilers today, there will be a lot of temperature swings in the DHW coil. We strongxly suggest the use of an automatic mixing valve as shown below. This serves two purposes. It will give you a more even & safe temperature at the faucets and extends the draw time for your hot water.
Even though this is a costly way to make hot water you can reduce the fuel bill slightly by turning the water temperature down as low as you can and still have enough hot water for your showers and sinks. An aquastat controls the water temperature in the boiler, maintaining it hot enough to provide plenty of hot water to meet your needs. The older controls will have three knobs. A high limit, a low limit, and a differential. The rule is that there must be a minimum of 20°F between the high and low settings. The differential setting should be up to 20F - 25F. Most cast iron and steel boilers will allow the high limit to be set at a maximum of 210F. Check your manufacturer's instructions. Many customers will turn the temperatures down in the summer and back up in the winter. This is not needed; once you have determined the settings to supply enough hot water, you can leave them unchanged, both in summer and winter. When we understand the operation of this control, you will agree.
Let's look at the wiring terminals first. There are terminal markers L1 and L2. On control wiring, this is always Line 1 (hot) and Line 2 (neutral). We also have B1 and B2. This is the burner operation. B1 is the burner hot, and B2 is the burner neutral. We have C1 and C2. These are the same as the B1 & B2 except they operate the circulator. The ZR and ZC terminals are probably the most misunderstood terminals on these controls. They should always be used when you are zoning with circulators and using add-on relay(s). This control acts as a circulator reverse control. What this means is that when the low limit setting of 10F is reached, the burner turns on and the circulator turns off, ensuring you have enough domestic hot water. As the water heats up again, the circulator restarts.
The ZR & ZC terminals, when wired in properly, will make all the other circulators start and stop as the water temperature changes. If the other zones of heat are not wired in this way, the circulators will run until the thermostats call for heat ends. You may experience fluctuations in domestic hot water or run out of hot water if you have larger water volume zones.
Let's understand the function of the three knobs on the older style aquastats. The high limit is active only when the thermostat is calling for heat. The low limit is set to maintain water temperature for domestic hot water use. The differential tells the burner how much temperature to make up on a call from the low limit. The operation of this control is confusing to many. Let's assume the control is set at 200F high limit, 170F low limit, and a 25F differential setting. The control has a 10ºf differential built into the high and low limits. This means that if the temperature drops 10F from the low limit, the burner will start. To avoid confusion, if the circulator is also running at this time, it will shut down the circulator at this point. If you are in a call for heat, the burner will shut off at 200°F (this will not shut down the circulator) and come back on at 190°F as long as there is still a call for heat. You can see the built-in 10F differential from Honeywell (in black to the left). The red numbers are our settings.
Example High, Low and Differential Limit Settings
The aquastat operates off the low limit for the hot water demand. It still has the 10f differential built into the low limit to start the burner. The burner will run until it brings the temperature up the amount of the differential, which should be set at 20f - 25°F. The action is with a low limit setting of 180F. The burner will fire when the temperature drops to 170°F (180°F - 10°F). The burner will run until the temperature rises by the amount of the adjustable differential. The control will cycle between 170F and 195F, with a 25F differential, until the thermostat calls for heat. The formula for this operation is (Low limit- 10f) + differential.
Low Limit Operation
When we get a call for heat, the burner will fire and run the temperature up to the high limit setting. In our example, it is 200ºf. The burner will shut off. When the temperature drops 10F, the burner will re-fire if there is still a call for heat. When the call for heat ends, the boiler water temperature will drop to the low limit setting minus 10F and re-fire, adding the differential temperature and shutting off again as shown in the above picture. It will run this way until another call for heat is activated to supply hot water.
High Limit operation
Now let's look at problems and troubleshooting
3. Water starts out hot or too hot but cools rapidly
4. Occasionally water runs cool
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Water is too hot
This is one of the easiest problems to resolve.
1. Turn the low limit down and adjust the differential until the water is cooler but still hot.
2. Install an automatic mixing valve and set it 20F less than the low limit setting. The reason for this is when you get a call for heat, the boiler will still go up to the high limit setting and the water will get hotter.
3. Defective control
4. The sensing bulb is not fully inserted into the well. See link
5. Mineral build-up on the outside of the aquastat well.
6. Defective electronic sensor (newer style)
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1. Verify the differential is set all the way up
2. Try turning the low limit water temperature up
3. Check the high limit is at least 20ºf higher than the low limit.
4. Check that the Inlet is cold water in and the outlet is hot water out.
5. Check the incoming water pressure. Most coils are rated for no more than 60 psi.
6. Check the size of the coil to the manufacturer's specs. If you cannot get that information, a rough measure is 50,000 BTUs per gallon of flow. For example, a 100,000 BTU would only produce about 2 to 2.25gpm of hot water.
If no mixing valve check the draw. Put a 5-gallon bucket in the tub. Turn on the hot water only. Run the water into the bucket for 1 minute. Check the amount of water against the flow of the coil. For example, if you have a 3-gpm coil, you should not have more than 3 gallons of water in the bucket in a minute. If you do install a flow restrictor that matches the flow rating on the coil.
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1. The low limit and differential knobs may need to be readjusted. The differential should be up. The low limit may be too low; try turning it up 10 degrees. If it doesn't resolve the issue, it's probably something else wrong. Read on.
2. Is there a manual bypass valve between the hot and cold pipes coming out of the coil? If so, the washer could be bad, allowing cold water to mix with the hot water, cooling it down too much.
3. Check that the Inlet is cold water-in and the outlet is hot water out.
4. Check the incoming water pressure. Most coils are rated for no more than 60 psi.
5. Check the size of the coil against the manufacturer's specs. If you cannot get that information a rule of thumb measure is 50,000 btu's per gallon of flow. Rules of thumb are not set in stone. Just as a rough idea.
6. A large water volume system (cast iron radiation system) not piped with a boiler bypass loop. See boiler protection for boiler bypass piping.
7. Coils are rated for a 100F rise in water temperature. This should be more than is ever needed. They may not be at exactly 100F rise but I would guess 90F - 100F Check inlet and outlet temperatures.
8. The faucet is overdrawing the DHW coil. A flow restrictor or should be installed to match the coil rating. If the DHW coil is rated for 3.5 gpm a 3.5 gpm flow restrictor should be installed. If the water pressure is too great, a flow restrictor may help, but it normally will not resolve the problem.
9. If there is an automatic mixing valve in the domestic coil piping, check the temperature of the pipe coming out of the coil. If that pipe stays hot exercise the mixing valve (move the adjustment knob back and forth). Sometimes sediment will collect on the inner workings, and this will release them and allow the mixing device to function again.
10. If the water temperature is cool coming out of the DHW coil piping, the DHW coil may be in trouble. If your potable water is above 5 grains hard, it would require a water softener. You will have a buildup on the internal walls of the DHW coil, causing the velocity of the water to increase in the DHW coil. The water is not in the DHW coil long enough to get the proper heat transfer. Without a flow, the water will heat up, and when you start drawing, it will move too fast. The same thing happens with excessive water pressure. To clean the interior walls of the DHW coil, a contractor will normally run an acid solution through the DHW coil in the opposite direction that the water normally flows. Precautions are taken to prevent the coil from being blocked as debris comes loose. A problem with cleaning DHW coils is removing the acids. The DHW coil may leak if cleaned a few times. Most contractors today are opting to change the DHW coil instead of cleaning.
11. Another potential issue that may arise if the pipe from the DHW coil cools off is a buildup on the exterior walls of the DHW coil. This is a more serious problem. If this is the case, the problem may mean a heating system leak, and the system is taking on more make-up water. The makeup water contains oxygen, minerals, and chemicals. Not only can this affect the output water temperature, collecting minerals on the exterior of the DHW coil, but it can also shorten the life of the boiler. You should always take care of any heating system leaks as soon as you can. To clean the exterior of the DHW coil, remove it from the boiler and clean it with an acid-based cleaning solution. Just a note, the gasket must be replaced when the DHW coil is pulled and reinstalled. Never reuse the coil plate gasket.
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1. This usually only happens when there are multiple circulators and the ZR and ZC terminals in the aquastat are not being used.
2. Could be a defective control
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