Domestic Hot Water Coil Troubleshooting

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 the way the control senses the temperature change to have the cold water to the inlet side and the hot from 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 very expensive 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 and tries to maintain the water hot enough to give you plenty of hot water to satisfy your needs. The older controls will have 3 knobs. A high limit, low limit and a differential. The rule is there must be a minimum of 20ºf between the high and low settings. The differential setting should be all the way up to 20ºf - 25ºf. Most cast iron and steel boilers will allow the high limit to be set at a maximum of 210ºf. 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, when you have determined the settings to supply enough hot water just leave them there summer and winter. When we understand the operation of this control you will agree.

Let's look at the wiring terminals first. There are terminals marker 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 burner hot and B2 is 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 when the low limit setting - 10ºf is reached, the burner turns on and the circulator turns off, so 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 domestic hot water fluctuation or run out of hot water if you have larger water volume zones.

Let's understand the function of the three knobs.. 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 200ºf high limit, 170ºf low limit and a 25ºf differential setting. The control has a 10ºf differential built into the high and low limits. This means if the temperature drops 10ºf from the low limit the burner will start and not to be more confusing but, 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 200ºf (this will not shut down the circulator) and come back on at 190ºf as long as there is still a call for heat. Let's take a look. You can see the built in 10ºf 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 20ºf - 25ºf. The action is with a low limit setting of 180ºf the burner will fire when the temperature drops to 170ºf (180ºf - 10ºf). The burner will run until the temperature rises the amount of the adjustable differential. The control will cycle between 170ºf and 195ºf, with a 25ºf 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 10ºf 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 10ºf and re-fire adding the differential temperature and shutting off again as 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

1. Water is Too Hot

2. Water is not hot enough

3. Water starts out hot or too hot but cools rapidly

4. Occasionally water runs cool














Water is too hot

This is one of the easiest problems to resolve.

1. Turn the low limit down and set the differential up until the water is cooler but still stays hot.

2. Install an automatic mixing valve and set it 20ºf 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. Sensing bulb not all the way 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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Water Is Not Hot Enough

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 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 manufacturers spec's. If you cannot get that information a rough measure is 50,000 btu's per gallon of flow. Example, a 100,000 btu's 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. 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 restrict or that matches the flow rating on the coil.

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Water is Hot But Cools Rapidly

1. The low limit and differential knobs may need to be readjusted. The differential should be all the way up. The low limit may be too low, try turning up 10 degrees. If it does not resolve the issue is probably something else wrong. Read on.

2. If there is 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 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 to manufacturers specs. If you cannot get that information a rule of thumb measure is 50,000 btu's per gallon of flow. Rules of thumbs 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 100ºf rise in water temperature. This should be more than is ever needed. They may not be at exactly 100ºf rise but I would guess 90ºf - 100ºf.  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 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. While there is no flow the water will get hot and when you start to draw water it is moving 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 the opposite direction the water normally flows. There are precautions taken to not totally block the coil as debris comes loose. A problem with cleaning DHW coils is getting rid of the acids. The DHW coil may leaks if cleaned a few times.  Most contractors today are opting to change the DHW coil instead of cleaning.

11. Another problem which could be encountered if the pipe from the DHW coil is cooling off, is there 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 make-up 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 is also shortening the life of the boiler. You always water to take care of any heating system leaks as soon as you can. To clean the exterior of the DHW coil is to pull the DHW coil and clean it outside of the boiler 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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Occasionally Water Runs Cool

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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Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or it's personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website.