There are a lot of boilers installed every year with a domestic hot water coil in them. I continually must ask myself why?????? Years ago, people thought it was free hot water. This could not have been further from the truth. The truth is, it is almost, if not, the most expensive way to make domestic hot water. Let's examine how a boiler with a domestic hot water coil operates. The boiler remains hot at all times, so the water surrounding the coil is also hot. The temperature of the boiler water around the coil is what heats the water passing through the coil. There is an aquastat control to maintain the boiler temperature. When the water temperature drops 10°F, the boiler will fire again to maintain the water temperature around the control set point. The amount of hot water produced by the coil is dependent on the boiler input and the GPM rating of the coil. As a general rule, it takes approximately 50,000 BTUs to produce one gallon of hot water as it passes through the domestic coil. There is no storage of hot water in this application, except for what is in the coil, which is usually less than a quart.
If the boilers are rated at an input of 100,000 BTU, it would equal approximately 2 gallons per minute of hot water. What if we took that same boiler and installed a 3-gpm coil? Would we get more hot water? We can expect to receive between 2 and 2 1/4 gallons per minute of hot water, depending on the water pressure. So, we spend more money on a larger coil and get the same amount of hot water. The downside to this is that more water goes through the coil with a 3-gpm coil, so the water spends less time in the domestic coil, and the temperature coming out of the hot water faucet is slightly lower. We have more flow but less temperature. How do we compensate? We increase the boiler water temperature. There goes more fuel. The rating on the coil is not always what you get. Sometimes, larger coils are used to achieve a satisfactory flow for home usage. The temperature rise will be less pronounced in larger coils with smaller firing rates compared to better-matched coils with the same firing rates.

We could add an automatic mixing valve to the system, which should be on every domestic hot water coil and tank-type water heaters anyway. Slow the flow down through the coil to compensate for the increased flow by switching to a larger domestic hot water coil. There are also other problems we have not addressed yet with the operation of the boiler.

We mentioned earlier that there was a control to maintain the water temperature in the boiler constantly. The low-side setting of the control is normally set between 150F and 180F. This means that the temperature in the boiler is always maintained at or above the low side of the control setting. This is just in case you want to wash your hands, rinse a glass, take a shower, or not use any hot water at all; it is still that temperature. A boiler that maintains temperature works by constantly pulling heat up the chimney from the hot boiler. The warmer the chimney, the higher the draft, which means more heat leaves the boiler. This is because the boiler is trying to maintain a sufficient water temperature to keep the coil hot enough in case you need hot water. Higher water temperatures result in higher fuel bills and increased draft. Sounds wasteful to me. Every boiler installed today should be what we call cold start. A cold start means the boiler will only run on demand for heat or hot water. Between demand cycles, the boiler may drop to room temperature. This cannot be done with a boiler that has a domestic hot water coil installed in the boiler. The hot water could be made in a standalone water heater, an instantaneous wall-hung water heater, or an indirect water heater tank. The boiler heats an indirect water heater, but the boiler only runs when there is a call for heat or additional hot water. The advantage of an indirect water heater is that it is very well-insulated, has extremely low standby losses, and typically comes with a lifetime warranty. Regardless of the method used to heat domestic hot water, as mentioned above, the boiler no longer maintains a constant water temperature.

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