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 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 look at how the boiler with a domestic hot water coil operates. The boiler stays hot all the time, so the water around the coil is 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 you get from the coil is dependent on the boiler input and the GPM rating of the coil. As a rule, it takes about 50,000 BTUs to make one gallon of hot water as it passes through the domestic coil. There is no storage of hot water in this application other than what is in the coil, usually less than a quart.
If the boilers are rated at an input of 100,000 BTU, it would equal about 2 gpm of hot water. What if we took that same boiler and installed a 3-gpm coil? Would we get more hot water? We may get between 2 and 2-1/4 gpm of hot water, depending on 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 flow that is satisfactory 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 around what the low side of the control is set at. 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 enough water temperature to keep the coil hot enough in case you need some hot water. Higher water temperature results in higher fuel bills and more 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 get as cool as 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. An indirect water heater is heated from the boiler, but the boiler will only run when there is a call for heat or more hot water. The advantage of an indirect water heater is that it is very well insulated, has extremely low standby losses, and normally has a lifetime warranty. Either way of heating the domestic hot water, as mentioned above, the boiler no longer maintains any water temperature.

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