There are a lot of boilers installed every year with a domestic hot water coils 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 take a 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 that water drops 10f it will fire the boiler again to keep the boiler water around the control set point. The amount of hot water you get from the coil is dependent on the boil input and the GPM rating of the coil. As a rule, it takes about 50,000 btu's 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 rated at an input of 100,000 btu's 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 & 2-1/4 gpm of hot water dependent on water pressure. So, we spend more money for a larger coil and get the same amount of hot water. The downside to this is 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 turn the boiler water temperature higher. There goes more fuel. The rating on the coil is not always what you get. Sometimes larger coils are used to get a flow which is satisfactory to the usage in a home. The temperature rise will not be as much in larger coils with smaller firing rates as 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 and therefore compensate for the increased flow by going 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 there was a control to constantly maintain the water temperature in the boiler. 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. What happens with a boiler that is maintaining temperature is the boiler is always drafting heat up the chimney from the hot boiler. The warmer the chimney is the higher the draft so more heat leaves the boiler which is trying to maintain enough water temperature to keep the coil hot enough in case you may need some hot water. Higher water temperature = higher fuel bills, higher water temperature = more draft, sounds wasteful to me. Every boiler installed today should be what we call cold start. Cold start means the boiler will only run on a 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 which 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 it is very well insulated, extremely low stand by 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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