Proper Piping of Cold Water Inlet to Indirect Water Heater
As we take the new indirect water heater out of the box and do a quick inspection, we see all the tapping's on the tank are probably 3/4", which includes the domestic hot and cold-water pipes, and also the boiler supply and return piping. So, we grab our trusty pipe wrench, pipe dope, fittings, and a piece of 3/4" copper tubing, correct? Well, maybe not. Most homes are plumbed in 1/2" pipe for the domestic water lines. I hope you installed a domestic expansion tank if you are on a public water supply. If you have a well, it would not be needed. The purpose of the expansion tank, located on the cold inlet side of the IWH, is to absorb the pressure produced when water is heated. As the water heats, it expands. Since water cannot be compressed, it expands into a tank that has an air charge and a bladder. Air is compressible and acts as a pillow, maintaining the proper pressure.
If the domestic water lines are 1/2", you can connect 1/2" tubing to the tank by using reducing bushings, bell couplings, or reducing male adapters as long as they are brass or copper. I strongxly recommend using an automatic water mixing valve. This will automatically mix hot and cold water to give you the preset water temperature of your choice. I would always pipe the domestic hot pipe down about 12" then turn and go back up. This is called a thermal trap. The trap will require insulation. If you pipe straight up from the hot supply, whether it is on the top or side, you will get thermal migration. These tanks have a low standby loss, but it will increase if you do not thermally trap the pipe. Thermal migration is when the hot water in the tank moves up the hot water pipe. As it goes up the pipe, it will give off heat. As the water cools, it goes back down the same pipe. It adds to the cool-down rate, which means the tank must run to reheat the tank sooner.
The boiler side gets a little trickier. We need to consider the pipe size to ensure it can carry enough BTUs and achieve the proper flow rate. Bigger pipes carry more water flow.
Let's look at the US Boiler Alliance indirect heater specs.
If you look at the last column, you see the flow rate required is 6 gpm on all the tanks except the largest tank. The tank tapping for the boiler piping is 3/4". We need to choose the proper pipe size. The industry has adopted a chart for pipe sizing that addresses BTUs carried and flow. View chart.
We see from the chart the pipe size required is 1" as 3/4" will only carry about 4 gpm. The question often arises: "Why not use bigger tapping on the indirect if we need 6 gpm flow?" When the water enters a smaller area, its velocity increases. The velocity is important for proper heat transfer. If we used 1" pipe and 1" coil, the velocity would not be where we need it, and it would also increase the cost, as a 1" coil costs more than a 3/4" coil.
The second bit of information needed for pump sizing is resistance to flow. The indirect manufacturers will supply you with that information. You need to add the pipe, fittings, boiler, and all other peripherals. The boiler choice today will change the pump size. Cast iron boilers have a minimal pressure drop, but most modern condensing boilers have a significantly higher pressure drop. For more information about pump sizing and pressure drops see pump sizing