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, in them for the domestic water lines. I hope you picked up that 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 which goes on the cold inlet side of the IWH is to absorb pressure produced when water is heated. As the water heats it expands and since you cannot compress water it expands into a tank that has an air charge and bladder. Air is compressible and acts as a pillow and maintains 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 would strongxly suggest the use of 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 will move 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.

12" Inverted trap  

Insulated Inverted Trap


The boiler side gets a little trickier. We need to be concerned with pipe size to be able to carry enough BTU's and getting 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's for the boiler piping are 3/4". We need to choose the proper pipe size. The industry has adopted a chart for pipe sizing which addresses btu's 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. Many times, the question comes up as "Why not use bigger tapping's on the indirect if we need 6 gpm flow"? When the water enters a smaller area it increases velocity. 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 minimal pressure drop but most of the new mod/con boilers have a significantly higher pressure drops. For more information about pump sizing and pressure drops see pump sizing

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