Many plumbers see this heat exchanger as four connections that need to be tied in, but there is actually a more in-depth science to this. In order to for heat exchange to occur, heat will always escape to the colder medium. If you mix hot water to cold water (or vice versa), what is actually happening is that the heat energy from the hot water is being absorbed by the cold water. As a result, we actually have warm water.

But before we dive into the science, it helps to understand what is a Primary Side and what is a Secondary Side. The Primary Side will always be the heat source side. In a Heating application, the Primary Side will always receive the boiler supply water. In a Cooling application, the Primary Side will always received chilled water from the Chiller. Within a Heat Pump, the primary side will always be the refrigerant side entering the heat exchanger.

The Secondary Side, by deduction will always supply to the target, client, or end equipment. In a heating application, the secondary side may supply hot water to Unit heaters (UHs), Force Flow Heaters (FFHs), MUA Heating Coils, Cooling Coils, etc). Similarly, in a cooling application, it will enter the Chilled Water coils of the MUA, or other equipment used for cooling (fan coil units come to mind).

Now that we got the difference between primary and secondary out of the way, it helps to understand that when a heat exchanger has primary hot water entering on the primary side (where the heat source is), then on the secondary side the cooler water will be warmed up – which is the obvious part.

But what many don’t realize when they pipe the heat exchanger, is that there is a difference in heat transfer when piped in parallel flow versus counter flow.

Below shows a parallel flow configuration. Here the water flow (arrows) are pointing in one direction (to the right). While the orange colour signifies the water from the primary side, the blue colour signifies the water from the secondary side. Let’s visualize this by using an example:

You have a heat exchanger supplying hot water to the Make Up Air (MUA) during the winter. Glycol Hot Water is being supplied to the heating coil (in the MUA) at 90 °F. After the heat is transferred to the surrounding air, the Glycol Hot Water coming back out of the MUA would be less than 70 °F. This glycol is pumped to the heat exchanger entering at 70 °F (The blue colour). The boiler is supplying hot water at 90 °F in an effort to heat up the glycol water before it is supplied back to the heating coil.

Here’s the important part: Because the piping is in parallel flow, the temperature exiting the coil is actually going to be roughly the average between your primary inlet water temperature, and your secondary inlet water temperature. This works out to be roughly 78 °F – 80 °F being delivered back to the heating coil. This would not be nearly enough to provide the adequate tempered heat for the MUA, and you can be sure that occupants will be complaining.

If the plumber piped the heat exchanger in counter flow configuration, then the problem would disappear because what you would be achieving is the highest form of heat transfer in the heat exchanger. Every part of the water entering on the secondary side is heated by the hottest temperature of the primary side, that is until the temperatures on both sides are equal (assuming no heat loss).

From the picture (below), you see that on the left side when piped in counter flow configuration, the secondary outlet temperature would be much higher than if the same heat exchanger is piped in parallel flow, experiencing the same circumstances.

It is always important to pipe heat exchangers in counter flow configuration. I have occasionally encountered installations where it is in parallel flow and have to bring this up to the mechanical contractors attention before water is filled and pressure tested. This saves a lot of aggravation when the engineers start questioning why the heating performance is inadequate to what was designed.