Head Loss

oldhousenut

Can anyone explain head loss to me? I'm looking at purchasing either a Trinity or Triangle boiler and been told head loss is better on the Triangle. But it looks like the turn down is better on the Trinity. Comparing the Prestige 175 and the Ti 150. Thanks..

Brad White

Feet of Head

Kind of a funny set of terms now that I think about it...

Feet of Head is a measure of a pump's ability to lift water at a given flow rate. A pump that can force a certain flow rate (GPM usually), up a column of piping a certain height in feet is said to be able to deliver X gpm at Y feet of head.

Hint/Tip: The term is arbitrary- it could be centimeters, furlongs, Smoots (for you MIT guys). But feet of head can also be converted to PSI. One PSI is equal to 0.433 feet of head; every PSI will equal 2.31 feet of head.

Now, "circulators" refer to a pump but in a closed system. Actual "lift" is irrelevant so the "feet of head" term remains but as a measure of resistance (friction and dynamic losses) in that closed system.

When a piece of equipment is said to have a "head loss", it means that water entering at a certain pressure will leave at a lower pressure. That difference is the head loss and is specific to a given flow. If something has a certain head loss it is meaningless without a concurrent flow rate.

If the entering pressure is 10 PSI (23.1 Feet of Head) and leaving at 8 PSI (18.48 Feet of Head) is said to impose a head loss of 4.62 feet. If you could stack open water tubes on either side of the measured device you could see the difference in water levels matching those numbers.

How this affects your choice of boiler: Some boilers require primary-secondary flow to guarantee a minimum constant boiler flow. Thus this is helpful (to know the head loss) so the circulator can be sized. Other boilers can rely on just one primary circulator. Knowing the boiler head loss in this case tells you how much "residual" head remains to deliver hot water to your radiators and back.