Just Because…

Don’t poke the mask off the ole’ Lone Ranger…

And you don’t mess around with Jim.  Croce, that is.  Miss this dude…

Ever wonder why pump curves look the way they do?  I mean, why is a 007 curve “flat” while the curves of a 0015 3-speed, or a Grunfos 15-58 3-speed are “steep?”

Well, it’s not “just because.”

In the pump world, a high-flow, low-head flat curve is often called an “American” pump curve, while a low-flow, high head steep curve is referred to as a “European” curve.


It has to do with the types of systems those circulators were designed for.

Last time we discussed how the typical European hydronic system – panel radiators, TRV’s and home-run piping – became the typical European system: it was the easiest, most convenient and least expensive system to retrofit into millions of old, existing homes that had no central heating.

And these old, pre-1960 structures weren’t very well insulated and leaked heat like a sieve.

When you design this kind of system, specific pumping requirements  take shape.

Consider an old, leaky cottage somewhere in Belgium or Germany. It gets quite cold, so let’s assume a heat loss of 100,000 BTUH.  When picking a circulator for a parallel piping system, you size for the total flow rate of the system, but only the head loss of the worst case piping loop.  What you select as a designed-for Delta-T makes a huge difference.

Let’s presume the longest piping run of  ⅜” (≈ 14mm)  PEX is 100′ total – 50′ there, 50′ back – and the radiator needs to deliver 16,000 BTUH at design conditions.  Here’s the required flow rate at a 20Delta-T:

GPM = BTUH ÷ (ΔT × 500)
GPM = 16,000 ÷ (20 × 500)
GPM = 16,000 ÷ 10,000

GPM = 1.6

So that one radiator needs 1.6 GPM. What would the pressure drop be?

Let’s look it up.

At 1.6 GPM you’d get .45′ of head per foot of ⅜” PEX.  Multiply that by 100′ and you’d have 45′ of head


As a result, European panel rad systems are designed around Delta-T’s of 30 or 40 degrees, not because it’s “better” or “optimal,” but because that’s what’s needed to keep circulators for those types of systems reasonable to install and reasonably priced.

At a 30Delta-T, the flow rate for that radiator would be around 1 GPM, but the S&R piping head loss would be around 20′.

At 400, the flow rate drops to 0.8 GPM, making the head loss only 11′ of head per foot of pipe.  At a total run of 100′, that’s 11′ of head.  Add for the TRV and other components and you’re looking at around 12′ for the worst case loop.

The total load was 100,000 BTUH, so at a 40Delta-T the required flow rate would be 5 GPM. Your pump requirement would be 5 GPM at 12′ of head.

See where it fits?  Right in the “wheelhouse” of the low-flow, high-head, steep “European” pump curve.

So, that’s why European pump  curves  are steep – because the steep curve fits the most common application in Europe.  Traditional flat “American” curve pumps – made famous by Taco and B&G – are flat for the exact same reason, as we’ll discuss next time.

In the meantime, stay away from those Car Wash Blues…

“…the man says we got all that we can use…”

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