Don’t poke the mask off the ole’ Lone Ranger…
And you don’t mess around with Jim. Croce, that is. Miss this dude…
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.
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 200 Delta-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.
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 300 Delta-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 400 Delta-T the required flow rate would be 5 GPM. Your pump requirement would be 5 GPM at 12′ of head.
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…”