There are some things you can’t cover up with lipstick and powder…
Dave Edmunds, Nick Lowe and the boys with “Girls Talk, ” a top-20 hit from waaaay back in ’79.
Online discussion forums are both fun and infuriating because everyone’s an “expert.” These “discussions” start with questions, but usually devolve into calling the other guy an idiot because he doesn’t agree the sound logic of your “fact-based” position.
Never mind the “facts” supporting those “fact-based” positions are only facts because the holder of that position constantly repeats them and proudly calls them “facts.” If someone asserts what they’ve just stated is a “fact,” it kinda closes off discussion, doesn’t it?
And you can always make a statement even more factual BY USING ALL CAPS, FOLLOWED BY MANY EXCLAMATION POINTS!!!
This online dynamic thrives in discussions comparing Delta-T and Delta-P variable speed circulators.
With that prelude, what follows is simply information – use it as you wish.
Well, here goes. You may be surprised.
Here’s a pump performance curve for a Wilo Stratos Eco. It’s a very nice pump curve. At full speed it very much mirrors a Taco 008.
See that diagonal line going down and to the left, starting at about 10′ of head and ending at 5′ of head? That line’s pretty important.
Delta-P circulators basically chop off the upper portion of the pump curve. How much is chopped depends on where you set the red programming dial. The 10-5 line is default.
This line is the Delta-P circulator’s new, fixed pump curve. No matter what happens, the circulator will operate on this line.
Those curvy lines starting in the lower left corner of the chart and flowing up to the right are sample system curves. No matter what the system actually requires in terms of flow or head, this circulator is going to force the system to operate at a point where the system curve intersects the pump curve.
For example, let’s say we have a 4-zone system that requires a total flow of 10 GPM (100,000 BTUH), with a maximum head loss of 5 feet under design conditions – not an unusual system. That’s Point A below.
Now the $64,000 dollar question: when all 4 zones call for heat, where will the system actually operate?
If you said roughly 11½ GPM at roughly 6½’ of head, you win! (Not sure why? Follow the system curve line)
And 1 bonus point if you also said the circulator will be running at full speed.
Why? Because this type of circulator – even though it’s called “variable speed,” still operates on a fixed pump curve. It’s a funny looking sorta upside-down V-shaped curve, but it’s still fixed. The heating system has to work on that curve, because energy in equals energy out.
Now let’s say our biggest zone, worth about 40,000 BTUH (or 4 GPM) satisfies and the zone valve closes. The total flow requirement of the 3 remaining zones is 6 GPM. Let’s presume we still have the highest head loss zone (5’) calling (this isn’t to prove a point, it’s just to make use of the existing lines for clarity).
So the flow/head requirement is Point B. For another $64,000, where will the system actually operate?
Did you say 9 GPM at 10’ of head? Ding-ding-ding! We have a winner!!!
And another bonus point if you said the circulator is still running at full speed.
And an automatic entry into our “Lightning Round” if you said the “variable speed” circulator has yet to vary its speed, and is delivering 50% more flow and 100% more pressure differential than the system requires.
And if you said that this circulator will deliver this same fixed flow rate and head pressure in October – when the heating load may only be 25% of the total, in December – when the heating load may only be 75% of the total, and in that 2-day stretch in January when the heating load is around 98% of the total (provided the heat loss calculations have no added “fudge-factor”), then you receive an automatic entry into our “Tournament of Champions.”
Will this system “work?” Of course! Shove 15-to-50% more flow than you need through a system, it’s gonna deliver BTU’s. If that’s you’re only requirement though, it’s doubtful you’re even considering a variable speed pump. My guess is you’re expecting more.
You’re expecting a variable speed pump to, you know, vary its speed.
Which hasn’t happened yet.
Yeah, it works. but what does it all mean? Why does it matter? We’ll look at these and a bunch more questions in the coming weeks.
And next time we’ll take a look at what happens when the pump does vary its speed.
Till then, take it away Dave…
Edmunds and Lowe in Rockpile – one great band!
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