Trouble Boys…

Another live classic from that Pile of Rock…

Billy Bremner, face to face with the “Trouble Boys” from Rockpile’s legendary 1980 Hamburg concert.

And now, back to the show…

Last time we showed how to estimate head loss in a zone valve system. But as anyone in the trades knows, there’s a big difference between estimating and calculating.

Estimating gets you in the ballpark.  Calculating gets you to the right section and row.

Estimating head using the “safe” method requires measuring the length, multiplying by 1.5, then multiplying again by .04.

Two things, however:

  1. This formula estimates head based on the highest flow rate for each size pipe (4 GPM for ¾”, 9 GPM for 1”, etc).  If the actual required flow is less than the max, the head will be overstated, sometimes by a lot.
  2. In a retrofit, we’re making an educated guess at zone length.

But we can drill down a little closer to the actual head loss.  Here’s a pressure loss chart from the Copper Development Association showing pressure loss in PSI/foot of copper pipe at different flow rates.

The zone in question is 125 feet long.  Multiply by 1.5 to allow for fittings and the total developed length is 187.5’.  The flow rate for the zone is 3 GPM, and the pipe size is ¾”.

Taking a closer look at the chart, the pressure loss per foot of ¾” pipe at 3 GPM is .009 PSI.  Let’s math this sucker out…

187.5 × .009 = 1.69 PSI

To turn that into feet of head, multiply by 2.31:

1.69 × 2.31 = 3.9’ of head

Call it 4’.

So instead of the 3 GPM at 7.5’ of head we estimated, the zone only needs 3 GPM at 4′.  Calcs for the other two zones show the 2 GPM zones have a little over 1’ of head each, instead of the 5′ we “estimated.”

What does this all mean?

It means we’re closer to reality, with new pumping requirements of 7 GPM at 4’ of head under design conditions.

Let’s look at how a Delta-P and a Delta-T variable speed circulator handle this new reality.

Here’s Delta-P when programmed to the “estimated” head:

“A” represents one of the small zones calling, “B” represents the two small zones calling together, and “C” represents all three zones calling together. The red A, B and C represent the requirements, the purple A, B and C represent where the system actually operates when the Delta-P circulator is programmed to the “estimated” head.

Note with just one zone calling (Red & Purple A’s), the actual flow is around 4.5 GPM, more than double the 2 GPM required.  When all three zones are calling, the actual flow of 9 GPM is roughly 1/3 more than the required 7 GPM.

But it’s when the two smaller zones are calling that this gets interesting.  If you calculate out the system curve of 4 GPM at 2′ of head (Red B), plot the points and connect the dots, you see where that system curve intersects the pump curve (Purple B).  Not only is actual flow rate more than 2.5 times what it needs to be (10.5 GPM vs. 4 GPM), the pump will be running close to full speed.

In fact, it’ll run faster with two zones calling than it will with all three zones calling.

Program the Delta-P pump more accurately, the situation improves, but the same dynamic exists:

The circulator is dead on with all three zones calling, but when the small zones are calling, the supplied flow rate will be double what it required, and the circulator will still go faster with two zones calling than it does with three zones calling.

And remember these are design condition requirements.  This circulator is going to run this same speed in October as it does in January.

How does the BumbleBee handle the job?

With all three zones calling, the ‘Bee gives you 7 GPM at 4′ of head.  Since both smaller zones have requirements below the ‘Bee’s minimum speed, the system runs on the lowest pump curve line.  With just one small zone calling, the pump delivers just over 3 GPM instead of the 2 required.  With both small zones calling, the  supplied flow rate is 5 GPM instead of the 4 required.  That’s only 25% more, compared to double with Delta-P.

Again, no one is saying Delta-P pumps don’t work, or are of poor quality.  The Delta-P pumps available here are excellent pieces of technology made by world class manufacturers.

Do they “work?” Heck yeah, but it seems pretty clear that in residential zone valve jobs, Delta-T gets you closer to the right flow and head pressure much more often.

And that means systems that’ll run more efficiently, effectively and economically.

And the reason is simple…

Arithmetic.

Love me some Rockpile…

2 Responses to “Trouble Boys…”

  1. Thank you for a detailed explanation of delta-P circulators John! I just got done reading your last 4 blog posts and learned a lot.

    How does the AutoAdapt feature work on the Grundfos Alpha? Does it change according to the heating load? What is the theory behind it?

  2. Autoadapt doesn’t adjust to the heating load. All is does is automatically create the funny looking inverted V shaped pump curve based on programmed algorithms. It has no way of knowing the BTU load. The theory, as I understand it, is to force the circulator to use as little electricity as possible. It speeds up and slows down as zones open and close, but it has no way of knowing the actual load.

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