Spin Doctors

For some reason this one's banging around the cranium today...

 "Ain't nobody gonna bow no more when you sound your gong..."

Why the Spin Doctors weren't bigger in the 90's remains a mystery.

Spin Doctors are still hard at work out there in the hydronics business, especially when it comes to variable speed circulators and this whole Delta-T/Delta-P thing.  All sorts of interesting questions come up during our training classes, and it's remarkable the kind of "spin" that's being fed to contractors and technicians by salespeople, reps, manufacturers and so-called "industry experts."

Over the next few blog posts, we'd like to examine some of that spin and try to give you some things to think about so that you may reach your own conclusions. One thing I've learned over the past few years is that none of the so-called "industry experts" out there are what you'd call "independent," and all have a vested (read: financial) interest in you choosing one over another. 

That said, you guys get I work for Taco, right? Do I have a vested interest? You bet!

And you also get that Taco sells both Delta-P and Delta-T residential circulators? 

And that we're happy to sell you either one?

So with that, let's kick off this series with one of the odder claims & counter claims in the Delta-T/Delta-P discussion:

Delta-T pumping doesn't make sense - the Delta-T in the system actually gets smaller as it gets warmer out in order to deliver enough heat in milder conditions.

There's a fallacy in logic called post hoc, ergo propter hoc (Latin for "after this, therefore because of this" - click here). It's based upon the mistaken notion that simply because Event A happens before Event B, then Event A actually causes Event B. 

In simpler terms, dinner happens before midnight, therefore dinner causes midnight. Sequence, however, isn't the same as causality.

And this particular "spin" has some serious post hoc, ergo propter hoc going on.

There are dozens of graphs that show beyond the shadow of a doubt that as the heating load goes down, the system's supply and return Delta-T will get smaller. And you know what? The system Delta-T does get smaller as the heating load goes down.

Post hoc, ergo propter hoc would have us believe that Event A - a decrease in the heating load (because it's getting warmer out) - is causing Event B - a shrinking Delta-T between the water going out to the heat emitters and the water coming back from them. 

But it all falls apart once you actually consider why the supply and return Delta-T gets smaller as the outdoor temperature goes up. 

It's the circulator.

Oh yeah, that...

The reason is fairly simple: a fixed-curve circulator is going to run the same speed in April and October as it does in January. That means the system will be getting the same flow rate in April and October as it does in January.  The heating load, however, is much lower in April/October than it is in January.

There are three rules at play here:

Rule 1: When it gets warmer out, the BTUH heating load goes down.

Rule 2: GPM = BTUH/(Delta-T x 500)

Rule 3: If flow rate stays the same and the BTUH load goes down, the system Delta-T has no choice but to get smaller.

Logic and basic math tell us that the Delta-T has to get smaller in this situation. It's not all that controversial - it's not even debatable - and it's even considered normal. Walk in any mechanical room in January and again in April and compare the supply-return water temperature differentials.

This isn't a matter of "the system's gonna do what the system's gonna do." Rather, this is the circulator doing what the circulator is gonna do, and the system doing what the circulator tells it to do. The circulator isn't just along for the ride. In many ways it's actually driving the bus.

So let's discuss the circulator for a second. It's important to note that fixed speed circulators and variable speed Delta-P circulators are both fixed-curve circulators. That means they operate on the same performance curve all the time. 

It's easy to understand that a fixed-speed circulator will run on the same fixed-curve all the time. After all, it only has the one speed. But Delta-P circulators are also fixed-curve, as shown here.

The 3 flat orange lines represent the 3 different fixed curves on which the Taco VR 1816 Delta-P circulator can operate, based on where the user sets the programming dial. This is typical for any residential Delta-P circulator with a constant pressure mode.

Even though the circulator changes speeds, the performance curve is fixed, and the system must work along that pump curve. 

In on-off zone valve applications, a Delta-P circulator (when programmed in the preferred Constant Delta-P mode), will vary its speed as zone valves open and close in order to maintain a fixed, constant pressure differential. It cannot, however, run faster when it's cold out and slower when it's warm out. It has to work on that fixed curve.

Constant Delta-P does wonders if you have a problem with banging zone valves (of course, a flat-curve fixed speed pump, like a Taco 007, would do likewise), and it may save a little electricity. However, the circulator only knows whether zones are open or closed. It has absolutely no way of knowing, or even inferring the actual BTUH load at any given point in time.

(Note: in the Proportional Delta-P mode, the head pressure differential created by the circulator will actually go down as zone valves close, which makes this operating mode a poor choice for zone valves. It's still a fixed curve, but you may not be able to provide enough flow in certain situations. Proportional Delta-P was designed for, and is best applied to, modulating thermostatic radiator valve systems with continuous circulation.)

So if the circulator runs the same speed and delivers the same flow rate when it's warm out as it does when it's cold out, and the overall BTU load of the house decreases as it gets warmer out, seriously, what would you expect the system's supply and return Delta-T to do?  

Where's Captain Obvious when you need him?

The decreasing heating load didn't cause the Delta-T to get smaller, supplying the same flow rate despite a decreasing heat load did.

Call it pump hoc, ergo propter hoc!

Remember, a Delta-T variable speed circulator uses natural changes in a heating system's supply and return water temperature as information to determine how fast to run.  It wants to maintain the designed-for Delta-T in the system for as much of the heating season as it can.

The system Delta-T will try to change as zones open or close (taking more or fewer BTU's out of the system), and it will try to change as the outdoor temperature - and the heating load - changes. So as it gets warmer out, in order to maintain your designed-for Delta-T, the circulator will naturally slow down. 

There's also an secondary post hoc, ergo propter hoc argument that gets tossed around by "industry experts." It claims that you still have to deliver enough heat even when it isn't very cold out, and since the system Delta-T gets smaller as it gets warmer out, you can only deliver enough heat when it isn't very cold out with smaller system Delta-T's. 

Again, the fallacy is that since smaller Delta-T's in the system happen before (or in conjunction with) delivering enough heat when it isn't very cold out, it must be that the smaller Delta-T's are required to deliver enough heat when it isn't very cold out. 

And twisting the logical pretzel even further is the asertion that if the systemDelta-T is not allowed to shrink as it gets warmer out, then you won't be able to deliver heat to the house when it isn't very cold out. 

Then you hire Ted Wells to write an "independent" report saying it's more probable than not that Delta-T variable speed pumping doesn't work because the Delta-T has to get smaller and flow has to increase as it gets warmer out in order to deliver BTU's when it isn't very cold. 

But once you know why the Delta-T really shrinks in the first place you can examine the post hoc, ergo propter hocs above and reply, in Latin:

Taurus excretum!

We'll be spending the next several blog posts discussing more misconceptions, applications and realities of variable speed pumping.  

In the meantime, you gotta feel for ol' Jimmy Olsen...

"A pocket full of Kryptonite..."