Logical Song

Humoungus hit for Supertramp back in ’79…

The Logical Song,” with one of my favorite lines ever:

“Watch what you say, or they’ll be calling you a radical…”

In hydronics, there’s no radical, cynical, liberal, animal, vegetable or mineral.

But there is logical.  In fact, it pays to be downright Spockian when thinking about your systems and how all the parts and pieces work – both on their own but in particular how they work together within the dynamics of the system.

Because after all, it is a system.

Let’s consider the logic behind variable speed circulators, both Delta-T and Delta-P.  The prevailing attitude is that they always give you the right flow, no matter what.

But that’s not logical.  As we’ve said before (here and here), they’re not that smart.

For either circulator to “always” give you the right flow, each would have to have some way of knowing what the BTUH load of the system is at any given point in time.  They’d also have to have an idea of what the outdoor temperature is at any given point in time, as well as how much radiation is installed.


As we’ve said many times in this space before (here, and here), Delta-P circulators are best suited for panel radiator/thermostatic radiator valve jobs. The trigger that tells a Delta-P circulator to speed up or slow down is resistance against the impeller.  More resistance (head pressure from the system), it slows down.  Less resistance, it speeds up.

So, When TRV’s close, that means the heating load is either satisfied or at least going down.  Closing TRV’s increase resistance against the impeller, ergo the pump slows down.  When TRV’s open, the opposite happens.

Pretty slick, right?

But Delta-P circulators, even with the vaunted “auto-adapt” function, still work on a fixed pump curve.  It’s a funny-looking, inverted V shaped curve, but it’s still fixed.  And  because it’s a fixed curve, the system will always work where the pump curves and system curves intersect, due to that pesky law of thermodynamics that states energy in = energy out.

In a zone valve application, a Delta-P pump only  knows when zone valves are open or closed, and in what combination.  It has no way of knowing what the actual load of the system is at any given point in time, so it will have to work on a fixed, inverted V-shaped pump curve.  It has no way of knowing the actual heating load, and therefore has now way of knowing how much flow is required. It only reacts to whether a zone is open or closed.  

Therefore, in a zone valve application, a Delta-P pump will run the same speed in October as it does in January as it does in March.

Delta-T, on the other hand, measures the temperature drop of the fluid as it goes through the system.  What makes the fluid temperature drop as it goes through the system?

Logically speaking, the fluid gives off energy to the heat emitters, which in turn deliver heat to the rooms.

And the colder it is outside, the more heat is needed.

Delta-T is designed with that logic in mind. By monitoring the supply-return water temperature differential (Delta-T), the circulator’s logic can tell when more or less flow is needed.  When it gets colder out, or when more zones are calling, more energy is being taken out of the fluid.  The Delta-T will naturally want to get wider.  The circulator senses that, and speeds up to deliver the required flow and BTU’s.

When it gets warmer out, or when zone valves close, the Delta-T will want to get narrower since the system needs fewer BTU’s.  The Delta-T control tells the circulator to slow down.

Therefore, we can reason that a Delta-T circulator will not only vary its speed as zones open and close (just as a Delta-P circulator will), but that it will also vary its speed as the heating load changes with the outdoor temperature.

That’s a darn sight closer to providing the right flow most of the time.

Sound logical?

Bloody well right!

Quite right…

Leave a Reply

You must be logged in to post a comment.