Watching The Wheels

We lost John 33 years ago this past Sunday…

My favorite Lennon classic!

Some random hydronic thoughts while watching the wheels

GPM = BTUH ÷ (ΔT × 500)

If there was ever a non-controversial math formula in all of hydronics, this is it.  It was good enough when Gil Carlson developed the very first System Syzer Wheel way back when, so it should be good enough now.

Time certainly doesn’t diminish the math.

It’s a wicked simple math formula that essentially helps you determine what flow rate you’ll need to deliver a certain amount of BTU’s at a designed-for Delta-T, or temperature drop between the supply and return water temperatures.  Hydronic systems have been designed using this formula for decades.

Say you have a 40,000 BTUH zone, and you’re designing the system to a 20* Delta-T (you don’t have to, but it is the industry standard for most residential hydronics – basically because it makes the math easy). Fill in the blanks for the formula to get the required flow rate:

GPM = 40,000 ÷ (20 × 500)
GPM = 40,000 ÷ 10,000
GPM = 4

If you make the Delta-T 30 instead of 20, the required flow rate would be lower.

If you make it 40, the required flow rate would be lower still.

In fact, wide Delta-T’s are used all the time when sizing pipe and pumps for variable speed injection mixing “mini-tube” systems. You can transfer a boatload of BTU’s through very small pipe with a very small pump through the power of Delta-T.   Say we had a 40,000 BTUH radiant system designed to a 10* Delta-T. The supply water temperature is 110*,  so the return water temperature would be 100*.  And let’s say the primary loop water temperature is 180*.

To size your injection pipe and pump, you’d want to find the Delta-T between the water going from the primary into the secondary, and the water coming from the secondary into the primary.

That would be 180 – 100, or an 80* Delta-T.  Let’s stick it in the formula:

GPM = 40,000 ÷ (80 × 500)
GPM = 40,000 ÷ 40,000
GPM = 1

Now, could you carry this out to the extreme and use an 800* Delta-T and deliver 40,000 BTU’s with .1 GPM?  I suppose you could, but to carry the argument out to that level is borderline ridiculous.

When we look at the reality going on in people’s basements, GPM = BTUH ÷ (ΔT × 500) is how we design systems, and it’s why the Taco BumbleBee and 00-VDT variable speed circulators use Delta-T technology to vary their speed.

It’s not a matter of “regulating” output by “imposing” a Delta-T on a system.  It’s simply a matter of using the designed-for system Delta-T as a meaningful and downright logical means of varying the speed of the circulator.

Let’s say you have a zone valve system with a few zones calling. The circulator is chugging along as a specific speed.  All of a sudden another zone opens, meaning the system needs more “heat.” The obvious immediate result is that the return water temperature is going to decrease, and the Delta-T in the system will get wider.

This happens all the time, regardless of what kind of circulator you have.

A Delta-T circulator will immediately recognize what is essentially a change in the BTUH requirement, and speed up.

When a zone closes, it’ll recognize that change in actual heating load, and slow down.


Because GPM = BTUH ÷ (ΔT × 500).

Not only does a Delta-T pump react immediately to zones opening and closing, it also goes faster when it’s colder out and slower when it’s warmer out.  As it warms up, the BTUH requirement goes down and the system needs less heat. That will manifest itself in a shrinking difference between the supply and return water temperature.  A Delta-T pump sees that, and does the logical thing: slow down.

Pretty simple, ain’t it?

As with anything, there’s a practical limit on the low end.  Check out the BumbleBee performance curve here. That line labelled #1 on the BumbleBee pump curve is the minimum speed, so that’s as slow as it’ll go.  So when it’s mild out, and in a reset system when the water temperature is fairly low, the circulator will operate on that minimum speed line.  The Delta-T won’t be 20 (or whatever you set it for), but you’ll have ample flow to satisfy any heating load.

GPM = BTUH ÷ (ΔT × 500) was good enough for Gil Carlson, and it still applies today.

Lots more blog posts on Delta-T operation.  Click here, here, here and here to check it out.

And here’s my second favorite Lennon tune ever…

33 years…

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