Rikki Don’t Lose That Number

You don’t wanna call nobody else…

Steely Dan hit #4 in the summer of ’74 with this classic.

And speaking of lost numbers….

“Hey Buddy!  I’m only getting a 15 degree Delta-T in my system.  You said it was going to be 20.  Get your a** down here and fix it, NOW!!!!”

Said no one, ever.

Pick a boiler room, any boiler room, anywhere in the country, any day of the winter and check the supply-return Delta-T.

I’ll bet you all of Warren Buffett’s money that if the system has a fixed speed circulator (or a Delta-P variable speed circulator, for that matter), it won’t be 20.

And I’ll double down and bet all of Jimmy Buffett’s money that it never will be 20, either.

And I’ll let it all ride, along with Little Miss Muffet’s tuffet, curds and whey that the system works.

A basic understanding of heat loads, system curves and pump curves explains why.

As we been discussing, hydronic systems work where the system curve intersects the pump curve (click here and here to review). As hydronics professionals, we painstakingly calculate the heat loss as accurately as we can, knowing full well any heat loss calculation is loaded with what engineers call “load calculation safety factor,” or what we regular people call “fudge.”

Then, using the University Hydronics Formula, we determine the GPM flow rate required to deliver the BTU’s to satisfy that calculated heat loss.  Next, we estimate – as best we can – the head loss through the piping system at the calculated flow rate.  Once we know the required flow rate and estimated head loss, we select a circulator whose pump curve is above that requirement point.

The calculated BTUH load is for “design conditions,” that so-called “coldest day of the year.” If we use the ASHRAE recommended design condition, the outdoor temperature would be at or near design temperature roughly 2% of an average heating season. We may even “fudge” the outdoor design temperature to be even lower, just to be “safe.”  Combined with all of the other “fudge factors” inherent in any heat loss calculation method and it’s very likely our heat load calculation is anywhere from 15-to-40% higher than the real heat load of the building at design conditions.

And since we’ve calculated the required flow rate based on that heat load, the calculated flow rate is most likely 15-to-40% higher than necessary.

And since the head loss has been estimated based on the flow rate, which was based on the heat load, it’s higher than necessary.

To top it all off, we’ve chosen a circulator whose performance curve is above the “exaggerated” calculated flow and estimated head.

That, friends, is called measuring with a micrometer, marking with a piece of chalk and cutting with an axe.

Let’s say our heat load is 60,000 BTUH at an outdoor design temp of 0*.  If we design the system around a 20* Delta-T, the required flow rate is 6 GPM. And based on that flow, we do some math and estimate the system heat loss to be 6 feet of head. If we use a standard 3-speed circulator, here’s what we have:


The system requires 6 GPM, but the system curve line intersects the pump curve line at around 7 GPM. So matter what, whenever that system turns on, the circulator will be circulating closer to 7 GPM.

So let’s say it’s 0* outside, and we go to this boiler room to check the system Delta-T.  What would it be? A little algebraic manipulation of the Universal Hydronics Formula tells us the Delta-T would be around 17*.

If it was, say, 20 degrees out, the heating load would only be around 42,600 BTUH.  The system is still getting nearly 7 GPM worth of flow.  The math above tells us the system Delta-T would be around 12*.

At 35* outside the load would be about 30,000 BTUH, or about 50% load.  The system Delta-T then would only be 8.5*.

And all this is assuming the heat loss calculations were 100% dead-on absolutely accurate, with no built-in or added-in fudge factor.

Is it any wonder you never see an actual 20* Delta-T out there? Rikki lost that number…

Will this system deliver heat?  Yep.  Will is “work?” Absolutely.

So what’s the problem?

The problem is one of boiler short-cycling.  When the Delta-T across any boiler’s heat exchanger gets smaller, the boiler tends to short cycle more.  And the more it short cycles, the more its economy of operation suffers.

Check two recent blog posts (here and here) to see how reducing short-cycling can dramatically affect fuel usage over the course of a heating season.

And until next time, try saying “ooga-chucka” over and over again before you start feeling silly…

Yet another unlikely 1974 smash from Blue Swede.

In case you’re wondering, it’s 20.

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