Breaking Glass

Any faithful reader of this blog knows it’s a Nick Lowe world and we’re just watching…

Oh change of mind, sound of breaking glass…

Last blog we discussed Delta-T, moving targets and aiming devices.

But we also mentioned that when flow to a heat emitter is reduced, its output is reduced as well.

If a variable speed pump (any of ‘em, not just the BumbleBee) reduces flow to a heat emitter, wouldn’t you run the risk of not having enough heat output in milder conditions, especially if the water temperature is lower with outdoor reset?

Another dandy question!

Let’s examine one scenario, using a simple zone of fin-tube baseboard. We’ll use Slant/Fin #30 as our reference product.

Looking at the manufacturer’s specs, we find the following:

Output with 170* AWT (average water temperature) at 4 GPM flow rate: 540 BTUH/ft

Output with 170* AWT at 1 GPM flow rate: 510 BTUH

170* AWT is 180* out, 160* back at a 20* Delta-T.

The zone is question is a big one – 40,000 BTUH at design conditions (let’s presume 0*outdoors at 70* indoors). The Universal Hydronics Formula tells us the required flow rate for this zone under design conditions would be 4 GPM.

Note the Slant/Fin specs show the output at both 4 GPM and 1 GPM.  The specs also say that if you don’t know for sure you’ll have a 4 GPM flow rate, you should size the zone based on the 1 GPM output. If we do that, we’ll need 79 feet of baseboard in the zone.  Let’s call it an even 80.

One thing we know for sure is that if we installed a BumbleBee (or it’s older brother, the 00-VDT), we would definitely have a 4 GPM flow rate under design conditions, because GPM = BTUH divided by (Delta-T x 500).

That’s the law.

Simple math says:  540 BTUH/ft x 80 feet (at 170* AWT and 4 GPM) = 43,200 BTUH worth of output.

So far, so good.

Now let’s say it’s 35*outside, and we’re at 50% load.  The zone now requires only 20,000 BTUH worth of output.  The boiler has outdoor reset, and is now supplying 150* AWT (160* out, 140* back). At that temp,  Slant/Fin #30 will put out 380 BTUH/ft at  1 GPM flow rate.

The output of the entire 80 foot zone at that point would be 30,400 BTUH.

In other words, enough.

And we also know the Delta-T pumps will be delivering 2 GPM (that pesky Universal Hydronics Formula thing again), not 1.

What if it were a mod-con boiler, and the AWT was lower at 35 degrees outdoors?

At 140* AWT = output is 320 BTUH/FT
320 x 80 = 25,600

At 130*AWT: output is 260 BTUH/FT
260 x 80 = 20,800

At 120* AWT: 210 BTUH/FT
210 x 80 = 16,800 BTUH

These outputs are based on 1 GPM, but a Delta-T pump would still be giving us 2 GPM.

We’re good.

Why are we good?  Bottom line is 80 feet of baseboard is 80 feet of baseboard.  Average water temperature changes the per foot output, but it doesn’t change how many feet are installed.

And flow rate changes output, too.  But how significant is it?

At 1 GPM and a 170* AWT, Slant/Fin #30 has an output of 510 BTUH.  At 4 GPM it has an output of 540 BTUH.

So math tells us that a 75% drop in flow rate slashes the per foot output of the baseboard less than 6%.

I think we can handle that.

How would panel radiators fare in this same scenario?  Next time, dear reader.  Next time…

And so it goes…

Nick with Rockpile from 1978.

Little known fact – Nick and I had the same hairstyle that year.

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