Let’s keep rolling the Rockpile, shall we?
Tapping toes everywhere agree!
Last time we examined the very real functional differences between a “properly” programmed Delta-P variable speed circulator and the Taco BumbleBee Delta-T circulator in a typical zone valve application. To get the most out of the Delta-P circulator that little red “Max Head” dial has to be set as close as possible to the correct head loss of the system. In theory, that would get your funny-looking-inverted-vee-shaped pump curve as close as possible to the system’s operating requirements at design conditions.
Two challenges though. The first one we discussed last time: that funny-looking-inverted-vee-shaped pump curve is fixed – so the circulator runs on that line all winter long. Not a big deal during the roughly 2.5% of the heating season spent at or near design conditions. The other 97.5% of the time, however, we’re badly over pumping.
The other challenge is less obvious, but potentially more dramatic:
How do you determine the head loss needed to set the dial in the first place, especially in a retrofit or boiler replacement job?
Calculating flow rate is easy: once you know the heat loss, divide the BTUH total for each zone by (DT x 500). That gives you each zone’s flow rate. Add ‘em up for the total flow rate for the job.
To estimate head loss, first find the length of the longest zone (sometimes an educated guess, at best) and multiply it by 1.5. This accounts for valves, fittings and other stuff – all of which add pressure drop.
Multiplying by 1.5 gives you the “total developed length.” Next, multiply that number by .04 (4′ of head for every 100′ of straight, properly sized pipe).
Here’s an example:
30,000 BTUH zone (3 GPM at design conditions)
Total run: 125′ (measured from the outlet side of the circulator all the way back to the inlet side)
125′ x 1.5 = 187.5′ total developed length
187.5 x .04 = 7.5′ of head.
Let’s presume that’s the highest head loss zone in the system. Let’s also presume there are two other smaller zones in the house, 20,000 BTUH or 2 GPM – each, and the math tells us the head loss for each zone is 5′. The job totals are 7 GPM at 7.5′ of head.
Let’s plot this on the Delta-P pump curve:
By adjusting the red programming dial to match the estimated head of the system (7.5′), it sure looks like we’re going to be in an under-pumping situation when we’re at or near design conditions. Most of the heating season this would work just fine.
Just not when it’s really cold out.
Same application, this time with a BumbleBee…
As you can see, the operating points fall within the shaded variable speed operating range. So at design conditions, the BumbleBee will give you the right flow, no matter what combination of zones is calling. The BumbleBee will also vary its speed as it gets warmer out, ultimately spending most of the heating season running at or near its minimum speed (line 1).
But estimating head loss is hard, especially in a retrofit when measuring the run can be, at best, an educated guess. This formula shown here estimates head on the high side, so it’s “safe.” What would all this look like if we were a bit more precise in our head calculations? Check back later this week for that one!
Filed under: Uncategorized