So, back to our detective story (to review, click here). Why did those zone valves bang?
The answer, friends, can be found between the flanges.
The old circulator was a 3-piece Taco 110 – a high flow, low head flat curve circulator. The total heating load of this job was 80,000 BTUH, and by applying the Universal Hydronics Formula (GPM = BTUH ÷ (Delta T × 500)), we know the total required flow rate for the job is 8 GPM. Let’s presume each zone is roughly equal in overall load, at roughly 16,000 BTUH apiece (1.6 GPM each).
With all zones calling, here’s where the system operated when the 110 was in service:
That’s about 8 GPM at roughly 7 feet of head pressure.
Now let’s look at what happened when only 1 zone called:
The flow rate changed drastically, from 8 GPM to 1.6 or so. But notice the head pressure – or pressure differential – created by the Taco 100 is only about 7.5 feet. Not much of a change, meaning the zone valves were closing against, at most, 7.5 feet of head – not enough of a pressure differential (3.25 PSI) to cause banging.
A new circulator was installed with the new boiler – a 3-speed Taco 0015 because after all, a 3-speed circulator’s the only one you’ll ever need, right? And if the job requires a total of 8 GPM at roughly 7 feet of head, the 0015 looks perfect, no?
It’s obvious the system would “work” with the 0015 set to either Medium or High speed. And you can bet dollars to donuts it would work most – if not all – the time at Low speed, as well.
That is, if we can ignore the banging.
Why does it bang with the 0015? Take a gander…
Remember, with a fixed-speed circulator the system will always operate on the pump curve. Always. Note where the operating points are with each of the 3 speeds, and note the corresponding head pressures. We know the zone valves didn’t bang at around 7.5 feet of head pressure differential, so it’s likely there was no banging with all zone calling and the 0015 set at Low Speed. After that, however, it was bang-city.
The installer clearly thought the 0015 was the problem, so he swapped it out for a Grundfos 15-58 3-speed. Here’s what he got…
With the circulator set on Speeds 1 or 2, we can safely assume there was no zone valve banging when all the zones were calling and one closed. Why? Look at the head pressures. At Speed 1 the circulator is only creating 2′ of head pressure, and only 7′ at Speed 2. If the Taco 110 didn’t make noise at those head pressure, neither will this one.
However, as with the 0015, the problems start as more zones close. The system moves up the pump curves and the zone valves close against greater pressure differentials. The acid test is that even at the lowest speed, zone valves banged. As you can see, with only 1 zone calling, the 15-58 was still creating roughly 11.5′ of head pressure, or nearly 5 PSI of pressure differential.
Which was enough to cause banging.
On the face of it, either the 0015 or the 15-58 would be a perfect fit for this job, since their performance curves cover the flow and head requirement. And all the sales guys say that since it’s 3-speeds, it’s the only pump you’ll ever need and it makes pump selection “easy.”
Well, the easy answer is often the cleverest. Unfortunately, it’s also often wrong.
Both 3-speed pumps are, quite simply, the wrong pumps for this job. There are 3-ways of dealing with this problem, which we’ll tackle next time.
Meanwhile, groove to the greatness that was Lowell George & Little Feat…
“…guess that guitar player sure could play…”
Filed under: Uncategorized