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Fire and flow are part and parcel when we break down the fundamentals of proper design, installation and troubleshooting in the world of hydronic space heating. If we have too much of one, too little of the other, none of either or any combination thereof, your phone is going to light up like a Caleffi zone valve relay in January.
And when we’re in the bowels of buildings figuring stuff out, we don’t have the time to be reading run-on paragraphs, so I’m going to hit the necessary facts with bullet points. I like to keep things simple and straightforward. Flow is where we’re going this month, so just go with it.
Qualifying or selecting a circulator
• Start with a Manual J heat loss calculation. You can’t size a pump properly without knowing how much heat you need to move, so start here — always. If the building has only one zone with one pump, you’ll use the heat loss for the entire building. If there are multiple zones, you’ll need a heat loss requirement for each zone and size the pumps accordingly.
• The next step is to establish a target flow rate in gallons/minute (gpm) using the following formula: GPM = BTUh Loss/(500 x delta T).
• The delta T in the equation is the number we use in our design to establish the temperature difference between supply water temperature and return water temperature. It will change based on the application: a 10-degree temperature difference is typically used for residential radiant heating; a 20-degree change for commercial radiant heating; a 20-degree change for high-temperature applications such as those using radiators or baseboard; and a 25-degree delta T for snow- and ice-melt systems.
• Our pipe size is based on gallons/minute with flow velocities between 2 feet to 4 feet per second. Why? If the water is moving too slow, the air bubbles in the water will be unable to move along horizontal pipe and cause all kinds of problems for you. Specifically, we won’t get those air bubbles back to our air separator where they can be expelled from the system. If it’s moving too fast, you’re going to have noise issues and potential pipe erosion.
• This next step can be as tricky, or as simple, as you want it to be. We have to find the head loss or feet of head of the piping system. The easiest way for me to explain this is anything causing resistance to our water flow (gpm) is part of the head loss. A tee, a 90-degree elbow, a flow check, y-strainer, ball valve, length of pipe and many other components are part of this equation. Each will have a value assigned to it called its equivalent length of pipe based on its description and size. Add up all the values and plug them into a pesky little equation that looks like this:
Hl = k x c x l x (f1.75 power)
Hl = the head of the piping system
k = value based on the pipe size
c = correction factor for something being used other than water and its temperature
l = total equivalent length of the piping circuit
f1.75 = flow rate raised to 1.75 power
Ugh! I hear ya! I apologize for dumping that on you. Seriously, the last thing I want you to do is to start daydreaming. I want you engaged. Reading that mess certainly did not spark enthusiasm in your desire to continue. But stay with me here.
The fact is, this method is almost impossible when you’re troubleshooting. Unless, of course, there are exposed walls and ceilings throughout the building which would allow you to count and itemize every fitting, measure every length of pipe, find the total equivalent length of every other component, plug them into the equation and do the math.
That has happened exactly zero times in my career. So, let's thank the really smart people again for making this easier on those of us who have a schedule to keep and continue to struggle for the legal tender. Those same really smart people came up with an insanely easy way to arrive at a number for head loss that any third grader who pays attention could solve. Here goes:
1. Measure the number of feet of the longest run of pipe to and from the pump. This will take an assumption or two on your part, but it will get us relatively close.
2. Multiply this number by 1.5.
3. Multiply that number by .04.
That’s it! That final number is our pump head or the head loss.
Is it as accurate as the first option? No, of course not. How could it be? In the first option, we’re dealing with concrete data based on real numbers and actual measurements. We had access to the entire system.
But we’re solving problems in a 50-year-old home under conditions that we deal with as is. And “as is” often involves a customer that wants answers and solutions sooner than later. The latter option will keep those customers happy and get you the correct replacement pump at the same time. Ignorant is not a good look and you’ll never have to worry about that with this plan of attack.
• Our final step is selecting a circulator based on the pump GPM and the pump head. These two numbers make up the target operating point, which gives a specific place to plot our needs on a graph of your favorite circulator manufacturer’s pump curves and see which pump best fits your application (see Taco pump curve graph).
• We’re looking for a pump curve that passes through our target operating point or the one that is directly above it. This point should fall within the middle-third flow rate range of the pump curve for maximum efficiency. In this case, my choice would be number 5, the 007, because it meets all the criteria above.
New pump vs. Do the math
Let’s take a quick look at an existing pump selection and size. We have an average-size, two-story home in the northwest suburbs of Chicago. The heating system is an older 80,000 BTUh cast-iron boiler serving cast-iron baseboard, piped reverse return, throughout the home. The homeowners called you because they’re having trouble heating one bedroom that has a single length of baseboard in it. In fact, the baseboard is ice cold.
They already tried another company, but didn’t have faith in them because they were told the existing Taco pump wasn’t big enough and they needed something with more power; his words, not mine. The other guy wasn’t even there long enough to take off his jacket.
So, you arrive at the scene and break out your tools: a pad of paper, a pencil and your iPhone’s calculator.
Customer says, “What are you doing?
“I’m double-checking to see if your Taco 007 pump is big enough. It will only take a couple of minutes.”
Customer: “Well, I guess if it’s only going to take a couple of minutes that’ll be alright, but I’ve never seen a service tech operate like this before.”
“I’m going to do the math and then I’m going to trust the math,” you reply.
Let’s see, we have an 80,000 BTU/h boiler using cast-iron baseboard, so we’ll be figuring a 20-degree delta T.
80,000/(500 x 20) = 80,000/10,000 = 8 GPM
After some quick measurements, you find that the longest length of pipe comes to around 140 feet.
140 feet x 1.5 = 210 feet total equivalent length, accounting for the fittings and pipe size
210 feet x .04 = 8.4 head loss
Result: 8 gpm @ 8.4 head loss
“I got it,” you say.
“You’ve got what?” the customer asks.
“I know if your pump is the right size.”
“Well, is it?” he asks.
“It sure is.”
“You’re certain of that? It only took you a couple of minutes,” he says.
“I’m absolutely certain. You need a pump that will deliver 8 gpm @ 8.4 head loss and your Taco 007 is fully capable of doing that.”
“Well, I don’t know what all that means exactly and what the heck was that other guy trying to sell me? A bag of goods?” the customer asks.
“No, sir. He was trying to sell you a pump you didn’t need.”
“Ok, that’s just fine and dandy, but our bedroom is still pretty dang cold, ya know,” he says.
“Yes, sir. Considering the way your system is piped and since it’s the only room that’s cold, I’m betting it’s a frozen pipe or just an air-bound length of baseboard. I’ll check for air first. It’s only going to take a minute or two as well.”
Lucky for the customer in this fictional story, it was just air.
And also lucky for this customer he called a qualified company the second time around. The tech didn’t get the pump sale this time but I bet he got a customer for life. If, you know, it was a real story and all.
I’ve mentioned it before. Don’t be the ripping-and-running type of contractor or technician. Learn your craft as well as you possibly can and use that knowledge to grow your career or your business. The dishonest and unqualified ones are a dime a dozen and easy to spot.
For further information and help, I’ve added these notes, tools and a pipe size/flow chart.
• When zone valves are used, use a flat curve pump or a delta P pump to account for the valves opening and closing. Otherwise, you’re going to be adding a bypass pressure differential valve.
• Use integrated pump check valves wherever possible because they are less restrictive than integrated checks. Less restriction, less head loss.
• Consider a larger pipe size if it falls within recommended velocities over a smaller pipe so a smaller pump can be considered.
• Pumps in series: Head doubles, gpm stays the same.
• Pumps in parallel: Head stays the same, gpm doubles.
• Bell & Gossett System Syzer Calculator Wheel.
• Bell & Gossett System Syzer Software.
• John Siegenthaler’s Hydronic Design Suite.
• John Siegenthaler’s Modern Hydronic Heating textbook, 3rd edition.
• Caleffi’s idronics Issue 16.
• Taco Technical Documents 9 and 10.
Next month, I’ll break out some more bullet points and we’ll dig into the other critical ingredient of a successful hot water space-heating system — fire.
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