Do you know the two items you need and what to do with them to size a circulator correctly? Without a doubt it’s these two items, after poor piping practices that give hydronics a bad name in many circles. If the pump isn’t right, the noise and operation can ruin even the best installed job. Here is an extract from my book Hydronic Systems and I’ll have more to say about that at the end.
Flow rate is the amount of heat in Btus converted into gallons per minute (gpm). The system circulator must move that flow rate and those Btus around the system. You will note that I didn’t say ‘out of the boiler.” That’s because, if the boiler water is warm or even one degree above the system piping temperature, heat transfer has already started. With as little as 1°F temperature difference, heat transfer will occur do to conduction. It’s what engineers like to refer to as Delta T; DT. Heat hates cold and cold hates heat and you will always have heat transfer from a cold surface to a hot surface. Keep in mind that heated air does rise but look at Figure 1 and think about what’s going on.
In Figure 1, we have a box heated to 70°F. Don’t worry about how it’s heated, just that it is! On the top of the box, the surface is exposed to a temperature of 65°F and heat transfer is shown as the number of arrows going in that direction. The left side of the box is exposed to a temperature of 70°F and there is no heat transfer. The right side of the box is exposed to a temperature of 72°F and heat is now transferring from that side into the box. Finally, the bottom of the rectangle. The temperature is 65°F and we have as much heat transfer leaving there as we do on top, or is it the south side? Aha, that’s just the point, it’s not whether it’s up or down, left or right, it’s where the cold is. Okay, so let’s get back to flow rates and look at some examples and how to convert Btus to gpm. Let’s say that you have a boiler with a 90,000 net output, and one 12-foot length of copper fin-tube baseboard rated at 550 Btus per foot.
The conversion formula is:
Net Btu Hr Load = Flow Rate in gpm
For the boiler example we are using the conversion would go like this:
90,000 Btu = 9 gpm
For the baseboard example:
12 x 550 = 6,600 Btu = 0.66 gpm
The 10,000 number that keeps popping up is a constant for a 20°F DT across the system. Think of it as 180°F supply water out, 160°F return water back from the system as an example. If you work in only residential equipment that number will do it in most cases, if not there are many places you can get the other factors, or give a circulator manufacturer a call. Check out the Web too. Most OEM’s have nice sites full of great information and a few toys too!
Pump head is the resistance of the system expressed in ‘foot of head.” Pump head and feet of head are also referred to as ‘pressure drop.” Are you getting confused yet? Call it what you will, but it’s the resistances created by the piping, fittings and most radiation types to the water being pumped through them. Many heating men get confused about this concept because they think they can fix everything with more pressure. That’s not even close to the truth, as you will see. Be careful to remember that a fan coil type heater such as a so-called ‘kick heater” can have a ‘high-head” of its own. It is not unusual and difficult to understand why some of these heaters will require their own zone. Sometimes a dedicated circulator with head pressures as high as 10 feet is the only thing that will do the trick. Be careful of both framed and ‘A” coils used in hydro-air applications. They can have some serious pressure drop, too.
In most cases, it is very easy to calculate system head and here’s all you have to do: Measure the longest run in feet. That’s the total length. Imagine you’re a drop of water and you have to travel from the supply tapping around the system and back to the return. That’s the longest run. Then you take that number and multiply it by 0.06 and that’s the pump head for your selection. Let’s look at a few examples, and remember by total run we mean the total linear length of pipe that water will have to run to get from the supply outlet of the boiler, through all the pipes and fittings, and back to the return inlet of the boiler. Remember, it’s the combined length of the run, horizontal and vertical dimensions.
In one system, the total piping run is 90 feet of run: 90 x 0.06 = 5.4 feet of head
In another system, the total piping run is 300 feet of run: 300 x .06 = 18 feet of head
Finally in a big system, the total piping run is 750 feet of run: 750 x .06 = 45 feet of head
Therefore, do you still think that favorite little inline pump of yours will work everywhere? Well, let’s see.
Another thing you have to know in selecting the right pump is how to read and understand a ‘pump curve.” In Figure 2, we have such a curve. It is for the Grundfos UPS15-58FC/FRC, Figure 3, and the idea here is to find a circulator that will match your system as close as possible. The perfect or near-perfect pump would be the one at the intersection of both the gpm and foot of head. You won’t believe how quiet a system can be with the right size pump. No cavitation noises, no water noise, no pinging, nothing! Nothing, but quiet hydronic heat. Notice I didn’t say it would eliminate baseboard clicking, that’s either a temperature or air problem.
To read a pump curve you first determine what your system gpm is. Then you determine your foot of head and then look at a pump curve. We have the following examples using five different systems to look at and let’s keep it simple and say that all of our Btu requirements are the same; 100,000 Btus.
100,000 Btus ¸ 10,000 = 10 gpm
In our continuing example the system lengths are as follows:
50′ pipe run x .06 = 3 feet of head
67′ pipe run x .06 = 4.02 or 4 feet of head
Hey, it’s not rocket science.
88′ pipe run x .06 = 5.25 or 5.25 feet of head
Maybe it is rocket science.
110′ pipe run x .06 = 6.6 or 6.5 feet of head
Just round it off to the nearest ¼ foot of head, you will be okay.
125′ pipe run x .06 = 7.5 feet of head
In Figure 4 you can see another pump curve showing a group of pumps called A, B, C, D and E. We cheated some to have fun and you’ll notice that all of them come right out on the line for the previous pipe runs.
1 = E 2 = D 3 = C 4 = B 5 = A
Okay, how about the real world? Let’s say that you had an output of 5 gpm and a head of 6 feet. Well, pump E would put you real close, but C would put you even closer, in fact it would put you right on the curve line and that’s about as good as it gets. If the choices are between a pump above the curve and a pump below the curve, go with the one below the curve. It means selecting a lower velocity pump which means you will eliminate a lot of noise. Another term that you hear a lot of people throwing around and some of us really don’t know the meaning of is the word velocity. Velocity is simply the speed of the water as it travels through the pipe. As you would expect, speed is also a companion of noise. The more speed, the more noise. Just a little common sense, that’s all!
I’ve just finished a major revision to Hydronic Systems (Design, Piping & Wiring) and it features the latest in controls and a lot more. It also includes a whole new section on Primary-Secondary pumping and piping in an easy to understand way and just what is the deal on low temperature emitters. It’s out now and can be found on my Web site.
*George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He operates www. FiredragonEnt.com and is the author of books on oilheating and HVAC subjects. He can be reached at 608 Moose Hill Rd., Leicester, MA 01524. His phone is 508-421-3490, fax at 508-421-3477 and his e-mail is FiredragonEnt@charter.net