The kid called with a pump selection question while I was waiting in my car at a job site. I was way early because of the lack of traffic, which is weird side effect of so many staying home. At least I think it was a pump question. I really don’t remember some things too well anymore. When I asked him what pump he would pick, I realized we have a long way to go. He was on the right track as I semi-recall, but needed a refresher.

To have a proper class, the teacher and pupil(s) have to have the same materials. He was at his home office, with access to everything since he is young and has tech skills. I’m in my car with my messenger bag that doubles as my utility belt (like Batman’s). It has my tape measure, level, legal pad, etc. Part of the etc. is my favorite tool, a slide rule; more specifically the B&G System Syzer Calculator. I asked the kid if he had his System Syzer with him. The kid did.

Now we had the common teaching aid to use for this specific teaching moment. The problem with his understanding of the pump selection seemed to be with temperature difference, or Delta T as we call it in the wet heat industry. The System Syzer is a slide rule turned into a circle by an icon of the industry named Gil Carlson. I was first introduced to this tool by my instructor at the B&G Little Red Schoolhouse, Bob DeWyze, in the mid 1980’s. I was very green and didn’t understand its importance at the time.

Mr. DeWyze had us using it for pipe sizing, which is only one of five scales, which all have multiple uses. It was years later that I started discovering the uses of the other scales. Today I feel pretty confident with all the scales, but understand that I don’t understand it like Gil Carlson did. He also is credited with developing the concept of primary/secondary piping and the all-important relationship of the expansion tank connection to the pump for best air control.

The hot water heating industry is built on the foundation of understanding that Gil Carlson laid in the 1950’s. I teach these concepts on a regular basis and use them to troubleshoot systems. I was going to start with the kid on the concept of Delta T, which is Scale #1 on the System Syzer. This is the scale that simplifies the equation for flow in GPM based on BTUs and delta T. If you can estimate BTUs and the temperature drop across the system, it estimates the flow. I say estimate, because everything can be an estimate. In this industry, estimating is okay.

So with the kid, I wanted him to start to understand how water flow has a relationship to the temperature difference the water experiences as it moves through the system. Basically, you increase flow to lower the temperature difference. Our industry has adopted a 20 degree F temperature difference because it is so easy to estimate. If you have a 100,000 BTU boiler, you need 10 GPM of flow, or 9.96 GPM to be specific. That’s a whole .04 GPM difference. Again, the estimate of 10 GPM works, if you want a 20 degree delta T.

I asked the kid where the pump was going to be used because different systems are designed around different water temperature differences. A radiant panel in a bathroom floor uses a different design delta T than a unit heater in a warehouse. He said it was going in a residential radiant system. That application uses a 10 degree delta T, not the typical 20 degree delta T. As water moves through the radiant panel, we want the water to experience a small change in temperature, so the panel itself feels about the same temperature as you walk around in your bare feet. You can argue the point of tubing layout to minimize surface temperature differences, but in a replacement pump situation, you don’t know how that tubing was patterned. In new construction, we still don’t know exactly how the installer will put it in.

He had the heat loss calculation for the space on his computer program, so I had him line up a ten degree delta T over that 50,000 BTUs loss. The System Syzer wheel shows a GPM flow rate of 10 GPM. I then had him line up the industry standard of 20 degrees over the 50,000 BTUs. Predictably it showed 5 GPM. Less flow equals more temperature drop. BTUs divided by 10,000 is the rule of thumb. Simple enough, but based only on a 20 degree delta T. Again, for residential floors we want less than a 20 degree delta T.

We now had the flow rate but were missing the head loss. Well Gil had thought of that with Scale #2. The kid said that the loop lengths for the radiant panel were about 250 feet average. So I had him line up ½” type L copper pipe in the upper window. Gil hadn’t thought of PEX pipe when he developed the System Syzer, so I use copper pipe as the best estimate. There were 10 loops for the 10 GPM, so each loop would see about one gallon per minute of flow. The piping friction loss per 100 feet of pipe shows 2.5 feet in the lower window. The 250 feet loop length is dived by 100 to give us 2.5, then multiplied by the 2.5 friction loss gives us an estimated head loss for the whole manifold of 6.25 feet.

The pump would then be sized for 10 GPM at 6.25 feet of head, plus the head of the rest of the circuit. The boiler was piped as primary/secondary, so the rest of the circuit would see minimal loss unless the distance to the manifold was high. In this case, the manifold was in the same equipment room as the boiler. We decided on a pump selection of 10 GPM at 8 feet of head.

I then had the kid work the math for the same application for a 20 degree delta T, to hopefully show how much things change. The flow rate drops to 5 GPM, and predictably the head loss drops to 1.75 feet for the manifold. Let’s call it 5 GPM at 4 feet for the pump selection.

The kid seemed to understand the relationship, at least at that moment. Like with me, it will take a few more moments to start to understand the real workings of the System Syzer Calculator, and therefore the real workings of a closed loop hot water heating system. Thanks Gil for being a genius.

*Patrick Linhardt is a thirty-five-year veteran of the wholesale side of the hydronic industry who has been designing and troubleshooting steam and hot water heating systems, pumps and controls on an almost daily basis. An educator and author, he is currently Hydronic Manager at the Corken Steel Products Co.*