A trip back in history
By Paul Rohrs, Radiant Expert
Biggerstaff Radiant Solutions, Lincoln, Neb.
Rod Serling in his best Twilight Zone voice: You’re traveling to another dimension, a dimension not only of sight and sound, but of mind — a journey into a wondrous land whose boundaries are that of imagination. Your next stop, the “Twilight Zone.”
Recently I was asked to write about the 10-year anniversary of Phc News, Plumbing Engineer’s sister contractor publication. In 1999, Phc News’ inaugural addition was distributed, so let’s talk hydronics from 10 years ago. But first, here’s some more information that will help you look back in time and see what was going on.
According to the Energy Information Administration, the average price of gas in July of 1999 was $1.19 per gallon. The Blair Witch Project was scaring people in movie theatres and there was a very prominent three-letter alphanumeric character on everyone’s mind: Y2K. What was going to happen? Were we going to experience a global shutdown of computers and peripheral equipment? Were stoplights going to stop working and cause gridlock? Or would the water supply shut down and basic needs not be met for people nation and worldwide? We all held our breaths as the ball fell.
Does all of that seem like it was 10 years ago? In 1999, hydronic design and sales were really starting to boom in the Midwest. As a wholesaler and radiant designer, things were going fairly well. I had bought a new software program called Visio so that I could view and alter Tekmar drawings that seemed to emulate an interesting industry standard of mixing called variable-speed injection-mixing.
Before the advent and market saturation of mod-con boilers, hydronic systems typically had atmospheric boilers that were subjected to high temperature boiler water that could then be mixed down to system requirements and even lower temps when not at design conditions. Radiant floor heating with high mass concrete slabs could have system temperature requirements in the mid- to lower 90°F ranges, and that is at design conditions. So, how to safely protect boilers that required 138°F boiler inlet water and produce lower system temps? Enter VSIM. It can provide minimum boiler inlet temperatures, full outdoor reset for the type of heat emitters you have in the system, and can meet the demands of residential, commercial, and snow melting systems.
Let’s look at the basic system set up of a variable speed injection loop and how we can use it to meet multiple zones of a single temperature system. If you’ve learned anything about me it’s that I like to use color to show differences in my drawings. In Figure 1, we see the primary boiler loop in the green square (also marked “A”) showing an atmospheric boiler that is designed to provide 180°F fluid. The P1 circulator is pumping away from the air-separator and expansion tank and pumping right back into the boiler inlet with a wye-strainer. To the far right of the boiler loop is a secondary loop with a blue background marked “B.” This is our load side that shows the P2 circulators providing heat to our panel radiators with thermostatic radiator valves (TRVs). Now the P2 circ is pushing our 130°F reset temp water thru the panel radiators giving up their heat and coming back at 110°F to be reheated and sent back out again. How much 180°F boiler water do we need to inject from section A into section B, to get it back up to the necessary 130°F water? Based on our math and design conditions, we need 2.85 GPM. Our variable-speed injection-mixing loop that connects the primary boiler loop and secondary loop of panel radiators is in the orange rectangle. The P3 circulator that connects our primary boiler loop (A) to our secondary loop (B), needs to spin fast enough to provide 2.85 GPM of 180ºF water back into the loop to bring it up to 130°F. I am setting a 100,000 Btu/H load at design conditions.
How fast does the variable-speed injection-mixing circulator have to spin to meet the load at design conditions?
Ps = Primary Supply (temp)
Sr = Secondary Return (temp)
GPM = Load
Ps – Sr 5 500
GPM = 100,000
180-110 5 500
GPM = 100,000
35,000
= 2.85 GPM @design conditions. That’s right, the maximum speed in which the P3 circulator will need is 2.85GPM.
With a proper deployment of a VSIM control, an outdoor sensor will send a signal to the controller so that it knows what the outdoor temp is. Additional sensors are placed on supply and return piping to monitor what the minimum and maximum system temperatures are. Minimum and maximum outdoor air temps are also programmed in. The reset curve is the net result of this programming and automatically adjusts the target temperature for the heat emitters; in this illustration it is the panel rads. A weather-responsive system is really appropriate for load matching in any condition, for any heat-emitter and boiler type. The feature I encourage you to explore further is that of boiler protection. With atmospheric boilers, we need a minimum of 138°F boiler return water to prevent sustained flue gas condensation. With VSIM, the boiler loop will now have 138°F water entering the boiler before the P3 circ will start injecting hot water into our secondary loop side. That is pure boiler protection that you can bank on if you choose atmospheric boilers or mod-con boilers are not in the budget. When this system is not at design conditions and there isn’t a 100,000 Btu/H load present, the target-reset temp is lowered and the P3 circulator does not need to spin the full 2.85GPM. Enter the variable portion of variable-speed. In anything other than design conditions, it is likely that the P3 circulator will spin at an even slower speed, still injecting only what it needs to reach the target temp. At 40°F, say we only need 110°F water to the panel rads, and at a 20°F ĘT, that equals a 90°F return water. Use the formula listed above to calculate the GPM that the P3 circ will need. Email me your answers: paulrohrs@neb.rr.com.
There are several recommendations you should consider when sizing and deploying this VSIM circulator. First, do not use a circulator with an integral flow check. It can cause surging and not provide a smooth secondary loop reset temp. Secondly, it is advisable to pipe a thermal trap on your injection piping if you have multiple VSIM loops feeding different temperature zones. Thirdly, consider using a ball valve for isolation or a globe valve for isolation and flow setting on the return leg. If you need to isolate the boiler loop from the secondary loop, this is an easy way to keep them separate. Finally, if you are a visual person and need proof that the P3 circulator is doing its job, you can opt for a visual flow indicator. This item has a spring and plunger combination that shows you the actual flow rate of the P3 circulator. When initially setting up a system, it is very easy after the purge and fill process, to hard wire the P3 circulator with the correct voltage and then set the flow with the adjustments provided. After the maximum flow is established, wire it in to your VSIM controller.
VSIM is a tried-and-true mixing method that can always provide a full-reset temp to any type of heat-emitter regardless of the type of heat source. Even though it was state-of-the-art ten years ago this method of mixing in systems are now very common to see out in the field and I will say that VSIM still has an important place in modern systems still being designed.
Why travel into the “twilight zone” when you can create your own zone of variable-speed injection-mixing and know that you can use your imagination to define new boundaries in hydronic heating. Take that Rod Serling.
Paul Rohrs welcomes your comments. Contact Paul at paul@biggerstaffradiantsolutions.com