Variable Speed Pump-O-Rama

With a modulating boiler and a constant flow rate, as the modulation of the burner decreases and the btu’s decrease, the Delta T decreases. Less input – Same flow – Lower Delta T. You only notice that if you’re a geek and the boiler gives you that info.

Mod-cons get their best efficiencies from condensing the flue gases, based on the return water temp. The lower the return water temp, the higher the efficiency. You all know that, but I want that fact to sink in because I will talk about that for years to come.

Anyway, we learn from the good stuff that happens, and how to repeat it. We can also learn from our mistakes. That industry rocking idea was to use a variable speed pump as the boiler pump for a mod-con boiler. The pump was new on the market and varied its speed based on Delta T. I was observing that mod-cons were rarely operating at 100% modulation for space heating, therefore in my misguided theory, since the pump didn’t need to be at full speed, if we slowed down the pump, it could help peak the efficiency from less electrical energy used and from lower return water temps to condense flue gasses.

Somehow I sold this idea on a couple of jobs. However, as Mick sings, you can’t always get what you want. We didn’t blow a fifty amp fuse, just caused numerous high temp lockouts. The mistake of applying a variable speed pump to a modulating burner seems obvious in retrospect, but most things do. The speed of the pump and the modulation of the burner have to be varied by the same control. Without that coordination, if the pump is slowing down as the burner is modulating up, the boiler control senses that sudden rise in temperature and locks out on high temp, like there’s no flow.

Some boilers now come with the built-in smarts to allow the pump speed to ride with the burner modulation rate. One control telling two varying devices to play nice together. That’s all I wanted, but I didn’t know it at the time.

Another application for a variable speed pump is multiple two-way zone valve systems, or a system with changing flow rates. Photo A shows a gaggle of zone valves to the right of a variable speed pump. This generation of variable speed pumps can internally sense how many valves are open or closed and adjust its speed accordingly.

At this shack, which looks like stately Wayne Manor as you drive up, each zone valve controls flow to back-up hot water coils in geothermal air handlers. Geothermal was all the rage in this high-end neighborhood years ago and during the previous gutting of the house, a steam system was removed—after I had spent my career keeping it working. Down in the Batcave, the old boiler room became a home theater, complete with a sloped floor since it was a half story below the basement level to allow gravity return of the condensate to the old steam boiler. I can’t make this up.

This fall, a contractor asked me to meet him at the job to discuss adding a pool heater to the recently installed high efficiency hot water system. I recognized the place when I got there, but was confused by all the construction activity. It turns out a new owner is remodeling the place, once again. You have to love rich people, all their remodeling keeps the economy going.

We determined that adding another zone valve to the existing gaggle would work. My concern was the pressure drop through the new pool heat exchanger, since sometimes that can surprise you. Fortunately, the existing variable speed pump develops enough head pressure to overcome the researched loss through the heater.

In another application at the opposite corner of the same tony village, we used a variable speed pump to replace existing zone pumps. See photo B for the pile of pumps that were taken out. At this job, the pumps were on the return side and replaced by two-way zone valves with end switches, wired to relay panels. On the supply side we added an in-line air separation device and the variable speed pump, with the expansion tank connection on the inlet side. (See the diagram for the piping concept.)

This variable speed pump was fully adjustable, with about six modes of operation. We started the pump in the automatic adaptive mode. It somehow “learns” how to speed up and slow down, but it can take a few days. After a week of weak performance, the pump was switched to the constant pressure mode. Where I don’t know the ins and outs of the auto adapt mode and its algorithms, I can say that multiple pump manufactures recommend the constant pressure mode for multiple zone valve applications. The mode switch did the trick.

So I took a broad view of variable speed pumps to get you excited about the wide application for this latest and greatest technology. Talk to your pump rep about what wiz bang variable speed pump they now offer the next time he or she stops in with the donuts. Next month we go back to the basics, static fill pressure. Gee that sounds exciting.

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.