The hidden cost of oversizing becomes apparent during the life of the system.

With extremely few exceptions, every new hydronic heating system has at least one circulator. Some larger residential systems might even have a couple dozen. There’s a pump for every purpose; it’s just a matter of defining that purpose and picking the right pump.

All those who design hydronic systems use some method of pump selection. Some make good use of resources such as published pump curves and data to estimate the head loss of a given piping circuit. They find the operating flow rate where the head produced by the pump equals the head dissipated by the piping. They might even consider how upsizing the pipe size might allow them to reduce the size of the circulator.

Others use less analytical approaches such as:

  • This pump worked in the last system, and today’s system isn’t much different, so ...
  • Pump sizing is easy; whatever size pump is in the van is what the system needs.
    Or my favorite:
  • This is the pump that came with the boiler, surely the boiler manufacture knows what pump the system needs ...
Sure, there are times when experienced hydronic pros have a good idea what pump to use without resorting to calculations. With rare exception a 1/25 horsepower wet-rotor circulator will work fine for a baseboard zone circuit consisting of 150 feet of 3/4-inch copper tubing. It’s also likely to fit the bill when connecting a cast-iron boiler to a nearby 30-gallon indirect water heater.

When the system includes radiant heating, larger indirect water heaters, snowmelting or other less routine subsystems, some hydronicians tend to get that “gut feeling” that a little 1/25 horsepower zone circulator may come up short. This feeling is not unlike the one that drives boiler sizes up. It’s based on fear of the unknown combined with the desire to avoid callbacks. The remedy is usually to select a bigger circulator (and probably a bigger boiler to boot). After all, what’s more American than the concept that “Bigger Is Better”?

The designer/installer usually can bury the extra cost of the larger circulator in the job. He doesn’t worry about being questioned on this by the owner. In fact, he might even point this out, expecting kudos for being observant enough to pick the bigger pump, especially if a competitor’s quote is based on a smaller pump.

Formula 1

Pay Me Now And Pay Me Later

In most cases the cost associated with specifying one component vs. another is the purchase cost, or perhaps the purchase-plus-installation cost. For example, a typical 1/25 horsepower circulator costs a contractor about $50 at present. The cost of a larger 1/12 horsepower circulator is about $220. Both wet-rotor circulators have about the same installation cost. The extra $170 for the larger pump, while more than lunch money, can usually be absorbed in the overall cost of the system, especially a large (read $20K-plus) radiant system.

Imagine a situation in which the designer simply assumes that the primary loop pump has to produce a flow at least equal to the sum of the flows of the secondary circuits. I’ve found this to be a relatively common (albeit flawed) assumption. Another misconception is that the primary loop circulator has to be the largest circulator in the system. Equally common, but equally wrong. Not wanting to take any chances, our ace designer opts for the 1/12 horsepower circulator, which draws about 200 watts while operating compared to 80 watts drawn by that wimpy 1/25 horsepower circulator.

Now let’s assume the system is installed in a typical northern climate where the primary circulator operates about 3,000 hours per year. What do you suppose the difference in operating cost is if the local price for electricity is say 10 cents per kilowatt hour?

The calculation is straightforward. Energy (what you pay for) is power x time. For the example at hand, the extra cost associated with operating the 200-watt circulator over the 80-watt circulator can be viewed in Formula 1

At this point you may be thinking that anybody who can afford a $20K-plus heating system can probably scrape together the extra $36 per year to operate the bigger pump. In fact, they’ll never even realize the bigger pump costs them more to operate.

Formula 2

Little Things Add Up

Suppose the pump lasts 20 years. What’s the total extra operating cost over that period? Simplistically (perhaps naively is a better word), one might assume the cost of electricity will remain the same over the life of the pump. In that case the total extra operating cost is $36/year x 20 years = $720! That’s over four times more than the extra purchasing cost of the larger circulator, and we’re not finished.

Let’s speculate that electricity will inflate at 3 percent per year over the next 20 years. Sure it’s a guess, but at least it’s leaning in the right direction. The cumulative cost factor for 20 years at a 3 percent increase per year is 26.87. The difference in operating cost over the 20-year period would be this factor times the difference in cost during the first year:

$36/year x 26.87 = $967

That’s almost six times the difference in purchasing cost between the larger and smaller circulator.

You can make similar cost projections for different values of first-year cost, years of service life and energy inflation rate using Formula 2.


i = rate of inflation (decimal percent) (i.e., 3 percent = 0.03 in formula)

N = number of years in the analysis period (Note: this is an exponent)

First-year cost = the extra operating cost in the first year ($)

For example, if the extra energy cost in the first year is $25, and the cost of that energy is inflated at 4 percent per year for 15 years, the total extra cost at the end of 15 years is demonstrated in Formula 3.

If you really wanted to get picky, you might even figure in the slightly higher mortgage payment to finance the extra $170 for the larger circulator.

Formula 3

Ignorance Is Bliss

I hope you’re convinced that the real cost of oversizing a circulator is not the extra first cost, it’s the accumulated operating cost over the life of the circulator. My guess is that less than one in a thousand of your customers would ever realize this situation. I would also guess that less than one in a million of you would ever choose to bring it to their attention. So, why play this numbers game?

Many of you reading this consider yourselves hydronic-heating professionals. Professionals seek to do the best job possible for their clients. Part of that responsibility is investigating the real cost of various design decisions and using that information to reduce their clients overall owning and operating cost whenever possible.

If you’re more of an environmental thinker, take the above situation and multiply it by however many thousands of oversized pumps you estimate are installed in all kinds of hydronic systems every year. You might even take the wasted electrical energy and back it up through a power plant operating with a fuel-to-electricity conversion efficiency of about 33 percent. I don’t know what those numbers are, but it’s likely the total would be eye opening.

Do The Curves

Will proper circulator sizing save the world from running out of conventional energy? No, but it certainly is part of the picture. A part we have huge influence over. A part that a surprising number of our customers would care about if they only knew.

Most pump manufacturers have low-cost or free software, slide rules, or other design aids that let you quickly evaluate the tradeoffs. Some of it is but a download away on the Internet. Take advantage of these resources. Look for situations where slightly higher temperature drops allow for lower flows, smaller circulators and, most importantly, lower pumping cost. Consider using one-size larger tube in situations where you drop a circulator size. Minimizing the “parasitic energy” required to push water flow through hydronic systems yields some remarkable savings over the life of the system. Many of us promote energy savings as a benefit of hydronic heating. Let’s be sure we deliver on that.

Siegenthaler At ISH NA

John Siegenthaler is a scheduled speaker at the ISH North America trade show debuting in Toronto. John will discuss "Injection Mixing: Concepts and Applications" on Friday, Nov. 1. Part I is at 10-11:30 a.m., and Part II runs 2-3:30 p.m. To register for the show, visit