In residential hydronic systems, circulator technology has been on the march in recent years. Newer ECM circulators are reliably delivering wire-to-water efficiencies double that of the older PSC circulators they aim to replace, and they bring modulation to the table as well, which slashes power consumption even further by turning the circulator into a reactive system that can slow down or speed up in response to current conditions. In many cases, a variable speed ECM circulator can promise energy savings of 75% or more compared to old-style, fixed speed circulators, and eliminate the need for pressure bypass valves to boot!
Does this mean every pump in America should be ECM? If the goal is to minimize energy usage at any cost, sure, but to most people economics plays a role, as well. To decide if you should use an ECM pump in a given application, I suggest doing a fairly simple analysis to settle the issue with confidence, so you can be sure you're doing what's right for the system and the client!
First, if using such a pump can eliminate a pressure bypass valve, it just paid for itself, so do it! Gosh, that was easy, wasn't it? Pressure bypass valves cost about the same as the upgrade, plus they take labor to install, so eliminating it is a win win.
Second, the same goes if you are willing to eliminate a multi-zone relay, taking advantage of the ability of these circulators to dead head at very low power usage indefinitely. You don't even have to turn them off… the 5 watt minimum draw of most ECM circulators won't overheat and is about the same as the parasitic electrical loss of a transformer in a zone relay. That's nice, but we prefer to keep our zone relays as they keep wiring clean and give simple feedback as to system operation.
If you didn't need a bypass valve in a given application, and you want to keep your zone relay (or didn't need one anyway), then you need to know:
1. Your cost per watt-hour for electricity.
2. The expected watt reduction from using the high efficiency pump.
3. The expected run time of the pump.
Cost per watt-hour is pretty straightforward: take your total cost per KWH from your electric bill and divide by 1000. Here in Maine, it's typically $0.13/Kilowatt-Hour, or a cost of $0.00013 per watt-hour. Make sure you get the whole cost including generation AND distribution if you are in an area where these are separate on the electric bill!
Expected watt reduction is more of a guesstimate: however, I would suggest assuming 50% reduction in a fixed speed application and 75% in a variable speed application, and most applications are variable speed. Given the ECM circulators usually available at this writing, we are usually replacing a roughly 90 watt PSC unit so you can usually figure either a 45 watt or 65 watt savings for round numbers.
Run time is the last variable, and this depends heavily on your application. For typical heating system pumps and boiler pumps, I would typically suggest run times of 50% for fixed temperature systems, 75% for outdoor-reset systems, and 100% for constant circulation systems: all of this for the duty season only, of course, either winter for heating or summer for cooling applications. Other pumps require different run times of course: pumps serving residential indirect tanks don't run much and probably are never worth switching out for high efficiency models.
So if you have a six month heating season with a fixed water temperature (50% run time), that's 6 months x 30 days a month x 24 hours a day x .5 = 2160 hours per year!
Once you know all this, the equation is simple:
Cash Savings = (Run Hours) x (Watt Reduction) x (Cost per Watt). Remember that's cost per watt, not the cost per KWH on your electric bill!
So for my fixed temperature, variable speed circulator, saving 65 watts in maine at $0.00013/watt, that would be:
(2160 Hour Runtime) x (65 Watt Reduction) x (0.00013 cost per watt) = $18.52/year. Not bad, that's about a five year payback for the upgrade. Put that same circulator on an outdoor reset water temperature system with a 75% runtime, and savings increases 50% to $27.38… less than a four year payback. And in a constant circulation system? $36.50, less than a three year payback!
Next time, we'll investigate when to choose variable-temperature vs variable-pressure ECM pumps. These two modulation methods have very different applications and each has a proper role in our systems moving forward. Until then, pump away!
Rob Brown is co-owner of Northeast Radiant Technology, a high performance hydronic design and supply company that has been in business since 2002 serving clients across North America. He currently enlists his 15 years of industry and design experience to work deploying low energy systems including NRT's home base, a net-zero radiant heated and cooled shop using 100 watts of distribution energy for all air, dehumidification and heating/cooling. Contact Rob through www.nrtradiant.com or via email email@example.com.