We have witnessed many technological breakthroughs in the past fifty (50) years or so. My engineering career began manufacturing the first transistors, through integrated circuits, personal computers, parallel-processing supercomputers and their logical applications. Now witness the plethora of personal, mind-bending “high-tech” devices available to almost everyone. Similarly there are advances in energy, transportation, communications, etc. that will derive our future.

OK. So what’s this Hydronic Revolution? Hydronics is the use of water as a medium in heating (or cooling) systems. Thus a Forced Hot Water (FHW) System is a hydronic system. As simple as they seem, FHW Systems need to be carefully designed to maximize their performance, both heating-wise AND electrically! Unfortunately most applications are incorrect hydronically and virtually all electrically, nor can they be.

Heated water generated by a boiler must be pumped through “radiation” (baseboard, radiators, heaters, etc.) at an idealized rate to suit the heating demand. Pump it too slow and you have a wide temperature variation and “lazy” performance. Pump it too fast and you create noise. In both cases you waste energy, particularly electrical. The pump (circulator) runs necessarily too long, or too hard. In any case it uses too much electricity to execute the heating demand.

Enter the Variable Speed “Delta-T” Circulator some years ago. (Reference our Blog Library topic item of two (2) years past.) It measures the input and output temperatures of radiation piping and adjusts its pumping rate (speed) to maintain a “Delta-T” (temperature differential), usually set to 15-20°F with further adjustability to suit varying applications. It was a great advance by Taco, the premium American Hydronics Manufacturer with variants produced by others (U.S. & Foreign). It has not really ever “taken off” though, finding a niche in radiant heating and other problem solving but little else to date, until now.

Taco has again “upped the ante”, and really upped it this time! Their newest offering, the Taco BumbleBee® is a marvel of Applications Engineering. (It even looks like a Bumblebee.) An “Intelligent Circulator” is now an apt, if not a modest description. Kudos to our fellow engineers!

Referring to the Taco BumbleBee® Specification Sheet you can appreciate the sophistication of this device, and its flexibility to suit varied applications. It can do so much in fact that we must limit our discussion to its application as a simple system circulator, the objective of this blog. It is also in this “Delta-T Mode” that this BumbleBee® really “stings”. (Pardon the Pun, please.)

Of equal or greater importance is what is not explicitly referenced in the Taco BumbleBee® specifications, how it attains its intelligence. Simply stated, the circulator’s logic “learns” its application pattern over a period of seven (7) days, placing it in memory and executing it in the successive period. While executing, it continuously “observes” the pattern variations and adjusts its memory accordingly. Thus your programmable thermostat(s) cycles and heating load variations can be patterned and satisfied efficiently.

Similarly, observing pattern changes such as the length of a heating demand allows the Taco BumbleBee® to empirically determine the relative outside temperature, emulating an Outdoor Temperature Sensor Option function. Being a passive measure, it would lag the correction time of an outdoor sensor, but to what overall effect?

Utilizing a single system and/or distribution circulator such as the BumbleBee® vs. multiple fixed-speed circulators does require controlling individual thermostat demands via Zone Valves. Here again Taco has met the challenge. Again a little background is helpful.

Taco is the undisputed giant of the residential hydronic zone valve market. Their “Heat Motor” Zone Valve has dominated for decades and is available in most hardware and “box” stores as is their Model 007 Circulator. The combination of these power and control most residential hydronic systems, however poorly executed they may be. Referencing our Blog Library on this site (and on the internet in general) will accent how much print is given to addressing hydronic distribution issues!

The new Taco “Zone Sentry®” Series Zone Valves are the “next generation” product to replace the old “Heat Motor” (energy hog) valves and the ultimate companion to its BumbleBee™ circulator. They are both energy-efficient and intelligent by design as well. Whereas the old “Heat Motor” and “007” dominated and created their renowned application difficulties, the new Taco Duo of Zone Sentry® and BumbleBee® are their long awaited successors.

We among many other heating bloggers have derided “Zone Valves-Circulator Systems” as being an arguably cost-effective alternative to individual circulator-driven systems, and with justification. We in fact chide that zone valves-circulator systems “do nothing well”. True for the old, but not for the new generation!

Thus, the titled “Intelligent Delta-T Circulator FHW System” consists of a Taco Bumblebee® Circulator and Taco Zone Sentry® Valves coupled to a high-efficiency hydronic boiler for maximum performance. We have now and for over a generation specified and used Weil-McLain Boilers in our applications with absolutely no regrets. Weil-McLain is recognized as the premium All-American Manufacturer, with a growing market share. Their Warrantees are fabled.

So, to quote the old adage, “How do we get there from here?” From our perspective it’s all about education, particularly within the residential and light commercial market sector. You, the now hopefully more informed consumer, DIY (Do-It-Yourselfer), contractor, “week-end warrior”, tradesman, property manager or developer must drive this point forward and in your interest.

System specification is now relatively simple:

  1. Your total heating demand as determined by a heat loss calculation.
  2. A boiler selection by size and fuel type to suit.
  3. An intelligent Delta-T Circulator sized to your total heating demand.
  4. Energy-efficient Ball Zone Valves for your systems distribution controls.


  1. If your boiler selection has a “system loop circulator” you will employ the Delta-T System in series with it, per the manufacturer’s specification. (High-mass boilers typically don’t require a system loop.)
  2. If you wish to generate DHW (Domestic Hot Water) with your system, an Indirect Water Heater will be required. (You’ll never regret it.)
  3. The new generation Zone Valves typically have built-in diagnostics and indicator lamp(s). You don’t need a Zone Valve Controller, as long as you can see the individual lamps. Redundant cost.

Your challenge will frankly be to find an installer (if required) to execute an effective solution. At our peril, we suggest our Blog “PLUMBING GUYS PLUMB, HEATING GUYS HEAT” as a reference source. Find an “old dog” or the “new dog” that has learned the “new tricks”. There are a lot of “Joe the Plumbers” out there that will cost you plenty in both installation cost and system performance. SPECIFY AND DEMAND. SHOP SMART.

Quoting Cesar Chavez 1927-1993 (American Activist and Labor Organizer): “We have seen the future, and the future is ours!”

Mercier Engineering has seen its future as well. We have and are developing a series of “Intelligent Delta-T Circulator FHW Systems” PRODUCTS built upon Weil-McLain Boilers, Taco Bumblebee® Circulator and Taco Zone Sentry® Valves that will provide the user with a very significant cost-performance advantage to built-on-site alternatives. They are a pre-built, assembled hydronic boiler package, ready for expedient on-site installation, with proven performance.

Our dedicated website is currently under complete re-construction to incorporate the latest technological innovations. It will be a significant resource to this end. Meanwhile please excuse us while we work and take advantage of this site’s Heating Blog Library.

Regardless, we have hopefully made you aware of hydronic FHW Heating progress and how it will definitely affect our immediate futures.