Beyond AFUE: Exploring the real meaning of heating system efficiency

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With the introduction of super-efficient, variable speed, ECM-driven Delta-T circulators for residential use, I’ve taken a keen interest in selectively applying them for use in radiant heating applications. The results have exceeded my expectations. Bear with me just a bit and I’ll explain what I found out after several installations.

As just a bit of background, I’m . . .

But before I jump into the an explanation of my “study,” I’ll add that I’ve also been aware but haven’t yet tried to use non-ECM, variable Delta-T circs. I know they’re out there, but I’ve simply not tried them.

I’ve always appreciated and prefer Delta-T systems. And, by using a Delta-T circ with a super-efficient ECM motor, you get the best of both worlds – the best pumping strategy with greatest efficiency.

With a ΔT variable speed circulator, the pump varies its speed to maintain the designed-for ΔT. That means the Delta T will always be 20 degrees – or whatever you dial it in for (5 -50°) – even with heating load or outdoor temperature changes.

A circulator changing its speed based on ΔP, however – whether the ΔP is dialed in based on estimated system head loss or is automatically selected – will vary its speed to maintain a fixed system pressure differential. The system ΔT will fluctuate, often decreasing.


How’s a reduced ΔT affect the system? Consider the impact on a modulating-condensing boiler. If the system is designed for a 20°F ΔT, but gets only a 12-15°F ΔT, the amount of run-time the boiler spends below the point of flue gas condensation will be affected.

If the boiler is supplying heat to radiators, and the boiler’s reset control is telling it to fire to a high limit of 142°F on a 20°F day, a ΔP circ programmed on an estimated system head loss may wind up sending 130°F water back to the boiler. That’s right at the condensing point, making the boiler work at, say, 86% AFUE.

But a circulator programmed to deliver a 20°F ΔT will send water back to the boiler at 122°F, creating more condensate, allowing a boiler to hum along at 89% AFUE.

For the jobs I’ve been doing lately, we opted to see how far and effectively the ECM ΔT circs could perform as a stand-alone system circulator, co-joined with zone valves to govern flow to any number of hydronic zones.

Having now designed and installed several oil-fired ΔT-based distribution systems initially Taco’s VT2218 circs, coupled with Zone Sentry zone valves, we’ve monitored and serviced them for close to two years now. They all live in variously-sized and aged single-family homes, from a 1,300 sq ft 50 year old house, to a 2,800 sq ft 10 year old home. All are identically-sized systems within our 7,000 degree-day New Hampshire zone.

There’s an on-going discussion within our trade community about AFUEs, particularly in what they do not measure related to system performance. “Idle time” or “stand-by” losses are logically presumed to be non-contributory and detractive. Focusing only on boiler AFUE (annual fuel utilization efficiency) as an accurate depiction of system efficiency is flawed.

AFUE is only one of many indications of system performance. Most heating systems are affected by a wide range of variables. We also learn, by reading about how these tests are performed, that accuracy is very difficult to achieve, so we’ve done some research of our own. And, yes – inconsistency plagues even the best attempts to achieve accuracy; we confirmed it through our own field tests.

From our observations, there are five key elements contributing to total system energy efficiency:

  1. The boiler (heat engine) energy conversion efficiency or AFUE.
  2. The physical attributes of the specific boiler complimentary to system operation.
  3. Efficiently moving heated water to the zone distribution point(s).
  4. The effective matching of radiation elements to heating demand.
  5. The control algorithm(s) to match energy creation with varying system demands.

All of our initial efforts have been with oil-fired hydronic systems and is the focus of this writing. However, much of this effort – what we’ve learned – is applicable to other-fueled hydronic systems.

The ability to vary the output (energy creation rate) of heating equipment plays an important role. This has been achieved in gas-fired boilers by “modulating” combustion with sophisticated valving and controls. Typically they adjust from 20 to 100% of capacity – from “idle” to “full speed,” using an automotive analogy.

But there’s a challenge: direct modulation of oil-fired systems isn’t feasible under normal circumstances. A fixed (capacity) firing rate via pressurized, nozzle induced fuel atomization is the norm. The only option is to adjust the operating temperature of an oil-fired hydronic boiler with controls to meet heat demand. This is reasonably well managed with modern “cold-start” boiler aquastats, external temperature sensors, etc.

Referring back to our five (5) elements to total system efficiency, circulation is number three (3) on the list, but really is the foundation of any hydronic system improvement. Taco reports that their system “Delta-T” circulator-only swaps yield up to 15% fuel and 85% electrical usage reductions.

Unfortunately, we do not have the benefit of data recording equipment, so our observations are admittedly empirical; that is: based on and verifiable by observation or experience rather than theory or pure logic.

I’ll point out, though, that we have had the benefit of developing and operating our personal dual-fuel (oil-wood) hydronic system for the past 40 years that features a wholly convective inter-system loop (no circulator) and manually controllable convective zones.

No electricity; no problem. This experience reflects into our near-boiler piping configuration that optimizes hydronic convection, complimenting and idealizing Delta-T delivery. Ironically it typically reduces floor space, piping, valving, wiring and controls to maximize element no. 3 as well.

The implication in our AFUE boiler attribute argument (element no. 2) appears to be the benefits of a high-mass boiler as related to overall system operation. Our boiler-of-choice has been the Weil-McLain Ultra Oil with the Beckett NX Burner for the past ten years. (Only one “no heat” service call, a failed Honeywell Aquastat. W/M has since replaced it with the Hydrolevel 3250-Plus.) It also happens to be “The Heavyweight Champion” at over 600 lbs for a 3-Section, 100KBTUH, 87% Triple-Pass Boiler. A 30+ year economic life would not be unreasonable from our experience with this and other oil Weils.

Our observations are:

  1. Dramatically reduced burner cycling, extending boiler and component service lives. “This thing seldom runs” is the first customer observation “and is so quiet.” (Compared to their prior unit, obviously.)
  2. Multiple individual zones cycling between burner cycles, drawing from thermal mass storage (iron and water).
  3. Reduced average boiler operating temperatures.
  4. A pressure-fired burner seems to stabilize operation under particularly “cold-chimney” conditions, a frequent event in our “frosty north” external chimneys.
  5. “Cold-shocking” seems to be a non-issue considering 3250-Plus operation and system circulator fail-mode convection.
  6. The “close-coupled” HTP SuperStor Ultra acts as an integrated boiler protection device, being the closest path in fail-mode convection.

Our fourth element, radiation has become particularly accentuated both by our personal and Delta-T Beta System experiences. Our “Beta” Customer called in May to advise that his home was gradually cooling and noted the Viridian VT2218 was flashing and indicating an error code, but he had adequate DHW. Upon arrival all (3) Zone Sentry Lamps were ON (2 Heating & 1 DHW) and supply lines were at system temperature. After a few minutes the DHW Zone Lamp went out and remained so, indicating demand satisfaction. The main level above return was above ambient @ approx. 110/120°F. The second upper level was at ambient.

Note: We do not use a flow-check valve in our system circulator or beyond. The Zone Sentries directly control all heat demands.

This “Beta” is the 10 year old, 2800 sq. ft., well-constructed and insulated 2-storey residential home in our field profile. A failed steel-plate boiler was replaced with our High Mass Delta-T System. The main floor radiation is a single ¾”-piped series perimeter baseboard loop that we would have split-looped, given the option. The second upper level is likewise an up-and-return ¾”-piped perimeter loop. It has a full-height 20 ft + exterior chimney that exhibited sooty startups.

For reference the system was installed in late December, 2014 and a follow-up in early January noted the following:

  1. Exterior (daytime) temperature was 15°F.
  2. The UO-3 Boiler & Beckett NX cycled 4 minutes an hour total.
  3. The main zone demanded twice an hour.
  4. The upper zone demanded one and one-half per hour.
  5. There were no DHW demands in the total three hour observation period.

Taco was advised of their VT2218 failure and approved an immediate warrantee replacement. The defective unit was directly returned for autopsy and Taco determined a “fluke” electrical component failure. The experience nonetheless was a blessing in gaining some very useful field data. These and other field issue notes were passed on to Taco, and in particular startup and ramp-up issues reflected in the “BumbleBee” HEC-2 and the Viridian VT2218. Taco advises some of these are incorporated into the ramping profiles of their VT2218-HY1-FC1A-01, re-released on 9-1-2015.

Thus far we have seen no purpose in deviating from the default settings of neither the Hydrolevel 3250-Plus (on the Weil-McLain UO Boiler) nor the Taco VT2218 Circulator Logic in Delta-T Mode. Despite being only passively coupled, they seamlessly satisfy heating demands. There is obviously some damping effect from the high-mass boiler. We likewise utilize no external control logic or relays, simplifying wiring and diagnosis.

Physical radiation and configuration has been outside our scope of development, yet ultimately completes the heating “package”. If our concern is finite efficiency then radiation schemes beyond basic series and split loops must be addressed. Ultimately it’s the market’s call — the more finite the distribution and control, the greater the materials and labor consumption, and the system life-cycle operating costs.

Residential Delta-T Hydronic Distribution is not only an evolutionary but as we express a revolutionary process from our developmental experience. It will benefit any FHW system in degree, and thus not a question of if it will become generally appreciated and applied, but when. Adding thermal mass into the equation further enhances system performance in our humble opinion.

The “AFUE War” between boiler manufacturers and their installers has been substantially nullified with the addition of the enhanced fuel and electrical hydronic distribution efficiencies provided by Delta-T Technology. With Taco Engineering concurrence we will be claiming the most efficient heating system from combustion to distribution points on the market, as installed.

To employ our quip, “We are putting ‘Automatic Transmissions’ on Boilers.” When will our trade brothers follow?

Author: Paul D. Mercier, Sr. d-b-a Mercier Engineering


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