WHAT IS A GRAVITY HEATING SYSTEM? – Gravity Convection Heating Revisited

What is a Gravity Heating System? A century ago all water-based hydronic heating (hot water and steam) employed the natural gravity convection attributes of heated water and water vapor (steam) to distribute energy. NO DISTRIBUTION ENERGY WAS REQUIRED! They were effectively single-zone systems that could only be modulated by varying the energy input of the boiler and the radiation outputs using register dampers or steam radiator vents, respectively.

The three (3) basic elements of hydronic heating are heat generation (boiler), distribution of energy (pumps) and conversion to area warmth (radiation). Of these hydronic distribution is typically the least understood, generally misapplied and desperately needs revisiting!

The introduction of electric circulation pumps in the 1920’s enabled forced hot water heating (FHW) and changed hydronic distribution forever. Gone were the large, pitched piping and radiators, replaced with zoned heating and finned radiation. So while natural (gravity) convection underlies all hydronic distribution, it is never considered in contemporary practice. Therefore check-valving is installed to negate its now undesirable effects. Theoretically still beneficial “free” natural gravity convection is lost.

Early electric circulation pumps (circulators) were large, power consumptive and constructed of discrete components, i.e. motor to coupling to pump. We “old-timers” have vivid memories of failed couplings of varied types, seized and leaking pumps and smoked motors. The advent of wet-rotor circulators was like manna from heaven, reducing circulator issues with greater longevity and reduced power consumption benefits.

The introduction of Delta-T (differential temperature sensing) ECM Circulators has now projected hydronic distribution management to an entirely new level of sophistication. Embedded control instrumentation features and operational data display of delta-t circulators provide us with finite attribute identification and application control.

The focus of our work has been to optimize that innate, suppressed contribution of natural gravity convection as both a distribution energy saver and a selective fail-mode feature in hydronic heating. As such the Delta-T ECM Circulator has been the crucial tool in the development of our “Neo-Gravity Delta-T ECM Hydronic (FHW) Heating Appliance™” (U.S. Patent 10,690,356, Canada Filing 2,964,131). We claim optimization of natural gravity convection within our boiler, near-boiler distribution piping with distribution energy requirements using a single, dedicated Delta-T ECM Appliance Circulator. Citing an automotive analogy, we refer to it as “putting an Automatic Transmission on a Boiler™”. This intelligent, variable speed circulator is effectively a hydronic CVT (Continuously Variable-Speed Transmission) in practice.

Now let’s go back again to that old gravity hot water heating system of a century ago. By comparison, contemporary hydronic heating systems have smaller piping with multiple zones for heating flexibility. The old “gravities” employed high-mass cast-iron boilers with larger piping to modulate heating supply, otherwise control was particularly difficult when using solid fuel firing such as with wood or coal. Generous boiler sizing, distribution piping sizes and radiation elements gravity convection worked fairly well, and again with NO distribution power requirements!

Properly piping a contemporary FHW system using a dedicated “Delta-T Moded Circulator” with complimentary low-energy ball-type zone valves yields great results! Transposing it onto the old gravity system layout and you functionally emulate its performance as depicted in the following figures.

The major advantage is in applying natural gravity circulation (aka “free energy) to the contemporary upgrade. We have seemingly lost all other trade skills in managing or enhancing gravity convection. No consideration is ever given to pitching, compacting and minimizing distribution piping in particular to advantage “Mother Nature”. Additional gains are available in radiation layout by using properly sized and configured perimeter series and/or split radiation loops. The 45° elbow fitting for example saves 30% of piping and reduces head pressure significantly over a 90° elbow run. All the increased pipe and fitting volume and head pressure reduces the natural gravitational convection effect, not to mention substantially increasing materials, labor and lifetime energy and maintenance operating costs of the system.

Our Delta-T Mode Circulator measures this head effect well via its wattage indicator. All of our single, dedicated system circulator Beta Site installs (currently up to 3,000 sq. ft. structures) exhibit an 8 to 13 watt distribution power consumption with typical 4 GPM flow upon a 20° delta-t (adjustable) differential attainment. Compare this to 80 watts minimum for each contemporary fixed-speed circulator. With delta-t stabilization established, you witness circulation wattage steadily decay to half or less as natural convection contributes. We refer to this effect like “paddling your canoe with the current”. Takes a lot less effort ….. and power!

A secondary effect of delta-t managed gravity convection is radiation heating profile modification, smoothing demand amplitude variation and increasing comfort. Some of the extended fuel savings we observe and the delta-t manufacturer claims seem to be due largely to this radiation profiling effect. Yet another contributor is the lowered system operating temperatures of using a very high mass cast-iron boiler vs. contemporary low-mass units. Burner operation cycles are significantly less frequent than the systems they replace, increasing component lives and lowering maintenance costs.

A personal cast-iron boiler observation: This author has never replaced a “cold shot” cracked or magnetite impaired cast-iron boiler in over sixty-five years of hydronic and steam installations! Perhaps a discussion for another day, but have we also “thrown the baby (cast-iron boiler) out with the bath water” to cite the old adage? Our commentary on the recent 2019 Annual Boiler Report would seem to support our contention. Read our “reading between the lines” blog on this report.

Finally, the combination of higher boiler thermal mass with enhanced gravity convection extends selective fail-mode heating continuity substantially. Recently and four years prior our Beta Site #3 experienced a fail-safe circulator interruption. The latter an over-current condition from a voltage surge “fail-safed” its operation. In both instances the condition was not discovered for an estimated 2 to 3 days, despite significant heating demand. Neither living area heating nor indirect DHW generation were affected. Second level heating reduction was eventually noted, as prior. The customer called and we reset the power switch over the phone to resolve. It is also noteworthy that we have had no system related service calls in over fifty (50) aggregated-operating-years and 150,000 operating-hours on our monitored Appliance Beta Sites!

In closing, the contemporary excesses and misapplication of hydronic distribution are very troubling to this author. If tradesmen are promoting their prolific system distribution piping efforts as efficiency measures they are most sorely misdirected and possibly even deceitful. Witnessing customers proudly showcasing excessively installed systems or trade supplier contests for the “prettiest system” installation pictures are particularly disconcerting. Trade practices and hence consumer perceptions need challenging. Are we selling parts and labor ….. or performance?

Perhaps it is time for our engineered “appliance” approach to rein in “The Plumber’s Playground©”.

Updated 06/24/2022 P.D.M., Sr.

About the Author

Paul Mercier

A Semi-Retired Manufacturing Process Development and Hydronics Engineer.
Following a stellar career in Computer Hardware & Software, Miniature Ball Bearing, Avionics & Panel Meters, Magnetics, Plasma, IC & Discrete Component and Product Manufacturing throughout New England, have re-focused on a second, parallel skill set in Hydronic Heating.
Received U.S. Patent 10,690,356 on an "ENHANCED CONVECTION, DIFFERENTIAL TEMPERATURE MANAGED, HYDRONIC HEATING APPLIANCE". It is America's first, true Hydronic Heating Appliance, enabled by Taco Delta-T ECM Hydronic Circulation coupled with our proprietary near-boiler piping system. The resultant is a simple, durable and efficient hydronic heating appliance that surpasses all contemporary built-on-site systems both in value and performance.

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