Gravity Convection Heating Distribution Revisited

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 needs revisiting.

A century ago all water-based hydronic heating (hot water and steam) employed the natural gravity attributes of heated water and water vapor (steam) to distribute energy. NO DISTRIBUTION ENERGY WAS REQUIRED. These 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. Natural (gravity) convection of heated water underlies all hydronic distribution by default, yet is not considered in contemporary practice excepting to install check valving to negate its effects.

The introduction of electric circulation pumps in the 1920’s enabled forced hot water heating (FHW) and changed hydronics forever. Gone was the large, pitched piping and radiators and in came zoned heating with finned radiation. The heating market never looked back, and justifiably so. Underlying this however remained the natural gravity convection effect that had to be controlled using check valving as needed within the system.

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 particularly failed couplings of varied types, seized and leaking pumps or smoked motors. The advent of wet-rotor circulators was like manna from heaven, reducing circulator issues by swapping out the older types with greater longevity and reduced power consumption benefits.

Now the evolution and introduction of particularly Delta-T (differential temperature sensing) ECM Circulators projects hydronic distribution management to an entirely new level. Integral instrumentation and operational data display of these circulators provide us with finite attribute identification and application control.

The focus of our work has been to optimize this hidden 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 our development of our “Delta-T ECM Hydronic Heating Appliance”. We claim optimization of natural gravity convection within our boiler, near-boiler distribution piping and distribution energy requirements using a 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 to that old gravity hot water heating system of a century ago. Contemporary hydronic heating systems have smaller piping with multiple zones for heating flexibility by comparison. The old “gravities” necessarily used high-mass cast-iron boilers to modulate heating supply, otherwise control was difficult particularly when using solid fuel, constant firing as with wood or coal. With generous distribution piping sizes and radiation elements gravity convection worked fairly well, with NO distribution power requirements!

Properly pipe a contemporary FHW system with a dedicated “Delta-T Mode” system circulator and complimentary low-energy ball-type zone valves in lieu of flow checks yields great results. Transpose this configuration onto the old gravity system layout and you functionally emulate its performance as in the following figures.

The advantage is the gravity performance of this contemporary upgrade. We have seemingly lost the trade skills of enhancing gravity convection. No consideration is given to pitching, compacting and minimizing distribution piping in particular. Additional gains are available in radiation layout by using properly sized and configured series and/or split radiation loops. The 45° elbow as an example saves 30% of piping and reduces head pressure significantly over a 90° elbow run. All this increased pipe volume and head pressure reduce the natural gravitational convection effect. Not to mention less materials, labor and lifetime operating costs of the system.

Our Delta-T Mode Circulator measures the head effect well via its wattage indicator. All of our single, dedicated system circulator Beta Site installs to date exhibit an 8 to 13 watt distribution power consumption upon a 20° delta-t (adjustable) differential attainment. Compare this to 80 watts typical for each 16gpm fixed-speed circulator or 20 to 25 watts each for the equivalent delta-t or delta-p install. Once the delta-t achieves the differential setting, you can watch the wattage steadily decay to half or less as the natural convection effect carries on. We refer to it as “paddling your canoe with the current”.

A secondary effect of gravity convection seems to be a modification of the radiation heating profile, 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 to this radiation profiling effect. Another contributor is the lowered system operating temperature effect of using a very high mass cast-iron boiler vs. contemporary low-mass units. Burner cycles are significantly less frequent and briefer than the unit it replaced.

Our personal observation is that in over sixty years of hydronic and steam installations this author has never replaced a “cold shot” cracked or magnetite impaired cast-iron boiler. Perhaps a discussion for another day, but have we also “thrown the baby out with the bath water” to cite another adage?

Finally, the combination of high thermal system mass with enhanced gravity convection extends selective fail-mode heating supply substantially. Recently and a prior four years ago our Beta Site #3 experienced a fail-safe circulator interruption. This time 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 gradually reduced and was eventually noted, as it was prior. The customer called and we reset the power switch over the phone to resolve. It is noteworthy that we have had no system related service calls in over twenty aggregated operating years on our multiple Appliance Beta Sites!

In closing the seeming excesses and misapplication of hydronic distribution are troubling to this author. If tradesmen are promoting their distribution field configuration efforts as efficiency measures they are sorely misdirected and possibly even deceitful. Witnessing customers proudly showcasing excessively installed systems and trade supplier contests for the “prettiest system” install pics are also particularly disconcerting.

Perhaps it is time for an “appliance” approach to rein in this “Plumber’s Playground”.