The topic of variable speed circulation in the solar thermal controls is remaining a mystery for many installers and designers of solar thermal systems.  Hey no blame here - that is a pretty new technology – solar heating. Even variable speed circulation control in hydronic heating is remaining unexplored for many professionals.

Let’s tackle that gap and get up to speed with the rest of the solar world.

The Universal Hydronic Formula: BTUH = GPM x Delta T x 500

You see – in the hydronic heating we deal with stable heat source output and changing load demands, or in other words we control the amount of BTUs to match the heat loss of the building. The purpose of a variable speed circulator in solar thermal circuit is to automatically adjust its speed based on continuously changing heating source output €“ solar collector. In both cases for understanding how it works we can use the universal hydronic formula, which states that energy (BTUH) is equal to amount flow (GPM) multiplied by temperature difference (Delta T) multiplied by 500.

The terms are defined as following:

BTUH (BTUs per hour) - that is amount of energy at any giving point of time (and that’s where the difference begins)


Hydronic heating system: required to compensate heat loss of the load (building or zone)

Solar Thermal System:harvested by solar collector and has to be efficiently utilized

GPM (gallons per minute) - the flow rate of the heat carrying media needed to move the required amount of energy (BTUs).

Delta T - The designed temperature drop across the load (in Hydronics) or rise across solar collector array and storage tank.

500 The shortcut coefficient calculated by multiplying the weight of 1 gallon of water (8.33 pounds) by 60 minutes in an hour, again multiplied by specific heat of fluid, in case of water is 1€.

If fluid is 40% solution of propylene glycol that number is 467. For simplicity of further calculations we will stay with water as heat transfer media.

solar_variable_speed_control_001Let u€™s take for example 1,000 ft2 “Energy Star Rated€ house with 25 BTU/ft2 of heat demand at 0°F design temperature. It will have 25,000 BTUH heating load designed to 20-degree Delta ”T. Single circulator is supplying homerun system with panel radiators.

Using the Universal Hydronic Formula, GPM= 25,000· [20 x 500], equals to 2.5 gallons per minute.

By milder temperatures (usually more than 90% of the heating season) the DeltaT will be higher and will change proportionally to the buildings heat loss. For arguments sake, let u€™s say that at 25°F outdoor temperature the heat loss decreases in 40%. Having our circulator sized for 2.5 GPM, what will happen to Delta”T required?

2.5= 15,000· (DeltaTx500); Delta”T= [15,000·2.5] ·500 = 12 degrees!

In our case it means that system return temperature will rise and as we all know from our experience - the boiler will soon short-cycling and system efficiency will reduce significantly.

To keep Delta ”T at desirable 20 degrees we need to decrease system flow to 1.5 GPM. That’s what a variable speed circulator with integrated DeltaT control is doing - it€ is modulating flow by keeping DeltaT steady.

In a Solar Thermal Circuit on the other hand we deal with variable collector output. The sun radiation is changing and during one day is varying from 0 to 317 Btu/hr/ft2. When we’re sizing solar circulator we are taking two things into consideration: system pressure drop and maximum amount of BTUH we need to move. The required pump head has to overcome the sum of pressure drops in the piping, collectors, heat exchanger and fittings based on minimum flow velocity of 2 ft/sec.

Let u€™s make another system example: 4 solar 4 x 8 ft collectors with thermal efficiency of 61.6%. The maximum amount of energy that array will produce is: 4 x 32ft2 x 317 Btu/hr/ft2 x 0.616 = 25,000 BTUH.

Using the Universal Hydronic Formula, GPM= 25,000· [20 x 500], equals to 2.5 gallons per minute.

solar_variable_speed_control_002If our collectors are installed by the book and facing true south with slope equal to latitude degree, that output of 25,000 BTUH will be available somewhat around from 1 hour before solar noon and 1 hour after. Only 2 out of 8 hours of typical system operation we need to maintain the design flow of 2.5 GPM! All other time if the flow is remaining constant, the Delta”T will decrease proportionally to decrease of solar array output.

In the case of a basic On-Off temperature differential controller as we know, the solar pump will short cycle, increasing electrical consumption and heat loss.

New differential temperature controllers from various manufacturers operate the circulator at speed proportional to DeltaT between the collector and storage tank. As  DeltaT rises, the solar circulator speed increases. By using that Target Delta”T technique the overall system efficiency can be increased by up to 20%.

However proportional variable speed control (1% Delta ”T increase resulting in 1% speed increase) does not always optimize system to 100% of its possible efficiency. Different collectors have different thermodynamics, every system has its own unique reaction time and behavior to temperature changes.

solar_variable_speed_control_003RESOL variable speed controller has more than Delta T-ON, Delta T-OFF and Target DeltaT settings. By adding into equation of control algorithm an additional constant DeltaT Rise (degrees of differential change) and tying it to pump speed output, RESOL Controllers make fine-tuning of every individual system possible.

The DeltaT based variable flow control is called in solar thermal industry “matched” flow control mode.  If the system runs at “matched” flow, the volumetric flow rate of the entire array is adapted to actual sun radiation by controlled circulator speed. The aim is to achieve high storage temperatures, even at low sun radiation, and reduce or eliminate the need for backup heating.

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