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.

Read more: Solar Variable Speed Control: Targeting the Delta T


Guessing is Not an Option

Before detailed design can be done, many important planning steps have to be made:

Collecting basic information   Rough dimensioning  Cost estimate   Simulation  Design optimization

and only then...

Detailed Planning

Collecting Basic Information

Let us start today by talking about the first step: Initial object data input - or Collecting Basic Information.

You see - because of varying energy supply from the sun, we need to know precisely the volume of hot water consumption, the consumption profile and desired water temperature. By having fewer variables in equation the result could be more accurate. To have that information is critical because all further dimensioning tasks are based on it.

The values for DHW consumption data given in different literature have been found too high in practice; and result of their use is over-sizing of the solar system with all other consequences. Often these references are old, and have been calculated to cover highest possible demand factor.

Imagine - you are just breaking-even finishing the job and the customer is demanding to fix the overheating issues on warranty... - nice prospect, isn't it?

Another very important sizing criterion is the consumption profile.

Similar buildings/institutions can have absolutely different load profiles. For example: 100 room student dorm of the Faculty of Music and 100 room dorm of the department of Physical Education. In the second one the students are taking showers after the gym, outside the dorm.

Once again - the consumption volume and profile are so crucial in getting your design done right that you don't need to even think of "guesstimating" it, but measure it!

Read more: Designing Large-Scale Solar & Hydronic Systems

For each and every one of your customers, Solar is a long term investment.

For that reason they are looking for an installer that they can rely on.

Solar Thermal or Solar PV is a major investment for each of your customers, and it has to be customized for every application and budget.

Whether  you sell small pre-packaged systems or custom-made complex all-in-one solutions, you need to discuss the benefits of your product  in detail with the customer.

Knowing What to Communicate

Make sure to communicate to your customer your superior  technical and overall knowledge of the rebate process, building codes, homeowner’s association restrictions and financing options. Solar is a long term investment and your customers are expecting a quality system that produces for at least 25 years; that's why they want to establish a solid relationship with you –the installer.

But unless you are an order taker, it is the way that you handle a prospective client that determines if you get the order.

Read more: Solar Installers! How to treat your customer Right

Is Solar Heating your lost sale?

Interesting statistic: – 78 % of people involved in selling and installing solar heating systems are not from the heating or plumbing trades. Where are they coming from? Many of them are electricians, auto mechanics, millwrights, handymen, educated do-it-yourselfers, or even home owners without any technical background at all.

It is very possible these days to buy everything including collectors, storage tanks, controls, condensing boilers and furnaces without ever setting foot in a plumbing/heating retail store or wholesale outlet. And why bother if traditional heating and plumbing suppliers are not even considering stocking any “high risk” or slow moving merchandise?

The Internet has replaced the salesmen and showrooms.

All information is retrievable in a nanosecond. The Internet has made trade buyers and consumers smarter than ever, even smarter than some salespeople in wholesale outlets.

Is the Solar Thermal industry facing the same fortune as hydronic heating? It took almost 25 years and hundred of thousands of dollars in investment for Viessmann North America to educate the industry about condensing boilers and low-loss headers. If it wasn’t for that missionary work of the Viessmann Academy, hydronic heating would still be roaming around the “black forest” of primary-secondary piping and copper fin boilers.

In the beginning it was too “techy” to understand, too difficult to sell, too challenging to get it done right and too hard to compete with the existing “industry standard”. But now it seems to be a matter of good reputation for every boiler manufacturer, wholesaler and contractor to offer, sell and install MODCON (modulating/condensing) boilers and radiant heating. Hurray! We now have the knowledge. It just took us 20 years to get it…

Read more: Second Best is a Loser

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