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 ΔT 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 (ΔT) 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 (ΔT) – 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.
Let’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 ΔT. 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 ΔT will be higher and will change proportionally to the buildings heat loss. For arguments sake, let’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 ΔT required?
2.5= 15,000÷ (ΔTx500); ΔT= [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 ΔT at desirable 20 degrees we need to decrease system flow to 1.5 GPM. That’s what a variable speed circulator with integrated ΔT control is doing - it’s modulating flow by keeping ΔT steady.