Flow rate in underfloor heating system

[TSN79]

Consider an underfloor heating system in an apartment building. Each apartment has its own distribution box with a number of loops and a thermostatic mixing valve set to 35°C. The loops are intended to work with a ΔT of 5°C (35-30), and the riser pipe supply temperature is 75°C. Say there are five loops, each with a flow of 2 l/min (0,033 l/s). Using the equation kW = l/s ⋅ 4,18 ⋅ ΔT each loop provides about 0,69 kW. The loops rely on a separate circulator in each distribution box.

I wonder what will happen if each loop's flow rate is doubled to 4 l/min (0,066 l/s). The loop's ΔT will naturally decrease as the water flows faster, but I'm tempted to consider it simply as an exchange where ΔT is halved as the flow is doubled. Am I correct? Does that also mean that the mixing valve will need to receive the same flow of 75°C water from the riser as when the loops had 2 l/min?

 

[russ_watters]

I wonder what will happen if each loop's flow rate is doubled to 4 l/min (0,066 l/s). The loop's ΔT will naturally decrease as the water flows faster, but I'm tempted to consider it simply as an exchange where ΔT is halved as the flow is doubled. Am I correct?

No. The relationship between flow, delta-T and heat transfer is just that: a relationship between simultaneous quantities. It is not a description/model of what actually causes the heat transfer.

What causes the heat transfer is the temperature difference between water and pipe, pipe and floor and floor and air. Modeling that is somewhat difficult, but at least we can answer the conceptual question easily: Since increasing the flow while keeping the supply temperature constant decreases the delta-T, the average temperature in your system is now higher...

 

[TSN79]

Since increasing the flow while keeping the supply temperature constant decreases the delta-T, the average temperature in your system is now higher...

Thank you. Regarding the last question - since the return temperature will now be higher, the floor surface will be somewhat warmer. And if that increases the temperature difference between floor and air, then more heat will be transferred. Does that indicate that the mixing valve will need to use a bit more of the 75°C water provided by the riser?

 

[CWatters]

Thank you. Regarding the last question - since the return temperature will now be higher, the floor surface will be somewhat warmer. And if that increases the temperature difference between floor and air, then more heat will be transferred. Does that indicate that the mixing valve will need to use a bit more of the 75°C water provided by the riser?

Yes according to conservation of energy. If more power goes into the room then the boiler/furnace must supply more power to the heating system.

However... Most systems have room stats. If these keep the room temperature reasonably constant then the losses through the walls and windows will be constant so the long term energy use will be constant. So it might use more 75C water when the stats are calling for heat but they will call for heat for less of the time.

 

[russ_watters]

Thank you. Regarding the last question - since the return temperature will now be higher, the floor surface will be somewhat warmer. And if that increases the temperature difference between floor and air, then more heat will be transferred. Does that indicate that the mixing valve will need to use a bit more of the 75°C water provided by the riser?

The setup isn't clear to me(a diagram would help...), but if this is a true primary secondary arrangement the two flow rates are totally independent of each other. If this "mixing valve" is really a flow control valve, it controls the flow. If the valve position doesn't change but the secondary flow rises, the mixing situation may reduce the supply temperature making the assumption i made in my previous post false. I can't be sure without seeing a diagram of the piping arrangement.