DANFOSS TRVs - flow rates

[Diggory]

I bought some Danfoss TRVs (RAS-B2).
They are (dynamic) valves with a preset numbered dial to save faffing with lock-shields to balance a system (lock-shields are all left fully open).
The spec states suitable for a flow rate 10-135l/hr; Danfoss kindly provided this formula to check flow rate.

I checked using the formula noting that some of my rads were coming out towards the lower end of range.
However, when I started to use the installer app to get the valve presets, the returned flow rates are significantly higher.
For example, a 1kW K2 radiator is more like 45l/hr and not the 12l/hr as per the worked example.

Using the app, my rads are closer to the upper end of the range; in fact one exceeds the 135l/hr.
Anyone suggest what’s going on here ?

Thanks D

 

[Johntheo5]

Well, @Johntheo5 will provide you my own 60 year old formula which must have been wrong for all this time but nobody noticed!!.
I work in LPM and kW but no problem.... (1000W=1.0kW)

LPM = kW*860/60/dT. = 1*860/60/20, 0.717LPM, 0.717*60, 43LPH.

 

[Diggory]

Thank you @Johntheo5 - that’s very kind of you.
May I ask; is Δt the difference between flow and return temperatures ?
Thanks D

 

[Johntheo5]

That's correct,

If you use Danfoss's result of 12LPH, 12/60, 0.2LPM then from
kW=LPM*60*dT/860, = 0.2*60*20/860, 0.28kW, 280W (Wrong)

 

[Diggory]

@Johntheo5 - can I just check something ….

If I take the same physical size of radiator, same Δt of 20 but now at a lower flow temperature; let’s say the 1kW example by was at 80/60 and I now run same rad at 70/50 and achieve a reduced 0.75kW output.

(0.75x860/60/20)x60 =32.25 L/hr

So this suggests that lowering my flow temperature will also reduce my flow rate ?

Thanks D

 

[Johntheo5]

If you reduce the flow temperature to 70C then YOU must also reduce the flowrate "manually" (via pump speed), TRV control reduces the flowrate ONLY which means the return temperature reduces, the flow temperature remains the same but the mean rad temperature reduces = reduced rad output. (Row 15, below). If the flow temperature only is reduced (like with outside temperature compensation), the flowrate remains the same, (Row 20, below).
But, if you reduce the flowtemp then the TRV (if installed) will take care of the flowrate, to give the same required 75% output. (Like Row 10, below)

I can't post a spreadsheet file but the bottom screenshot shows the simple calcs, if you wish to use it, just configure one row and copy and paste for any more required rows.

 

[Diggory]

Thank you @Johntheo5
I’ll need to look/digest that
Many thanks D

 

[fixitflav]

I bought some Danfoss TRVs (RAS-B2).
They are (dynamic) valves with a preset numbered dial to save faffing with lock-shields to balance a system (lock-shields are all left fully open).
The spec states suitable for a flow rate 10-135l/hr; Danfoss kindly provided this formula to check flow rate.


View attachment 400014

I checked using the formula noting that some of my rads were coming out towards the lower end of range.
However, when I started to use the installer app to get the valve presets, the returned flow rates are significantly higher.
For example, a 1kW K2 radiator is more like 45l/hr and not the 12l/hr as per the worked example.

Using the app, my rads are closer to the upper end of the range; in fact one exceeds the 135l/hr.
Anyone suggest what’s going on here ?

Thanks D

That's wrong! It should be l/s = kW/4.18/DT, l/h = 3600*kW/4.18/DT = 860*kW/DT, lpm = 860/60*kW/DT, as John said.

Worrying that Danfoss forgot to correct from second to hour, will somebody tell them?