Sizing Circulation Pump

[dpmiller]

On some, the pump is under PWM control to help achieve the dT

[MortarThePoint]

On 07/07/2023 at 09:39, MortarThePoint said:

Loop: I've taken the longest loop and take that as a fraction of the overall loop length of the system and it works out as 6% (yes, I have a silly number of loops). That means it will have a flow rate of 6%*1.92m3/h = 0.12m3/h = 0.032kg/s. I don't have data for 16mm UFH pipe but 15mm Hep2O is probably a good proxy and they publish that. In other considerations I worked fitted a line to that data which gives Head_Loss_KPA_Per_M = 79.9*(FLOW_IN_KG_PER_S^1.76) = 0.187 kPa/m. My longest loop is 110m long so that works out as a pressure drop across the loop of 110m * 0.15kPa/m = 20.6kPa = 2.1m head loss.

 

One aspect this neglects is all the bends in the UFH pipe. They aren't sharp bends, but with a bend radius of around 12.5x bore if a 90 bend or 6.3x bore if a 180 bend. Using this tool [1] I get pressure drops due to bends of these sizes of equivalent added length of 67mm and 96mm respectively. There is one such bend every ~3m so that's a pretty small addition of around 3%.

 

[1] http://www.pressure-drop.com/Online-Calculator/

[JohnMo]

5 hours ago, MortarThePoint said:

@JohnMo When you are running less than peak power, the flow temperature goes down, but does the flow rate go down too? I guess it can go down since the water dT between room and water is lower and so to get the same flow vs return temperature difference, the flow would need to slow down.

 

I expect to be optimal, you want the dT of at the ASHP to be the same as the dT at the UFH manifold. That wouldn't be the case though since I think the ASHP circulation pumps will run at constant speed.

My ASHP basically runs a constant dT, this is controlled by varying the circulation pump speed. Not really had much time running in heating mode except a bit of commissioning, but cooling dT is 5.4 degs, from what I remember heating was similar. Which is fine for the UFH, will just fiddle with the compensation curve to get the room temps correct.

 

While I was looking I found this, looks like it brings down the load fact as outside temp increases, kW output only drops by 40%.

[PeterW]

Seriously over thinking this ..!! But ignoring the basics of fluid dynamics in that the fluid will flow through path of least resistance and you’ll have a hell of a job balancing this lot with no manifold pumps. I would fit a pair of Wilo Pico 25/1-6-130 on the manifolds and use self adjusting heads and then tune the flows to and from the LLH with the heat pump side pumps. 

[Nickfromwales]

1 minute ago, PeterW said:

Seriously over thinking this ..!! But ignoring the basics of fluid dynamics in that the fluid will flow through path of least resistance and you’ll have a hell of a job balancing this lot with no manifold pumps. I would fit a pair of Wilo Pico 25/1-6-130 on the manifolds and use self adjusting heads and then tune the flows to and from the LLH with the heat pump side pumps. 

Yup. Defo only needs a single primary pump IMO, and defo better to keep 1x pump per manifold, when over 2 floors and with 2 different values (head and temp).