Theoretical questions regarding low loss headers (or close-coupled tees)

[sammathias]

It seems way OTT for a domestic set-up. Why would you want to install an additional pump?

Large domestic system which require an additional pump..or a domestic property with with UFH, The LLH steals temperature from the return before it gets back to the boiler so the return to the boiler tends to stay at condensing range 50-55 degrees increasing efficiency. Expensive tho!

Even Some of the higher output vaillants will tend not to work correctly without a LLH.

 

[fixitflav]

The cold feed would be at point B as I have added to one of the diagrams in the above post. If the system is open-vented, the cold feed and vent logically have to be somewhere on the LLH or the primary circuit, but the system need not necessarily be open-vented, in which case...

The cold feed at point B gives the reference head (neutral point) for both circuits. Looks bad to me. Assuming the secondary circuit is pumped clockwise (as shown by the arrow on the pump), the minimum head is between heating load and pump suction. If say the header tank static is 2m, and the pump head 5m, head at pump suction is minus 3m.
Did you mean ".....vacuum all the way from B to the pump inlet?"
The head between pump outlet and B is the header tank static (2m in my example) plus a tad of pipe friction.

 

[chris watkins]

Hydraulic separation using a LLH (expensive) or close coupled tees (cheap) allows modern boilers to receive the minimum flow rate required by MI's, their control systems can then make adjustments to the output (burner modulation) & anti-cycling routines based on the heat being used against a typical 20degC difference between the F&R. In conventual designed UK systems the flow rate changes as zone valves, TRV's & automatic by-passes open & close, they can't make sense of what the system actually requires.
This is more apparent the larger boilers & systems as the burner can not go low enough & turns off as heat is not being carried way. Then they fire back up after a sort while (cycling) this is very wasteful so many have a program which stops it, limiting the stop/starts per hour, trouble is areas of the building then go cold or HW cylinders don't heat up in.

 

[Ric2013]

Did you mean "...vacuum all the way from B to the pump inlet?"

Err, yes. That was a good typo, wasn't it. I meant inlet.