External bypass valve with modern pump

[MNW67]

A overrun bypass is pretty essential for gas boilers as they have a very small HEX, 2 or 3 litres, vs ~ 20litres for a oil fired boiler, Also the boiler may be running at high output when the last zone valve closes so the HEX has a lot of residual heat. I got the installers of my daughter's now 9 year old Vokera Vision 20S to install a NO 2 port valve close to the HW cylinder which is a good distance from the boiler so the pipework dissipates the HEX's residual heat quite well during the boiler's 3 minute overrun period.

Worcester Bosch seem to have copied your idea! The attachment has the full document with more details, but I have copied and pasted a small section.

 

[Johntheo5]

I was wondering if the pump is running at constant pressure, would it slow right down in that situation. Maybe almost to a stand still. So that the pressure never actually rises above the level it is set at. Even when all the TRVs and zone valves have closed and it is dead heading. The horizontal pump curve at 3.0m shows it maintaining the same pressure all the way down to zero flow. I don't know whether that literally means zero flow! It is very confusing, because it is still using about 10W at zero flow at that setting.

View attachment 367051

The hydraulic power required by any pump at 100% efficiency is (LPM*M)/6.14, (if I have converted correctly from my old imperial units to SI units)
So a pump circulating 20LPM @ 3M requires 20*3/6.14, 9.77W, the pump hydraulic efficiency (see Link) is 61.44%, 9.77/0.6144, 15.9W, the motor+mechanical efficiency is 65.15%, total power, 15.9/0.6515, 24.4W. A pump circulating 1.0LPM @ 3M requires 1.0*3/6.14, 0.4885W, the hydraulic efficiency is only 7.66%, 0.4886/0.0766, 6.38W, the motor+mechanical efficiency is still 65.14%, total power, 6.38/0.6514, 9.79W.
Pump hydraulic efficiencies probably havn't changed in 70 years but the advent of these permanent magnet DC? motors has dramatically reduced the power required, the old type motor efficiency could be down to around 19% or so, the power demand between no flow & full flow scarcely changed.
The hydraulic (in) efficiency is absorbed in heating the water as it passes through the pump, on very large pumps, the impeller condition and the condition of the wearing rings etc is determined by measuring very accurately the dT between the pump suction and discharge, if you ran your own Wilo or UPS3 or whatever with the discharge valve shut then the temperature will rise and rise, the pump will then start cavitating and can eventually get damaged. Very high head pumps like multi stage boiler feed pumps with a discharge head of say (like the ones I had 30 years experience on) 600M, 60Bar, had a automatic leak off valve to allow adequate flow through the pump while running up (and stopping) until the discharge pressure was sufficient to overcome the 45bar boiler pressure and establish adequate flow. Even pumps of fairly modest discharge head of ~ 100m, 10bar will have a simple manual leak off valve, permanently locked open.

https://wilo.com/ie/en/Products-and-expertise/en/products-expertise/wilo-yonos-pico/yonos-pico-25-1-6

Grundfos will have similar data.