equal pressure on flow and return..how to make it different?

[old duffer]

When a radiator loop from the main flow and return pipe has equal pressure, there can be no flow across the radiator.
What is it you professionals do to reduce the pressure on a single loop and create the differential so as to enable the flow?

 

[Swwils]

Why would there ever be equal pressure each side of the pump.... Apart from when off.

 

[old duffer]

Why would there ever be equal pressure each side of the pump.... Apart from when off.

well, lets say that you have zones and the first zone is near the boiler so the feed and return manifolds are then a few feet apart.

 

[Swwils]

Then you would need to balance the system, an exaggerated example would be that closest rad to the boiler seeing a massive pressure differential giving it a very high flow rate.

The system resistance will always build to match the pump head... So the furthest circuit will get less flow to balance at the same differential pressure of the head.

 

[old duffer]

shall I put that down as a don't know?

 

[Swwils]

Your question doesn't make sense. If your furthest rad is getting no flow because there is no pressure differential then the answer is to restrict flow at the other rads and balance the pressure across the whole system... This might mean that you add so much resistance to the closest rad that the overall head at the pump needs to increase.

 

[old duffer]

lets assume that you deal with non ideal situations for central heating. So the pipe runs for flow and return are not the exact same length. The radiators are not the same size in each room and each room does not have the same heat demand. Lets also assume that each loop to feed each radiator is not the exact same size. So in other words, without balancing the entire system to try and achieve what will probably be impossible (and the fact that some rads might not heat at the same rate as others is easy to live with) and you have to handle individual radiators on individual zones, how do you adjust the pressure on that individual rad so as to enable flow.

 

[Swwils]

Someone made a design mistake then and didn’t size pipes right. Otherwise just wait till the other zones satisfy themselves and the low DP rad is the only circuit left... which is silly.

This is kind of exactly why we have variable pressure pumps, to deal with hydraulic changes TRV etc cause.

 

[old duffer]

combi boilers were the get out of education card. Plumbers put them in and they did a job. What size do I need Mr plumber "what size have you got it Mrs...does it work OK? Then you need a one of the same size Mrs.
Mass flow rate calculations based on given delta T?
how about starting with heat losses and environmental needs of each room and positioning of the radiator accordingly?
Well having calculated these variables and decided on the 90 percentile figure (or whatever percentile you decided) you then calculate the flow required and the pipe diameter and minimise resistance with solder joints and no tees or 90 degree bends...yeah right LOL.

So back in the real world....

So the answer is probably "don't know" but here goes. If the pressure is equalised (common occurrence) on the loop of a single radiator (common occurrence) and you accept that in the world of hydronics, circulation can only be achieved when there is a flow of higher pressure to a lower pressure, how do the professionals achieve this pressure differential.

If the answer is we can't or we don't, then say so.

I scratched my car bumper today...about a 4 cm scratch......having the whole car resprayed tomorrow....

 

[Swwils]

Your question doesn't make sense. If your furthest rad is getting no flow because there is no pressure differential then the answer is to restrict flow at the other rads and balance the pressure across the whole system... This might mean that you add so much resistance to the closest rad that the overall head at the pump needs to increase.