Im struggling with the pump term "Pump Head". when used in central heating application.
Is it the distance between the lowest point of the central heating to the base of the pump.
Or
The distance between the highest point of the heating circuit to the base of the pump.
Or something else.
Thanks
garry
Something else!
It's the pressure generated by the pump to overcome the friction losses in the system, and thus to allow the water to circulate. If you look at a pump's performance graph you will see that the head is highest at very low flow rates, and lowest at high flow rates. [Think of the back pressure you generate in the mains by putting your thumb over the cold tap spout].
The pressure is often expressed in terms of an equvalent static 'head' of water (1 bar is approx 10 metres) but it has nothing to do with the physical dimensions of your system.
On Mon, 11 Feb 2008 07:01:52 -0800 (PST) someone who may be Garry wrote this:-
It has nothing to do with the height of anything. That is the static head. The pump head is a dynamic head which only exists when the pump is running.
Imagine a heating system where all the pipes are horizontal, other than those to the F&E tank. The heating pipes would then be under the static head from the F&E tank. Assume that the pipes from the F&E tank are connected to the right place, the suction side of the pump. When the pump is turned on it will develop a dynamic head at the outlet. The pressure in the pipe at this point will be the sum of the static and dynamic heads.
As the water flows round the pipes some of the dynamic head will be "used up" overcoming the resistances in the pipes and the various fittings which form part of the pipes. By the inlet to the pump this dynamic head will be reduced to zero and the pipes will just be under the static head.
How much water the pump can push around a particular heating system depends on the dynamic head it can create and the resistance the pipes provide. Pipes are generally sized so that they are as small as possible to carry the necessary amount of heat without the water velocity rising so high that the system would be too noisy.
Indeed. By contrast to the pump, a CH system's radiators and pipework (including the boiler) will have a head (pressure) versus flow curve where friction causes the head to *increase* as the flow increases. Head is approximately proportional to the square of flow in pipework.
If you plot the two head versus flow curves on the same graph the intersection shows the head and flow values where the combined CH/pump system works.
Changing the pump setting will alter its head / flow curve and so change the operating point.
I always fit the 5m head pumps rather than the 3m ones as I find they can be run on a lower speed setting without stalling.
A 5 metres head is a common term. Have a pump at ground level and a 8 metre high plastic tube on the outlet straight up in the air. The inlet off a tank next to the pump. A 5 meter head will pump the water up the plastic tube 5 metres.
In water terms, head = pressure
Having a pump that runs the CH system properly at its lowest setting is probably a good idea to reduce pump noise.
However, for small systems a pump capable of 5m dynamic head, even on its lowest (fixed) setting, may result in too fast a flow which means noise from the pipes.
My small system only needs a 3m head pump on its lowest setting and still the flow is fast enough to keep the drop across the radiators low-ish at about 8 C. It is quiet enough almost everywhere in the house but flow noise is just audible in places. Perhaps I will get a Grundfos Alpha or some other adaptive pump some time to see if I can get both a quiet pump and a quiet flow everywhere.
A fixed setting 5 m pump in my system, even on its lowest setting, might well produce more flow and therefore more flow noise than a 3 m pump.
I had a 5m one in my 2 bed flat, on the second setting and it was fine but it was a pressurised system and they tend to run quieter.
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