Could someone please explain to me what is meant by the term "head of water" with respect to an open vented header tank / direct cylinder hot water system. I thought the head of water was the vertical distance between the top surface of water in the header tank and the taps but I'm told this is not the case. :?
Water has volume (quantity) and, thank`s to the Earth`s gravity it has weight. Water will also find it`s own level - hence the success of water in toilet u-bends or basin traps - or in ditches that the Ancient Egyptians used to set their levels around sites or more recently the lengths of tubing with coloured liquids that we used to set levels across e.g. foundations on a building site.
You can see the effect of a "head of water" if you take an empty pop bottle then make three equal sized holes spaced between top and bottom. Put a bit of tape over each hole and leave a turned over section to get hold of. Fill the bottle with water then stand it in the bath or where you can water the garden.
Quickly pull the tapes off and you will see the top hole gives one jet of water, the middle a further jet, and the bottom one the furthest jet. The different lengths are due to the head of water or distance between the bottom level of the CWST AND the outlet. (If you measured from the surface of the tank then the head would vary according to the falling height of water as you ran the taps, albeit the falling ball valve should trigger an inflow).
If you have a CWST on a platform across rafters in the loft and you have a shower unit in any room below where the shower head is e.g. 2 feet below the ceiling then the only head of water available to you would be that 2 feet plus the few inches to the base of that tank. To increase the pressure one would have to increase the head of water and the easy way would be put the CWST on a raised platform (or go for a pumped system).
The amount of water available to the tap is important when choosing which tap to buy. The cold water supply is at mains pressure so is
generally not of importance; the tap however will require the correct amount of hot water pressure to perform satisfactorily.
Water pressure can be measured in three common units, bar, psi and Head (m).
1 bar = 10 metres Head = 14.5 psi.
Hot water pressure
Domestic hot water systems generally fall into two categories pressurised and un-pressurised.
Pressurised water heaters (instantaneous gas water heaters or modern combination boilers) deliver a continuous large volume of hot water.
For Combination boilers or instantaneous water heating systems the boiler output is typically around 20 psi = 1.8 bar = 18m head. This means
most taps should give good flow rates with these types of system.
Un-pressurised water heating systems found in older houses generally have a cold water storage tank in the loft and a heater tank on the first floor. The vertical distance between the header tank and the tap outlet gives an approximate calculation of the available hot water pressure.
An un-pressurised system, for example if the vertical distance was 5m this would roughly equate to 0.5 bar maximum available pressure.
Note: if the route the pipes take is not direct, has lots of bends or
long horizontal runs the available water pressure will be reduced.
In general terms most single story houses or bungalows with un-pressurised systems do not suit â€˜highâ€™ pressure taps such as single lever mixers for example.
Alternatively a booster pump can be fitted to increase the available
water pressure, allowing a wider choice of taps.
If you have a CWST on a platform across rafters in the loft and you have a shower unit in any room below where the shower head is e.g. 2 feet below the ceiling then the only head of water available to you would be that 2 feet plus the few inches to the base of that tank.
Why the base of the tank? Surely that would only be the available head if the tank was empty? Hopefully a shower would not use up the water fast enough for the header tank to ever be emptied completely. In fact I would not expect say a 5 or 10 minute shower to consume a whole header tankful of water even if it was not being topped up by the ballcock.
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