How much fall required for a hot water header tank?

[Lower]

I will be in the process of refurbishing the bathroom in my 1930's semi in the next month or so.

It needs a complete new bathroom suite but we also want to move the hot water tank out of the bathroom and up into the loft.

The current hot water system is a standard vented hot water tank fed by a header tank in the loft. I want to move the hot water tank into the loft but how much head do i need for the header tank above the hot water tank?

I estimate that if i leave the system as is and just move the hot water tank into the loft space (and yes there is sufficient support in the loft space for the weight of the tank) i would get roughly half a metre of head above the hot water tank.

Is that enough or is that going to knacker flow from the hot water tank? There would then be another 1500mm or so between the bottom of the hot water cylinder and the shower head or highest tap in the bathroom and i'd be planning to put a pump in anyway.

Thanks in advance.

 

[D_Hailsham]

It is the height from cold water tank to tap which is the important factor. The HW cylinder does not come into the equation.

You need the CW tank above the HW cylinder, but that's all.

 

[Lower]

The fall from the cold water header tank to the tap will remain unchanged and we have reasonable pressure at the moment.

So the fall from the header tank to the hot water cylinder doesn't matter as long as the header tank is above the hot water cylinder? Seeing as the hot water is taken off the top of the hot water cylinder i thought that the fall from the header tank would be significant.

But if not, then great. Makes moving the tank a whole lot easier!

 

[D_Hailsham]

So the fall from the header tank to the hot water cylinder doesn't matter as long as the header tank is above the hot water cylinder? Seeing as the hot water is taken off the top of the hot water cylinder i thought that the fall from the header tank would be significant.

The HW cylinder is a closed vessel; water only comes out, when you turn a hot tap on, because water is going in from the cold water tank. The pressure is therefore determined by the height between CW tank and tap. As an example: my HW cylinder is in the airing cupboard and my mixer shower is well above the top of the cylinder, but I still get very good pressure from the shower head.

The only thing which may affect the pressure is the size of the pipe from cold water tank to HW cylinder and from cylinder to tap. If this is too small for the distance, you may get some pressure loss due to friction in the pipe.

 

[Diyisfun]

You will need to raise the cold water tank to height higher than the hot water tank.
You will get a good pressure for a shower as well without the need for a pump.

 

[D_Hailsham]

You will need to raise the cold water tank to height higher than the hot water tank.

That's what I said

You need the CW tank above the HW cylinder, but that's all.

 

[Lower]

Thanks for the above. Makes sense and makes my job of moving the tank much easier!

 

[SimplePimple]

I am planning to move my hot water cylinder into the loft. The cold water tank is already raised high into the apex of the roof, however the base of the tank will still be slightly lower (only about 200mm) than the feed from the top of the proposed hot water cylinder. The top of the cold water tank (i.e. the water level) will be about 500mm higher than the top of the cylinder, so I assume this will work fine in normal circumstances, but what what will happen if a heavy draw of hot water from the cylinder causes the level in the cold water tank to reduce to the same level as the top of the cylinder? I'm assuming that the hot water pressure will reduce dramatically (down to whatever volume of cold water the ballvalve can continue to deliver into the tank presumably?), and I'm guessing that air may be sucked in from the vent pipe too? Are these going to be serious problems? How often in normal use does a cold water tank actually get drained down to within 200mm of it's bottom anyway?
If there are likely serious ramifications then I suppose I could just raise the tapping from the tank to a point slightly higher than the top of the cylinder. I would lose some of the tank's capacity obviously, however this would avoid the drawing-in of air from the vent wouldn't it?
Any advice welcomed.
(I've re-read this and it sounds more complicated than it is! - All I'm trying to say is that there is a 200mm overlap between the water levels of the CW tank and cylinder!)
Thanks.

 

[kirkgas]

i've read your post quickly so apologies if i have misunderstood.
you seem to understand what is going on, so only a couple of things, how often a tank empties will depend on how fast it is refilled(ie poor inlet pressure will allow the tank to empty, so how good your pressure is answers that one) on the point of repositioning the outlet of the tank, i wouldnt do that as it wont stop the tank emptying if the inlet pressure is low as the water level will drop as you draw hot water and will either drop till level with the top of the cylinder or your new connection, (which will be the same level) also raising the outlet of the tank could lead to stagnent water being left in the 200mm section of tank under your new connection

 

[D_Hailsham]

How low does the water level drop in the tank before the ball valve opens to let water in? You may be able to make the drop smaller.

Provided the cold water flows into the tank faster than it leaves, preventing the water level dropping to the critical level, you should not have any problems.

 

[SimplePimple]

Thanks.
I've just moved the cylinder up to the loft and it's working OK.
The cold water tank seems to refill pretty quickly (very nearly keeps up with the drain, even with all the taps on) so I think there's very little chance of the level in the tank ever dropping to the level of the top of the cylinder. Pleased about that.
However, some air is being drawn in through the vent pipe, which is not the end of the world, but it is causing a bit of "spluttering" at the taps. Would I be right in thinking that if I changed the feed pipe from the tank to cylinder to 28mm (currently 22mm) then this may stop the air being drawn in (because the tank will be more able to keep the cylinder full, thus avoiding the suction that must be drawing the air in)?
I assume this would work (or at least help), however I am aware that this solution may perhaps reintroduce my original worry (i.e the tank level may then drop faster than the feed can keep it up!).
Your thoughts?
Thanks.