Garavity Heating / hot water : Straddle Tank ?

bob_a_builder

Has anyone heard of a 'straddle Tank' ?

Have a low pressure gravity heating system, had new bathroom fitted with single hole tap and flow rate is very poor.

Found this Q & A on a building website which refers to same problem

"Another solution if you should like to keep with the new taps is to add a straddle tank to the hot and cold supply in the loft, effectively increasing the head by doubling the amount of water pressing down on the supply.

I have used this method several times as an economical solution to poor water pressure and an essential requirement on certain installations of power showers"
https://www.mybuilder.com/questions/v/6491/poor-flow-on-new-bathroom-mixer-tap

Not heard of a straddle tank before and even google is not coming up with much

Got visions of my existing tank overflowing ?

Can anyone throw any light on how such a thing might be fitted, does it even exist ? - is it possible ?

Gloomendoom

Unless you fit a pump, the only way that you will increase pressure on an indirect system is to increase the head. Height is the issue, not capacity.

bob_a_builder

Yes - I know, and thats why if trying to understand how the straddle tank which seems to raise the water head (assuming it actually exists) works & is installed

Will be having the next bathroom done soon, which will have a pump for the shower and I suppose we could run the pumped flow going to shower to the taps too
but that wont fix the room which is already done

The room we've just done has a 'digital' shower - and I now know why they are called that - they are either full on or full off
no control of the flow rate at all !

Gloomendoom

bob_a_builder wrote: »

The room we've just done has a 'digital' shower - and I now know why they are called that - they are either full on or full off
no control of the flow rate at all !

Noughts or ones. :rotfl:

MrBluesky_2

Your monoblock tap is designed for high water pressure and who ever fitted it should know better.

Uk gravity hot & cold water supply is considered low pressure and yes low pressure tap's are available,so the tap should have been the first consideration.

Fitting the wrong tap's happens all the time,the glossy internet adverts dont always mention the minimum flow rate required for the tap to work correctly.

A good old fashioned plumbers merchant would be worth a visit to get the correct taps.

Good luck,

Geoff1963

I know some basic engineering, which might help you :
"Straddle" means "sit across", suggesting it sits across the hot and cold supplies.

The delivery rate of water into a house, or out of a tap, is limited by two things :
Pressure
Restriction
Think of a battery for starting your car. If the voltage ( pressure ) is low, that won't work ; but neither will two dozen AA batteries connected in series, because the resistance ( restriction ) is too high.

If the feed into the house is too restrictive, the flow will be low ; an easy check is to run one tap, then turn on some others, and see if its flow reduces. It is a real problem with simple electric showers, because flushing a toilet makes them run hotter.
A pump might not help very much, because it will be trying to suck water out of the mains, and that causes cavitation, which is noisy and inefficient.

The hot supply is worse, because it also has to go through the boiler. Try the same test on some hot taps.

If you only need water at intervals, the fix is to store it up slowly, then release it quickly, like the cistern of a toilet.
With a gravity tank, the pressure of the storage is limited by the height of the building ; but imagine putting a lid on the tank, so the air pressure increased as it filled up. That could store water, and give as much pressure as the supply, like having a tank on top of a flagpole ; but as the tank empties, the air expands, and the pressure drops. It's how Hydrolastic Suspension works.

Now I'm guessing a bit ( more ). What if instead of pressurised air, the tank had two halves, separated by a rubber sheet, with the cold mains supply feeding the other half. When hot water is used quickly, the heating system has a large restriction, so the flow into it is low ; but the pressure from cold side can push the hot water out of its half of the tank, and with cold side being fed directly from the supply, the flow is higher.

When you next use cold water, the rubber sheet tries to return to mid-position, so some of the cold water is delivered from the cold half of the tank ; and water gradually flows through the boiler, to "inflate" the hot side, until the tank is again split 50:50.
Perhaps instead of a rubber sheet ( which would chill the hot water, and warm the cold ) it is two tanks, with a common air-space above them. As the hot water drains out, the cold level rises, and the air volume and hence the pressure, stays the same.

It doesn't "double the amount of water pressing down on the supply" ; but builders are artisans not scientists, so we should forgive them finding a way to understand the devices they install. I couldn't make a brick wall.

I'm betting it also has another name.

bob_a_builder

Uk gravity hot & cold water supply is considered low pressure and yes low pressure tap's are available,so the tap should have been the first consideration.

I know that too, and the tap was quoted as suitable for low pressure down to 0.1 bar
but as you say "adverts dont always mention the minimum flow rate required for the tap to work correctly."


1 Grohe Eurosmart Cosmopolitan Basin Mixer Low Pressure - GHTAPB202

http://www.plumbworld.co.uk/grohe-eurosmart-cosmopolitan-18008-29788

EachPenny

bob_a_builder wrote: »

Has anyone heard of a 'straddle Tank' ?

I believe it is another name for a type of steam locomotive more commonly called a "saddle tank". but I'm not sure you'd want to have a steam loco in your loft

bob_a_builder wrote: »

"Another solution if you should like to keep with the new taps is to add a straddle tank to the hot and cold supply in the loft, effectively increasing the head by doubling the amount of water pressing down on the supply.

What this sounds like is where space restrictions in a loft prevent the use of one large tank one solution is to use two smaller tanks connected together with pipes between them near the bottom. The supply enters one tank and the feeds are taken from the other to ensure the water stays fresh. There are various names for this kind of arrangement, but 'straddle tank' isn't one I've heard before. However, I could see how the name might be used because in a way the tank(s) might be straddling (in an inverted sense) a beam or other obstruction in the loft.

Unfortunately though I think this might be a case of misunderstanding of the physics of plumbing. The 'amount of water pressing down on the supply' will only increase pressure if the amount is in a vertical column. A larger volume of water alone won't increase pressure at the outlet - as Gloomendoom says it is about height (head).

It is rare that increased storage volume (capacity) will help. The only time that capacity might become an issue is if the flow of water out of the tank significantly exceeds the rate it is being replenished by the mains. If the mains cannot keep up then the level of the water in the tank will drop. As it drops, the head will also reduce because the top surface of the water is no longer as high, and therefore the outlet pressure will reduce.

Installing a larger tank, or a parallel tank (possibly this 'straddle tank' arrangement) would mean for the same rates of flow in and out, the top surface level of the water will lower by a smaller amount - so the 'head' will reduce more slowly and therefore the pressure at the outlet will remain higher for longer.

So if the 'straddle tank' is what I think it is then it might (in the right circumstances) overcome a loss of pressure issue - but not for the reasons given in the source link.

Apart from increasing the available head, the only other thing which can help with low pressure/flow rates is to increase the size of the pipes feeding the tap. This won't increase the head, but using say a 22mm pipe rather than 15mm pipe the losses due to friction within the pipe are reduced. Reducing the number of bends and making them a larger radius also helps reduce loss. Also bear in mind that modern taps are usually fitted using flexible coupling pipes which are often of a tiny bore size - these act as a significant flow restrictor in gravity fed systems.

To be honest it is unlikely that any of these changes will significantly change the flow/pressure and it would make more sense to simply change the tap for one better suited to a gravity fed system.

bob_a_builder

it would make more sense to simply change the tap for one better suited to a gravity fed system.

Yes - I think it a combination of lack of head and the small flexi pipes.

But I had thought I had bought a tap suitable for low pressure (see above)

Given I only have a hole for one tap, can anyone recommend a more suitable tap ?

EachPenny

bob_a_builder wrote: »

Yes - I think it a combination of lack of head and the small flexi pipes.

But I had thought I had bought a tap suitable for low pressure (see above)

Given I only have a hole for one tap, can anyone recommend a more suitable tap ?

It might be a silly question, but I notice the tap is fitted with an internal adjustable flow limiter. The manual says the tap is supplied with the flow rate set to maximum - have you checked to make sure the limiter is actually set to maximum flow?

tired_dad

Our high flow hansgrohe taps were fitted to a low pressure system. The tap outlet had a high pressure adapter that was swapped out for a low pressure one and all was fine

We have lots of bathrooms. The Gravity fed is fantastic in that there is no pressure drop when multiple showers are taken. Hardly any moving parts. Hardly ever needs servicing (if ever).

I'm v happy running taps and sinks on low pressure system. Then have a 4bar pump (Stuart Turner) rinning hot and cold to each of the showers/baths.