Improving hot water pressure for mixer tap

[BenjyK]

Hello everyone. This is my first post on the forum after much lurking.

I have very low hot water pressure in my upstairs basin mixer tap. Being in a maisonette there is hardly any head height from the tank (about 0.8 metres). However there is decent enough flow from the bath tap (separate hot & cold taps) which only slightly lower (a further 0.4 metres). This has led me to conclude that whoever fitted the tap probably didn't choose one suitable for a low pressure system.

The minimum pressure tap I have been able to find is 0.1 bar. It has M10 connections and the flexi tails included - but they look like they will still be restricting flow somewhat as the aperture of the tails is pretty small especially at the compression nut end. So I have been considering using some copper tap tails instead. I have also been searching for a monobloc mixer tap which has larger M12 connections for the best chance of maximising flow, however, I've not been able to find any... in fact you practically never ever see thread size included in specs!!

If anyone's had a similar problem perhaps you could advise? Am I on the right lines... or getting a bit carried away? I don't want to fit the new tap only to find the flow is still poor. I'd also like to exhaust all other options before fitting a pump on the whole hot water system.

Thanks!

 

[terryplumb]

The fact is seperate taps will always out perform monobloc taps on low pressure gravity systems. If you have isolation valves on basin Pipework ensure they are full bore. Monobloc with copper tails give marginally more flow than those with Flexi tap tails. Don't know exactly what you are expecting ,but if you expect to get the kind of flow that you have from cold mains kitchen tap ,you will need a pump !

 

[Madrab]

Yup, the only way you can really maximise flow on a gravity system is to use single full flow taps (no monobloc cartridge taps) and fully sized pipework to the taps. That will be what the bath uses and therefore it maximises the flow.

Monoblocs will always restrict the flow. The only other thing you can really do is source a monobloc tap with screw down taps, not 1/4 turn type.

 

[BenjyK]

Thanks, I appreciate the input. I didn't know that monoblocs were more restrictive than single taps.

There are no obvious restrictions being caused by the connecting pipework. I don't need to get it anything like mains pressure, but just enough improvement to avoid having to fit a pump. As it is now, I just measured a measly 1.71 l/m.

I've had a good search and found that some brands list flow rates down to 0.1 bar, which is helpful. I didn't see any correlation between given flow rates and whether 1/4 turn or not. But it's a bit of a minefield actually as I've found that Victorian Plumbing and Tap Warehouse seem to list much higher flow rates than what the manufacturer's data sheets say in many cases. I'm investigating this further.

I'll update again once I've fitted a replacement to say if it worked.

 

[terryplumb]

1.7 L/ minute is dire. Can you show pics of pipework going to tap ?
I assume the hot and cold are both gravity fed ??
Does the cold give the same flow rate ?

 

[oldbuffer]

1. Monobloc taps have to fit a standard (generally) 35 mm hole in the basin / sink.
2. Inside the threaded shank which fits into that 35 mm hole, there is not enough room to fit 2 x 15 mm pipes after allowing for the wall thickness of the shank.
3. They are therefore always fitted with:
3a. Very restrictive flexible hoses, which typically have a 10 mm internal bore. OR
3b. Copper tails, which are narrower where they go through the tail, increasing to 15 mm at the pipework connection end. Less restrictive than flexis, but no as good as 15 mm pipes.
4. Ceramic insert (1/4 turn) taps have two ceramic discs inside, each with two diametrically opposed slots. The faces of the discs are grounds extremely flat so that they form a water tight seal. The slots each occupy just under 1/4 of the diameter of each disc, a little way in from the outside. When closed, the slots in the upper disc are covered by the unslotted portion of the lower disc, and no water flows. When open the slots of the upper and lower discs are in alignment, so water can flow through. However, the area through which the water can flow is much less than that offered by a traditional tap washer being raised off its flat seating, so flow rate is less.
5. If your basin is designed for a monobloc tap, it will have a single tap hole, so you don't have the option to convert to separate taps unless you change the basin.
6. Finding a tap set genuinely capable of operating at very low pressure is your only hope without installing a pump.
7. If you measure from the base of your cold water cistern to the floor, and from the outlet of your tap to the floor, subtracting the second measurement from the first (in metres) and dividing by 10 will give you a rough approximation of the pressure you have available. (Assuming the floor levels are the same!). So if the base of the cistern is 2 metres above the floor, and the tap outlet 1 metre above you will have ((2 - 1) / 10) = 0.1 bar.