Boiler / Hot Water Pressure advice needed

DettaWalker

Hi,
first of all thank you for reading and merry Christmas
We've just moved into our new 4 bed house. We're planning on installing a new ensuite to the master bedroom as the house only has 1 family bathroom. We have engaged a local business to do the work and are awaiting their quotation. They have given some initial advice (which is fitting an unvented cylinder) but I'd like a second opinion.
We currently have a boiler that feeds a vented hot water cylinder in a gravity fed system with a cold water tank in the loft. The hot water cylinder is relatively small, maybe 140/150 litres. When I run a bath, as it gets to the top, the water gets cold. The house never had a water softener fitted and we live in Reading where water is terribly hard (we just had our old water softener installed to remedy that). I imagine there is a lot of limescale in the bottom of the cylinder reducing its capacity even further. The house is quite old, built in 1965. 
We don't know the water pressure at the moment and I'll ask the company to measure that next time they are round but I did measure the flow at the kitchen tap which is right next to the CW supply coming in (and off the water softener due to drinking water regulation). The flow is 8l/minute.
Having asked people who live up the road (uphill from us) they were unable to fit an unvented cylinder due to insufficient water pressure. This makes me wonder whether it is even possible to do this.

Here are my questions:
1) Is the kitchen tap on mains supply a reliable way to measure flow? We do have an outside tap, but I don't know how this is plumbed.
2) I'm sure we won't be able to do an unvented cylinder as we probably lack pressure, and after reading up on combi boilers, I don't think we have sufficient flow in the house to run two showers simultaneously, let alone heating + bath in winter. Any other opinions?
3) Is there any way to improve hot water pressure on all taps with a vented cylinder? I don't want to move the CWT in the loft as a metre only gives you 0.1  bar pressure and  our loft isn't that big to begin with - so it may not even be possible. The kitchen tap is a problem as it has mains pressure for cold water and there is a struggle to get warm water out of it as the cold water pushes past the hot water giving you a very narrow range to achieve anything but cold or very hot water.
4) Is there any other way we could still have an unvented cylinder? I read the minimum pressure is 1.5 bars, is flow important?
If there's any other advice.. appreciate it.
Thank you for your help!
Regards
DW

southcoastrgi

Flow & pressure are 2 different things,
test from the outside tap 
if you fit unvented most if not all your water will be off the mains & 8 ltrs/min is no where near enough flow for unvented or a combi 
if you want more hot water then fit a bigger cylinder (although you may need a larger roof tank as well to supply it) 
you could fit a pump to give you better h/w pressure but just bare in mind this will empty the cylinder faster as well 

grumbler

Start with reading these recent threads

Boiler advice

Water Pipes

Kitchen and outdoor taps, most likely, are the only accessible taps in your house to measure the flow. The only other place is the tank in the loft.  Just keep the ball valve open and measure how fast it fills (in cm/min).

southcoastrgi

grumbler said:

The only other place is the tank in the loft.  Just keep the ball valve open and measure how fast it fills (in cm/min).

You can't test from here 🙄

grumbler

Why?.......

Jeepers_Creepers

Hi Detta.

8lpm is below the minimum that WBs are obliged to provide (at street level), so it can be considered completely pants by any standard. You'd need to have that flow rate confirmed by a plumber, but if it's anywhere close to that, then both a 2-outlet combi and an unvented cylinder are pointless - unless you also fit a mains booster tank (such as Grundfos and Challis).

Let's assume that your incoming mains is 8, 9, 10 or even 11 lpm and cannot be improved by running a new incoming pipe. Ie, that's it - you are stuck with it. That is completely rubbish flow and will be dreadful to live with. The only thing it's any good for is filling your kettle at the kitchen sink. You'd therefore be nuts to consider a combi of any size, and certainly not a high-flow one. You would similarly be bonkers to contemplate an unvented system for the same reason.  

That leaves your choice as;

(a) improve your current vented system; once you fill that tank in the loft, you have all that lovely water to play with - you can do lots with it. Or,

(b) fit a Grundfos or Challis or Salamander in your garage which will take the mains water, store it, and pressurise it to 3 or more bar. This will give you awesome pressure and flow, more than enough for a super-combi or unvented system.

For (a), you can undoubtedly improve what you currently have by raising that CWS as far as possible. 0.1bar improvement might not seem a lot, but it's coupled with large pipes (22mm) going to your baths and showers, so should actually provide you with a good flow; you choose good quality shower mixers designed for low pressure systems. You also check that the supply pipe from the CWS to the vented cylinder (which you will increase in size) is as direct as possible, nicely downwards-sloping at all points, and could even benefit from being upped to 28mm. Ditto the pipes from the CWS and vented cylinder to the baths and showers - 22mm, but carefully routed. Finally you can add booster pumps, tho' I personally hate them...

For (b) you'll need the extra storage space for booster pump, but they do work, and really well. If you manage to almost empty it, it'll still refill from the cold mains at the max allowed rate or 12lpm to keep it topped up, and that itself is better than what you currently have!

Finally, yes, system boilers with a storage cylinder (vented or unvented) have far fewer moving parts and are far less likely to go faulty.

DettaWalker

Thank you Jeepers - very helpful indeed. According to people in the street, water pressure is poor, especially as you go uphill. Someone at the bottom of the street has an unvented cylinder and it works great for them, so I will definitley have my plumber confirm flow & pressure to see whether it isn't possible. I do wonder if poor flow is caused by limescale deposits in our pipes (we have one of the hardest water in the country). Anyway, on to your suggestions.

A) Sounds like the cheaper option (correct me if I am wrong) - all I'd have to do is upgrade the vented cylinder to a bigger one to provide more hot water, potentially upgrade the loft tank to a bigger one and fit a pump to the new shower to pressurize it.

I had a look at these systems - I couldn't find a mains booster tank but an accumulator for cold water storage - is that what you are referring to? it mentions you need a charger pump if pressure is poor but doesn't specify poor in bar... , so here I'd have to get the accumulator and potentially the pump (which is probably noisy?), upgrade the water tank to a bigger size or fit a combi boiler. prices I've seen are at 2k for the accumulator alone + new boiler at around 1500 or unvented cylinder at around 2000 + pump

sound about right? Again thank you for your advice

Jeepers_Creepers

They combine cold water storage along with a pump.

I think they operate in one of two ways; most (Grundfos, Salamander, Pump World etc) fill their tanks normally with mains water, and will then pump it on demand to the house's supply. Others - Challis comes to mind - works differently by pumping the mains water at the required max rate of 12lpm into a pressurised accumulator (which has an expansion vessel inside it). This then stores the water under pressure, so it's released to the house under this 'unpumped' pressure.

The only one I have any experience of is the Grundfos*, and this was used in a 6-storey flat where water trickled out the kitchen tap. With the Grundos, the flat owner reported showers to be almost 'painful', and he actually had a pressure reducing valve fitted to its outlet to calm the 4+ bar pressure!

I personally prefer the 'idea' of the Challis where the tank is pressurised in advance, so the water is then released in a more natural way without a pump firing up every time to drive it like with the Grundfos.

For either, the very best solution would be to fit it outside the home, in the garage or perhaps a purpose-built insulated 'shed'.

* https://www.anchorpumps.com/grundfos-4-5-bar-home-booster-240v

https://www.challisshowers.com/about-the-company

https://www.pumpworld.co.uk/Our-Products/ProdDetail/catid/23/eid/45240/smart-rectangular-fixed

southcoastrgi

grumbler said:

Why?.......

Because the ballvalve will have a mains pressure seating in it which has a 3mm hole in it so that will in no way give you an indication of actual flow rate available 

grumbler

southcoastrgi said:

grumbler said:

Why?.......

Because the ballvalve will have a mains pressure seating in it which has a 3mm hole in it

Is it really the same as in the toilet?

For toilet 3mm looks plausible, but I thought it was a bigger valve in the loft.

southcoastrgi

High pressure (mains)

Low pressure (fed from roof tank)