Plumbing Question

Options for change of hot water system and increasing water pressure

Hi everyone,

I currently have an old and tired vented indirect hot water system in my Edwardian 3 bed terrace. I want to increase the space in the bathroom to put in a separate shower and bath (where the hot water cylinder is installed) and also increase the hot water pressure, that is currently VERY poor due to the low head of water from the header tank in the loft.

Everything is old, worn and rusting so I am happy to have it replaced.

I have around 2.5bar static and 2 bar dynamic pressure on the mains but suspect I have an old narrow gauge supply rather than a modern 25mm MDPE. I can't even find a stop-tap or the incoming supply, other than out in the street to check this at the moment.

So, I doubt that retaining the vented system will work very well. Options as I see it are:

1. install a new combi boiler
2. install a unvented system (megaflo or similar)

Has anyone done this and got first-hand experience? Obviously the combi is the simplest system but do you get decent pressure, as I assume you can’t add a pump onto this?

Any thoughts and comments welcomed.

Many thanks

-----------------extra info--------------------


Thanks for the response and for the tips on pressure check, power flush and magnetic filter - great stuff!

I measured the flow rates this morning in the bathroom to compare what I'm getting from the bath taps and shower. Results below:

Cold bath tap 0.2 L/s (12 L/min)
Cold shower head 0.15 L/s (9 L/min)

Hot bath tap 0.2 L/s (12 L/min)
Hot shower head 0.05 L/s (3 L/min)

The tank is sitting on the joists about 0.5m above the height of the shower head, which in turn is 1.5m above the bath taps. Taps have 2m of head on the hot, shower has 0.5m head. Cold water ruyns off mains. There is a small reduction in flow on the cold, I assume due to the reduced shower hose diameter.

Would these figures be okay for a combi? Would it help to have flow rate from the bib tap outside where I measured the pressures?


2 Answers

Best Answer

It would be of benefit to get a flow rate as well while at it, ie liters per minute.
That would give you some idea of possible pressure loss at the loft level.
If not much go with combi.
Combi is more cost effective of two but it does have pressure fluctuation when two taps open at same time. This can be minimized with a good pressure balance shower valve like Grohe G100 or similar but it will not cure it just minimize it a bit.
Megaflow would not have this issue but it is costly and requires more space then hot water storage tank, green one on 1st floor.
Also very important to isolate and pressure test your central heating system at 2.5bar for minimum of 48 hours to check for leaks, keep in mind pressure relief valve at boiler side is set to activate and release at 3.0 bar.
Also a must, proper power flush prior to new boiler, chemicals to old boiler and magnetic filter fitted to new boilers 22mm return pipe of central heating system. Flush out new system with clean water to get rid of any residual flux and etc, prior to opening boiler heat exchanger valves and filling with inhibitor.
Tip: pressure gauge on boiler is to measure radiator pressure only and has nothing to do with mains water pressure; common misinterpretation.
Hope it was informative

Answered 19th Mar 2013


Member since 12 Feb 2013

You need to make sure that the flow rate you are measuring is mains or tank fed, on a bath you can have mains or tank fed supply. The cold supply on the kitchen sink should always be on the mains. 12l/min isn't sufficient in my opinion to warrant having an unvented cylinder. Make sure you are measuring your water main, not a tank feed. A good combi will give you 12l/min of hot water but when you open 2 taps you will struggle too maintain that flow. If that is your main supply, then there is an option for a Home Mains Booster Pump. I think Grundfos do one which is essentially a 200l storage vessel which stores water from the main and then boosts it too required pressure and flow which will support more options.

Answered 18th Sep 2013

Broughton Plumbing & Heating

Member since 18 Sep 2013

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