Water flow for combi

drummer_666

Hi,

I need to replace the boiler in the house I've just bought. It's seriously old, floor standing one with corroding steel pipes etc

Anyhow, I really want to replace with a combi, so that I can lose the tank in the bathroom and rearrange the layout upstairs. The room that currently has the tank would become a bedroom.

The water pressure in the street is bad, neighbours said they have had problems, the water board has done major works a few years back to improve the pressure, which it did, but it's still relatively low.

The max flow rate on the boilers I've been recommended is between 11.5 -13 litres per min.

When I had people out to quote it was 5.6 l/min. We've since taking an internal pipe out which was clogged. Increased the flow to 8.5 l/min (this is after the boiler people came around).

One person quoting said pressure/flow is fine (ignoring him coz he lied about stuff, swaying us to combi), another said it is def too low, third person said it was similar to his a few streets away and he had combi and it was fine. They all tested by eye only.

I'm confident that if the flow was 10 litres per min it would be fine for a combi. But is 8.5 litres per min good enough, when the max for the boiler is 11-13?

What does less flow actually mean? There would be one shower/bath using it. Second shower would be electric.

Thanks a lot

gas4you

8.5 is very low.

I was under the understanding that the water authority had to supply a minimum of 9l/m and 1 Bar pressure?

You could investigate having a new larger mains supply fitted from the stop c o c k in the street. This would improve things as long as the flow is there in the street main.

Get your water authority to check what the flow and pressure are in the street.

From what you have now, there would be no point in getting anymore than a 24kw combi, and these are no good at filling baths in the winter.

If it can't be improved, I would recommend a traditional tank fed system as you have now.

You may find that what you have now is not enough for an electric shower either, as these need around 1 Bar working pressure to function.

ListysDad

There is another way... but.

The op didn't mention the actual pressure achieved either static or dynamic. If you have at least 1bar dynamic then it is possible to fit an accumulator where you store a reasonable quantity of mains pressure water for showers and the like. Typically they work best when coupled with either a thermal store or unvented cylinder. That way you get over the combi issue and also have as good a flow as you can get at best pressure.

BTW one can also get a 'pressure set' to up the accumulator pressure. Unfortunately, none of these solutions are low cost though they do deliver.

HTH

drummer_666

ListysDad wrote: »

There is another way... but.

The op didn't mention the actual pressure achieved either static or dynamic. If you have at least 1bar dynamic then it is possible to fit an accumulator where you store a reasonable quantity of mains pressure water for showers and the like. Typically they work best when coupled with either a thermal store or unvented cylinder. That way you get over the combi issue and also have as good a flow as you can get at best pressure.

BTW one can also get a 'pressure set' to up the accumulator pressure. Unfortunately, none of these solutions are low cost though they do deliver.

HTH

Do you also think 12 litres/min is low? That's what it is in my rented house and that has a combi.

How do you measure the pressure?

You say they are not low cost, are we talking a say £200 or say £500+

Thank you

drummer_666

At the moment the cylinder is in the bathroom and the tank is above this (still in the bathroom)

Would I be able to move the cylinder and the tank to the loft? I guess this would make pressure in the upstairs bathroom better, and the downstairs pressure would remain the same?

I guess if I moved it, it would prob be best to replace the cylinder and perhaps tank also?

How would I know if the loft can take the weight?

If the loft wouldn't take the weight and it's not tall enough, another option would be to move the cylinder into where the new upstairs bathroom will be.

ListysDad

Firstly, flow is a complex equation of using among others things pressure, resistance and orifice size but the two key items pressure and flow can be measured reasonably simply. There are two pressures we are interested in; static (the pressure your system rises to when there is no water flowing) and dynamic (essentially the pressure measured when two taps are open).

The flow rate I'd look for for a good shower is 18 litres per minute. Get a big container and fill it from say a 1litre jug marking each litre as it fills. Then open a tap into it for 60 secs - that gives you your best flow. Do it again but with another tap running - that gives you your worst flow.
You can buy a pressure gauge from screwfix and attach it to your OS tap. When all other taps are off open teh OS tap. That will be your static pressure. Open another tap and measure again - that's your dynamic pressure. Bear in mind that all static pressures rise overnight as the system is used far less.

As I've said, 18l per min will give you a very good shower.

If you only end up with say 10lpm and less than 1 bar dynamic pressure then you're going to have to run round a lot to get wet. Then you might consider the accumulator idea. Basically the tanks sits there and fills up to whatever the static pressure is or less - if you have a pressure reducing valve in the supply line. Then when you use the shower water is pushed out at a pressure more akin to the static but gradually dropping and at a better flow rate as there is far less restriction to flow.

At the moment you have beggar all pressure! 1 meter between the underside of the cold water storage and the position of the shower head equates to 0.1bar. This means you'd have to lift your cold water storage tank to 10m above the shower head to get 1 bar. This is why people fit pumps.

Personally, and depending on access to teh loft, I'd fit two tanks and lift them as high as I could (whilst being able to maintain them) in the loft. Properly constructed the platform is no problem in terms of weight. Do NOT bodge it tho as you'll end up wearing it otherwise!

By fitting a good quality shower (Aqualisa or Mira) you'll get pretty reasonable flow rates without pumping. The shower internals have to be very well engineered to reduce internal resistance as resistance is teh enemy of flow... Also, if you want to squeeze the best flow out of it, use 28mm pipework to the cylinder and to as close as you can to the mixer valve before reducing down. If using plastic use stainless steel inserts as they are less restrictive to flow or copper only and ensure all the ends have NO internal chamfers.

HTH

Mr_Ted

No one has yet mentioned pressurised DHW Cylinders?

You could put one in the loft space as long as the loft will withstand the load, and you will get almost mains water pressure from it and a good heat recovery rate from a Modern one so that you will have good pressure at all taps and be able to run a shower easily and fill a bath consecutively with no problems!
There much like a conventional system but without the CWS tank to feed it in the loft!

They do need some maintenance and install by a Qualified installer for Pressurised Systems!

As for plastic pipe, if you use that you lose all earth integrity if you have any associated electrics with shower, boiler or Cylinders, which you most certainly will have?

Oh and why use a word like Static and DYNAMIC, when it so much more understandable to say STANDING and WORKING pressures???

Think maybe a Solicitors gobbledegook, lol!

ListysDad

Mr_Ted wrote: »

No one has yet mentioned pressurised DHW Cylinders?!

Except here, "best when coupled with either a thermal store or unvented cylinder"

I use static and dynamic cos that's what they are. It's all about edgeyamakashun...

It wasn't picked up on tho as you have demonstrated!

LD

Mr_Ted

lol yep sales talk again eh lol

Why make it simple when you can baffle lol

ListysDad

you can still buy me a beer!

drummer_666

ListysDad wrote: »

You can buy a pressure gauge from screwfix and attach it to your OS tap. When all other taps are off open the OS tap. That will be your static pressure.

I'm pretty sure I don't have an outside tap... how do I measure it without one?

I have the waterboard coming out on Wed to check the pressure etc being fed from pipes into my house. They said 7sec per litre was borderline for them... meaning they think 9 litres per minute is good enough...

Aren't unvented cylinders expensive, and as you say the require maintenance... how much maintenance? From my reading, they seem less reliable than vented cylinders

How will I know if my loft can withstand a cylinder?