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Ardent Engineering
Ardent Engineering, Engineer
Category: UK Plumbing
Satisfied Customers: 91
Experience:  Registered Gas Installer
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We have low water pressure from the mains and a combi boiler

Resolved Question:

We have low water pressure from the mains and a combi boiler (so no water tank). We can have showers no problem however if someone uses another tap in the house, the water pressure in the shower disappears. How should we fix this? We especially need a fix since we are having another shower installed in our second bathroom and would like the pressure to be good and also for two showers to be on at the same time without issue.
Submitted: 4 years ago.
Category: UK Plumbing
Expert:  Ardent Engineering replied 4 years ago.

Ardent Engineering :

In-depth knowledge of electrical and mechanical engineering services. Vaillant boiler questions welcome.

Ardent Engineering :

Hi there, this is a common problem and there a few different means of addressing it.

Ardent Engineering :

Firstly, you have to consider how capable your combi boiler is in delivering hot water, because they are inherently limited in this respect.

Ardent Engineering :

The throughput capacity of a combi boiler is anything from 8 to 16 litres per minute

Ardent Engineering :

You generally need 12 litres minute for a good shower

Ardent Engineering :

If you provide the details of your boiler I can identify its capacity

Ardent Engineering :

There is little merit in spending too much on boosting your Mains Cold Water if your boiler is limited, so we should work from the boiler's capacity.

Customer:

We are getting a new boiler so will get whichever boiler is best for this.

Ardent Engineering :

In that case, for two showers, a combi boiler is not the best option

Ardent Engineering :

You would benefit more from having an Unvented Domestic Hot Water cylinder, indirectly heated by a separate boiler.

Customer:

I think we are limited with storage space though so would need a similar sized boiler and can't really get a water tank?

Ardent Engineering :

I see, space limitation is usually the crux in domestic situations

Customer:

What are we talking in terms of space required for a cylinder and boiler?

Ardent Engineering :

OK, you could stick with the combi boiler option - say a Vaillant EcoTEC 831 which should be good for 13 litres/min

Ardent Engineering :

The problem is, if you have low Mains pressure, you're likely still going to need to boost it, so you're likely to need: Boiler, Cylinder and Booster

Ardent Engineering :

At least a cubic metre for each, and that's a very tight squeeze

Ardent Engineering :

If you want the best performance from either system, you may need to boost the Mains - see this product:

Ardent Engineering :

But this costs more than a boiler

Ardent Engineering :

If your mains pressure is less than 15 litres/min at 1 Bar, there's no other way to honest

Customer:

Just to be clear: the fix would be to get a boiler, cylinder and booster. Would the booster fit in the 2 cubic metre space as well?

Ardent Engineering :

No, you would likely need a separate self-contained plant room

Ardent Engineering :

We have carried this out often enough to know there is no short cut

Ardent Engineering :

It really depends on how bad the Mains supply is

Customer:

Ok, so say we don't have enough space for a plant room (we don't), what then?

Ardent Engineering :

If the mains supply is marginally less than the capacity of the boiler, then you could also get the booster...

Ardent Engineering :

but the booster supplies way beyond a combi boiler's capacity, so there's a high surplus of performance

Customer:

I am lost. What do you mean by "If the mains supply is marginally less than the capacity of the boiler, then you could also get the booster..." I think the issue now is that we can't fit the booster in?

Ardent Engineering :

If the boiler's capacity is only marginally under-supplied, you could add an expansion vessel to the supply

Customer:

I see. What are the figures in terms of mains supply?

Ardent Engineering :

You will need 15 litres min at 2 Bar to supply the boiler

Ardent Engineering :

It sounds like you don't have that, but it would be best to confirm what you do have, then calculate the measures required to increase it

Ardent Engineering :

The Homebooster's dimensions are 600 x 600 x 1000

Ardent Engineering :

The Vaillant EcoTEC models are all (bar one or two) ~ 400 x 350 x 800

Customer:

Ok, we will look into the mains supply. I have read on other sites that if you just have a cylinder this won't help. It's down to the mains supply. I guess this is where the booster comes in?

Ardent Engineering :

Absolutely - the cylinder is passive, it only lets the pressure and flow through that's available on the inlet

Ardent Engineering :

The Grundfos Homebooster is the most compact solution you're going to find, and the best balance of cost too

Customer:

Ok. Does it matter where the booster goes in the house?

Ardent Engineering :

No, not really

Ardent Engineering :

Within reason

Ardent Engineering :

It's possible to have it separate from the other plant

Customer:

Ok, thanks.

Ardent Engineering :

If you can get it immediately downstream of the Mains Water Stop Tap then that's the best location schematically

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