Great water pressure; it should be a requirement in any home, shouldn’t it?

From reading my previous blogs, you may already know that this is probably the biggest benefit when choosing a system boiler with unvented cylinder over a combi boiler.

But will you always be guaranteed an incredible flow rate with an unvented cylinder?

In theory, yes.

In this blog post, I’m going to explain all you need to know about water pressure and the flow rate to your taps – and what could interfere with it.

The best part about unvented cylinders

Firstly, let’s go back to the initial question; in theory, you should always get a higher flow rate with an unvented cylinder, because of two things:

  • The storage capacity the cylinder provides will always give you a great flow rate
  • The expansion vessel fitted inside will ensure a consistent, steady flow

Yes, so we’re all fine and dandy on the unvented cylinder.

However, there is one more consideration: an unvented cylinder will only be as good as your mains pressure.

When your water pressure is low

If your mains water pressure (or flow rate) is particularly low and only allowing for 15 litres per minute, there’s not much point in installing a boiler with an unvented cylinder capable of giving you 25 litres per minute – you won’t experience the benefit.

It’s possible you may experience this problem if you’re living in an area with low water pressure, or own an older property or flat with much older lead cold water main supply pipe

However, if you’re in an area where the mains water has been upgraded, you should expect an average of 17 to 25 litres per minute of water to your home.

To find out if your mains pressure is to blame for your low flow rate, ask a plumber or engineer to perform a flow rate test and pressure test.

Don’t want to put up with low water pressure? The good news is, I’ve actually provided some potential solutions for low flow rates in this blog.


In theory, you should always get a superior flow rate when you have an unvented cylinder installed, due to the storage capacity and expansion vessel which does a great job of providing a consistent, high flow rate.

However, your unvented cylinder’s performance will only be as good as your mains water pressure. So, if you’re living in an area with only 15 litres per minute, you will not be able to experience a 25 litre per minute flow rate as promised by your high-capacity boiler. Which is a great shame.

Ask an engineer or plumber to perform a flow rate or water pressure test for you to determine if this is the problem. I’ve also provided some solutions to low water pressure in this blog.