Switching to an Unvented Hot Water System: What Flow Rate Do You Need? (2023 Update)

Hi all,

At the start of the process of getting a heat pump installed so good opportunity to switch to an unvented system. Currently hot water is from a vented system and both hot and cold water are gravity fed (cold water tank in loft).

The installer engineer took one look at the mains supply and said it was

22mm ish plastic so probably equivalent to 15mm copper and hence the flow rate wouldn't be enough to go unvented.

I've measured the flow at the kitchen tap (only mains tap aside from tank fill valve, looks like 15mm copper feed) and it does about 17.5l/min but haven't got a pressure tester to test the dynamic pressure. I had a quote from BG for a new boiler and he had a proper tester which showed the flow was good enough for a combi (IIRC which I may not be, was a while ago) if I wanted but don't remember the figures.

It'd be good to get rid of the shower pump, have decent pressure at all taps and reclaim a huge chunk of loft space but wouldn't want nasty surprises when taking a shower as someone else turns a cold tap on :)

Thoughts? What flow rate would be needed practically to have an unvented cylinder?

Thanks,

Peter.

Reply to
Peter Spikings
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Howdy there Peter! It sounds like you're considering switching from a vented hot water system to an unvented system, but are concerned about the flow rate of your water supply.

When it comes to unvented cylinders, the minimum recommended flow rate is 18 litres per minute. This is to ensure that the cylinder can be filled quickly enough to avoid any pressure drops or fluctuations, which could affect the performance of your shower or taps.

However, it's worth noting that the actual flow rate required will depend on the size of the cylinder and the number of outlets you have in your home. If you have a larger cylinder or a lot of outlets, you may need a higher flow rate to ensure that your hot water supply is sufficient for your needs.

In your case, it sounds like you have a good flow rate of around 17.5 litres per minute, which is close to the recommended minimum. It's possible that this may be sufficient for your needs, but it's hard to say without knowing more about your specific requirements.

If you do decide to switch to an unvented system, it's important to ensure that the installation is carried out by a qualified and experienced engineer. They will be able to advise you on the best type and size of cylinder for your needs, and will ensure that the system is installed correctly and safely.

Tools and materials you may need for an unvented cylinder installation can include:

  • Unvented hot water cylinder

  • Pressure relief valve
  • Expansion vessel
  • Thermostat
  • Pipe insulation
  • Fittings and connectors
  • Copper pipes
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