unvented cylinder installation current combi vs. new system boiler

8 Mar 2024
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United Kingdom
We have recently bought a 5 bed detached which has a combi boiler servicing the ensuite, kitchen and the radiators. there is also a vented cylinder which serves the downstairs cloakroom and family bathroom where the pressure is very low. we wanted to move to a unvented cylinder but are getting varied opinions on the boiler. Some say we need to rip out the old combi and get a system boiler and other are saying combi can be used with the unvented cylinder with no problems. Not sure what the way forward is. I'd like to not throw away a working combi. Pressure in mains is good to get an unvented cylinder but not sure of how to proceed on the new boiler vs. keep combi to heat the unvented cylinder.
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1. Combi can be used perfectly well with a hot water cylinder, vented or unvented. It is set up as "another radiator" on a separate zone.
2. However, you need to make sure the incoming main is sufficient to provide at least 20 litres per minute at a dynamic pressure of around 2 bar to make it worthwhile. For a combi alone you need more like 15 lpm at 1.5 bar.

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