Combi or pressurised system

20 Jan 2021
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United Kingdom
I currently have a 4 bed converted bungalow which has 2 showers, a bath and 10 radiators. The property is being converted to have 5 bedrooms, 3 showers, a bath and 12 radiators. The boiler is a Worcester 24Kw combi. It works ok but we do get pressure issues sometimes when someone is in the shower and a toilet is flushed or the washing machine is on.

One of the builders who has quoted for the job has said that the combi needs to be replaced for a pressurised system and a tank. Another builder says that a combi will be ok but a bigger one will need to be fitted. I mentioned it to a plumber who said that combi's can be used for larger properties but its not recommended due to the pressure issues.

I am not sure what to think. A bigger combi will be the cheaper option but if there are pressure issues once installed, we are stuffed.

Any advice greatly received.
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1. The key questions relate to the incoming mains cold feed, and are:
1a. What is the static pressure. E.g. pressure at open outside tap with nothing else running.
1b. What is the dynamic pressure. I.e. pressure at outside tap with kitchen tap running.
1c. Flow rate taken from mains connected tap (usually kitchen tap) with all valves between meter / water board connection and tap fully open and non-restrictive.
2. If the dynamic pressure is less than 2 bar (and preferably 3 bar) OR the flow rate is less than 20 litres / minute (and preferably a bit more) then as its stands, it would not be suited to an unvented (pressurised) hot water system.
3. If the pressure is less than 1.5 bar OR the flow rate is less than 15 lpm, then a big combi will struggle to achieve its maximum hot water output.
4. If the above tests "fail" you can look at ways of improving flow / pressure, such as larger mains connection, accumulator.
5. The above test should be made before any decision is taken with respect to unvented or combi.
6. If you go the combi route, unless it is a very big combi (30 kW +) you are unlikely to get two simultaneous high volume showers.
7. If you go the unvented route, you will probably need to upgrade the cold feed into the unvented cylinder to 22 mm pipe.
8. Don't let builder's specify your system, you need a plumber / heating engineer.
8. Don't let builder's specify your system, you need a plumber / heating engineer.
Agree with absolutely everything that Oldbuffer has said, but this is most important and bears repeating. If you let us know where you are (nearest town or first half of postcode) we can usually recommend someone
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Adding to other advice , you may be able to use the existing combi to do some outlets and also heat a unvented cylinder

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