Oil combi with electric unvented

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I have an oil combi 12/18 which doesn't really cut it for hot water having 2 bathrooms. Can i tee in an electric element only unvented to the hot supply? Or do i have to disconnect hot supply and leave one tap on boiler then pipe in unvented separately. Thanks
 
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1. If you add the output from an unvented cylinder to (effectively) the hot water output of a combi you would run the risk of pushing water backwards through either the combi or the unvented cylinder. So you can't do that. I don't believe non-return valves would help and could well be dangerous.
2. You would have to separate the two hot supplies.
3. Why not use the combi to heat an unvented cylinder? Say run kitchen taps directly off the combi, install zone valve(s) on the heating side, and run an unvented cylinder off that.
4. Be aware of the pressure / flow requirements to run an unvented cylinder (2 bar, 20 lpm minima, properly measured) and the probable need to have a 22 mm cold feed to the cylinder.
 
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Convert the heating circuit into a Y or S plan and install a cylinder to feed your bathrooms, keeping kitchen etc on the combi hot water. Whether you go unvented or conventional depends on cold supply factors but an experienced G3 registered plumber can advise.
 
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I have an unvented cylinder already from a job, it only has 2 elements and no coil. I was thinking of leaving the kitchen on the combi and putting the unvented on the bathrooms?
 
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Possible, but heating with electricity will be far more expensive than using the oil boiler.
 
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