Changing a valve on a combi boiler system

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Have read posts on this and understand that by closing the service valves on the flow and return to the boiler then opening the valve this will create a vaccum? (have already done this on my own system although its a conventional boiler). The problem is I can see no valves on the pipework whatsoever nor a drain nipple, the boiler does sit very low, could it be within the casing of the boiler or can we do it some other way?
 
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If by a conventional boiler you mean a heat only boiler on an open vented system (header tank, hot water cylinder etc ) then you won't have isolation valves on the flow and return pipes at the boiler. If you want to create a vacuum on this type of system you will need to bung the feed and vent pipes at the header tank and then open a drain point somewhere on the system. Expect a pint or two of water to come out before it stops. Make sure any auto air vents are shut before you start.
 
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You say you want to change a rad valve on a combi system?

What make of boiler is it?

The valves can leak when operated, I think closing the auto air vent on top of the boiler has the same effect but a plumber will need to confirm this.
 
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