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I have a victorian fireplace with a modern gas fire installed. The fire place has a hatch at the top to open/close the chimney, which the fitters have connected to a wire to force it to be fully open constantly. However, it loses a lot of heat through this. I believe it is partly ventilated from underneath. How much should this kind of hatch be open? can I close it completely? Can I close it and have a CO monitor? Surely as long as this is OK there is nothing to worry about?

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I presume that if the flue is "forced" open, then you have a vented fireplace system, is this correct? – Aaron Dec 13 '11 at 22:29
@Aaron Forgive my ignorance, but aren't all fireplaces vented? – Paul T Davies Dec 14 '11 at 8:54
Is it a fireplace unit or a log set or something else? I know they make ventless log sets (they burn very efficiently) so that you can use them in a "nonfunctioning" fire place. – Aaron Dec 14 '11 at 12:05
Is the control for the flue accessible? – Tester101 Dec 14 '11 at 14:34
With any gas appliances or fireplaces in your house, you should have at least one CO monitor regardless. – gregmac Dec 14 '11 at 21:39
up vote 4 down vote accepted

Your flue should always be open when the fireplace is operating. It is a fire and smoke risk to close the flue while the fire is lit.

When not burning, the flue should be closed to prevent heat loss.

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