Replace Window with French Doors, how much heat loss or gain?

David28

I have a 12mm double glazed unit in a wooden frame.
The window is 145cm by 145cm.and 68cm from the floor.
The wall is dab and dot on breeze block then a filled cavity then brick exterior.
If I replace the window with Upvc French doors with a 20mm gap between the panes how much heat will I lose or gain with replacing the window?

Jeepers_Creepers

I doubt you'll notice the difference.

There will be difference, but I don't even know in which direction! The improvement in current patio door insulation levels but with a larger glass size, versus your very narrow 12mm DG units in timber frames.

I doubt you'll notice.

shinytop

OP, it's likely your modern DG unit will be similar to your wall for heat loss because brick and block are poor insulators.
Try something like this and see what you get. 
https://www.uvalue-calculator.co.uk/
Your new 20mm unit will be better than the old window.  I'd guess you'll be better off but as Jeepers says you probably won't notice, as long as the doors seal properly.  

twopenny

I have huge patio door/window and there's no particular heat loss.

On snowy or freezing nights I close blinds and curtains to keep the heat generated by radiators in but for most of the year I don't draw them. Also any day there's some sun it warms the room a treat so I make sure it gets in.

This was the same with a wood frame and the UPVC

Not sure how that works but the sun is strong when coming through double glazing. So I think you may gain.