Lower coil spring of tandem

A handy person from San Francisco, CA says:

Hello I run a handyman business and need help to determine lower coil spring of tandem.

180 Coil Springs

Bottom sashes weighs 15.6lbs. Now has a tandem coil spring I need to replace.

Top spring, I can read at 4# but cannot read bottom spring. Should I just change this to one coil spring? 8lbs.?

What should I use for bottom coil spring?

Thank you

Joe 

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Emily from SWISCO responded:

Hi, Joe.

We sell tons of coil springs that look like what you show here so I'll need more information to help narrow down your options. Can you show us an undamaged pivot shoe removed from the window? That will help me identify the compatible coil springs. 

A handy person from San Francisco, CA says:
Here you go. I know the smaller window is 7# springs.

The two larger windows are tandem with top spring being 4# but cannot read bottom spring. Window weight is 15.6 lbs.

Thank you for your help
Really need to get these on order to complete repair.

Joe
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Emily from SWISCO responded:

Perfect, thank you very much! It appears this is a match for our 15-087 shoe that's designed to use the Series 180 coil springs. Assuming the 15.6 pound window has a total of 4 springs, you can use (2) S180-4 4 pound springs on each side of the window which would total 16 pounds of support. This weight setup should be fine for you as long as the weights are within a pound of the sash's actual weight. 

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