Mismatched balances

A quick learner from united states says:
I am repairing a single hung window. I removed the channel balances, and they are not the same. No wonder why it broke. A single spring on a wide window is less than adequate in my mind. I would have preferred two springs, one on each side, I presume they should be weaker springs since there are two of them. My window sash is 52 inches wide & weighs about 25 lbs. But, I cannot figure out what series, (380 385, etc) I have. Presume it has to do with the size of the channel but I don't know. Judging by the markings I can see, I have 2940 EL and a 2950 BS balances. I prefer to get two of the same, unless you recommend otherwise. Please advise.
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Casey from SWISCO responded:

Hello,

It looks like you may need our Series 377 channel balance. There should be identical balances on both sides of the window. 

I would recommend that you weight the window sash to see which would work better for both replacement balances:  

2940- 23 to 28 lbs

2950- 28 to 32 lbs

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Take a look at the specifications on our S377-30 product page to determine what will work best for your window. Thank you!

A quick learner from united states says:
I stuck my bathroom scale under the window & the sash weighs ~25 lbs..
But it is split over two balances, L & R

Should I be looking for half the spring force? Like 12 lbs each side.
Thanks again.
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Casey from SWISCO responded:

You would not split the weight of the sash between the two balances. Both balances need to be able to support the full weight of the sash, so you'll want 2940 options for both balances to support a 25-pound sash. 

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