Rivco windows

A quick learner from Nashua, NH says:
Hi, I need to replace the channel balances on several Rivco windows. I've identified them as 751 because they are tilt out, ribbed and have a 15-014 balance knife lock shoe. They are all stamped with 28 but they measure 29 inches.

The upper sashes weigh18 lbs. The lower sashes weigh 22 lbs. Oddly I have one pair of balances for a 22 lb lower sash that still works, that is stamped 28HD. Finally I have one window that has 751balances stamped with 20 that measure 21nches with a sash that weighs 16 lbs. What do I order to replace the channel balances?
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Tom from SWISCO responded:
Good afternoon! Thank you for posting in our discussion board. Let's take a look at the first set of balances, which each measure 29". They would be our S751-29.

For the 18 pound window, I would recommend the 28R stamp, which can carry a window sash weight range of 15 through 22 pounds. For the 22 pound sash, you have two options: you can either go with the 28R, or the 28HD, which can carry 22 through 28 pounds. Which one would you prefer?

Now, for the 21" balance, you should look at our S751-21. The stamp I would recommend is 20R, which can carry 10 through 17 pounds.

Please keep in mind that the weight range 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. Suggested sash weights are approximate.
How to install a ribbed tilt channel balance with knife lock shoe
How to install a ribbed tilt channel balance with knife lock shoe
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Classifying Channel Balance Fittings
Classifying Channel Balance Fittings
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