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Discussion Starter · #1 ·
Hi. On my Lennox TXV-controlled HP the nameplate shows a factory charge of 11 lb 13 oz. The manual indicates to add 1 lb 12 oz for the indoor unit matchup. And my line set is longer than 15' so--also according to the manual--7.2 oz is required. This of course assumes I'm accurately interpreting the manual. So, I have a couple resulting questions:

1. The nameplate calls for 8* +/- 1* subcooling. Should that be the resulting value after the additional charge, or could the value be different as a result of the additional R-410a?

2. My Lennox documentation makes no reference to superheat. Do you look at superheat on a TXV system? If so, how might I determine what is an acceptable range?

Thank you
 

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There is a charging chart on the back of the elec box cover or in the install manual. Final readings after taken after the proper amount is added.

Lennox also uses a "approach" method to figure out their charge amount. Have to read up about it in the install manual. I am not sure if they use it for heat pumps as it is too cold for them where I am but I imagine they do. If charging for AC they do. Not sure for heating. If you have the install manual go to the end of it and the new ones have pictures of the charging methods. Way too complicated to describe over the net and a pic is worth a thousand words.

This site has their manuals


http://www.completeheating.ca/index.php/manuals1/lennox-heat-pumps
 

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Discussion Starter · #3 ·
Yes; I forgot to mention their approach method is noted in my manual. My nameplate shows a target of 8* +/-1* in cooling mode. When I've weighed in the past (and added the additional amounts for the indoor matchup and line set length) I've usually seen higher subcool values of 10 or even 11. So I thought I'd ask on here if 8 should be the final number, or if it could be higher by adding the additional recommended charge. My high side pressure has always been within the +/- 10 PSIG after the additional charge, so now my lack of training has me scratching my head at the higher subcool readings. Low side has always been approximately 15 PSIG lower than the nameplate value, and I even recall the original installer noting that point years ago. Thanks.
 

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There is a whole complex set of factors like indoor blower speed, coil matchup etc than can affect your readings. Like I said heatpumps are uncommon where I am as it gets below -15 or -20F a LOT where I am so I don't see them. I am not familiar with your climate so will have to bow out. Maybe Been~ knows more or some of the other US techs.
 
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