under size condensing unit

[fiveboroughac]

i have old air handler capacity 5 ton 10 seer and i want to change the existing condensing unit that is 5 ton 10 seer with a 4 ton 13 seer.

is that going to work? and if not what is the problem with it?

[Marc]

It is going to work? Hard to say, you might get lucky. Is it proper to install a 4 ton condenser on a 5 ton coil without changing the orifice or TVX on it? Absolutely not. What about using a 13 SEER condenser on a 10 SEER air handler? Nope. It's a lousy gamble with your money. Do it right.

The 4 ton has a lower refrigerant flow rate than the 5 ton. And a 4 ton 13 SEER likely has a lower refrigerant flow rate than a 4 ton 10 SEER. The 13 SEER's have larger condenser coils, smaller compressor capacity but same tonnage when joined to a matching inside unit. Your evaporator (cooling) coil will be starved for refrigerant, resulting in inadequate head pressure, bubbles in gas, reduce cooling capacity (from 4 tons), etc.

Marc

[RSpermo]

The first thing you should do is have someone do a Manual J calculation so you know what size your house really needs. There is no need to guess on unit size when calculations will give you the correct answer.

[Marc]

Out of curiousity, does anyone on this forum actually complete Manual J calculations as an inspector?

I'm thinking of buying the ACCA Manual D and Manual J books and learn how to do these two calculations because I'm seeing lots of lousy duct system installations on new construction and I suspect that the tonnages aren't the best choice either.

Marc