Manual J question

[acobra289]

Is it possible that he is slightly over sizing the heat pump to get the needed heating btus?

When he gave me the sheet and pointed out that it would cool to 75* on a 95* day. I asked him what if I wanted it to cool to 72 on an upper 90's day and he said it should do that. He said the 75 on a 95 was basically a "worse case" scenario day. Like if we had people over and there was cooking in kitchen and what not.

I hope this guy isn't full of crap. He is actually my best option so far. (And no, he is not the lowest bid.)

[beenthere]

Oversized A/Cs tend not to remove moisture real well. How did your old 3 ton do.

[acobra289]

Oversized A/Cs tend not to remove moisture real well. How did your old 3 ton do.

I guess it does ok. It's really hard to judge as it is probably about 10 years past it's prime. I'm pretty sure it's original to the house and the house was built in 73 or 74.

Right now the humidity is around 53% in the house. But the outside humidity hasn't been bad that last few days. It will get much worse come July and August.

[beenthere]

So did teh humidity get worse in the house last year, or was it able to keep the humidity below 55%.

[acobra289]

I can't say for sure. I have only been using the hygrometer for the past 5 or 6 months. I wanted to keep tabs on the humidity in the house before I installed hardwood floors.

[beenthere]

You can always get a whole house dehumidifier if it doesn't emove enough moisturre.

[BigJon3475]

69% RH right now where I'm at in Charlotte, @ PTI 73*, 67% RH, 62* dew point.

Where did he get those outdoor design temps. I grew up in Greensboro and all my family is there. It's always had lots of humidity.

I see 19* for heating and 90* for cooling, coincident wet bulb is 74, 50% RH leaves you with 37 design grains, M (DR)

[BigJon3475]

Sorry I made a mistake......

Winston: 23* heating, 89* cooling, 74 coincident wb, 40 design grains for 50% RH, M daily range.

[acobra289]

Sorry I made a mistake......

Winston: 23* heating, 89* cooling, 74 coincident wb, 40 design grains for 50% RH, M daily range.

So does that mean the numbers he used were pretty close?

And thanks to everyone that has responded.

[sparksandfan]

The design temperatures are close enough, it's the other inputs that could throw this one way or the other.

If the analysis is correct, then you would be better off with the 2.5 ton. You have a 3 ton now, hence the questions about your comfort with it.

Usually you would not put in a 3 ton in place of a 2.5 ton to up the heating when you have a furnace. You try to get the best fit of air conditioning, then see where the balance point is for heating. Notice that the cooling and heating capacities are close to each other for heat pumps. That is the nature of the beast.

Do you believe the manual j or do you believe your experience with the 3 ton? Would you believe another independent manual j? That may be the way to ease the anxiety.

[dcmcm5]

So does that mean the numbers he used were pretty close?

And thanks to everyone that has responded.

Whoever did your calc used 10 and 95, which are the wrong numbers for Winston Salem. As previously mentioned, the 1% and 99% values are 89 and 23. This is a common error, usually intentional, by contractors. They will say "it got down to 10 degrees here last winter" and that is what they use. That is not how you do a Manual J, and that is why you "happen" to come up with the same size unit that is already installed.