help with manual j results

[toireht]

home was built in 1980
location: baton rouge, louisiana
built with 3 ton central a/c with electric heat.

did manual j and need help determining what to replace with

sq footage 1662

results
sensible gain btuh:259334
laten gain btuh: 6462
total heat gain btuh: 32396 (2.5 ton)
total heat loss btuh: 36360

what size do I need?
a/c with electric heat?=
heat pump with b/u heat =
geothermal heat pump =

thanks
james

[RyanHughes]

Are you sure you input all of the data correctly? A load calculation is only as accurate as the data input into the program. What design temperatures did you use?

Did the 3 ton cool and dehumidify well before? If it did, then I'd stick with 3 tons. 3 tons isn't oversized because the next size down wouldn't handle the load at design conditions (30,000 BTU/H = 2.5 tons - nominally).

Are you considering going with a two-stage air conditioner or heat pump? I wouldn't consider using electric heat as backup when you can heat more cost effectively with a heat pump. There are two-stage heat pumps that run at a lower capacity in first stage. Many manufacturers offer such systems. You can use electric heat strips as backup with the heat pump. Generally you size the heat strips to match the entire heat loss figure incase the heat pump breaks. In that case, you'd need 10-15 kW of backup heat strips. This also depends on how much power you have to begin with. How many kW did you have before?

Have you gotten any estimates yet? You could call some companies out for estimates and share your results with them to get their input. More goes into a replacement. You'll also need to make sure your ductwork is sized and laid out properly for the system to perform well.

[toireht]

Hey Ryan Hughes

i used hvac calculator residential version.

summer: 93
winter: 29
grains: 116
daily temp: medium

the 3 ton did ok as far as I can remmeber, it has been out for several years. I question the size because the house was originally smaller, i enclosed the carport which adds about 360 sq feet more than original, perhaps the original ac was oversized. I believe the original heat was 10KW.

I can't seem to get any good techs in my area, non mentioned a manual j calculations, just went off my old system and got a recommendation of 4 tons from one tech and 3 1/2 tons from another, they just looked at what I had before and added my additional carport I enclosed.

I am looking at going with a heat pump, maybe a water source geothermal heat pump also. would you recommend a 3 ton on these.

thanks
james

[RyanHughes]

If you go with an electric heat pump, which I would recommend, then I'd go by your load calculation results assuming they are accurate. I'm not sure about geothermal sizing so I won't answer that question (someone else can I'm sure). Geothermal isn't cheap.

Oversizing isn't that uncommon unfortunately. If they didn't size the system correctly, then can you be sure that they sized the ductwork correctly? I hope the person who recommended a 4 ton looked at the ductwork.

[toireht]

the 4 ton tech did not look at the ducts, I will look into making sure they are sized correctly. I think I will probably go with a 3 ton heat pump.


thanks for the input
james

[RyanHughes]

I would meet with the contractors (or new contractors) and show them your load calc results and also explain your concern with ductwork sizing (making sure you have enough supply and return, ductwork sizes, etc.). Then get their input and post back here to go over the estimates. Find a good, reputable dealer who is familiar with such calculations.

[dash]

Is your carport conversion better insulated then the original home?

What indoor design temp did you use and is that what you want to maintain ?

[beenthere]

Your sensible load puts a 3 ton border line if your ID design temp is less the 75°F.
Your latent load seems low to me.
What infiltration rate did you use.

As Dash alludes to, the addition may not be adding much load to the house.

[toireht]

carport is same or better insulations
used 68 summmer and 72 winter

thanks
james

[beenthere]

68 ID temp is a bit low for a summer temp.

See what is says at a more reasonble temp, say 72, and 74.
What infiltration rate did you use.

[toireht]

infiltration
summer .42
winter .85

[dash]

carport is same or better insulations
used 68 summmer and 72 winter

thanks
james

Three tons won't cool it to 68°.

The standard btu equip. ratings are at 80° indoors,large deduct of sensible btu's ,to run it at 68°F.

[toireht]

Three tons won't cool it to 68°.

The standard btu equip. ratings are at 80° indoors,large deduct of sensible btu's ,to run it at 68°F.

dash,
don't understand what you mean. can you explain please?

thanks
james

[dash]

dash,
don't understand what you mean. can you explain please?

thanks
james

First this applies if your load calc is correct.

Your calc came up with sensible and latent btus,you need equipment that is rated to cover each of those numbers


ARI and manufacturers ratings are for what the equipment will produce at specific indoor and outdoor temp.

Charts usually show various OD temps and the capacity for each.

Indoor is at 80° F,footnotes on the chart may tell you this.

For every degree indoors below 80 the sensible capacity is to be derated ,835 sensible btus for each degree below 80 ,per 1000 cfms of air flow.

So, 835 btus X 12 degrees X 1.2(for 1200cfms)= 12,024 deuct in sensible ,and 12,024 increase in latent.

With the new latent number,half the excess latent capacity(over the latent load calc) can be added back to the reduced sensible.

Seems you calc is off since you have a 3 ton(I think) or you weren't running it at 68° F,in the past.

[toireht]

First this applies if your load calc is correct.

Your calc came up with sensible and latent btus,you need equipment that is rated to cover each of those numbers


ARI and manufacturers ratings are for what the equipment will produce at specific indoor and outdoor temp.

Charts usually show various OD temps and the capacity for each.

Indoor is at 80° F,footnotes on the chart may tell you this.

For every degree indoors below 80 the sensible capacity is to be derated ,835 sensible btus for each degree below 80 ,per 1000 cfms of air flow.

So, 835 btus X 12 degrees X 1.2(for 1200cfms)= 12,024 deuct in sensible ,and 12,024 increase in latent.

With the new latent number,half the excess latent capacity(over the latent load calc) can be added back to the reduced sensible.

Seems you calc is off since you have a 3 ton(I think) or you weren't running it at 68° F,in the past.

thanks for the info, I guess what I am needing to do is change the 68 degrees in the software i am using. can you tell me what I should use. I am trying to determine what tonnage to size a new system. I can't get any techs in my area to do a manual j cal. I have double checked my measurements, insulation and variable that I put in the software. the software list tonnage but not sure how to read it or if it listing for ac or heat.

thanks
james

[beenthere]

Use 72 for winter, and 74 for summer.
When it list gain, that is for cooling. loss is for heating.