• 06-12-2005, 06:32 PM
fishin
load cal says, heat loss of 64,000btu and 34,000 of heat gain. what would be the recommended sizing based on this for a split system, how many tons a/c and btus of gas heat?
in reguards to a/c tons what are the guidelines for the number of tons -vs- amount heat gain.(15,000=1ton, 20,000=2ton)?
• 06-12-2005, 06:42 PM
cem-bsee
you will now have to check each product for specific capacities -- be sure to understand what the criteria was for the rating -- what ambient/ outdoor temp
• 06-12-2005, 10:55 PM
beenthere
12,000 = 1 ton.

So it would appear that a 3 ton a/c, and a 80,000 input 80% furnace may do your house.
• 06-12-2005, 11:55 PM
Irishmist
fishin: Just curious. I appreciate your initiative in determining what size your home is calling for both in heating and in cooling. Are these the first steps in a do-it-yourself project, or are you going to have a contractor come in and provide you with a proposal?

Ideally, if your load is 34,000 btu for the cooling, you would want to come up with a 'match' between indoor and outdoor unit that gets you as close as possible to that number for the maximum efficiency. for example: you might find a "3" ton condensing unit when matched with a particular "3" ton coil, yields a cooling capacity of 33,500 btus. That would be a good match for your needs.

Also, keep in mind, that the recommendation provided you on an 80% afue furnace would not be the same as on a 90% afue furnace. That is, if your heating load was 64,000 btu, then the 80,000 is probably correct using an 80% furnace. However, to meet the same load using a 90% furnace, the btu input rating would be about 71,000, ideally.
• 06-13-2005, 07:03 PM
fishin
this is new construction, with a 90% VS furnace. contractor feels a/c should be sized up to 3.5 ton and 100,000 btus would be needed to push that much a/c.(i don't know any thing about that). my thinking based on the load cal. is 3.0 ton and 75,000 btus. can't an oversized furnace short cycle like an oversized a/c? as for the coils the one being used can be used with a 2 ton to a 3.5 ton compressor by changing the pin settings.
• 06-13-2005, 07:35 PM
beenthere
An over sized furnace will short cycle and use more fuel.

Don't let them over size your equipment, you'll be the one paying the price for years to come.

• 06-13-2005, 07:56 PM
Panama
Quote:

Originally posted by beenthere
An over sized furnace will short cycle and use more fuel.

Not according to ACCA Manual S (1995). Manual S has a graph in Appendix 4 that shows negligible loss in efficiency if a modern furnace (AFUE > 75%) is oversized up to 5 times the heating load.

• 06-13-2005, 08:45 PM
beenthere
I seen that some years ago, and still don't beleive it.

• 06-13-2005, 09:16 PM
dan sw fl
Quote:

Originally posted by fishin
this is new construction, with a 90% VS furnace. contractor feels a/c should be sized up to 3.5 ton and 100,000 btus would be needed to push that much a/c.(i don't know any thing about that). my thinking based on the load cal. is 3.0 ton and 75,000 btus. can't an oversized furnace short cycle like an oversized a/c? as for the coils the one being used can be used with a 2 ton to a 3.5 ton compressor by changing the pin settings.
Location: ______ State
Floor Area: ______
Glass Area: ______

Design Temp. _____'F Heating
............______'F Cooling
Desired Inside Conditions
............______'F
............______'F

What happens when outside conditions are NOT at THE Defined Design Load?
• 06-13-2005, 10:51 PM
Panama
Quote:

Originally posted by cem-bsee
you will now have to check each product for specific capacities -- be sure to understand what the criteria was for the rating -- what ambient/ outdoor temp
fishin,

Here is an example of what cem-bsee is talking about:

Lennox G50UH-36B-090 furnace.
Lennox C33-36B-2 indoor coil with TXV.
Lennox HSXA12-036 condensing unit.

The furnace is rated at 72,100 Btuh output. Enough for you.
The blower is rated at 1,300, 1,135, 965, or 830 CFM at 0.5" wc.

The coil is rated as follows, assuming 95 °F outside, 75 °F inside, and 50% RH (63 °F WB) inside.

1,020 CFM: 32,200 total cap, 24,800 sens, 7,400 latent.
1,220 CFM: 33,200 total cap, 27,200 sens, 6,000 latent.
1,420 CFM: 34,100 total cap, 29,700 sens, 4,400 latent.

High speed will give your more cooling capacity but less dehumidifying capacity. You need to match this to your cooling load calculation, making sure you cover both the sensible and latent loads.

You told us your heat gain was 34,000 Btuh, but you did not give us the breakdown.

This is the information you need to select the right equipment for your needs. All manufacturers provide it to their dealers. Some, like Goodman/Amana, make it available on the web sites.