sealed combustion equipment

[salv97]

This issue of sizing right on the money. What good does it do? Trust me, I realize most of what's out there is way oversized. I'm not saying that's a good thing - especially with standard efficiency equipment. With atmospheric burners hooked to chimneys, short cycling was bad for efficiency because the unit seldom would see full steady state operating conditions, AND the chimney would be heated up frequently, promoting infiltration because the chimney would do what its designed to - which is DRAFT AIR out of the house! I'm saying this problem is all but gone with sealed combustion equipment. So guess what, if your furnace is oversized by 20% or so - who cares?! It's cheap insurance. Really, what is the capital savings of a 45mbh furnace compared to a 60mbh model? Especially spread out over the life of the equipment. Sure makes me sleep better knowing if we do get a really crazy out of the norm cold snap my heating system can do the job.

[ACFIXR]

states that 40% should be the max % of oversizing and that "Oversizing is not an issue as far as fuel efficiency is concerned............can be oversized by as much as 100 percent without causing a significant increase in the operating cost" page 2-1

[beenthere]

Short cycling of sealed combustion equipment makes it less efficient.

Oversized 2 stage sealed combustion, is less efficient yet. Since in low/first stage, its less efficient then in second stage.

Sleep well knowing that your not saving what you though you would.

Keep taking that placebo.

The smaller furnace would have been oversized, and gave you safety.

Now your even more oversized.

Yep. You saved yourself 49 bucks, buy not buying HVAC Calc and using it.

[salv97]

i had 2 professionals come to my house telling me 80000 btu was the way to go,lennox doesnt have an 80000 btu furnace so i went to the 88000btu they both spent a couple hours here thats why i chose thebigger furnace

[beenthere]

Did they do a load calc? No.

They just went by sq ft. And were oversizing it also.

[salv97]

well they asked about insulation,they saw all the windows,i dont think its as complicated as everybody has been saying,also would you guys agree that 2 and a half ton air compresser was sized right for 1400 square feet

[beenthere]

And since you never saw the load calc. They didn't do one.

And its doubtful anyone will say that 2.5 tons is the right size A/C for such a small house.

[salv97]

let me ask you a question how the hell do you know if you oversized the air conditioner,this was the size the previous owner had in the house,it seems to work perfectly,if its 90 degrees out which it is my house is comfortable with the thermostat at 78

[big sky hvac]

Sure, you can oversize your equipment if you want to. You can oversize your furnace, but can your ductwork handle it? If you oversize the furnace and you have inadequate ductwork, you're going to end up on the upper end of the designed temp. rise of the furnace or above it, which could lead to premature failure of the heat exchanger. If the ductwork is too small for the blower in the oversized furnace, you're going to have whistling. You can oversize the A/C, but it may not dehumidify very well because it's going to have shorter run cycles. Do what ever makes you feel warm and fuzzy inside. The proper way would be to have a load calc. performed. Unless your house has a lot of southern exposure, a lot of windows and no shade, 2.5 tons seems a little big for 1400 sq. ft, but with a load calc. you'd know for sure. It's impossible for anyone to know if it's the right size without seeing the house.

[skippedover]

salv97, you've missed the whole point of 'right sizing' equipment. It's got very little to do with energy efficiency but a whole lot to do with comfort. Let's get a little extreme for an example. Suppose the AC system in your home is 5-tons (60,000 Btu's). Further suppose that's 100% oversized, your home requiring only 30,000 Btu's or 2.5-tons. Now let's also suppose it's a design temperature day, let's say 91F. According to the needs of your home there are 30,000 Btu's/hour entering the home and a 30,000 Btu AC unit would be just holding temperature, running constantly.

Now let's look at the 60,000 Btu AC system. The home gains 30,000 Btus in a given hour or 7,500 Btu's/15-minute segment. At 60,000 Btus/hr = 15,000 Btu's/segment the unit comes on, runs 7.5-minutes and shuts off. It never ran long enough to pull the evaporator coil down to the air dew point so the place is as cold as an icebox but the humidity level is equivalent to outdoors. The 5-ton units stays off for 7.5-minutes, then repeats the cycle. Same timing, same result. Every hour. Every day. The 2.5-ton unit though has thoroughly dehumidified the home and was running constantly. Energy difference, negligible. Comfort difference, like night and day. Indoor air quality difference, mold and mushrooms growing in the 5-ton home. Proper sizing does matter.

With the absence of humidity control, we'd see the same results with a large furnace when a smaller one will do the job. Short cycle, hot/cold/hot/cold. A nice 3-stage, properly sized, would produce hot/hot/hot from about 45-degrees OAT and lower, never turning off at all. Pure comfort. Energy efficiency? No difference between the properly sized unit and one twice it's size. Just comfort. Read the posts on this website. It's not efficiency most are complaining about. It's comfort. Comfort. Comfort.

[beenthere]

let me ask you a question how the hell do you know if you oversized the air conditioner,this was the size the previous owner had in the house,it seems to work perfectly,if its 90 degrees out which it is my house is comfortable with the thermostat at 78

I didn't oversize the equipment. I didn't size your equipment.

Your in Philly. Your weather isn't really any different then mine.

[Chuck]

How many threads will it take to convince yourself you have the right size unit????