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Old 03-13-16, 04:12 PM   #11
stevehull
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MN - you have never seen the Navy build a shed! Foam sheets when sloppily applied, do little to block wind - especially when it it 20+ mph almost constantly.

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Old 04-14-16, 01:57 PM   #12
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Hello stevehull,

I read your description of thermal mass with great interest. I have about 85275 pounds of sand that I use as a radiant slab in my office. I think it stores about just shy of 400,000 btus of usable btus. I'm in the process of trying to figure out exactly how to estimate the value of this in order to calculate the lowest usable water temperature I can operate at.

I'm thinking what I may do is this winter set-up a buffering hot water tank and see how low I can set it's thermostat while tracking the temperature along many points of my system.

Short of that I don't think there is why to estimate.
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Old 04-16-16, 07:11 AM   #13
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AC oversizing penalty has 4 major factors:

1: Poor humidity removal during part load conditions. Those humid days in the 80's people will turn their AC down to 70 or below to get humidity relief. A smaller AC will pull the humidity without overcooling as much.

2: Uneven temps in between cycles. A cold blast of air followed by a stuffy humid feeling. Happens each cycle, customers end up fiddling with the thermostat constantly.

3: Ductwork unable to deliver needed CFM. A big one, system will only deliver the amount of cooling the ductwork can handle. Customer stays comfortable because all the capacity isn't needed to begin with. They just pay higher utility bills.

4: Cycling efficiency. It takes 5-10 minutes before pressures stabilize and the system is operating at it's rated EER. oversized systems cycle more often and spend less time in the efficient operating zone.
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Old 04-16-16, 10:48 AM   #14
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Actually, I found using heating/cooling degree days PLUS utility data was very accurate for determining my house's loads. For a one month example,

123 therms = 12,300,000 btus at 882 degree days is about 14,000 btu per degree day or 581 btu per degree hour. My design day outdoor temp here in North Carolina is 20*F. So 70*F - 20*F is 50*F delta. Then 50*F * 581 btu / (degree hour) = 29,000 btu/hr at design temps.

Of course it is not exact as it does not include internal heat gain or the fact that those therms also heat water. But I refined the numbers by running a simple linear regression of two years data. The results matched what I expected from using a stopwatch to time how long the system runs on extreme days. The Manual J (with all assumptions made to overstate the load) came back 50% higher at 42,000 btu/hr. At the time, the system output could not go higher than 32,000 btu, and it heated our house without any trouble on an actual design day.

If I had time I would make a simple webpage that could run these calculations for different areas. But I have some toddlers to run after now. Cheers!
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Old 04-17-16, 07:53 AM   #15
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An easier way is to clock system run time on a design day. Do so after thermostat has been at the same set point for at least 1 hour. In most houses you will find the furnace runs less than 50% (aka 2X the size needed).

My old furnace was an 88k, never ran more than 5 minutes before hitting setpoint unless it was recovering from setback. New 44k furnace runs about 20 minutes out of 30 at 17f design conditions.
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Old 04-28-16, 04:57 PM   #16
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I wish I still had the info from my laptop. I took the local temps and did my own calculations to come up with what man j uses. It was quite a bit different than the published temps they have. Here the book design temp is 99*. When I ran the local temp data it was 105*. Which is why so many houses here are hot in the summer. It's very common to up size units on relativly new houses. A friend of mine does quite a few. Also some city's have started to spec design temps that are higher than in the man j calculators as they know they are off.

As for the muggy days in the 80s. Well here there are so few that it doesn't matter. It goes from nice and the windows open to mid 90s like a light switch then summer gets here and it's over 100* for 8 hours a day with highs up to 117 some summers and often for weeks on end with those highs.
Man j is a great tool but you have to keep a skeptical eye on it and don't believe the numbers provided are correct for your locality. It even says as much in its own documntation.

Our old system was a 3 ton (dying compressor)/100k furnace which just wasn't enough in the summer. The new one is a 3.5 ton 16 seer with a 100k two stage furnace 96%. Should be a night and day difference this summer. I also had to move 3 registers. The one in the kitchen and dining room were in really bad spots and there was only one in the living room. They didn't understand much about airflow in 1960 or leikely didn't care...

What still surprises me is the gauge of the wire on furnaces now. They are using 18 ga wire from the j box in the unit to the board. That means a max current of about 8 amps. Much lower than the older models. The efficiency increases have made quite a difference in amp draw. Also the change to scroll from recip compressors. However I would have gone with a lower seer as we will be moving. Higher seer doesn't help home value and spending more on super high seer units never pays back unless your Elec is super pricey or you are solar.
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Old 04-28-16, 07:39 PM   #17
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Odds are VERY HIGH the 3 ton systems was only delivering about 2 tons. This is why the house was hot, it wasn't because the 3 ton wasn't big enough. To get the full 3 tons ductwork and system charge has to be right. 90% of the time they aren't right and system performance suffers severely. Hopefully they get the 3.5ton right and it delivers it's rated performance. If you notice a HUGE difference, you can bet the old system wasn't delivering anywhere near it's rated capacity/efficiency.

100k furnace is nuts for a house in Texas that's under 3,000sqft. Unhook the 2nd stage, you wont be needing it. Yes, you basically just overpaid for a 70k single stage. Surprised you spent the $$$ for 16 SEER/96% AFUE if you plan on moving. The 96% will NEVER pay for itself in TX. the 16SEER MAY pay for itself in a few years, depends on how much more it was vs the 14SEER.

Let me guess, the contractor used 500sqft per ton sizing. It's worked since the 1960's, why change now?
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Old 04-28-16, 07:59 PM   #18
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Our old system was probably in the 2 ton range or little better due to a bad compressor. I forget what the pressures etc were last time I checked it.

The system I put in was actually the cheapest system I could get. A 14 seer was actually more as would have been a 3 ton system. The system all in worked out to 3k. I put the unit in a different place as the old spot will be going away for a master bedroom remodel. I used to do commercial refrigeration and a friend of mine has an hvac company. I could have gone with a 13 seer as there are a few the warehouse had that were within the grace period but I didn't want a recip compressor.

Oh and yes we do need the 100k btu... The old one was a 100k and a couple years ago when we had a week of 17* for the high it was running pretty much non stop. Also the furnace I used was the cheapest that didn't need combustion air.


All the duct work came out.. All done with 3' sections and poorly placed registers. M&M is close and they had some stuff that wasn't picked up I got a deal on. I hate doing metal duct in a 100* attic so my friend need up doing it... Already that hot up there this time of year.
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Old 04-28-16, 08:32 PM   #19
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How big is the house? Unless its 3,000sqft or REALLY leaky, a 100k furnace shouldn't have to run wide open with a 17f outdoor temp.
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Old 04-28-16, 08:53 PM   #20
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Quote:
Originally Posted by Elcam84 View Post

Oh and yes we do need the 100k btu... The old one was a 100k and a couple years ago when we had a week of 17* for the high it was running pretty much non stop. Also the furnace I used was the cheapest that didn't need combustion
Are you on the high plains? If so, I'm sure the wind isn't helping.

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