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12-05-2011, 09:30 AM #1Regular Guest
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- Dec 2011
question on load calcs and equipment capacity
What is the proper ratio of equipment capacity to load calculations?
Is it really desirable to size the equipment to exactly match the load calculations?
I assume oversizing is more common. What is an acceptable range? 10% oversized?, 50% oversized?
It seems that a system that is sized to exactly meet the load would not have enough capacity to change the temperature of the house fairly quickly - and that seems at odds with how many homeowners want to use their HVAC systems.
For example, at night many people drop the thermostat into the low 60s, or even turn it off completely. In the morning it's cold inside. My neighbors house was 56 yesterday. A system that is sized to match the load, would not be able to raise the temperature to a comfortable range before going to work.
While at work, or on vacation, many homeowner again change the thermostat to keep the HVAC system from running. But again, when returning home, they expect the heat or AC to return the house to a comfortable temperature within a reasonable time (15 to 30 min is my guess).
How do contractor deal with this? Do you try to educate the homeowner, or do you oversize the system?
12-05-2011, 09:42 AM #2Regular Guest
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- Dec 2011
Here is an example of how oversizing a bit can, I think, save energy.
We live in southern california foothills, where there are hot days and cool nights. Most summer nights are in the low 60s. We circulate cool, night air through the house cooling it to 68 by morning, then close the windows and let the temperature slowly increase throughout the day. Most days, the outside temperature cools off before the house gets uncomfortably hot (80 to 82), and we open the windows again without ever turning on the AC. I think we save a lot of electricity by not running the AC on most days.
Those days where it doesn't cool off, are typically hotter, or more humid than the design day. On those days, an HVAC the is sized to match the load calcs may be able to maintain the temperature, but not drop it to a more comfortable level. As a result, we would likely run the AC more often, starting it earlier in the day to ensure the house never reaches 80. This would often mean running a smaller AC system on days we normally wouldn't. It sure seems like this would increase overall energy use.
Back east, where humidity is much higher, I assume AC units run a much higher percentage of the time, simply for moisture removal. Since the unit will run many hours anyway, sizing it to match the load calcs makes a lot more sense to me.
What are the opinions of the pros on this?
12-05-2011, 10:02 AM #3
The whole purpose for load calcs is to size it properly.Don't size for extreme days it's not the norm, you will create more problems designing it that way.
12-05-2011, 10:13 AM #4Regular Guest
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- Dec 2011
I understand the theory, and the problems associated with short cycling.
I wonder if you could talk about the specific scenarios I presented?
I note that you live in Fl. There I would expect AC to run almost constantly to remove moisture. But I just told you we only run the AC on extreme days. There is no need to run it on normal days. So why would I size it for a day when it isn't being used?
12-05-2011, 11:01 AM #5
A 2 ton unit running for 60 minutes will use the same or even slightly less energy (for a range of resaons) than a the same model 2.5 ton unit running for 48 minutes.
If a can only maintain 80F indoors, then you are above design conditions or it was actually undersized. But it will use less energy maintaining 80F than a larger system will to cool it down to 78F. Why, because you cooled it down and therefore by basic laws of physics, it used more energy with all other factor being equal.
Also, oversized units are often on undersized ductwork and operate less efficiently. Every time you shut a system down, some condensation is left on the evaporator coil and will reevaporate. A larger system has a larger coil has more surface area so more condensation reevaporates. Most systems need 5-8 minutes to reach optimum efficiency. A smaller system will therefore spend more time in a more efficient operating conditions. Shorter cycles result in larger temperature swings. A larger system turns on and off more often. Start-up wears equipment out more than long runs times do.
Here's an analogy... People alwasy comment on how cars made 20 years ago often got better meleage than many new cars in real driving conditions. There are 2 reasons for that. 1) older cars of hte same model were smaller and lighter, lacked all that heavy steel and equipemnt for safety, and 2) made a lot less horsepower. Look at test sheets. An average family car in 1990 struggled to reach 60 mph in under 12 seconds. Now even 4 cylinder models can get to 60mph in less than 8 seconds.... something most V6 models from 20 years ago could barely manage. Most drivers are more than happy to use that pwoer available. So even if an engine is lets say 20% more efficient, if you use 25% more power on average... you will still consume more fuel.
12-05-2011, 11:15 AM #6
If you want a real world example....
In 2007 (before I further educated myself and got involved in the industry) I had a 17 year old 8 SEER 2 ton AC with a mismatched 30 year old coil on a mismatched 13 year old furnace. Sounds pretty inefficient right? It had long runs times, and my highest elec. consumption on one of the hotter summers in the last 10 years was around 700kw-hr. Keep in mind I was comfortable at 79F with that system... and we had 40 year old single pane windows... although windows only really amount to 10% of the energy lost in your home, so they are for most practical purposes are mostly irrelevant to overall energy savings if they aren't leaky.
fast forward to 2008. I've tightened up the home by sealing leaks with spray foam... installed brand new Low-E argon filled double pane mid range quality windows and I had a 16 SEER 2 stage 3 ton Trane system with a vairable speed furnace installed. Actual SEER rating was shown as about 15.25.
Now you're expecting my cooling bill should drop right? WRONG! It now needs to be 75F to be comfortable. A 3 ton scroll puts out around 2.2 tons in low stage so the system short cycles, and can't remove much moisture compared to my 17 year old mismatched syste that while ineffiient on paper, it only put out around probably 1.7 tons and had long run times. My electric consumption during one of the coolest summers in the last 20 years reached 1000kw-hrs. I used over 30% MORE energy with a supposedly more efficient system because I had to keep it cooling indoors and it short cycled.
12-05-2011, 11:26 AM #7
Very smart airbare, to take advantage of your climate. Apparently you also have some "thermal mass" in your home that you are taking advantage of. A whole house dehumidifier along with some ceiling fans may allow you to eliminate the use of the a/c even on the above design & humid days.An answer without a question is meaningless.
Information without understanding is useless.
You can lead a horse to water............
12-05-2011, 11:38 AM #8
First the cars, new cars are lighter and safer (well not GM) and make more horsepower and torque than older cars and therefore are more fuel efficient by far. As far as your home system you had a 2-ton made improvements and then put in a grossly oversized system are surprised that it cost more to operate and you need a lower set point. By your own admission the unit makes 2.2- tons on low stage and your load is probably below 1.5-ton.
The op ask a valid question but also answers it himself, every geographic area is different and requires different design criteria. What works in Houston or Miami will not be the right answer for Sacramento or Denver.
12-05-2011, 02:41 PM #9Regular Guest
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- Dec 2011
we had plaster walls and a large interior chimney for thermal mass. Those are now gone, so we are trying really hard to eliminate direct solar gain to compensate. I think diret sunlight is the main thermal load here.
thanks to classic,
I ask because I have two very different quotes. One, which represents good ol boy standard practice, would put a 3 ton unit on our 1300 swft first floor, and a 2 ton unit on the 750 sqft 2nd floor, for a total of 5 tons. All contractors but one have proposed this approach, with some refusing to bid the job if I insisted on smaller AC units because they don't want complaints later.
The other quote, which hews closer to HVAC orthodoxy, would install a single 2 ton unit for the whole house and operate as a single zone. A 300 W fan would operate almost continuously to maintain uniform temperatures and comfort. The problem is that a 300 W fan runnin yr round uses 2,600 kWh/yr - which is about 75% of our current annual usage, and that is for circulation only. Actual AC would increase that substantially.
My conclusion, and I'm here to get opinions from people who actually know something, is that:
1. HVAC design orthodoxy makes perfect sense for humid regions, where the AC should run many hours a day to remove moisture and have lower diurnal temperature differences.
2. doesn't make as much sense in arid regions with large diurnal temp swings where a whole house fan or equivalent can provide much of the cooling
I'd be interested to hear from pros in the west to see if they agree.
12-05-2011, 05:26 PM #10
It is almost always best to adhere to the actual load evaluation but you need to temper that with local knowledge and the lifestyle of the homeowner as well as the finances.
I doubt you would need to run a fan 24/7 365 days just on a few days each year and that would be relatively inexpensive. If you need to run the fan a significant number of days an ECM motor would be much less expensive and effective than a standard PSC motor. Depending on your budget a two-stage system with variable speed ECM motor and zoning would be an option but very pricey. An ECM motor can be set to run very low CFM,s in continuous mode which will will help eliminate air stagnation and stratification.
I seriously doubt from what you posted a whole house dehumidifier could ever be justified expense. In a mild very humid environment but not in a semi arid area.
From your description it would seem difficult to justify the expense of an A/C system.
12-06-2011, 12:52 AM #11Regular Guest
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- Dec 2011
Thanks, classical. I think about these things much more than any of our contractors would like me to.
So these posts have helped clarify things for me. Reinstalling the old 3 ton unit downstairs and a new 2 ton unit upstairs will cost about $6k less than a 2 ton system for the whole house.
There will be a 36 sqft hole between the floors with no door or means to control stratification. Therefore, I'm thinking about a tweak to the system to try to improve temperature uniformity. Please let me know if this is crazy or dangerous.
The 3 ton AC / 80kBTU/h furnace will be installed downstairs. I have learned on this forum that there is no such thing as too much return air. So the downstairs would have a properly sized return air duct downstairs, and a duplicate return duct in the upstairs ceiling.
During winter, I would block the downstairs return, so all the return air would come from the 2nd floor ceiling. This should mix the hottest air back downstairs, reducing temperature stratification. During summer, I would block the upstairs return. In this case stratification would be a friend - with the cool air staying in the more occupied downstairs. Since the furnace is oversized, I could probably even get away without installing a furnace upstairs. Essentially, this would be a hybrid of the two proposals.
Do you foresee any problems with installing an extra return?