AC size?

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We have decided to go with a 14 seer heat pump with back up propane. Now the question is what size? Different contractors have said anywhere from 3 ton to 4 ton. Only one contractor actually did a Manual J. He determinded we need 41,000 BTU for cooling and 75,000 BTU for heating. He recommends we go wtih a 4 ton unit. There are actual BTU's and sensible BTU's, and given our area, we should size to 4 ton to match the "sensible BTU's" Does this make sense? This contractor has come with very good reviews. We just want to oversize the unit.
I live in southern maryland about 35 miles south of D.C. We have very hot and humid summers.
Thank you!
Jessica
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Since only one did any calculations, the others are just guessing. And what do you expect from the newgroup? We have no way of knowing anything about your house, how big it is, how well it's insulated, etc.
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Our house is 2700 square feet, and insulated well for a 20 year house. Our ducts are R6. What other information would be useful?
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Windows - size, type, how many, which way they are facing (north, south, east, west) Construction- wood, stucco, vinyl, brick. 2 by 4, 2 x6. how high are the ceilings, what kind/color are the shingles, how much insulationg in the walls/ceiling/attic, on a slab? Crawlspace? basement? Rooms - how many, how big, how many people/appliances in each Location- which direction does it face, do you have a lot of trees.
There are a lot of other things to consider, too.
Did you call my company? We do all of St. Mary's, Charles, Calvert, and parts of PG........
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wrote:

Read the other posts about some homeowners liking their homes kept at 65 degrees when it is 95 outside.
Are you one of those? If so, you may want to oversize substantially. Personally, I am NOT one of those and want my system to struggle to keep the temperatur at 77 on a 95 degree day. That means I want it to run for about 4 hours without shutting off on every hot afternoon of summer and if someone leaves the door open or cooks a roast or ... , then I expect the temperature to creep up to 80 with the ac running constantly.
Which one are you?
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Hi jessica, hope you are having a nice day
On 14-Jun-05 At About 02:15:32, jessica wrote to All Subject: Re: AC size?
j> Our house is 2700 square feet, and insulated well for a 20 year j> house. Our ducts are R6. What other information would be useful?
There are just too many variables and way too much info needed to do a manual J over the net. the only way is to have someone on site who can see the room layouts and get measurments etc..
-=> HvacTech2 <=-
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We'd like to know the heat input, in the form of appliances, and windows, and other environmental sources. Also the daily operation, if the doors are opened frequently (I had this happen, a house that never quite got cool. The two elderly residents were smokers, and the door was open all the time, as they went out to smoke).
I doubt you can get all the relevant details over the net. cause we won't know to ask, and you won't know to say.
--

Christopher A. Young
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Stormin Mormon wrote:

Perfect example of a typical dead, dumb and blind mormon asking another typical deaf, dumb and blind mormon for truth concerning the cult he's a slave to.
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"We just want to oversize the unit." Is this really what you want to say? Over sizing a unit is just spending more money than you need. Just making the compressor bigger is the mark of the unexpended. Bigger compressors usually come with bigger fans. Can your duct work handle the added CFM's?
I do not have a clue what a "sensible BTU" is nor how to measure it.
Is humidity an issue for your environment? If so contact the guy that did the manual J and ask him specifically about a VFD air handler, and removal of humidity. I installed on my last home a 12 seer 5 ton American Standard with the VFD air handler. It worked excellent for humidity control.
The installer is more important than the equipment. Have you considered having the ducts cleaned before the change out? I would also recommend that the air filter be sized larger than necessary (1.5x) That way you can use higher efficiency filters and not have the static drop of the filter bother the air handlers performance. Dirt and dust is a big deal in the desert of Arizona, and I have a dog.
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I ment 'We just do NOT want to oversize the unit.'
Humity is an issue here. What is VFD? We are getting a propane furnace with a variable speed blower so we can get the higher SEER out of the heat pump. Does this have anything to do with VFD?
He will be enlarging the cold air intake and putting the filter there.
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Since he is enlarging the return, ask about making the filter area at least 1.5 times the area normally used. Example, if the normal filter assembly is a 20x25 use a 20x30 filter cost is the same. but there is more surface area for the fan to draw from. My new York a/c has some serious issues with static air pressure. Basically give it a dirty filter and it will not cool my home. The unit loses most cooling at 0.70 inches of static. Not great when the filters I use CLEAN start out at 0.28.
Also ask about a tube return grill instead of the stamped grill. Again less static air pressure. Just suggestions for better performance.
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Really? A sensible Btu can cool a pound of water 1 F. A latent Btu can condense 1/1000 of a pound of water vapor.
Nick
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True but that does not tell how to measure it. How can I tell how many sensible Btu are in a given mass?
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Toss it into a bucket of water.
Nick
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jessica wrote:

Only one meets the requirements. He did the numbers and comes recommended. Frankly the most important part of the whole thing is the installer. It appears (and I have no way of knowing for sure) you have a good one. A good one will consider local conditions (like your humidity) when designing a installation.
--
Joseph Meehan

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Jessica -
here is the problem with getting a unit that is larger than you need
when the A/C kicks on, because it is oversized for you house, it cools it so fast that it just runs for a brief spurt.
the problem is that the OTHER forgotten function of A/C in the summer is dehumidification, if you want to be comfortable
the oversized A/C unit runs such a brief time to get the house cooled down, that there is not enough time to adequately dehumidity.
so folks who buy an "oversize" unit, while their intention is to be more comfortable, end up LESS comfortable in a cool, but clammy home.
get the correct size, and no more
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cowboy wrote:

It needs the runtime to ring out the humidity and achieve the Human Comfort Zone. - udarrell
--
http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html


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Size doesn't matter.

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Oscar_Lives posted for all of us... I don't top post - see either inline or at bottom.

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Tekkie

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Sensible heat is the heat energy contained in the contents of the house. Stored heat, I guess is a good term for it. A chair at 80 degrees contains more heat than the chair at 70 degrees. I don't know how you go about measuring it. Once you reduce the sensible heat in a building, it will not gain as long as the AC continues to run and remove any additional heat coming into the house.
--
Ed
http://pages.cthome.net/edhome/




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