I am trying to size a hvac system

[clark]

I have had a few bids on some systems and some guys our wanting to use much larger systems than others. Any idea why the differance. I am building a 6700 sq ft home in san diego and I have had guys want to put in 3 -3.5 ton units and some want to put in 2 -5 ton and 2 -2 ton units they wanted to zone the 3 -3.5 ton units. I started to think I didn't want to have so many outside units taking up so much space. I saw a TV program where they put in a "chiller" and it looked small and could handle the home. Does anyone know much about "chillers/boilers and air handlers" and "furnices and condensers".

thanks for any help

[suemarkp]

You size your HVAC needs by doing a "Manual J" calculation. You can buy a book from the ACCA for Manual J, or do the shortcut summary version I posted here:
http://www.selfhelpforums.com/showthread.php?t=3106
http://www.selfhelpforums.com/showthread.php?t=3107

Many contractors don't do a Manual J calculation, they just go by rule of thumb based on similar things they've built before. Others have an abbreviated one they do. If no one measured your perimeter, windows, and doors, they didn't do a Manual J. If you've insulated well and have good windows with Low e glass and inert gas filling, you may need less HVAC than "normal".

HVAC units typically only come so big, especially in single phase 240V residential units (but a chiller style may come huge). Having multiple units (e.g. one per floor) is better than a single large one with huge ducts and a maze of zoning dampers (those dampers will eventually fail).

And after you do a manual J, you may want to do a Manual D if using forced air. This will tell you how large your ducts need to be.

[clark]

thanks for the info, I looked over the link you sent it looks like a lot of work. I think thats why no one I had bid the job did the calcs. I am going to try to do the calcs and figure out what I need. On the link you said you could give me the outside temps for my area to input in the calcs. I live inland of san diego CA in La Mesa CA. Also you talked about a manual D do you have info on that, is it a different book or a part of the manual J. I want forced heating and air so I would like to make sure the guys will use the right size. I guess I didn't understand what I saw I thought 1 chiller unit could run multiple air handlers, oh well.

Thanks for the info, I will be up late with the calculator

[sloooo]

If you go with a chiller, you run into a new set of problems. Chillers require a VAV (variable air volume) or a CAV (constant air volume) to operate. The biggest drawback to a chiller for residential use is that in the summer, you'll always have to maintain cold water. Depending on how cold you want it, you'll be maintaining usually about 45* water and the circulation pumps to go along with it. With a VAV, you'll have a main air handler and coils in each room with control valves to control the temp of the water in the coil. With a VAV system, you'll need an air handler with a VFD (variable frequency drive) or as others call it, a VSD (variable speed drive). As a room satisfies, a damper will start to block off air to that room and you will start to increase your static pressure in your ducts. A VFD will slow your motor down based on a pre determined static pressure and maintaine that pressure.

Now, let's talk about a CAV system. It's basically the same thing, but your chilled water is throddled back and not the air. You will always have the same amount of air moving through the system, and only your chilled water valve is moving. You don't need a VFD since you'll always have the same amount of air circulating.

Regardless of either system, you will always run into a problem. Let's say in the morning, you have sun shining into one side of your house. You'll have a heat load and the chiller can handle that. Let's look at the other side and you have guess's that want heat, not cooling. Now you'll have a need for a boiler to control your house also. Now you have VAV or CAV boxes for each room, and a heating and cooling coil. It can get very expensive to operate that kind of a system. You also have to factor the cost of keeping the water chemically treated year around. That's not cheap either.

Your best bet is having a gas furnace (if avalible) and standard condensing units heating and cooling your house. You can divide it up into zones and have as many as you need. This way you can cool and heat only the room your in at the time. As far as space, the air handlers can be placed in your attic and ducted and the condensing units can be placed on the side of the house where guest will not go anyways.

As far as sizing your unit, standard rule of thumb in San Diego is about 1 ton for every 600 to 800 feet. Your close enough to the water where you don't have freezing temps and the summers are not 120* like other places.

[suemarkp]

From Manual J, I have the following listed for both San Diego and El Cajon:

winter design temp = 44F, summer design = 80F, enthalpy = 20 to 24

There is a separate post for Manual D here (its a separate ACCA book too -- each is about $40):
http://www.selfhelpforums.com/showthread.php?t=3145

I've never been to San Diego, but I'm surprised at how similar it is to Seattle on paper. I'm a little cooler (and a lot cooler in winter), and you're a little more humid. I have a 4 ton AC in my 4800 square foot house (but 1600 of that is a daylight basement and I have a nice big tree to the west). I would think two 3.5 to 4 ton's would work for you if using modern windows and tight construction. Consider heat pumps too so these will work year round. Should be competitive with gas since your winter temps are rather warm. I don't know your gas or electric rates though. Two stage compressors are nice too, especially if you're a little oversized.

[sloooo]

Heat pumps in California are nice if there is NO gas available Mark. The problem in California is that when Enron went under, California Governer at the time signed electricity pacts with the electric companys and we are still held to them. Most Californians get an electric bill with 2 different rates because of it. Got to love California ;-)

[clark]

Thanks for all the info and help.
I am still working out some of the details.
This has been a great web site.

Clark