Central A/C general questions


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Old 07-02-12, 08:22 AM
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Central A/C general questions

We are in process of building on an extension to a house we just bought, roughly doubling it in size. One of the open questions is whether to go with central air. As we've never had any kind of central air in our current house, this is something new to us, leading to 2 fundamental questions:

1) What kind of central air to go with? I'm aware of at least 3 types - the normal one, with large ducts running everywhere, another one with small pipes and small outlets pushing higher-pressure air, and the "ductless" which seems to be like a mini-split system writ large. A lot of folks recently are pushing the ductless.

One of my concerns is room-by-room control of the a/c. Having stayed in houses with conventional central a/c, the room-by-room control is pretty terrible - fiddling with registers and hoping something works. I'd vastly prefer a solution that just lets me have a thermostat in each room/zone to set the temp. Note that there are a _lot_ of rooms - 5 bedrooms, a home office, plus LR/EIK and bathrooms. Not sure if any of the options would let me have this much control over zones (for the heating system, I am planning on 8-10 zones!).

What are the pros/cons of the different system types?

2) How big a difference in cost would it be to install the central air now, while the construction is being done, vs. retrofitting it later? For budget reasons, we'd prefer to do central air in a year or two, but if it would save a ton of money on installation, perhaps we should do it now. Note that while the new half of the house is being built from scratch, the existing half will be almost completely untouched.

Thanks,
Juliean.
 
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Old 07-02-12, 10:56 AM
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Retrofitting central a/c into a house can be messy and expensive unless there's an unfinished basement or crawlspace that ducts can be run through.

Putting ductwork in almost always involves some demolition, so it must be done during other renovations.

Some contractors may suggest putting a system in the attic, but I don't recommend that due to...

1. Poor service access
2. Risk of water damage from condensate
3. Air leakage from house into attic - holes need to be cut for vents, ducts if two story
4. Higher operating costs due to duct leakage + conductive heat gain (55F air going through a 110+ F attic always picks up heat, even when the ductwork is insulated)

One of my concerns is room-by-room control of the a/c. Having stayed in houses with conventional central a/c, the room-by-room control is pretty terrible - fiddling with registers and hoping something works.

Read more: http://www.doityourself.com/forum/ai...#ixzz1zUMOdVHS
That's caused by poor design.

If the duct system is designed correctly, it will cool fairly evenly.

Ask for a room by room load calculation (used to size the runs to each room); ask how the duct system will be designed. (google "manual D")

Not sure if any of the options would let me have this much control over zones (for the heating system, I am planning on 8-10 zones!).

Read more: http://www.doityourself.com/forum/ai...#ixzz1zUN0GKK2
How is the house heated now?

Forced air is not suitable for that kind of zoning.

Hot water heat might be the way to go in your case, since it's easy to zone.
 
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Old 07-02-12, 12:26 PM
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Heat is hot water. I have no questions about heat, I'd just like a central a/c that's as flexible and controllable as the heat.
 
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Old 07-02-12, 12:27 PM
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I'm no HVAC guy but the control you're seeking seems like it would only come with a window unit in each room.
 
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Old 07-02-12, 03:10 PM
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Residential a/c systems can't be zoned to that extent.

Your best option is to stick with hot water heat and install mini-splits. (cheaper than ductwork)

It's possible to have several heads connected to one outdoor unit.
 
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Old 07-03-12, 08:30 PM
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Old 07-03-12, 08:49 PM
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Residential systems have to move a certain amount of air to avoid evaporator freeze-up.

There are two stage and modulating systems out there, but the minimum capacity is 66%/40% of full capacity respectively.

Bypass dampers can be used to relieve the pressure in the supply duct (when one zone needs cooling) and improve airflow, at the expense of efficiency.

It's not advisable to split a residential system into more than 2-3 zones regardless of what controls manufacturers offer.
 
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Old 07-04-12, 11:36 AM
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What about a ductless system like this:
Performance Series Multi-Split Ductless System - Carrier

Seems I can get 4 heads per outdoor unit. Each of these is independently controlled, right? Effectively giving me 4 zones? So 2 outdoor units can create 8 zones? Or am I missing something?

Also, with these ductless systems, is putting them in during construction vs. later a big deal?

PS Or for that matter this: MXZ-8B48NA Split Air Conditioning and Heating 48K Btu - Up To 8 Indoor Units :: Mitsubishi Electric - Ductless and Ducted Systems :: AC 4 Life, Inc. which already has 8 indoor heads?


Thanks,
Juliean.
 
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Old 07-04-12, 11:46 AM
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Seems I can get 4 heads per outdoor unit. Each of these is independently controlled, right? Effectively giving me 4 zones? So 2 outdoor units can create 8 zones? Or am I missing something?

Read more: http://www.doityourself.com/forum/ai...#ixzz1zgH0Sq1Y
Yup.

Sounds very expensive though - I would probably go for 3 or 4 zones if the floor plan allows for it.
 
 

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