single family residence in southern calif

[hanque]

Hi All,

I'm an owner/ architect working on my own house in Los Angeles.
It's a multi level hillside house partially buried:

I'm planning 3 zones based on how I predict the house will be occupied.
I'm assuming that a split system is the way to go but I really don't like the fact that cool air & warm air come through the same registers.
Two solutions come to mind:
1. Install duplicate ducting to each conditioned room one high, one low (not really liking this idea).
2. Install radiant flooring for heat and ducting for cooling only (radiant flooring seems excessive and is rare in southern cal).

Any other ideas out there?

Thank you!!

 

[cdxx139]

Whats wrong with warm and cool air using the same register? It is more comfortable to provide warm air low, but Im not sure if its worth is at your added first cost expense. (but this is your home, and I knopw I dont always do what has the best ROI for my home)

knowledge is power

 

[willard3]

From "Forum Policies":

>>>These fora should not be used to bypass your own in-depth research on the issues that affect you, nor is it intended to be a substitute for appropriate professional assistance within your field or geographical region.>>>

You may have to consult an Engineer who designs heating/cooling systems.

 

[hanque]

I agree w/ the forum policy & thanks for the reminder...as an architect I understand and appreciate the need for paid professionals.
I've solicited help from a number of engineers in my area and they're either not interested (single family) or they just want to run it through their manual j load calc software as a split system & send me an invoice.
No one seems to want to address the bigger picture, so that's why I posted.
For some reason the image in the original post didn't show so here are a couple of links:
Street View
Ducts!

Thanks again!

 

[MintJulep]

What is the basis for your dislike of using the same ducts and registers for both heating and cooling? There are millions of homes where this is done and it's "good enough".

Sure, you could improve performance a bit with separate distribution, but the improvement will likely be imperceptible.

Cooling will dominate I suspect, so I'd design for cooling and take what you get for heating.

As an alternative to completely separate distribution, how about a common distribution system with separate registers only. Switch from high to low distribution with change of operating mode.

Or look into high-pressure distribution systems. It might offset the cost of installing separate conventional duct systems.

Won't all those extra registers be unsightly?

 

[Drazen]

Use DX-terminals in each conditioned room, you can buy wall units that can change flow (upward supply at summer, downward at winter), and use separate ventilation-only system sized for minimum outdoor air, and equipped with heat recovery.

And of course, hire professional to do calcs and specifications, even if it is not mandated, calcs can save you much on initial investment compared with "rule-of-thumb" sizing.

 

[hanque]

Thanks for the feedback!
Individual dx terminals...sounds similar to "ductless systems?"
Looks like you're replacing ducting with plumbing (which makes my job as an architect easier).
I think something like this would work for the bedrooms w/ compact size, but how about an largish open area w/ multi levels?

 

[Drazen]

i was thinking about dx multi-split system.

you have number of different indoor units to use, i believe wall parapet units are best for residences, but living rooms might need some other, like concealed, cassette or so.

you still need ducting for ventilation, but ducts will be much smaller.

 

[KiwiMace]

In LA and the valley, most homes have a central DX/gas furnace combo. Because the cooling season is long and does have extremes, you are not just wanting to condition two or three bedrooms and the living area - Ductless splits will most likely work out too expensive for every room in the house. You'll probably find that zoning out a central system will cost you more than the life cost savings over just conditioning the whole house. The exception to this is if the house is large (say >2500 SF), you might be looking at two DX systems anyway, so you would do one system for the living area, and one for the upstairs bedrooms.

LA has a reasonable heating season, not extreme in terms of outdoor design temp, but it is sufficiently long to want to do it right. Radiant floors are super comfortable and quiet, but this will mean a whole new system to buy and maintain if you already have planned a central cooling, hard to justify unless you really want the floor warm through winter. However, if it was my house, I would be looking to heat the floor as I hate the noise of the central air.

I also want to add that decent double glazed windows will do just as much for occupant comfort in all seasons as a well designed HVAC system.

 

[Shophound]

I also don't quite get the objection toward one air distribution system for both heating and cooling modes. Truth is, do the building envelope right and the need to push air around at high volumes decreases. Comfort levels increase. Comfort is more than just correct temperature of the air. It's your surroundings and humidity levels as well as air movement and temperature. If the envelope is construed to not present widely varying surface temperatures to the occupants of any given room within during outdoor temperature extremes, the occupants will be more comfortable, year round. Humidity control is easier when ventilation and building leakage is managed well. "Build it tight, ventilate it right" isn't just cliche.

 

[berkshire]

Shophound (Mechanical)
The biggest objection I see is stratification. A system optimised for cooling does not do so well for heating because of that.
Hence Mint Juleps comment"Cooling will dominate I suspect, so I'd design for cooling and take what you get for heating."
B.E.


The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[IRstuff]

Generally, most houses in So Cal are suboptimized, even for cooling, simply because the ducts tend to be routed through the roofing spaces. This winds up with the A/C spending calories cooling the ducts, and the house getting a blast of warm air while the ducts have not yet cooled down, which the A/C must also account for as heat load.

However, from a practical point of view, there are two reasons why dual ducts are undesirable for me; they take up way more room, and you never heat and cool at the same time, and usually not even during the same season. This would suggest that having a single set of ducts but with two sets of registers might make more sense. One could contemplate even having manual controls that switch between heating and cooling, since there's rarely a time when you need both in the same season. Another reason for not having two sets of ducts would be that you probably are using the same air handler, so mucking with duct sizes and pressure drops will likely drastically ding the efficiency of the air handler, and I certainly can't imagine running two sets of air handlers.

TTFN
faq731-376

 

[Shophound]

I lived in San Diego for eight years, so I'm familiar with So. Cal winters. Not harsh, to speak directly to the point. Less than 20% of the year for heating degree days. Lots of shoulder weather. Pick your climate based on how far you live from the ocean. Further inland, the hotter it becomes.

So yes, design for cooling and build a good house. You'll be fine.

 

[hanque]

Thanks to everyone for your observations & suggestions.
Seems like a conventionally ducted split system w/ two sets of registers is a reasonable way of dealing w/ the problem.
It's wood frame construction so we can use wall cavities to send air up or down w/in each conditioned space.

Drazen's idea of atomizing the mechanical equipment is a compelling way to keep the ducting down but I worry a bit about more units to buy/maintain.

 

[Shophound]

Using wall stud cavities for air transfer is most often a BIG mistake. Unless you can guarantee the entire run of that cavity is airtight to anything outside of the conditioned air moving through it, you will encounter building pressurization/depressurization concerns, which often translate to increased heating and cooling costs.

Additionally, often the selected stud cavities aren't by default correctly sized for the amount of air that should move through them to be good choices.

The problem with most residential HVAC design is the supply and return distribution system robs the a/c or heat of capacity. You end up spending money to heat or cool ducts, money that should rather go toward controlling space temperatures.

 

[hanque]

Shophound, thanks for your observations.
I think I can design around the shortcomings you're pointing out: In every case this transfer would take place in a sealed duct located between studs, and would occur only on interior walls. In general I'm trying to keep ducts away from the exterior envelope.

 

[hanque]

Shophound, I'm not sure I'm reading your critique correctly, but are you suggesting that ducts in general are problematic?

 

[IRstuff]

I think that it's a question of how much constriction there is in the final leg of the ducting. I was in a house that had an upgraded air handler, and when the A/C was on, the ducting or the registers whistled because of the airspeed. So, that's certainly something to consider, i.e., whether the downsizing of the duct will cause a higher speed airflow that might have some audible noise impact.

TTFN
faq731-376

 

[berkshire]

One of the big problems of putting ducts between the studs in drywall construction is that very often there is no room for insulation.
This makes it sort of ok for return air, but does not work very well for supply ducts.
However that said it may be one method of sorting out the heating/cooling. Use a diverter flap on the same return duct with two grilles in the space, one high,and one low. The diverter flap would seal off the low grille for cooling, allowing the upper grille to draw. Then while heating open the lower grille and seal the upper.

I believe this is what Mint Julep was saying"" As an alternative to completely separate distribution, how about a common distribution system with separate registers only. Switch from high to low distribution with change of operating mode.""
However I think he was talking about supply ducts.

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[willard3]

Which Engineer is going to accept legal responsibility for this design?

Anybody...............