Help with AC choice for Northern CA

dljmthNovember 1, 2012

I could really use some help from your wise ones!

We are adding 1000sqft and are sizing an HVAC. This 1000sqft space is a kitchen/family room with vaulted ceilings, a small bedroom and bath. Think of it as a 1-bedroom apartment. This new space connects to the rest of the house in a very open way to the dining/living room (about 550 sqft). The old part of the house is on a 13 year old furnace with alumaflex ducts run mostly through the attic but under the crawl space (rodents) for the master bed/bath (the furnace was installed when we added the MBR).

Standard 2x4 framing (yes, we probably should have gone with 2x6 but we are adding to an existing 2x4 house). The new remodel will have a combination of cellulose and foam insulation at R-19 (ceiling) and R-13 (walls and floors). We live in a moderate climate in Northern CA where the need for AC is just a few weeks a year. We don't have AC in the current house. We do have one person in the house who has some moderate-severe allergies (living in the house while the addition is being built is causing problems because of the dust).

Originally, our spec called for hydronic radiant heat, but for various reasons (primarily budget) it fell off the list. Instead we are opting for gas forced air and want to take advantage of air filtration that the system can offer. So now the question is what is the best way to do it.

We think we should add the 550sqft of living space to the new furnace and keep the old furnace just to heat the bedrooms (we keep those rooms cold at night and they are rarely used during the day anyway) so will have a new furnace serving about 1550sqft. The HVAC contractor recommended a TRANE XV95 80K 3ton, 2 stage variable speed system with one of their higher end air filter systems.

While in our climate the system will mostly run in the 1st stage, there are times when we would use the system at the 2nd stage for air cleaning - essentially running the unit to clean the air during high allergy season.

For AC, he recommends a Cold Point system

So here are my questions:

1) is that the right size system or is it overkill? It seems from what I've read our requirements are right on the line and we could possibly go for a smaller unit - although the cost differential is only about $150.

2) Does anyone have experience with the TRANE air filtration systems? Specifically their Clean Effects?

3) We've never had AC so I don't really know the differences, but this system seems outdated. Plus it appears it uses an R22 which is being phased out. The reason he is suggesting this system is because we have very strict city codes for noise and can only have a unit that is less than 64dB. Does one even exist that could meet this requirement? We have lived without the AC for 20 years and don't really need it, but as we age the allergies seem to worsen so we thought we'd take advantage of the switch from radiant to gas forced air by getting AC. Should we scrap traditional AC?

Any advice is greatly appreciated.

Thanks in advance!

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Tell us about your typical weather/temps for your location, both winter and summer.

Can't believe you need an 80 KBTU furnace and don't care at all for the idea of mixing brands. In fact, I would go farther and say it is irresponsible.

As far as Clean Effects, you need to have a clear understanding of a homeowner's responsibility for regular cleaning/maintenance that this air
cleaner demands. If you are not willing to take on this issue, then I would stay away from it. Personally, I would not have one and would opt for a good pleated filter media cabinet like the Perfect Fit that matches to the furnace.

I would like to see in writing the city code that demands a 64 db rating. I can only think of several that would be meet that standard. My Trane condenser is rated at 74 db and I can not hear it inside and have to be close outside to hear it. In fact, I hear my neighbors which is louder than mine.


    Bookmark   November 1, 2012 at 8:38AM
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tigerdunes - thanks for your reply. Not sure I understand your comment about mixing brands. Are you saying we should have the same brand AC as furnace?

As for the 80K furnace, yes, it does seem like overkill, but our energy calculations show that the new space alone required just under 40K for 1000sqft. Now adding 550sqft the requirement would likely go up to 60K or slightly higher given the orientation of the house. The new part is Southern facing and has been designed to take advantage of solar heat gain. The old part is North and the shadiest and coldest part of the house. I think the logic from the HVAC person was that we could go for a 60K system but the incremental cost is $150 so size up. Not sure that is valid logic but I'm guessing that's the reasoning.

As for the db ratings, yes, I have a copy of the city code requirements and it is true. This low requirement is based on the location of the unit. We have to put it on the side yard because we are at setback in the back. Side yard placement has stringent noise requirements. Alternatively, we could place the unit in front of the house but it would be aesthetically unappealing.


Thank you!

    Bookmark   November 1, 2012 at 11:27AM
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You did not provide typical weather/temps for summer and winter. Pls post.

I would go no higher than a 60 KBTU furance at 95% efficiency.

And yes I would stick with one brand both furnace and AC.

For as little as you will use the AC, a good basic 13 SEER condenser
would be fine. However it will be problematic to find one with a low Db rating.

I personally would contact your City Manager and see what procedure if any is available to get a variance on such a stupid and restrictive code.

I do know Lennox makes a low Db rated AC condenser but it would be expensive.


    Bookmark   November 1, 2012 at 12:29PM
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The winter temps will go down to mid 30's at night but up to low 50's during the day. During the summer, mornings are always cool in the 60's and for 1-3 weeks a year we will hit the high 80's. Else it will be in the mid-high 70's.

    Bookmark   November 1, 2012 at 12:39PM
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"...there are times when we would use the system at the 2nd stage for air cleaning - essentially running the unit to clean the air during high allergy season."

Has anyone seen any data that indicates doing this helps with allergies? With forced air ducts on the outside of the living space, I'd be more concerned with keeping positive pressure inside the house with filtered, controlled venting if outdoor allergens are involved. "Seasonal" indicates outdoor, but problems during construction might not. If you need to control allergies, the first step is figuring out what that person is allergic to.

    Bookmark   November 1, 2012 at 2:24PM
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Allergies are both seasonal and dust/mold related. But your point is well taken. Need to know exactly what I have to combat. Thanks.

    Bookmark   November 1, 2012 at 3:34PM
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Doing a follow up post as we need to make our choice in the next week. We are going to scrap the AC. Have lived in the house for 18 years without it so we will be fine especially since we are upgrading insulation, putting in new windows, etc. Further refinement of the heating loss/requirement calculated as part of our CA Title 24 req's yields a need of 61,000BTU. So the question goes back to sizing of the furnace.
Here are some other (corrected) details:
-high window/wall ratio (in fact there are 2 of three exterior walls in the new space are almost all new windows)
-R38 cathedral ceiling
-R13/19 floors/walls
- day time temps up to 55 nighttime down to 36 (yesterday it got up to 70 during the day!) so sometimes we crank the heat in the morning and then wind up opening the windows in the afternoon.

From the TRANE site:
Model Nominal Capacity Output (BTUH) Stage1/Stage2 AFUE
TDH2B060A9V3VA 37,000/57,000 95
TDH2B080A9V3VA 49,400/76,000 95

So it seems like we are between sizes. It seems typical (I didn't say "right") that people size up (bigger is better).



    Bookmark   November 25, 2012 at 2:56PM
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Stick with the 60 KBTU model and look for opportunities to lower your heating load through insulation improvements, etc.

And it's important to get a true two stage thermostat that controls a two stage furnace rather than a timer on the control board. Big deal!

And bigger is not better.


    Bookmark   November 26, 2012 at 12:22PM
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