how many units do I need?

michoumonsterMarch 1, 2012

Hi all,

I am getting HVAC bids right now. I have a new construction 2 story with basement. We are in the SF Bay Area. We want to have A/C for the first and second floors, but don't think we need it for the basement, but we need heat for all 3 floors. Should I get 5 units -- 3 furnaces, and 2 A/C? Its seems like a lot of units. Or should I get 2 heat pumps combo units for first and second floors, then a separate furnace for the basement? One HVAC contractor told me that If I do the heat pumps, then I can only do electric heat for the basement. Is this true?

Thanks for any advice!!

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What is the area of each level? Do you have access to natural gas? What is your average rate for electricity?

    Bookmark   March 1, 2012 at 6:46PM
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Hi mike_home,
we do have access to natural gas. Our basement is about 1000 sq ft, second floor is about 1000 sq ft and main floor is about 3000 sq ft.

Electricity rate ranges from $.095 to $0.17 per kwH depending on tiered usage.
Gas rates range from $1.38 to $1.96 per therm dependign on tiered usage.


    Bookmark   March 1, 2012 at 7:22PM
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My suggestion would be to install two furnaces and ACs. The furnaces should be 95%+ efficiency, multi-stage variable speed. The ACs should be two stage.

The second floor would have one furnace/AC system. The other furnace/AC would be set up as two zones for the first floor and basement. This would be 4000 sq ft of space, but I think you will be OK given the basement does not present a big heat and cooling load. In addition the SF Bay area does not have severe winters or summers.

You want a contractor who will do a detailed load calculation and duct design. The zoning of the basement could be difficult so you want someone who has experience.

    Bookmark   March 1, 2012 at 8:07PM
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I might consider a dual fuel unit. But I guess it depends on whether you ever will hit the higher tier with your NG. How many therms are you allowed at the lower rate and what else are you doing with NG?

And how well insulated is the home?

An idea might be to use heat pump upstairs and furnace/AC on the first floor and basement so that you avoid the highest tiers on either.

Where in SF bay area - the variability locally is pretty impressive?

    Bookmark   March 1, 2012 at 8:38PM
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thanks mike! do you know if I would need to do some sort of A/C shutoff to the basement if the main floor and basement share the units?

is it better to have the main floor and basement share versus having the main floor and second floor share, or does it not matter?

usage-wise we will use the second floor much more than the basement.

    Bookmark   March 1, 2012 at 8:41PM
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david, our home is new construction on the peninsula. winters and summers are fairly mild-- we do get maybe a week of below zero days in winter, and a couple of weeks of 100 degree days in july/august.

we haven't put in any insulation yet but plan on doing regular fiberglass batts. we have 2x6 framing. windows and doors are all dual paned.

other natural gas appliances will be gas dryer, gas range, and a gas fireplace in basement, and another gas fireplace in main floor family room.

the electricity rate is 10kWH per day for tier 1, and tier 2 is $0.13 per kwH if up to 200% more, then tier 3 is $0.17 per kwH if we go over that.

    Bookmark   March 1, 2012 at 9:02PM
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Since both units are in the basement, then it may make sense to tie the basement into the second floor. Usually it is a more difficult to move air up two floors, so the basement is normally part of the first floor duct work. In your house it may work better since the second floor is only a third the size of the first floor.

You don't have to shut off the AC to the basement. The zoning systems would close a damper to the basement so it maintains the proper temperature.

Another option is to install resistance heat in the basement. Perhaps this is what the contractor was suggesting. You should not need to provide any cooling.

Your gas rate seems high. Are you sure they are correct?

    Bookmark   March 1, 2012 at 11:24PM
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Now your climate surprises me. How do you get above 100 surrounded by 55 degree water. And ditto the below zero. My father lives nearby and never gets below freezing or above 90. That sounds like a horrible midwest climate.

If your basement is truly below grade, a fireplace is probably all you need.

    Bookmark   March 2, 2012 at 5:45AM
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michoumonster- do you mean 0 degrees C? I lived in the bay area for 10 years and NEVER got close to 0 degrees F. david_cary- 5-10 100 degree days do occur during the summer when there's an offshore flow.

Also, be careful on the electric rates. PG&E electricity tiered usage as of 3/1/12 is: baseline $0.13/kWh, Tier 2 $0.15/kWh, Tier 3 $0.30/kWh, Tier 4-5 $0.34/kWh ( With 4000 sq ft on your main and second floor I can't imagine how you will stay out of Tier 4 and that's with NG heat. Think you may feel some serious PG&E pain if you go the heat pump route.

    Bookmark   March 2, 2012 at 9:16AM
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So what is baseline?

With a new house, you might be just fine staying out of the high tiers. 4000 sqft of new is probably better than 1500 sqft of typical CA house.

Different climate (although not that different) with similar size house including basement and my usage is about 800 kwh baseline and as much as 800 kwh for heat pump (in a hybrid setup). My impression is that our climate is much colder than yours (NC) and I have 2x4 walls. And electric backup on the solar hot water and electric heated tile which is all part of that 800 extra in winter. So truly, I'd expect your heat pump use to be 400 kwh. Now AC is a different story but there isn't much option there.

I think given the tiered rates, a hybrid setup on the 1st floor might be best. But again it depends on what the kwh baseline is and what the NG baseline is. Normally NG is used in CA but I had no idea that it was tiered also.

The biggest advantage of a new house is usually on the heating side. Cooling is not generally that much better since we tend to put larger windows in newer houses. I am down to about $400 a season to heat 5000+ sqft in an area that isn't cold but does freeze and snow at times. I've had apartments that cost almost as much. Last house was 1/2 size built 1993 and cost more.

    Bookmark   March 2, 2012 at 10:13AM
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From PG&E website:

0-351 kWh
$0.13 per kWh

Tier 2
352-456 kWh
$0.15 per kWh

Tier 3
457-702 kWh
$0.30 per kWh

Tier 4
703-1053 kWh
$0.34 per kWh

Tier 5
1054+ kWh
$0.34 per kWh

    Bookmark   March 2, 2012 at 10:31AM
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Rmrc, thanks for the correction! Yes i meant zero degrees c. We never get snow but do get some ice in winter. Do you think going all gas will be better? I was thinking doing some electric might set us up better if we decide to switch to solar in a few years when it becomes more affordable, but maybe it is too far out to plan like this?

Mike-home, i didn't think about the issue of moving the air up two stories to cool the second floor-- perhaps we need to rethink where to put the units. Would a good air handler be sufficient for that? Or even two air handlers?
Unfortunately, the gas rates are correct. After all of the city fees are tacked on top of the pge transportation fees, it gets pretty high.

David, i think nc weather is very similar to the bay area, except your winter is a little harsher. Also we really only have two seasons where you guys get to enjoy four. do you know how much the dual fuel systems cost? They sound like basically getting a heat pump plus a furnace in one package. Can the homeowner easily adjust when to switch to the gas versus electric depending on commodity prices, or is it something that can only be set once by a pro? Our gas rates are two- tiered and seasonal-- tier 1 goes up to 0.667 therms per day in summer and 3.2 therms per day in winter.

    Bookmark   March 2, 2012 at 11:30AM
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You could put the furnace for the second floor in the attic. However since you do get freezing temperatures it makes installing a 95% efficiency furnace more challenging. I would rather have both furnaces in the basement. The duct work can be designed so there will be good air flow to the second floor. This is the set up I have in my house.

You could upgrade the ACs to heat pumps. It chould be a few hundred dollars per AC unit. There are programmable thermostats which can control the heat pump and furnace based on outside temperature.

    Bookmark   March 2, 2012 at 1:15PM
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thanks everyone for all of the food for thought! One other question came up on where to locate the units-- my GC suggested to build a little enclosure in our side yard across the driveway and put all of the units there instead of the basement. This enclosure would probably be 10 to 15 feet away from the house, though it would be at the same level as the main floor. Would this make heating less efficient? I imagine there would be some heat loss? does it take less power to push air across rather than up?

    Bookmark   March 2, 2012 at 3:02PM
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Yes - it is easy to change the furnace/HP changeover. I just switched from 35 degrees to 40 when my therm price dropped to $.85.

3.2 therms in the winter is a decent amount for your area. We used 1 therm a day this year. I suspect if we were just gas, it might have been 3 therms. With water, you might be over but it is warmer there and your insulation is better.

My thinking (and some large bank just agreed) is that solar is at a temporary low price wise and only has up to go from here. 300 companies down to 20 in the last year d/t bankruptcy.

    Bookmark   March 2, 2012 at 3:06PM
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rmrc12, the OP is not on PG&E, obviously by the rate tiers described. There are one or two cities on the Peninsula that have their own utility companies, not hard to figure out which one this is.

At the given tiered rates, I think hybrid heat could make some bit of sense, but most likely a 95%+ efficient furnace will still be cheaper as electricity baselines here don't seem to accommodate hybrid scenarios (i.e. a heat pump would push into the highest electricity tier too easily). It's often 50-60 degress in the winter here on a nice day, a heat pump might be really efficient running around mid-day. But I notice my furnace mostly kicks in during the morning and late evening hours.

    Bookmark   March 2, 2012 at 4:04PM
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wwu123, you are right! my city handles its own utilities. for gas, they outsource to pge and tack on their own fees though.
i will ask all of our hvac bidders about heat pumps. it does sound like it might be efficient for most of the day, except for early mornings and night. it certainly would give us more options and help us hedge our bets against the energy rates.

    Bookmark   March 2, 2012 at 4:58PM
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