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Old 12-29-19, 02:11 PM   #3
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Originally Posted by dfhuynh View Post
Hey all,

I'm a newbie, thinking of how to heat/cool a new house to be built in Northern California where the temperature ranges from 45F to 105F. I'm hoping to get some opinions here about my plan--whether or not it makes any sense. Thanks in advance!

The new house will be 2-storied, around 2800 sqft including a 300-sqft Accessory Dwelling Unit for rental. The first floor has a "big room" and a guest suite, and the ADU. The second floor has 3 bedrooms of various sizes, from 150 sqft to 400 sqft.

(Of course, I'll have good insulation, so this is on top of that.)

I'm thinking of having a gas furnace to heat mainly the "big room" on the first floor (with some effects on the other rooms). The furnace should help get the other rooms to about 64F ~ 66F in the winter. In the summer, I plan to have an A/C unit to cool mostly the big room for family activities; it should help get the other rooms down to 76F ~ 78F. Then, in each of the bedrooms, I have a small 9000-BTU 700~900W heat pump to get the room's temperature to a comfortable level (68F ~ 72F) only when it's in use. I expect just 2 of the 4 bedrooms to be in use year round, whereas the other 2 will be used by visitors for maybe 2 months per year.

The ADU will be on a separate power sub-panel and completely electric (no gas). It also has a 9000-BTU 700~900W heat pump that's good for 400 sqft, for both heating and cooling.

Does this plan make any sense, environmentally / financially? Any concern if by chance all of the heat pumps are on at the same time?


I'm coming from a 1985 built to minimum code 2100sq ft house with 2x4 construction R13 fiberglass walls with a R5 polyiso wrap and leaky double pane windows. With the design temp based on the San Francisco international airport weather station of 41 degrees, my 60k furnace would run 5 minutes out of every hour, or only 2 hours per day. Leave all the doors open and the whole house will be at about the same comfort level on a cloudy "cold" day in your area with the center of the house heated. The smallest common size furnace is 40k BTU but then you've got a 105f outdoor temperature and tons more sun which usually means you need more air conditioning and there are limits to how much cooling a smaller furnace can provide, so often these get upsized. Heat rejecting glass is your friend as well as finding a contractor to do a proper cooling load so they don't upsize the air conditioner too much.

I don't know the ongoing financial costs of gas connection charges, along with the cost per therm or ccf of natural gas compared to your electric cost. If you've got sky high electrical costs in California and want a gas stove and gas dryer in your house, I'd say the combined system makes sense. However if the connection charges for gas are expensive and heating with gas is spendy out there along with electricity, than I'd just go with mini split heat for the whole house and skip the ductwork entirely. Personally, for 41f design load, I'd just do the mini-splits throughout the space and call it good and skip the gas connection charge, as much as I hate cooking with an electric stove, I'd probably get a induction cooktop.

Financially - I think you'd be better off with a zoned damper system if they can manage to properly size the ductwork to make this work appropriately, it would be cheaper than multiple mini-split heads. The trouble with this is most HVAC contractors don't size their systems so they can run on just a few zones so then you run without enough air flow for cooling to be efficient. If you are going with a custom home contractor, they will likely have a low bidder HVAC company that will butcher this job. You need the ductwork within the building envelope, not above insulation in an attic or laying in an uninsulated crawlspace, should be done with rigid metal ductwork. Cost cutting through competition and low bidders caused HVAC to become a garbage operation starting somewhere in the 1990s.

Regarding the "Radiant heat is way more comfortable then scorched air." This will be the most expensive way to heat your house and then you'll need a separate system to cool the house. In an area where you have such a warm winter temperature and a low heat load, this is massively overkill. Don't do hydronic. Even in a well insulated house in Minnesota radiant heat doesn't make financial sense compared to putting the extra hydronic cost towards going beyond code minimum insulation, I can't believe an expensive hydronic system is being recommended for an area where you barely need to heat a well insulated home.
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