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Old 12-28-19, 05:42 PM   #1
dfhuynh
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Default Mini heat pump per room?

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?

Thanks,

David

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Old 12-28-19, 10:31 PM   #2
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If you are building new, consider radiant floor heat throughout the entire house. I would also look into a geo exchange for any heat pumps so you can go water to water with the radiant. If your building, now is the time to consider it while everything is torn up.

Radiant heat is way more comfortable then scorched air.

If I was building new, the house would be designed around solar heating directly from the sun as a base. Well insulated. I would not do any air ducting but instead would have a central masonry heater as backup heat.

I think multiple heat pumps would be over complication. Its so easy to just move water around from one source. Plus, it may be hard to find a mini heat pump small enough for one room so you would be looking at DIY.

I'm a DIYer and am working on a couple of heat pumps but ultimately I am going to buy a small heat pump down the road that is UL listed for the house. DIY for the greenhouses is fine for me. I'm trying to find a small 2 ton water to water and they are not common in that size.
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Old 12-29-19, 03:11 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?

Thanks,

David
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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Old 12-29-19, 06:30 PM   #4
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Thank you, gadget and MN Renovator.

@gadget, I did consider a masonry heater, but I couldn't find a place for it in my current schematic. As for radiant floor heating, I'm personally nervous of water damages where I can't see; but I do like the concept and love it in other people's houses.

@MN Renovator, as you noted, I'm more concerned about cooling than heating, especially cooling during the really hot days when there's a planned or unplanned power outage. I'm expecting more frequent planned power outages in the coming years conducted by PG&E to supposedly lessen the risks of wild fires.

Just FYI, the energy costs here with PG&E are:
- for electricity, 22.981 cents / kWh
- for natural gas, 8.425 cents / kWh

However, I do plan to have plenty of solar panels and a battery hopefully good for 3+ cloudy days when there is a power outage. My thinking is to get the most out of "free" solar energy throughout the year, while not draining the battery too much during power outages.

I don't think I can go completely electric: my wife still wants a gas range and I think I still want gas for the water heater (but could be convinced to switch). (The renter gets an induction cooktop in the ADU for sure.)

For the "big room" (15f x 45f = 675 sq ft), I figure ductwork would get the space heated / cooled faster and more uniformly. On the other hand, is it possible to have multiple "air handlers" per "air condenser" in one mini-split setup? If so, then maybe I'll go completely electric with heating and cooling and no ductwork.

(Yes, I have had issues with ductwork before. In my previous house with just 1680 sq ft, the *new* ductwork was pretty bad that there was a 3F ~ 4F difference between the two ends of the house.)

Thanks!
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Old 12-30-19, 07:20 PM   #5
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Your guesses for sizes of heat pumps are about right for a poorly insulated house with reasonable (less than 10%) window area.

A well insulated house would have at least R60 ceiling, R30 walls, and R3 windows. My last house was insulated like that and had a one ton central air conditioner. In a stretch of hot weather where the daily highs were 100 to 104 deg F, the AC ran only 58% of the time. We normally heat to about 70 deg F and cool to about 78 deg F. It takes two or three days of 90 deg F or hotter to get the house from 70 to 78 deg F. A well insulated house is nice in an area where power outages happen.

Heating/cooling systems do not heat/cool by square feet floor area, but by heat loss or gain. The heat loss or gain is calculated from square feet of wall, window, and ceiling, plus internal heat gain. An example heat gain calculation for a 400 square foot corner bedroom on the top floor:

Wall area (20 + 20) X 9 feet high / R30 = 12.0
Ceiling 400 square feet / R60 = 6.7
Windows (3) at 3 X 4 feet / R3 = 12.0
Total = 30.7
Heat gain at 105 F outside 75 F inside = (105 - 75) X 30.7 = 920 BTUH
Heat gain from two people at 400 BTUH each = 800 BTUH
Heat gain from a big TV set at 100 watts = 340 BTUH

Total heat gain = 2060 BTUH. This is the load that should be used to size the AC.

Similar calculations apply to the other rooms and the entire house. If the heating and cooling systems are properly sized, they will keep the house at the desired temperature, and you will not even notice their operation. And the total electric cost for AC in hot weather will barely show on your electric bill, even at $0.23 per KwH.

Insulation gives you a more comfortable house, a house with low heating/cooling bills, and a house where the power can go off all day on a hot or cold day with minimal (about 3 deg F) effect on inside temperature.

Insulation also helps the entire house to be at the same temperature. My brother likes cold bedrooms. When he came to visit in winter at about 0 deg F outside, he blanked off the heat register, closed the bedroom door, and could only get the temperature down to 60 deg F.
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Old 12-30-19, 09:54 PM   #6
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I can't believe an expensive hydronic system is being recommended for an area where you barely need to heat a well insulated home.
I don't know, after experiencing how much better it feels to live in a house with hydronic, I couldn't imagine going any other route

OP I am curious where you plan on building in N Cal. I have lived there and winters are much colder then where you live now. Unless you are looking at Humbolt area.
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Old 12-31-19, 06:17 PM   #7
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@gadget, I'm in the SF Bay area on the peninsula. It gets down to the high 40s F this winter, and I'm expecting more multi-day heat waves in the coming decades.

@JRMichler, thanks for the calculations! I didn't know that before. Very useful!

I think I'll put my money into triple-glazed windows (R9), or even quad-glazed (R17) in one or two places. I will have a lot of big windows but fortunately they face the North-East direction and only get about 1 hour of sun in the early morning. For the windows facing South-West, I'll install external solar shades and pull them down in the summer.

Overall, it sounds like I can just go completely electric for heating/cooling and even have fewer heat pumps (3 x 9000 BTU) given excellent insulation. I'll run two air handlers on one condenser for any pair of rooms that don't usually get used simultaneously, like the "big room" downstairs and the master bedroom upstairs.

Thanks, everyone, for chiming in. I now have the answers I needed!

Happy New Year!
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Old 12-31-19, 09:36 PM   #8
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What you speak of is a "multi-split" or "city multi" set up. Main idea is one big outdoor unit (with a variable speed compressor) that stars out to a number of independent zone units indoors. Look on fleabay or scamazon for the lowest prices in the universe, and the good brands for double cheaper than dirt. Beware of the quickest cheapest sale! There exists a local, factory authorized guy who is awesome but not super cheap.

Actually, if you are having a site built home designed from the ground up, the HVAC guy should have an insulation and energy audit crew anyway. If not, hmmmm...

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Old 01-01-20, 11:14 AM   #9
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Originally Posted by JRMichler View Post
Your guesses for sizes of heat pumps are about right for a poorly insulated house with reasonable (less than 10%) window area.

A well insulated house would have at least R60 ceiling, R30 walls, and R3 windows. My last house was insulated like that and had a one ton central air conditioner. In a stretch of hot weather where the daily highs were 100 to 104 deg F, the AC ran only 58% of the time. We normally heat to about 70 deg F and cool to about 78 deg F. It takes two or three days of 90 deg F or hotter to get the house from 70 to 78 deg F. A well insulated house is nice in an area where power outages happen.

Heating/cooling systems do not heat/cool by square feet floor area, but by heat loss or gain. The heat loss or gain is calculated from square feet of wall, window, and ceiling, plus internal heat gain. An example heat gain calculation for a 400 square foot corner bedroom on the top floor:

Wall area (20 + 20) X 9 feet high / R30 = 12.0
Ceiling 400 square feet / R60 = 6.7
Windows (3) at 3 X 4 feet / R3 = 12.0
Total = 30.7
Heat gain at 105 F outside 75 F inside = (105 - 75) X 30.7 = 920 BTUH
Heat gain from two people at 400 BTUH each = 800 BTUH
Heat gain from a big TV set at 100 watts = 340 BTUH

Total heat gain = 2060 BTUH. This is the load that should be used to size the AC.
You seem to have calculated the gain/loss related to the difference between the interior and exterior temperatures correctly but you haven't factored in solar gain through the windows. This is the reason my house uses 15000BTUhr to cool on a 83f when it's sunny when 15000BTUhr can heat my house to 70f when it's 8f outside at night without solar gain.

The rest of the math checks out. I think I'd aim for windows with a U-value of 0.2 or better(R5) if I was going to go with R30 walls and an R60 ceiling. Heat rejecting glass especially on any Southern and Western exposures in sunny California.
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Old 01-01-20, 11:28 AM   #10
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I think I'll put my money into triple-glazed windows (R9), or even quad-glazed (R17) in one or two places. I will have a lot of big windows but fortunately they face the North-East direction and only get about 1 hour of sun in the early morning. For the windows facing South-West, I'll install external solar shades and pull them down in the summer.

Overall, it sounds like I can just go completely electric for heating/cooling and even have fewer heat pumps (3 x 9000 BTU) given excellent insulation. I'll run two air handlers on one condenser for any pair of rooms that don't usually get used simultaneously, like the "big room" downstairs and the master bedroom upstairs.
I think you'll find the price difference between energy star 2 pane windows and 3 pane windows R5 to be high but could be worth it, but aiming for glazing that is R9 or higher might be very cost prohibitive. Not sure I'd go with any sort of quad glazing or suspended film stuff. You don't have a large temperature spread like what can happen in Minnesota(our design heat load is based on -13f) which is 83 degrees difference between 70 and -13. I'd personally aim for R5 or better triple pane (0.2 U-value or less) and if I find better for a similar price I'd take that.

I agree that you could go completely electric for heating and cooling, if you use ductless mini-splits you've just removed ductwork and/or hydronic lines entirely and inverter mini-splits will have no trouble heating efficiently in your climate and you probably won't see them defrosting on anything but the worst days which is a major bonus.

I like the idea of two separate outdoor mini-split units, if one fails you've got a backup. I'd put one mini-split on it's own for the largest area and if I wanted more than 2 total indoor heads I'd use a multi-split off of that unit. I think these units are generally reliable, especially big names like Mitsubishi, but the cost of two single head units and a single mutli unit isn't too much more money and the smaller single head units seem to be slightly more efficient. This would be my personal preference, but I recognize it might not be the most popular idea.

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