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Old 11-29-2015, 03:59 PM
 
Location: portland, me
798 posts, read 1,906,097 times
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I am in the process of designing a new house. It will be passive solar, about 1000 sq. ft. and fairly well insulated (r50 roof, and r40+ walls). My builder/architect has suggested using a mini split for our heating needs, but he is not 100% sold yet since this house will be in Maine. I have researched these systems quite a bit, and they seem like a viable solution for our house.


My question is this: can we enclose the outside condenser to make it more efficient during winter? My idea is to fab an enclosure that has a south facing wall that is a trombe wall. This would allow the winter sun to heat up the inner space of the enclosure allowing the unit to operate better. Even a 5 degree temp increase can make a big difference for these units in the winter. In the summer, I could just vent the trombe wall to the atmosphere for cooling, or keep the unit off. Any insight is appreciated.
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Old 11-30-2015, 07:08 AM
 
Location: DC
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I think you want to work out the heat balance on this proposal. A small enclosure would cool down really fast in operation, especially on an overcast day. You might at least need a thermostatically controlled fan so that the inside of the enclosure wasn't colder than the ambient air. By far the best system in a northern climate is a ground coupled heat pump.
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Old 11-30-2015, 10:24 AM
 
Location: portland, me
798 posts, read 1,906,097 times
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Quote:
Originally Posted by DCforever View Post
I think you want to work out the heat balance on this proposal. A small enclosure would cool down really fast in operation, especially on an overcast day. You might at least need a thermostatically controlled fan so that the inside of the enclosure wasn't colder than the ambient air. By far the best system in a northern climate is a ground coupled heat pump.
Are you saying that the expelled air from the compressor (now colder) would stay inside the enclosure making it cooler? I was thinking that it would cycle back through the wall to be warmed up, but I see your point that on a cloudy day, there would be no heat to offset that. Good point, and something to think about.

As for geothermal, I disagree. The expense is far too large to offset the benefit in a house the size that I am building. We originally looked at having radiant heating put in under the slab, but the house will be too tight and well insulated making the floors more expensive than they are worth. We do plan on having a high efficiency wood stove for backup.
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Old 12-01-2015, 04:28 PM
 
Location: Vermont
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A Fujitsu 15RLS3H, Mitsubishi FH18 or FH15, whatever is the latest and greatest is going to be plenty for 1000 sq ft with R40 walls and R50 ceiling (I think the Fujitsu is the lowest in cost).

The one thing I like about the Mitsubishi's is the option for a WIRELESS REMOTE THERMOSTAT (which I Do not have on my Fujitsu).

I think if you enclose the compressor, you need a way for the colder than ambient air coming off the fan to get out of the enclosure unless you think the sun will heat it up quicker than the cold air is expelled. Maybe you could duct it to the outside???

I think the compressor needs an air supply too, so you'd have to put vents in your walls, unless there's enough air in the wall/enclosure to cycle though. If you restrict access to the compressor it will be less efficient.

Thinking about this in another way... I had toyed around with the idea of putting mine in my attic.
The problem there would be that at first , the air inside the attic would be warmer than ambient. The mini-split would be more efficient than it would be outside. As the minisplit does it's job, it would most likely cool down the air inside of the attic below ambient and become less efficient. I think , unless you had provisions to work around this, you would have similar issues .


You might check more on greenbuildingtalk.com

Also, I've seen recommendations to mount the mini-split inside head unit as low as practical to allow cooler air to enter the head unit, rather than already warm air. It is supposed to be more efficient this way.

Good luck, I don't think you will have any problems with the cold climate air source heat pump.
Also might want to try the "set it and forget it" method of heating with these units. The "experts" do not recommend setting back.

Is your dwelling 2 stories?

Mine is 2 stories and my upstairs is about 4-5 degrees colder than downstairs (so far) where the head unit is located. For the most part, this works fine. I have down set at 68 and up is about 64-65.
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Old 12-02-2015, 08:08 AM
 
Location: DC
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Quote:
Originally Posted by dubthang View Post
Are you saying that the expelled air from the compressor (now colder) would stay inside the enclosure making it cooler? I was thinking that it would cycle back through the wall to be warmed up, but I see your point that on a cloudy day, there would be no heat to offset that. Good point, and something to think about.
Exactly

Quote:
Originally Posted by dubthang View Post
As for geothermal, I disagree. The expense is far too large to offset the benefit in a house the size that I am building. We originally looked at having radiant heating put in under the slab, but the house will be too tight and well insulated making the floors more expensive than they are worth. We do plan on having a high efficiency wood stove for backup.
The major issue is the cost of installing the ground loop and that varies dramatically depending upon the geology of your site. If you happen to be sited next to an accommodating body of water, you can just weigh down the loop and sink it to the bottom of the lake/stream etc. It's hard to get a more economical installation than that. If you are building on top of a granite outcropping, drilling will be expensive. With that said, since you can roll the cost of the GSHP into a conventional mortgage, you can spend quite a bit on capital cost and come out ahead at the end of the month.
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Old 12-02-2015, 09:42 AM
 
Location: portland, me
798 posts, read 1,906,097 times
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Our building will be one story, but with an open, low angle cathedral ceiling. Mostly open concept with two bedrooms, and a separate bathroom and mech room. The roof is supported by trusses and beams that are exposed. We don't plan on having walls above the beams so we will get good circulation of air flow between rooms.

I have seen a picture of one compressor that pulls air from an enclosed porch while dumping air to the outside. The idea is that the sun warms up the porch, and there is plenty of air that comes in from below through the floor boards. I definitely think that any enclosure will need the bottom open at the very least. Still kicking around some ideas in my head.

As for geothermal, sadly we don't have enough water close by, and the ground work is cost prohibitive. I have seen some installs that place the loops under the slab, but I don't know enough about the costs on that yet.
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Old 12-02-2015, 10:57 AM
 
Location: DC
6,530 posts, read 6,466,290 times
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Putting a loop under your slab is not a good idea. It becomes inaccessible for any reasonable repair. If there is no body of water available, the next cheapest installation is usually a trench with the so called "slinky" installation. This can be dug with a trenching tool like utilities use for underground services.
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Old 12-02-2015, 11:02 AM
 
Location: Vermont
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I think it will be hard to justify the cost of an in ground system over a $2700 mini split that will probably average 3.0 COP.

What are you doing for a water heater and clothes dryer?

We are using the latest and greatest GE Geospring and Whirlpool hybridcare (both heat pumps)
So far so good, at least , functionally . I do not have a way to monitor these appliances individually yet and, they are a little loud.
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Old 12-02-2015, 06:57 PM
 
Location: portland, me
798 posts, read 1,906,097 times
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Joe, how loud would you say your GE water heater is? I've been looking into heat pump water heaters lately, and I think we will go that route. I plan to install some sound deadening into the walls of the mech. room to keep thinks quiet. Currently, we have an all-in-one, ventless washer/dryer that works by condensing. It takes a lot longer than normal to dry, and it quite loud. We are planning on using a more traditional stack setup unless I find something better.

I agree with you on the costs of the ground system. I just can't justify it when a mini split is so cheap. A 3.0 COP will only be achieved in our neck of the woods for part of the year. There will be some days where the temps are close or below the cold weather ratings bringing the COP closer to 1.0.
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Old 12-02-2015, 08:58 PM
 
Location: Bend Or.
1,126 posts, read 2,460,159 times
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I would recommend the Mitsubishi Hyper heat, especially when the temps get really low. I would not recommend you enclose it. The air movement through the coils is pretty massive, and it would not pick up enough heat from the wall to counter the amount you are emitting. I just had one installed and we placed it on the south side of the building where it will pick up some reflected heat from the building but still have good air flow. Because we have 24" eaves, placing it under the eave was not an option for that same reason. Especially in cooling mode in summer. another consideration is snow, which is why we mounted our outside unit 24 inches off the ground, and are already glad we did.


We considered the HP water heaters, but you have to be careful that your mechanical room is big enough, I think it is about 1000 cubic feet. If not, you will cool the room too much and the efficiency goes right our the window.


The heat pump dryers are another animal, they don't cool the room because of the way they work. We too have the Whirlpool hybridcare at it rocks! The only issue we have had is in 6 months the door has locked itself shut twice and the controls were dead. But as anything with a computer in it a quick power cycle makes everything OK>
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