Home greenhouses: Worth having?
m...@privacy.net
greenhouse attached to side of home.
I would use it to capture heat in winter to help heat
the home...as well as grow some of my own food.
Anyone doing that out there and can advise abt the
feasibility of this idea?
Rod Speed
It does work for heating the house.
Not ideal for growing food tho, greenhouses need heating to
stop them getting too cold overnight for whats growing in them.
And you cant grow that much in a small greenhouse
anyway except seedlings for planting out outside the
greenhouse and maybe tomatoes etc.
VFW
"Rod Speed" <rod.sp...@gmail.com> wrote:
A "Sun-Space" might be an idea to look into. tho.
Some are nice places to have the morning coffee on a day off. Facing a
little more Eastwardly. some add thermal storage .
--
Karma, What a concept!
m...@privacy.net
>And you cant grow that much in a small greenhouse
>anyway except seedlings for planting out outside the
>greenhouse and maybe tomatoes etc.
was thinking of growing thing vertically using
hydroponics
m...@privacy.net
>A "Sun-Space" might be an idea to look into. tho.
>Some are nice places to have the morning coffee on a day off. Facing a
>little more Eastwardly. some add thermal storage .
>--
yes point taken
how would once calculate the correct direction and
angle to install one thou?
Rod Speed
> Rod Speed <rod.sp...@gmail.com> wrote
Sure, but that doesnt fix the overnight heating problem.
I was involved with a fancy solar greenhouse that used hydroponics
that way, but the overnight heating problem involved quite literally
a couple of rooms full of gravel that was heated using a big solar
air heater during the day and was used to heat the greenhouse
at night, all computer controlled. The rooms full of rocks just arent
practical for a small greenhouse.
Rod Speed
> VFW <geor...@toast.net> wrote
>> A "Sun-Space" might be an idea to look into. tho.
>> Some are nice places to have the morning coffee on a day off.
>> Facing a little more Eastwardly. some add thermal storage .
> yes point taken
> how would once calculate the correct direction and angle to install one thou?
It isnt that hard, the sun position is obviously completely predictable.
There are calculators online.
Rod Speed
>>> I've entertained the idea of building a small
>>> greenhouse attached to side of home.
>>> I would use it to capture heat in winter to help heat
>>> the home...as well as grow some of my own food.
>>> Anyone doing that out there and can advise abt the
>>> feasibility of this idea?
>> It does work for heating the house.
>> Not ideal for growing food tho, greenhouses need heating to
>> stop them getting too cold overnight for whats growing in them.
>> And you cant grow that much in a small greenhouse
>> anyway except seedlings for planting out outside the
>> greenhouse and maybe tomatoes etc.
> A "Sun-Space" might be an idea to look into. tho.
> Some are nice places to have the morning coffee on a day off.
Yeah, I designed my entire house that way, works very well indeed.
> Facing a little more Eastwardly. some add thermal storage .
That last is a problem with his requirement for a small one.
No big deal for house heating, but just not practical for growing
veggies, they need the heat all night so they have to be substantial.
The fancy greenhouse I was involved with the control of had quite
literally a couple of rooms full of gravel, what we call road aggregate,
heated with a big solar air heater during the day and used to keep the
temperature of the greenhouse up during the night, computer controlled.
Thats just not practical for a small greenhouse.
Even say a masonary wall heated by the sun during the day
wont keep the greenhouse warm enough during the night.
m...@privacy.net
>Sure, but that doesnt fix the overnight heating problem.
Ok point taken
So you are saying that a small say 10 foot by 10 foot
greenhouse cant contain enough "mass" in the concrete
floor to radiate heat at night?
Rod Speed
> Rod Speed <rod.sp...@gmail.com> wrote
> Ok point taken
The problem is more that not enough of the floor thickness
gets to a high enough temperature during the day.
I have that approach in my house, with lots of concrete floor
heated by the sun thru 8 big patio doors on the sunny side and
while they are quite warm when the sun is shining on them,
they get quite cool quite quickly when the sun goes off them.
Even if you had say solar water heaters you wont find that they
will have enough heat in the water to keep the greenhouse warm
enough right thru the night unless you have a hell of a lot of water
and the associated solar water heating panels, particularly as
you are further north than is ideal.
Bob F
Of course it can, if you can reduce losses suficiently. That might not be
trivial.
Rod Speed
>> Ok point taken
Nope, I do that with my house, and the short story is that not
enough of the thermal mass of the concrete gets heated enough
during the day to provide enough heat during the entire night.
Thats why the fancy system I was involved in controlling used
quite literally rooms full of rocks heated with a large solar air
heater, to get enough heat stored in the rooms full of road metal.
> That might not be trivial.
Yes, but that alone isnt enough even with say good insulation blankets
or thick sheets of polystyrene etc covering the greenhouse at night.
There just aint enough joules stored in the concrete last the night.
Jeff Thies
> I've entertained the idea of building a small
> greenhouse attached to side of home.
>
> I would use it to capture heat in winter to help heat
> the home...as well as grow some of my own food.
Think sun space. You can open any windows connecting to the house and
get a little heat.
Otherwise it will be losing heat. Heating this at night for a
greenhouse will be expensive.
Thermal storage, is difficult. A drum of water will be the cheapest
and most effective, but it will take a lot to temper through the night.
I'd leave it as a daylight sunspace and close it down at night. You can
insulate anything not south facing or with a face that does not get much
light.
My current thinking, as far as frugal, is something like SunTuf as
the shell with mylar storms. Mylar is cheap and very transparent, but it
needs the UV shield of the SunTuf or whatever UV protected glazing you
use. That is what I will be doing with my solar cabana. It lived last
winter glazed in "clear" poly and with bubble wrap insulation. Balmy and
a delight when it was windy and freezing as long as the sun was shining.
Solar resources:
http://builditsolar.com/
Jeff
Jeff Thies
Yes. You can calculate this all yourself.
Calculate your losses. If you have 200SF of single glazing that is
about R1, and if you have a 30 F temp difference then that is 6000
BTU's/Hr. As much as (and more than) an electric space heater on high.
That's 144,000 BTU's a day
Concrete has specific heat of about .2 (water is 1). So to store
100,000 BTU's at 30F temp difference would take 17,000 lbs of concrete,
if you could get all the heat into the concrete. That is about 5 cubic
yards of concrete, 540 SF of a 6" thick slab.
Jeff
m...@privacy.net
>I'd leave it as a daylight sunspace and close it down at night.
This sounds like the best plan above.... and what I
should do.... thanks!
m...@privacy.net
>
>Solar resources:
>http://builditsolar.com/
Nice and CLEAN web page there Jeff!!
Rod Speed
>>> Sure, but that doesnt fix the overnight heating problem.
>> Ok point taken
>> So you are saying that a small say 10 foot by 10 foot
>> greenhouse cant contain enough "mass" in the concrete
>> floor to radiate heat at night?
More strictly that not enough of that concrete gets heated during the day.
> Yes. You can calculate this all yourself.
> Calculate your losses. If you have 200SF of single glazing that is
> about R1, and if you have a 30 F temp difference then that is 6000
> BTU's/Hr. As much as (and more than) an electric space heater on high.
> That's 144,000 BTU's a day
> Concrete has specific heat of about .2 (water is 1). So to store
> 100,000 BTU's at 30F temp difference would take 17,000 lbs of
> concrete, if you could get all the heat into the concrete. That is
> about 5 cubic yards of concrete, 540 SF of a 6" thick slab.
And you will find that you dont heat the entire 6" slab thickness wise during the day.
You in fact heat much less than 6" of it.
m...@privacy.net
>And you will find that you dont heat the entire 6" slab thickness wise during the day.
>
>You in fact heat much less than 6" of it.
what abt painting or dyeing the slab a flat
black...would that help?
Jeff Thies
Black, of course, would be a better solar absorber. But that isn't the
real problem. Only part of the slab will be heated directly by sunlight,
the rest will have to be conducted or convected in. Concrete does a poor
job of that. That is why Rod had those rooms full of gravel for air to
flow through and convect heat in and out.
Forget thermal storage in the slab. As pointed out elsewhere, even if
you could heat the slab throughout it would take more slab than you had.
If you really have excess heat to store, store it in water. Either
passively in stacks of bottles to maximize surface, or actively by an
air to water heat exchanger (radiator). But you will have a hard time
getting there.
Jeff
m...@privacy.net
>Black, of course, would be a better solar absorber. But that isn't the
>real problem. Only part of the slab will be heated directly by sunlight,
>the rest will have to be conducted or convected in. Concrete does a poor
>job of that. That is why Rod had those rooms full of gravel for air to
>flow through and convect heat in and out.
>
> Forget thermal storage in the slab. As pointed out elsewhere, even if
>you could heat the slab throughout it would take more slab than you had.
>
> If you really have excess heat to store, store it in water. Either
>passively in stacks of bottles to maximize surface, or actively by an
>air to water heat exchanger (radiator). But you will have a hard time
>getting there.
Good explanation thanks!!!
Rod Speed
Not enough to matter. The surface of the slab does get nice and hot.
The problem is that the thermal conductivity of solid concrete isnt that
great, so you dont heat much thickness of the slab.
m...@privacy.net
>Not enough to matter. The surface of the slab does get nice and hot.
>The problem is that the thermal conductivity of solid concrete isnt that
>great, so you dont heat much thickness of the slab.
How would one INCREASE the "thermal conductivity" of
concrete?
Or.... is it just best to forget abt using concrete as
a storage mass and use something like barrels of water,
etc?
Rod Speed
Rod Speed
> Rod Speed <rod.sp...@gmail.com> wrote
>> Not enough to matter. The surface of the slab does get nice and hot.
>> The problem is that the thermal conductivity of solid concrete isnt
>> that great, so you dont heat much thickness of the slab.
> How would one INCREASE the "thermal conductivity" of concrete?
By breaking the slab up into small bits of concrete
and heating it with air from a solar air heater.
In practice its more practical to just use a room full of gravel instead.
> Or.... is it just best to forget abt using concrete as a storage mass
Yes.
> and use something like barrels of water, etc?
Water has some advantages, but using rooms full of gravel has
some too, particularly being able to heat the gravel with solar
air heaters which are much simpler than solar water heaters.
The solar air heaters we used were just black painted metal under
a transparent poly carbonate cover, with big radial air fans driving
the air, one per room full of gravel. The rooms full of gravel had a
strong metal grid under the room full of gravel so there was a decent
air chamber at the top and the bottom of the room full of gravel.
Thats a lot easier to make than a water based system.
Takes up a lot of space tho when its on the side of a house.
With the greenhouse, the rooms full of gravel were just part
of the whole greenhouse with a room for the computer controls
on the non sun side of the greenhouse, on the south side in our
case because it was in the southern hemisphere.
m...@privacy.net
>The solar air heaters we used were just black painted metal under
>a transparent poly carbonate cover, with big radial air fans driving
>the air, one per room full of gravel. The rooms full of gravel had a
>strong metal grid under the room full of gravel so there was a decent
>air chamber at the top and the bottom of the room full of gravel.
>
>Thats a lot easier to make than a water based system.
Ok.... but too complex for home heating..... sounds
like a home greenhouse should be counted on to "heat"
ONLY during day
Rod Speed
> Rod Speed <rod.sp...@gmail.com> wrote
>> The solar air heaters we used were just black painted metal under
>> a transparent poly carbonate cover, with big radial air fans driving
>> the air, one per room full of gravel. The rooms full of gravel had a
>> strong metal grid under the room full of gravel so there was a decent
>> air chamber at the top and the bottom of the room full of gravel.
>> Thats a lot easier to make than a water based system.
> Ok.... but too complex for home heating.....
Nope, the problem isnt the complexity, its amazingly
simple, just a big radial fan and a decent design.
It doesnt even need fancy computer controls, a simple thermostat system works fine.
The main problem is the space lost to the rooms full of gravel.
Most would prefer to live in those rooms instead.
> sounds like a home greenhouse should be counted on to "heat" ONLY during day
Yeah, its very hard to justify the capital cost for a home greenhouse.
Although its a rather unpalatable truth, buying the veg commercially
at the best prices you can find is much better value. I only pay about
$1 every fortnight for the cheapest tomatoes they sell at the growers
market thru the winter and have to go there every week in summer
for the nectarines I eat a lot of thru the summer.
Summer veg has better economics, particularly with the tomatos, lettuce, strawberrys and potatoes.