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Heating a greenhouse at night... - Page 2

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Posted by john on September 3, 2008, 1:24 am

Neon John wrote:

Assuming I did the math correctly, my solar array should heat 300gal of
water to about 160F and I attempted to figure this conservatively.  The
plan is to use an over sized tank so mass can be added if excess
capacity remains.  Still looking for a solution for the 3 day ice storm
we occasionally get.

I like this idea, but the reason for putting the piping in the floor is
to get it out of the way.  The same with the pots of water (I actually
have two big barrels for this, but they take up too much space.)  Also,
some of the plants are tropicals and need to stay above 50F.

Wonder if a commercial quality hot water rated hose would be durable
enough to use as a radiator, then we could move it around as needed and
it would handle a little kicking around.  That wouldn't send as much
heat into the ground.

Any idea what temperature water I should plan on running through the
loop?  nominal pipe size, etc?  I suppose a lot of this will become
evident after I install everything, but would like to have a clue up
front to keep from having to rework everything.

Are you referencing heat trace tape used on exposed water lines to
prevent freezing?

I would be interested in the phase change setup if for no other reason
than intellectual curiosity.  (Works great for transferring heat in my
A/C.)  Please, tell me more.

Posted by Neon John on September 3, 2008, 3:45 am
On Tue, 02 Sep 2008 20:24:20 -0500, john

I would think so.  I used that kind of hose in my restaurants for 180 deg
wash-down water and it lasted well.  Life was usually terminated by one of my
hired monkeys cutting it or rolling heavy equipment over it.

You'll want to get the fairly thin wall, uninsulated hose.  Some hot water/low
pressure steam hose has a foam layer that reduces heat loss and makes it more
pleasant to handle.  You don't want that.

The embedded tubing in slab installations that I've worked on are typically
3/8 to 1/2 inch, depending on the length of the loop.

A major decision that you have to make is whether to run hot water from the
tank directly through the loops or to temper the water.  Hot water is simpler
but it requires distance between the tubing and the plants.  Plus it'll dry
out soil that it's placed upon or next to.  Tempered water is more complicated
and requires more tubing but eliminates the possibility of burning up plants
or overheating the greenhouse.

No, these were metal sheathed elements (Calrod-style) perhaps 10-15 ft long.
Very low energy density.  The ends were bent and extended up out of the dirt
where the power was connected. Undoubtedly these were made for greenhouse
applications.  Unc was VERY serious about his breeding and spared no expense
on his greenhouses.

I don't see why self-regulating type heat trace tape (NOT the cheap
unregulated stuff that they sell in the big box stores) wouldn't work as well.
It would be vulnerable to a spade hit, of course.

I know almost nothing about camellias or orchids but I do know that he kept
the greenhouse environment hot and saturated with humidity, even in winter.  A
true jungle environment.  I didn't enjoy being in there at all.

Google around for "phase change energy storage".  That's what I did when I
started the water heater project.  If you get to the point where you want to
duplicate my work I can dig out a backup disc and retrieve the project
details.  What brand and specifics of balls I used and so on.

As far as architecture, my client wanted to eliminate all but diesel fuel from
his motor coach and he wanted more hot water than the 10 gallon heater could
supply.  We did two things.  One, we removed the propane burner hardware from
the heater and had a single U-tube welded to the aluminum tank.  Antifreeze
circulated through this loop.  Then we filled the interior of the heater with
storage balls designed for 160 degree operation.

He installed an Espar diesel-fueled micro hot water boiler and plumbed it and
the water heater loop in with the engine coolant loop.  We also installed a
radiator and a homemade controller to supply comfort heat.  

The plumbing was arranged so that when the coach was underway, engine coolant
circulated through the water heater loop, melting the paraffin in the balls
and through the radiator to supply comfort heat to the coach.

When the engine was turned off, a pair of valves isolated the engine block
from the loop. (manual over-ride provided for warming the block in cold
weather).  The Espar supplied heat for both comfort heat and the water heater.
The Espar is factory-programmed to 180 deg F and is not changeable.  The 160
deg phase change media melting point provided 20 deg of headroom to ensure
that it melted completely while still providing very hot water.  The water
circulated around the balls, ensuring intimate contact and good thermal

We designed a custom PID temperature controller around a BASIC stamp for the
comfort heat controls and used a small programmable logic controller to
sequence the valves and fire the Espar and generator as needed.  The PLC only
cost about $25 and operated directly on 12vdc.  I think that it was a Siemens
but I'd have to look to be sure.

All that is way more complicated than what you want.

John De Armond
See my website for my current email address
http://www.johndearmond.com  <-- best little blog on the net!
Tellico Plains, Occupied TN
You have a magnetic personality... That must be why all your mental floppies are

Posted by phil-news-nospam on September 2, 2008, 7:46 pm
 On Mon, 01 Sep 2008 22:57:53 -0500 john

| I built the wife a small 8.5 x 12.5 greenhouse to winter her plants in.
|    I am building a small solar thermal collection and storage system to
| use for heat on cold nights; trying to ween away from electric and gas
| (seemed cheap at first, but project has developed a life of its own...)
|  Someone suggested using underfloor radiant heating which seems like a
| good idea as opposed to forced air, except I am familiar only with the
| concept radiant heating, not the application and most of the
| installation web sites I have visited show installation in either pier
| and beam or slab; my little greenhouse has a floor backfilled with about
| +12" of wood chips.  Putting the pipe in is not an issue, but where do I
| start on figuring the pipe diameter, composition, spacing, flow rate,
| distribution manifold (if needed?) water temp, etc.  Are balancing
| valves needed on the return manifold? etc.  I see a lot of opportunity
| for "I gotcha" and the repercussions of freezing the wife's plants are
| just too horrible to consider...!

Is your goal to merely prevent freezing, or to maintain some specific
temperature.  If it is to prevent freezing, then a lot of water based
thermal mass can achieve that.  It would collect heat in the day when
sunlight brings it in, and expel it at night when the ambient temperature
drops below the mass temperature.  It would also have the added ability
to expel more heat when the ambient temperature tries to go below freezing
due to the heat of phase change.  If the plants can handle a few degrees
below freezing, to account for the variations of temperature within the
air in the greenhouse at night when the outer surfaces are colder, then
this is a way to do it.

Are the plants on tables?  If so, put containers of water under each table.
The more mass (water) you can put in there, the better the results.

| Any help or suggestions for this or alternative methods would be greatly
| appreciated.
| BTW, the electric heat size calculated for 20F night is about 3kw.

That seems like something to be avoided.

|WARNING: Due to extreme spam, googlegroups.com is blocked.  Due to ignorance |
|         by the abuse department, bellsouth.net is blocked.  If you post to  |
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| Phil Howard KA9WGN (email for humans: first name in lower case at ipal.net) |

Posted by john on September 6, 2008, 1:41 am
 Ideally maintain 60F+ at night, I think most of her tropicals can
survive above 50F.  There are a couple of tables, but I can envision
every bit of available flat surface utilized as well as baskets hanging
from above.  (Our own little 8.5 x 12.5 section of jungle...)


phil-news-nospam@ipal.net wrote:

Posted by somebody on September 7, 2008, 5:13 pm
 You are in for some rude surprises.  Greenhouses in winter can be 80
degrees on a sunny day and drop to well below freezing at night.  The
insulative power of the plastic is nil and the cost of keeping the
plants healthy excessive.

One thought is to make a barrel stove in the greenhouse, bury it and
the stovepipe in sand, and stoke the thing up on days when you know it
will be excessively cold.

Oh yeah, be prepared for rain in your greenhouse.  All the moisture in
the air will condense out on the plastic, leaving plants in a
desert-like atmosphere and wet walls and sill plates.

If there are any areas of the structure that won't have the sun
beaming directly in, insulate them and forget about any minimal excess
light gain.

Good luck, BTDT, going to try something much smaller and convenient
this year.

On Fri, 05 Sep 2008 20:41:25 -0500, john

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