Radiant Heat Matrix Other Than Concrete.

DanHolohan

(I'm posting this for a member who is having difficulty getting online. Dan)



I’m building a small greenhouse (6’ X 12’) and plan to heat with a radiant system (1/2” PEX tubing), over insulation panels. Water will be circulated through a 500-gallon underground tank (550 F), which may be supplemented by solar heat.



Exterior radiant systems typically use PEX embedded in concrete, which will not work for my greenhouse. I considered pea gravel and sand, but these (apparently) have poor heat transfer properties. Would crushed limestone (1/4”- minus) work?



How thick should the matrix cover be over the PEX? (2 or 4 inches?). A thin cover will have a quicker response time (PEX to air temp.), but less heat storage capacity and insulation. Should I add “fins” to increase heat transfer?



Any ideas or comments are greatly appreciated!

Scott

Zman

Wall..

Scott,

It sounds like an interesting project.

Have you considered turning a wall of the greenhouse into a solar collector by day and  radiant heater at night? Greenhouses are usually way to hot in the daytime and cool rapidly at night. If you built a wall using pex and aluminum radiant plates and paint it  black  you could store the excess energy in your tank and pump it back at night.

Carl

hot_rod

run some numbers

to see what type of heating load the greenhouse presents.



I've never been able to make the numbers work. Mainly a lot of low insulation value walls and not enough floor square footage to cover that load.



Possibly some root zone heating, and some additional back up heat source for high load condition days.



I put an 80 gallon 1/2 insulated tank in my 6X8, warmed it with an evac tube solar collector. A 500W heatlamp is enough to keep the space above freezing, if the solar is not available for a few days. I have two glass walls, the rest is 4" foam insulation and the roof also, that helps with the loads, compared to all glass or plastic walls and ceilings.

SWEI

Thrrmal mass helps

Dark blue or black barrels along the south wall and perhaps the north wall depending on layout.  Fill with water.  Let the sun do its thing.

Gordy

Questions

Underground tank temp? Typo 550*



Buried tank location? Under green house hmmm one big emitter

Or remote?

Plants grown elevated, or in the ground?



A 500 gal tank would be darn near the

Size of the green house.

DanHolohan

Posting answers from Scott:

Radiant Heat Matrix Other Than Concrete?

Questions



Underground tank temp? Typo 550*

Buried tank location? Under greenhouse hmmm one big emitter

Or remote?

Plants grown elevated, or in the ground?

A 500 gal tank would be darn near the

Size of the green house.







Reply:



Temperature is typo; should be 55 degrees Fahrenheit.







The top of 3-ft diameter X 10-ft long tank, is 1 ft. below ground level (4 ft. total depth). So, tank water will be about average temperature of St. Louis = 55 degrees F. Tank is below greenhouse.







I’m raising ground surface by 2-feet with retaining wall and crushed rock (in-part over tank) to create pad for greenhouse, which has footing for granular material (not concrete). Building-up, so it seemed logical to install radiant PEX tubing. Tank water temp can be raised with heat-pump, solar, etc. Small area to heat, or cool (6 ft. X 12 ft.). Plants will be grown on shelves, or in raised beds.







Key questions:



1. Will crushed limestone (1/4” minus) work for heating matrix, using 1/2 inch PEX tubing? Will sharp rocks cause leaks?



2. Will crushed limestone transfer heat adequately to heat/cool greenhouse? [assuming other parts of system are working]. What about pea gravel or sand; which have more dead–air space?







Hope to avoid a concrete matrix due to greenhouse design and expense.



Thank You



Scott

Zman

Insulation

The tanks sounds great! You really need to insulate it.

I am still in favor of a combo solar absorbing/emitting feature in the green house. I think a heat loss calc and some calculations of potential emitting surfaces is needed.  You need to store water in the tank at a higher temp, giving up the heat to the ground does not make sense.

Carl

Dave H_2

Another variable

That variable would depend upon how you plan to grow in the greenhouse.



If you are growing directly on the ground, than yes concrete is difficult, in pots, not so bad. I would use a concrete base for the pex but make sure it is pitched and drained.



If growing on benches, then you can build it into the benches on the bottom side the radiant heating system.



Also what is your goal for the radiant system? Typically for greenhouse radiant, its used to maintain a root temperature to promote growth. The air temp will be cooler.



Dave H

hot_rod

Conduction

Is the most powerful way to move the thermal energy from pex tube. If you are considering a base rock type gravel, which has "fines" or sand that would transfer better than washed gravel



Really base rock is concrete without the Portland cement in the blend, and water to activate the mix



I too would use a concrete slab, gravel over if needed.



Be sure the tank you put in the ground is rust resistant. If it is steel, it needs to be coated like the underground LP tanks Also anode protection.

SWEI

If not concrete

then I would bed the tubing in densely compacted sand, possibly with an oil or other additive to increase both density and conduction.  Can't remember the name of the stuff, but we used something like this on a fiber job some years ago.  Darker and finer than regular sand, came out of the truck slightly wet -- and stayed that way.  Compacted really tight, but you could dig it out if needed.

Gordy

Polymetric sand

Expensive but once down and wetted when dry is hard and flexible. Main use in filling cracks for laying pavers. Polymers and cement are added to the sand for adhesion, and flexibility weeds won't grow through it.



Next thought would be a flow able fill sand with cement additive different mix designs usually richer (more cement) are the difference facilitate flow.



Your talking 1 cy 4" thick of what ever media you choose. Biggest hurdle is short load charge for redi mix

Mix it's gotten brutal around here.



Your cheaper ordering the 5 1/4 cy cut off than ordering 1 cy



Another polymer modified cold patch sell it by the bag, or go to the state or county highway maint yard. I like this option

SWEI

That's the stuff

Looks like most of it comes premixed, but there are a couple of options for field-mixed additives.  http://www.groundtradesxchange.com/forums/hardscaping/7697-polymeric-sand-additive.html discusses several options. 

hot_rod

Bio-Therm green house systems

I installed one of these systems in a hydroponic green house many years ago. Fired with a wood boiler, here in Missouri.



The tube was installed under the gravel floor, with an additional unit heater for extreme days. You might look around their site for sizing and load info.



The heated benches make a lot of sense, keeping the root zone area warn not the entire structure.



http://www.trueleaf.net/index.php/en/trueleaf-products/biotherm-tubing/products-biothermmicroclimate

Gordy

Tubing in sand

Here is something to ponder if your going to do tubing in sand.



That is movement of the tubing expanding and contracting over time working its way to the surface. So an anchoring system needs to be implemented. I would be Leary of staking into soil as the same thing could happen over time working the staking out of the ground.



Thinking this through I would favor elevated beds with the radiant to the root system. Let solar gain heat the atmosphere with in the green house.



I would insulate the tank at all sides but the top backfill with sand allowing the tank temp to radiate into the green house. It may take some trial and error as to what tank temp would allow an acceptable air temp in the green house, and to the root system tubing.



The tank would be heated solar, and what ever alternate means.



I think a vapor moisture barrier on the floor before final floor media is laid to prevent additional water from plant watering in the green house to go in the tank area is critical along with a tank coating. Unless the tank will be plastic not a bad idea.



If this idea did not work out the tank only being a foot deep could be dug down to insulate the top in the future.



If tank is not yet purchased I would opt for a rectangular plastic tank. 67 cubic feet will give you 500 gal. A tank 6x12x1 yields 72 cubic feet or 538 gal. Insulate bottom sides cover top with sand one big emitter with probably pretty low tank temp. If my idea fails dig out the 1' of sand and insulate the top.