radiant heat in greenhouse soil

john_27

Sarah...I am rebuilding a 25x45 turn of century Lord and Burnham....try to insulate your dirt as far down as possible, use reflecting material on sides of greenhouse to capture value of radiant heating....look at your 'house as a heating sink, and insulate as much as possible...I went with nat gas for a reliable burn....wood was too much work in Pa.
You may want to reconsider the tile because of condensation and slippage problems....we went with concrete(you can go patterned concrete as well).....sherry's greenhouse on the web is a good source.....good luck!

sarah

radiant heat in greenhouse soil

I have just put up a 24'x60' greenhouse, 8mm double wall polycarbonate, 10 ft at eaves, 16 at peak, and will have beds rather than benches. I have figured this to make a heat loss of about 157,000 btu at a temp. diff. of 70 (I'm in NE MAss.)(Did I calculate that right?)For heat, I bought a Tarm Excel 2000 (102,000=wood. 140,000=oil). I haven't taken delivery of of yet and can still upgrade to the 2200 (140,000/180,000) if I need to. Since it will run primarily on wood (oil backup), I will have a heat storage tank of 600/880 gallons (depending on the boiler model).I have a few questions and hope someone might have some answers.

1. I read a heat loss calc. formula which referred to "Delta T". I assumed this was the difference between desired inside temp. and actual outside temp. Is that correct?

2. I haven't been able to find anywhere any information regarding actual diffusion of heat in the soil, or for that matter along the tubing. Is there any formula or whatever for how fast and efficiently heat will pass from tubing into soil and then be lost to the air. I have read that warming the soil allows you to keep the greenhouse air temps. 10-20 degrees lower than would otherwise be required (=if you were doing all your warming by starting with the air). I know cold waqter can be mixed in with water (180 degrees) leaving boiler so as not to overheat root zone, but I don't understand how the valve works to reheat/preheat the returning much cooler water to bring it to 160 before contacting the boiler (which I've read is necessary to preserve boiler health). I was thinking I might run metal pipe from the boiler through the airspace, for one length of the house say, so it would radiate and convect some heat that way, then be cooler by the time it switched to pex and went into the soil. OR: I could maybe bury the tubing quite deep in the raised beds.

4. What are the relative properties heatwise of sand vs. good soil. Should the tubing be in a layer of sand below the rootzone soil?

3. I understand oversizing of boilers is a common problem and can ruin the efficiency and lifespan of a boiler. I sometimes think I'd be stupid NOT to get the larger boiler, but then am worried that I won't actually use it fully to capacity. (Does having a heat storage tank affect this one way or the other?) And what if the boiler IS too big - what is the actual problem that occurs? Although the btu loss is 157,000, at a diff. of 70, it is not zero degrees that often, and I might not need it to be 70 then. Also, I don't know how to factor passive solar gain. And I don't know how to factor the difference in btu's required by heating the soil rather than the air.

4. ALSO: adjoining shed (will be insulated) 20'x20'. Will have radiant floor heat. Currently bare ground floor. I'd like to do it right in crushed stone (Or is it stonedust?...) and put tiles over it, with stone dust in joints instead of mortar. Any advice about that? I'm looking for less pricey than residential tile - concrete tiles, more industrial type. Do they exist?

As you can see,I have much to figure out. Any help anyone can offer would be greatly appreciated. Thanks. ~ Sarah.

Joe_10

Heat loss

Your passive solar shouldn't lower your heat loss calcs because when you are going to have your highest heat loss will be at night. Somebody whith an E-10 heat loss program can show you predicted temps in the space for any day of the year using local weather data. It takes into account solar gain or heat loss through windows. I believe that the black twin tube was made for heating greenhouse planting beds but it does not have an oxygen barrier being a rubber based material. So needs to seperated from your boiler water.

Joe

hot_rod

Boy you are full of questions

I'll give it as try

#1 delta t basically means temperature difference. Yes it could be inside temp to outside. often refers to the temperature drop through the radiant loops, generally listed on the calc sheet. Also used to size boilers., piping, and pumps as the delta T through a heat exchanger.

#2 Can't lay my hands on it right now, but I had a research paper from Rutgers, I believe the went into root zone heating and soil stuff. Try their website under the ag research button.

Modulating, condensing boilers are a nice match for low return temperature radiant. It pretty much does away with the sizing issues and boiler shock and protection. High efficiencies also.

Yes a buffer on the wood burner would be a must, in my opinion. Hard to match the fire to the loads exactly. Any wood burner does best with long HOT burn cycles. Then store any extra BTUs in a well insulated storage tank.

Another idea for the shed would be a concrete floor that is saw cut to resemble tile and grouted. A very inexpensive way to get a "tile look" Insulate under the slab also. Although this would be an ideal "dump zone" for an over ambitious wood boiler

In wet areas, greenhouse applications, pervious concrete can be ordered. Basically it is concrete with the sand left out (low psi rating) it allows water to filter through the concrete for clean up and frequent watering. Be sure the sub base is designed to carry away the water

Lots of greenhouse info on the www, do some searching. Plenty of university research in this area. Hit the ag schools first. Good luck!

hot rod

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jerryb

Sarah, Go to manufactures list at the left and check out watts

coop

electric hot water heater for radiant flooring

I am building a new home and want to use a electric hot water heater for my radiant flooring .Iwould like to know if I could heat my domestic water to without having a mixing valveon the heating side of the system or do I need a seperate water heater THANK YOU

Floyd_3

Hey coop...

start a new thread.......

You'll get a better response and get noticed..

Troy_3

Radiant in greenhouse

Sarah: we installed a 30,000 sq. ft. greenhouse about 12 year ago. This is how we did it. 2" of polystyrene insulation,5/8" pex, covered with 4" of sand, and then 2" of gravel. They grow potted flowers on the gravel. the reason for the depth is that the heat will come up in a V pattern. You will actually see this in the plant growth if you don't give the heat enough depth for the Vs to cross. They really maximize growing room by only leaving a foot path about every 8'. They also tell me that the additional height helps the light quality.??? Lower air temps keep disease down. I'm told warm roots make for a happy plant. Not so different from humans! Wealso have seen significant growth increase in growth rate from tempering the water used tofeed the plant. Many growers have reported up to 30% faster growth. The assumption is the plant goes dorment after watering till the roots are warmed back up to what ever the particular plant type requires for growth. Once again- sounds like my wife!

Mike T., Swampeast MO

Buffering & Mixing

It certainly sounds like you have a decent handle on the situation and are being well-advised as to equipment.

That large tank (the buffer) really is the key to getting the best performance out of your system.

As wood or other solid fuels are more difficult to control accurately than other fuels, the tank give a big reservoir of BTUs to the heated space in times of high demand (but low fire) and conversely a big reservoir to the boiler in times of low demand (but higher fire).

By isolating the boiler from the system (the other purpose of buffering) the boiler will be seeing the return temperature from the tank--not the system. A four-way mixing valve (likely very appropriate to the system) will keep the water going to/from the system itself in an appropriate range by both mixing some of the supply water with the return and some of the return water with the supply.

I'm not certain of the requirements when heating greenhouse soil or the affect of different soil types, but do believe there is very good data out there. As I recall, soil heating not only allows you to keep the greenhouse significantly cooler than with air heating, but it can be wonderfully useful in starting certain plants/seeds very early in the season.

I'm not sure how elaborate, automatic (and expensive) you want your system to be but my greenhouse reading tells me that (particularly with a wood-fired system and lots of valuable plants) that you should have a low-temp alarm system and likely some form of automatic backup heat (LP, natural gas, oil, etc.).

As your floor zone and soil zones may have different temperature requirements, one of the easiest ways to do this with your system (assuming a single 4-way mixing valve that could possibly be weather-responsive) would be with simple 2-way proportional valves that will individually regulate heat to the floor and different soil "zones". These are strictly flow regulation devices (you can think of them as an adjustable BTU "gate") that will allow a continuous supply of heat to each zone just sufficient to achieve the desired setting. They are simple in operation, fairly inexpensive and considered highly reliable.

I wouldn't even consider dry laying ceramic tile--they are thin and light so they would move easily and would likely be highly prone to breakage.