Soil warming cable - Propogator

Discussion in 'Propagation This Month' started by Steve R, Dec 3, 2011.

  1. JWK

    JWK Gardener Staff Member

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    I've used a warming cable for several years. I used to have it set up on staging like shineys, but now I use it the soil border. I take it up in the sprjng and re-lay it in the autumn. In recent years I've dug out the soil and laid the cable on top of polysterene sheets with old bricks to keep the cable apart at the right spacing and retain a bit of heat. Then pile the soil back on and bring in my cloches for the more tender plants. I think insulation and thermostatic control help keep the costs down.
     
  2. Scrungee

    Scrungee Well known for it

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    Shiney - Am I right in thinking you have your propogator at an end of your greenhouse where the light is obstructed - possibily because that's the best use of such space for germination of seeds? - the end of my greenhouse is against a tall fence and that's where I was thinking of siting mine, purely for germination of seeds, not for rooting of cutting, or growing on of either.
     
  3. shiney

    shiney President, Grumpy Old Men's Club Staff Member

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    Hi Scrungee,
    Not really. It's at the end because that is the most convenient place to have it - out of the way and where I have the powerboard etc.

    It also happens to have the most light obstruction :thumbsup: but it gets plenty of light. There is my neighbour's conifer hedge about 4' or 5' from it but that is to the west so the greenhouse is in full sun most of the day - except early and late in the year when the sun is low and my willow and silver birch keep a lot of the sun off. It's still in full brightness, though.

    It can get pretty hot in there and I have six top vents and six lower vents and a thermostatically controlled extractor fan near the propagator. The fan is set to draw air in all the time. That has also been working continually for 35 years! :rolleyespink:
     
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    • Lolimac

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      I tidied up because i'm a girl:D
       

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      • Dave W

        Dave W Total Gardener

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        Steve, the temperature achieved within the propagator depends on two things: the heat output from the cable and the heat loss from the propagator. Regulation of heat is obviously down to a thermostat.
        You could calculate the electrical energy required if you know the temperature desired and cubic area and thermal loss. But if you don't want a load of brain strain and a few hours researching the afore mentioned, decide on the approximate base size of your propagator, work out how long a cable you'd need for about a 2 or 3 inch spacing and then build one with maximum insulation and a decent thermostat.
        You should be able to reduce heat loss through the base to next to nothing with thick polystyrene and reflective foil (baking foil or radiator insulation). If you can get hold of twin wall polycarb for the sides and top you'll reduce heat loss considerably.
        The volume heated does of course matter, less costs less to heat, but once your plants start to get big and require less heat you can increase height by making hoops and covering with bubble wrap.
         
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        • Steve R

          Steve R Soil Furtler

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          Thank you all for your input, photo's and technical know-how, I now have a pretty good idea of how to proceed with this and what materials I will need.

          Steve...:)
           
        • davygfuchsia

          davygfuchsia Gardener

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          Hi Steve ,I will take photo of mine Sunday ,but it will be untidy ..I do have two benches but only one in use at the moment .
          As long as you use some sturdy timber 3in x 2in or 4in x 2in) as a frame you will have no problem with the weight . My base on one is decking and I think the other is galvanised sheets..

          Dave
           
        • shiney

          shiney President, Grumpy Old Men's Club Staff Member

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          Loli, you're only trying to make me ashamed of the mess :( - but it didn't work :heehee:

          When Mrs Shiney has time to tidy and sweep under the bench (I can't do it) we then put some of our less hardy plants to over-winter under there. They pick up some of the heat loss from the propagator. A typical example is our 30 year old lemon verbena who has his winter holiday under there :D and his friend Myrtle loves being near him.

          We're able to put our larger plants under there as they have 3' headroom. Many years ago I made a heavy duty plastic wall, with overlapping flaps as a doorway, 7.5 foot from the end and lined the sides with bubble plastic. with the propagator in there, and reduced heat loss, it made a noticeable difference to that end of the greenhouse. With everything else, it became too much hard work for me to keep it up.

          Dave, I suppose I could fix some polystyrene to the underside of the bench to help insulation - never thought of that before :DOH:. I'll have to find where I can get some - probably try Wickes, B & Q or Homebase as they are near here. What thickness do you think I should get? Also, being a DIY idiot, how do I fix it to the underside of the bench?
           
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          • Dave W

            Dave W Total Gardener

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            The thicker the better though an inch or 3/4 inch would do.

            I'm a real cheapskate - any thick polystyrene packing that comes with new household appliances I keep. Apart from anything else, if you break it up it's handy for using as drainage in the base of pots and tubs.
            Mastic will stick it, in fact most gloopy glues will work. I used cork tile cement when I stuck some big sheets on the shed roof. The polystyrene is very light so you should be able to stick it easily.
             
          • shiney

            shiney President, Grumpy Old Men's Club Staff Member

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            Thanks, Dave. :dbgrtmb:

            I'll look for gloop! :heehee:
             
          • Kleftiwallah

            Kleftiwallah Gardener

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            I have made my own 8 foot by 2 foot. Polistyrene sheeting against the wood (to save the heating bill) and then heavy duty plastic. Sealant round the thermostat entry hole.

            This following tip got me £20 in vouchers. :dbgrtmb:

            There is a possibility of the sand/grit drying out in the bottom of the prop' bench but can still be damp on top. So, cut some lengths of plastic tubing and block the lower end with fine mesh to allow water through but not sand/grit. Stand two or three of these up at each end of the prop' bench as you fill in the sand/grit.

            'Buy' some suitably sized fishing floats and pop them into the plastic tubes. When you test to see if water is needed these floats will be grounded if dry, if these floats are 'floating' no water is required. Simples. :yess: Cheers, Tony.
             
          • shiney

            shiney President, Grumpy Old Men's Club Staff Member

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            I'm a little puzzled about all this talk of keeping the propagator damp/wet :scratch:. Apart from keeping humidity levels up what is the purpose?

            All our plants in the Propagator are either in pots or trays and we water them regularly. Is there further need for moisture?
             
          • Kleftiwallah

            Kleftiwallah Gardener

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            Water is a far better conductor of heat than air. Plus there is a chance of a section of the heater cable becoming overheating and either blowing the fuse or burning through the insulation. Cheers, Tony.

            My baby plants are in plastic coffee cups and are sunk into the sand/grit about 1 - 2 inches. Cheers again, Tony.
             
          • shiney

            shiney President, Grumpy Old Men's Club Staff Member

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            Thanks, Tony :dbgrtmb:

            Forgive me if I'm being rather thick about this :DOH: (I've already admitted to being useless at DIY :heehee:).

            I understand that water is a better conductor than air but are you referring to the evaporated water in the air?

            The medium that the heating cable is in is sand and therefore, almost, solid. That should retain the heat very well (I'm basing that on the old night storage heaters that had concrete blocks as the medium).

            If it's because of the evaporated air it's possible that the moisture might conduct the heat better but doesn't the act of evaporation cool the medium (sand) that it's in?

            If the sand was continually being allowed to cool down (switched off) and then heated up I would understand that the water in the sand would even the temperature out quicker. My propagator is never turned off so retains an even heat all the time.

            Like you, my small pots are sunk into the gravel. :thumbsup:
             
          • Kleftiwallah

            Kleftiwallah Gardener

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            I've read and re-read your post, but I'm still a little stumped just what your asking.
            There should be 'wet water' around the heater elements all the time.

            The job of the thermostat is to keep the temperature within a pre set range. It does this by switching the supply off and on again.

            Keep trying, I'll keep trying at my end. Cheers, Tony.

            (no daft questions, just daft answers).:thumbsup:
             
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