Waterbed heaters...

I was wondering if anyone has ever used a waterbed heater in a large DIY seed germinator and if so how effective it was? I was wondering how they cope not being under a thick bed of water (i.e. do they get too hot even on the lowest setting) when they often recommend a minimum thickness of about 100mm of water above them? I was thinking of making a large wooden box I can sit on my verandah, out of direct sun but with plenty of incidental light, lined with black plastic under which I'd put the waterbed heater with a thick bed of gravel or perlite on top to sit trays of cactus seeds.

Nice idea...I don't know much about the waterbed heaters, or how'd they behave without the mass of the water to heat up...or if they're capable of sustaining a given temp...

As for you design idea though, I can vouch from my own experience that it works...although I went with a thermostatically controlled horticultural pad and added artificial lighting. But otherwise it's what you're thinking, perhaps just a bit bigger.

Yep, and trust me, you don't want my electricity bill....

Gigantor - King of the propagators !

That's probably a little bit bigger than I was thinking of LOL I'd like a propagator that can fit about two rows of 4 seed trays. It would have a clear acrylic/perspex lid of some description to keep the heat and humidity in but allow plenty of light in. Do you cover your black plastic with gravel, or some other form of bedding, to help distribute the heat?

Yes it's kind of big, but it's not just for seed raising but growing on and cuttings etc as well. I use a layer of vermiculite on top of the plastic. Gravel might be okay, but probably a bit heavy and not as absorbent and I think perlite would be a little more likely to harbor algae. The vermiculite works okay, but needs to be replaced as it breaks down over time. On a smaller scale, gravel or propagating sand would work..

This is probably more the size your thinking. Same deal, although in this one I put the pads above the plastic. They're the rubberised Hort type pads without thermo controls. And I put a sheet of polycarb over the top when doing seeds...

Then there's this one with the acrylic hinged lid which works fine, but be warned that when the acrylic is exposed to warmth and moisture it has a tendency to warp a little bit, so it gets a little harder to keep a good seal.
The box alone -



And with its' twin in their stand with lighting etc...




Anyway, hope it gives you some ideas !!

Yeah it does They are awesome! I like the idea of sand too... I might make a wooden frame to sit the acrylic sheet in to prevent warping. I don't like how acrylic (etc) scratches so easily and seems to cloud as it ages but with three young rough-neck kids around using glass is out of the question. I wonder if silica based sand would act like silica gel desiccating beads and suck the humidity out of the air?

great propagators navajoa there are some good ideas there that i might have to steal for my next humidicrib. i saw some people discussing a heater made out of a underbed storage container filled with water and an aquarium heater which would work out pretty cheap if you could pull it off.

darren

Had another thought TasV, about using the sand. I remembered that I actually used propogating sand the first time I set up the acrylic lid boxes and it worked, but not long after the sand had pretty much 'set' and was a bit like concrete ! I think it was due to all the minerals salts etc left after evaporation - bonded it altogether. Rinsing first may help ?? Anyway, that's why I use vermiculite now !

Darren, yep I've seen that discussed somewhere too. Should work, as long as you keep the water up and get the right kind of submersible heater. But I'm more inclined to like the heat right up close to the base of the pot, not sure how you'd accomplish that with the water reservior method ???

Yep... makes sense... I think I too will either use vermiculite or fine(ish) gravel. A few questions about the vermiculite: How well does is distribute the heat? Do you end up with cold spots around the edges? I'm kinda leaning towards something like small scoria so that as it heats up it radiates it evenly.

Haven't done any really testing regarding the heat distribution. Hasn't been a real concern as my boxes were made to the size on the pads, so I'm not really relying on the Verm to spread the heat (other than up!). Also, in an enclosed area I'd tend to think that the temp would be reasonably even spread... but I could be wrong!
I can see how scoria might be advantageous in that respect, but it might be a little hard to work with I'd think, unless you can get some fine stuff. I like to 'nestle" my pots and punnets into the Verm layer, it could be harder to do so with scoria or lead to punctures in the plastic layer and then water seepage ??
Worth giving a whirl though !

Water bead heaters are great but pretty power hungry, good insulation will help here. They work best if you keep the sand bed at least partially flooded.

The silicone pet heat cables or mats are probably a more manageable size for small beds & you can get really accurate thermostats for them. Or you can get horticultural silicone 12V heat cables.
Aquarium heater placed in a reservoir under the seedling bed can work, but as i discovered the hard way, unless you insulate the sides & base of the reservoir really well your heater will be working flat out for little effect where you want it.

Silica sand wont suck any moisture but it does act as a really good thermal mass to keep heat even especially when its a good 4" thick & when partially flooded it helps keep things nice & moist. A layer of geo-fabric over the top will keep algae at bay & when it does grow on the geo-fabric just lift it out give it a good hose, a dunk in some bleach & back in the seed bed.

I prefer to use plastic in aluminium flyscreen frames, they are light, cheap, easy to make & kids cant hurt themselves on them. A length of double thickness weather strip gives a good seal.

Shortly, If I remember rightly, a few years back you were putting together a propagation bench with low voltage cabling. Did you ever finish this?

That I did, 3 of them 2 - 8' long X 2' 6" wide made from 2" gal angle & 3/4" ply then fiberglassed, both heated with 12V silicone heat cable from helios & 1 - 6'L x 2' 6"W that was unheated. they are all moth balled at present, I should be able to dig my to the back of the shed & take some pic's this weekend

Sounds like you have a shed like mine