comparisons of heat mat costs and performance
 

I'm building an outdoor propagation box with insulation. I will be rooting cuttings and starting plants from seeds. My coldest overnight temperature in the winter is typically 33 F (1 C), but more typically 40 F (5 C). My goal is to keep the rooting pots in the mid 70's (23 C) in the day and early evening and above 55 F (13 C) at night. I will use heat mats to warm the 45 sq.ft. surface and a thermostat to regulate the mats.

Here are the mats I'm considering:

Hydrofarm 48" x 20" @ 107 Watts, $55 each --> 16 W/sq.ft. and $8 / sq.ft.

Pro-Grow 22" x 60" @ 200 Watts, $155 each --> 22 W/sq.ft. and $17 / sq.ft.

Stanfield 3' x 6' @ 400 Watts, $376 each --> 22 W/sq.ft. and $21 / sq.ft.

Do you have experiences with these or alternative systems I could consider?

Re: comparisons of heat mat costs and performance
 

Did you see these?

Redi-Heat Propagation Mats - 12"W x 10'L $132.25 /EA

Output for 12"W mats is 20 watts per linear foot. Output for 17"W mats is 30 watts per linear foot. Output for 21"W mats is 40 watts per linear foot. Mats used on GFI outlets must stay under 500 watts to avoid tripping GFI.

Re: comparisons of heat mat costs and performance
 

Thanks, this model is interesting:

Redi-Heat 21"W x 10'L @ 400W, $132 each --> 23 W / sq.ft., $11 / sq.ft.

Re: comparisons of heat mat costs and performance
 

Have U consider electical underfloor heating system. Its much cheeper and better than heating mats and in my opinion its easier to insulate all system.
Grrrrh, my lack of words.....Hope U understand me:)
:woohoonaner:

Re: comparisons of heat mat costs and performance
 

Yes, heating cables are less expensive to purchase but very expensive to operate. They consume a lot of electricity which is expensive here in the U.S.

Re: comparisons of heat mat costs and performance
 

Think twice about that. If they are used properly with quality ceramic they can save U lot of money.
And U always pay Watts per Hour!
It isnt named "low energy heating sys" for nothing ;)
:woohoonaner:

Re: comparisons of heat mat costs and performance
 

The cables I've looked at would consume 50 W per square foot. I'm only trying to raise the temperature from 2 C to 15 C. For this I believe 15 W to 20 W per square foot is sufficient.

My area is also small (see first post). For a larger surface, it would be more cost effective to use hot water pipes under the flats of plants, with water heated by a gas boiler.

Re: comparisons of heat mat costs and performance
 

That is what i was reffering with watts/hour. More watts-less time needed, that why u use thermometers. True is that this sys needs more hardwork but in my opinion if U want to do something more proffesional than it is everthing clear. Heat mats are good and simplest solutions for areas that we dont use only for germinations and rooting.
:woohoonaner:

Re: comparisons of heat mat costs and performance
 

With heating mats and cables, the first 5 minutes of operation uses nearly the same number of Coulombs (~watt x hours) as the first 20 minutes. This is why I am concerned about power density (watts per sq.ft.).

Following your suggestion I have found this efficient cable here in the U.S.: Gro-Quick 140' and 200' foot cables at 700 Watts and 1000 Watts. If I bury one in a shallow bed of horticultural sand it should be able to keep 18 flats (288 4-inch pots) warm enough for propagation. The 700 W system would be affordable to operate at 17 W / sq.ft.

:woohoonaner:

Re: comparisons of heat mat costs and performance
 

The point is to get quality heating media as tile ceramic. Cables in sand are almost useless but whan cabels heat tiles thats completly different story. Agree? And U also get clean work surface.

Re: comparisons of heat mat costs and performance
 

I agree about sand, but horticultural sand is a different material, grain size, and heat capacity. There is an overhead misting system controlled by an electronic leaf. Water that runs off into the cable area will be heated and could provide mass transport of heat. Placing ceramic tiles on top of the cables and sand would reduce this to radiant transport, possibly keeping the pots too dry. This sounds like something to experiment with.

Attached is a worksheet showing the performance and costs of various options I've considered so far ...

Re: comparisons of heat mat costs and performance
 

But if tiles are glued as in pools or bath tubs your pots can allways be well watered by misting only?
Well after all this is just an idea :)
:woohoonaner:

Re: comparisons of heat mat costs and performance
 

Yes, these are good ideas!

If a work surface is important, then glued tiles are worth considering. This propagation table has side walls and a hinged top. It is not a work surface.

I will have cuttings in the pots and the electronic leaf is to prevent over-watering. The media in the pots needs to be less than moist but not dry. The "bed" they are sitting on has drainage because it is poor for the pots to sit in water. Having the pots sit directly on tiles without warm humidity rising underneath could cause them to dry out too fast -- I don't know. It is something for me to experiment with. I believe it is a good idea to have the cables in horticulture sand and allow the run-off water to be heated -- thus taking advantage of the heat capacity of water and heat-transfer by evaporation.

Re: comparisons of heat mat costs and performance
 

One way or another this is going to be an interesting project.
Keep us posted about progress and add tons of pix ;).
The most important is that U enjoy it.
:woohoonaner:

Re: comparisons of heat mat costs and performance
 

The sand bed idea will be a bad one for sterilization purposes I'm thinking. If you want to do multiple loads of cuttings in this table, it's going to need to be sterilized between batches. How will you get the sand clean?

Re: comparisons of heat mat costs and performance
 

1 drop of triple-ammonium per 5 gallons of water.

I will not be planting directly in the horticultural sand (1/8 to 1/4 inch silicate). Its only purpose is to distribute heat laterally. Any cuttings or plants will be placed in 4 inch pots, which sit in 4 x 4 ring-flats. For rootings, I will likely use 50% coir + 50% perlite.

Re: comparisons of heat mat costs and performance
 

Thanks for bringing this subject up, Richard. I am considering buying a heating mat myself.:woohoonaner:

Re: comparisons of heat mat costs and performance
 

I finally got a 20x20 Hydrofarm heat mat. I'm considering getting another.:woohoonaner:

Re: comparisons of heat mat costs and performance
 

From a different angle: what about cooling, or more specifically, keeping the the temps reasonable? Cincy and SD or different (for one thing, your football team beat mine!), but one major concern I have run across in spring is the wide variety of temps. Overnight, it may well be 35 degrees, but then during the day 60, maybe 70. Inside a grow box, if it isn't open, it could easily get to 135, frying your seeds/seedlings.

Mike

Re: comparisons of heat mat costs and performance
 

Wouldn't the sand cause water to go stagnant due to it holding water? The sand and tile would be heated to same temp as tile would be in basically mortar causing direct contact and radiating heat off. What you lining this with to keep the water from destroying the building material due to direct contact with the water and sand, as tile installed properly would make its own seal to water out.