Greenhouse Heating

I need to have heat in the greenhouse to get a good enough start to sell starts! This last February was 6* colder than average, and the average is around 20* at night with many below 10* (we'll just ignore the record of 29* below).

I don't have a lot of bucks to invest so let me tell you how my system came about: The first piece fell into my lap back in the summer when I saw a 60 gallon water heater sitting in front of my neighbor's milk barn. Being the curious junker I am I ask what was going on and the story went "the bigger chiller unit we installed caused the main breaker to keep tripping so we replaced the electric water heater with a propane unit"


So after I did him a favor and took the WH home, I stuck it in the shop thinking I would do something with it one day. Well one day came a few weeks ago when it dawned on me I could make that water heater into a radiant hydronic heater for my greenhouse benches. Here is a shot of 2 benches, each with it's own "curcuit" of tubing for hot water to cycle through:

I attached coils of PEX tubing to the underside of the bench tops and sealed the bottom with 1/2" rigid foam insulation - foil side up. The pex is plumbed to 3/4" pvc pipe, a supply and a return, and ran around to this work of engineering mastery:

Here you have your basic washing machine motor and pump assembly protected by a preciscion fitted plastic pot. Affixed to the top of the pump is the fully programable control unit (in the Folgers can). The control unit contains all the guts of the washing machine wiring because I couldn't figure out which of the 9 wires coming out of the motor would make it run. I just left them and the timer all connected and after a slight adjustment to the timer clock rendered it not a timer at all everything basically became an on/off switch.

If you look back at the pics of the seed cabinet, I think you can see behind it a sheetrock wall - you can see a bit of it behind the heater in this shot too. This building was a garage built sometime early last century and so it has a stemwall foundation. I'm building walls inside the old north wall and insulating them as I scrounge the materials, I'll do the north half of the roof as well. Here is a section through the wall so you can see I have about R-30 insulation with the part at the floor being rigid styrofoam.

Yet to be accomplished is connecting the pump to a line voltage t-stat I had hooked up to heat lamps for the previous incarnation of the greenhouse and completing the tubing installation on the last 2 benches. The washing machine pump setup is overkill since the motor is way overpowered for this job, if things work out I'll replace it with something more appropriate – when something turns up more appropriate that is...

My costs:
Benches, water heater motor/pump, wiring, breaker box - $0
200 ft PEX - $50
"Gator" fittings 10 @$6 = $60 Ouch!
PVC $40 +/-
40A 220 breaker - $20
 $170 total

I could have bought a Blue Flame propane heater and installed it for about the same but I'm thinking this will be much more efficient since I'm heating the underside of the trays with radiant heat and not the air of the entire greenhouse. Placing a cover over the benches using a cage structure (you can see the fence wire I'm experimenting with in the top photo and previous post) I can trap the heat right at the bench. Also in the back of my brain is plumbing the black water barrels as a solar collector / heat sink during the day.

The added benefit is that when starting seed or propagating from cuttings the soil temperature is what's important – which is why the seed mat in the starting cabinet, but with this setup we could theoretically start seed right on the benches.