car radiator for heating a greenhouse; sizing water tank

I have (finally!) got my high tunnel greenhouse built and covered. It is a plastic, uninsulated structure, partially sunk in the earth, 16 feet long and 10 feet wide, with the aboveground height being 5 feet. I don't want to heat it through the winter, but instead want it to moderate the large daily and seasonal temperature fluctuations in Colorado. To this end, a scrap radiator connected to a tank of water sounds like a good idea: see here: https://northernhomestead.com/car-radiator-for-heating-and-cooling-a-greenhouse/

The big difference is that I plan to use water in the tank, but Polyethylene glycol antifreeze in the radiator, circulated through a coil of pex pipe in the tank.

I realize that you can't give me exact answers, but I'd like to get a ballpark idea on the following:

How big should the tank be to keep the greenhouse at 25 F when the outside temp is at 0 F? 150 cubic feet? 50 cubic feet? 300 cubic feet?

That's a tricky question to answer.  Kind of like asking how long a stick is.

I'd use the video in that blog as a starting point.  How's their air volume compare to yours?  How's their glazing R value compare to yours?  How's their climate compare to yours?  How will your underground design compare to hers?

Then figure out the size of their tank and go up or down from there.  That fan running on high all the time would bug the hell out of me.  

But the idea is cool, capture heat on sunny days and feed it back out proportionally to the temperature difference.

Hi Gilbert

This might help you, They have about 1200 gal of water they worked with.

https://www.builditsolar.com/Projects/Sunspace/LowCostHtStorageNathan.pdf

Mike

I'm liking this idea more and more.  I was going to dig trenches in my greenhouse and route 4" drain tile that is connected to fans and ducts off the ceiling.  Plan being that I'd suck hot air down and store it 1' deep for it to bleed back into the room over the long haul.

BUT, the missus doesn't like the idea of burying all that plastic in our soil.  And I don't like the idea of digging that much

So, what other methods of heat collection, storage and distribution are there?  One I came up with involves this radiator design.  

I have an 18' ceiling so I initially didn't think this radiator system would work for distribution unless there was one radiator up high for collection and one down lower for distribution.  My insight was to collect air off the ceiling with a large duct and have it route down to ground level where the radiator would be located.  The same fan that blows the air through the radiator would draw the air down through the duct.  Then the water/glycol lines would be shorter and the distribution of heat would be at ground level.  Plus when distributing the heat, it would pull warmer air off the ceiling to heat the plants closer to the ground.

With all that said, I don't want a loud box fan running all the time (as in the Northernhomestead video).  I found a Honda Civic aftermarket racing radiator and fan on Amazon for $66 that comes with a 12" fan.  I'm not sure if that's overkill or underkill.  I also don't know how loud it would be and if I could put it in a box to muffle the noise.  I'd also have to rig up power from my AC wiring to run it.

I figure this system could be very modular.  If I needed more heat collection, add a radiator and duct.  If the water barrels won't hold enough heat, get more or change them over to phase change materials.  Or have different phase change materials in sequential tanks so one holds heat at 100F, the next at 80F and a third at 60F.  Keep adjusting until the maximum heat storage, collection and distribution is attained.

Any other thoughts on these systems?  Especially regarding the size of the fan and noise?

Miek Jay: I'd try a VERY small fan to start with, I have seen 4 inch bathroom fans running 4 inch duct do very well. You don't want a tornado in there, you just want the high heat pulled down as it gets there. Doesn't take a lot of air flow to do that. Think circulation fan, not tornado.

The fan system I'm thinking of was a cooling system in New Mexico, about 10 foot ceiling on the guy's greenhouse, less glazing than you need up north, more covered space. The little 4 inch bathroom fan kept it quite nice in there in the New Mexico summers. He was pulling heat off the roof, taking it down, it came back up through a swamp cooler type filter system. That was his entire system, and it worked nicely. Try a small one. See how it does in your world. Big fans are good for some things, but they are quite often overkill.

Thanks Pearl, that's interesting.  How big was his greenhouse?

Mike Jay wrote:Thanks Pearl, that's interesting.  How big was his greenhouse?

Hmm... based off memory, I'd say 30 feet by 20 by 10 foot high. Glazed area was patio doors standing on the ground, not quite the whole length of the sun side, so maybe 15 feet by 6 foot tall, once the structural spacers etc are taken out.

Part of the problem in NM in summer is if you open vents, you probably are sucking in air just as hot, and a lot dryer than what you are removing, so circulating it works better.

Wow, that's bigger than I would have thought a 4" duct could handle.  I was basing my assumptions on the video Gilbert linked above (box fan on "high" blowing through a radiator).

I have a small squirrel cage fan that I could use as a test.  No radiator on hand.  I wonder if I could find a baseboard heater core at the Restore and put that inside a 4" corrugated drain tile and blow air down the drain tile...

it more comes down to how much air do you really have to move? If you are just circulating it around, you don't have to do it at gale force. To take air from here to there doesn't require huge fans. If you need to turn all the air in the place every 10 minutes, that's one thing, if you don't, consider going smaller. He wasn't hauling around huge amounts of air, he didn't have to, he just needed a slow steady flow to pull the high temps off the roof, and wet them down.  

I'd need to pull 120 degree air off the ceiling and pass it through a 60 degree radiator to store the heat.  I'm assuming I'd want run all the air from the ceiling through the radiator every 10-20 min but I'm not really sure.  Let's say the volume of the upper part of my greenhouse is 4000 cubic feet.  I'd need to move 200-400 cfm to process all that air in 10-20 min.  This little muffin fan can push up to 110 cfm.  So two or three of those could do the trick...

Thanks for all the discussion!

I really like the idea of ductwork; I'd been trying to think up ways to get the fan and radiator suspended up near the peak, and how that would work with headroom, vents, etc. Ducting the air to a fan at ground level would create much better airflow.

I may install one with a temporary tank to get the sizing worked out before finalizing anything.

Mike J
lets say you did go the pumping hot air underground route
from what i gather you want it at least 4 feet below the soil or you will dry out youre soil too quickly
i have heard of several depths
4 feet for more short term heat
an 8 foot one and a 12 foot deep one for heat storage in summer when you dont really need the soil to get so warm

you could possibly make it by digging a hole and lining the bottom with clay
and clay on the sides as you build youre way up then a large bed of gravel/rocks for the air to flow through
then maybe a woven reed mat to add the clay layer to cap it off then build up the soil for growing in
possibly bamboo or something with the segments burnt out to make pipe to get the air from the top of the greenhouse to the gravel heat sink
just a thought ignore if not applicable

Thanks S, I had not thought about the hot air drying the soil out...  I kind of thought it would moisten it.  I imagined the hot and somewhat humid air would condense underground.

In any case, I think I'm sold on the radiator design.  Unless something better comes along.

What are we all thinking about the fans for the radiators?  Go with the stock radiator fan and power it somehow?  Might it be moving much more air than needed (per Pearl) and maybe we can rig it to run slower?  Or try to use smaller fans and make a shroud to mate them to the radiator?

Unlike in the video at the top of the thread, I'm planning to have a heat exchange coil in the tank so that I can run a propylene glycol mixture through the radiator. I'd like to use PEX instead of copper. How long of a PEX tube would I need to get the maximum heat exchange?

Or would this be just too hard on the pump? Should I just run the tank water through? Buying enough propylene glycol to fill the tank is prohibitively expensive.

My wild guess would be 100' of 3/4" pex would do the trick.  Should be big enough to not overload a pump.  That size would also move the glycol slower through the heat exchanger so it has more time to transfer.  But it is just a wild guess.  I bet a coil of soft copper would work much better, maybe 3x.  But also be much more expensive...

I have a totally different radiator idea that you may want to consider (or laugh at and discard).  What about stealing the copper tubing and fin arrangement out of an 8' baseboard radiator to run your glycol through.  Around the outside of the radiator put a 6" black corrugated drain tile (unperforated).  Put a fan at the end of the drain tile to blow hot air through the tube and around the fins of the baseboard radiator.  Send the hot air in one way and the liquid in the opposite direction for maximum heat transfer.  

Mike: thanks for the advice. I'd much rather use 100 feet of PEX than 30 feet of copper.

Do you think the baseboard radiator in a tube idea would transfer the heat well? I was wondering if the air would just flow around and past the whole assembly instead of between the fins, thus minimizing heat transfer. But it is something to think about.

I really don't know if it would work.  But I think it would.  Between the corrugations of the tubing and the fins, there would be lots of turbulence in the air flow = good.  Even if the air doesn't get down between the fins particularly well, the ends of the fins would be doing plenty of work.  I'll be trying it myself but I'm many months away from that part of the project.  

Has everyone seen this version?
BuildItSolar:Storing excess daytime solar energy to heat greenhouse at night!
It seems to be the same idea we're talking about.

Thanks Kenneth, that IS what we're talking about.  Looks like he used a radiator that's kind of like a car radiator but designed for our application a bit better.  Down in the comments he mentions what the specs are:

I bought my heat exchanger on ebay - search ‘12x12 Water to Air Heat Exchanger’. I used the 12x12, but they have other sizes too. the 12x12 is rated at 50,000 BTU, 700 CFM and 6 GPM. These are the three things you will have to consider in choosing a size. First, how much heat you think you can harvest, what the air flow across the exchanger will be, and what your flow rate for your transfer fluid (water?) should be. As long as the exchanger meets the minimum requirements of your system, you’ll be OK.

I found one on Ebay as well and it kind of looks like something from a furnace.  Doesn't that just make perfect sense?

In the HVAC world, this is a "fan-coil" (a water:air heat exchanger with a circulating fan), as compared to a "fin-tube" or "convector" (baseboard hydronic heaters powered by convection).
One benefit, as Mike noted in his quoted material, is having accurate ratings/specifications, versus spit-balling a homebrew contraption.
Another benefit is that it is "plug'n'play" versus "tinker'n'pray", and since it's only really unconventional in its application, you could easily hire a professional plumber/HVAC contractor to install it.