It will be a little more complicated since your heat is not automatically controlled. Also you will need to determine how many BTU's you will get from the 400 gallons of water on average.
I don't know anything about water for heating but common sense tells me 400 gallons probally would not do it even at the boiling point.
I am just going by our outdoor spa that is 550 gallons and we keep it near 106 degrees in the winter and we rarely go below 20 degrees and it can cool off fast.
The type of covering you have will make a lot of difference.
Double layer or single layer?
We have some very knowlegable guys on here. Maybe they can figure it out.

Here is a link that might be useful: heat calculator

I have never seen nor heard of a pyramidal gh, but assuming I understand you correctly, I'm going to guesstimate an outside square footage of approximately 1200 square feet. This is not exact, but we'd need a lot more information to get to a closer estimate.

When you say, "...poly on both sides," I will make the assumption that you are talking about poly on the inside and outside of studs, such that you have an airspace.

When you say, "...several nights in a row of sub-zero," I'm going to guess -10F? If your target temperature inside is 50F, for example, that's a +60F differential.

Given the above assumptions, you'd need about 51,000 BTUs to maintain a +60F differential on the coldest nights.

I'm sorry, but I can't help you with the BTU output of your proposed heaters.

-Bruce

I have a central boiler with a maximum 500,000 BTUs. The firebox is 5'x4'x3' and the 400 gallons of water surround the firebox. I am only heating the ghs with this stove.

I am in zone 3 and the coldest morning of the winter is often near -30 and only rarely -40, but I am quite sheltered from the wind and we usually have enough snow for some insulation around the bottom.

Yes, I have poly on both the inside and outside of 4" walls. A 30'x30' pyramid is 900 square feet and the cubic feet is found by multiplying 1/3 of the 20 ft. height. So the volumn of air is much less than a building with vertical sides.

The 51,000 BTU figure is something I can work with. I'll see what the 8' finned tubes are rated at or what a plentum in a furnace blower is.

A was going to try the first one with manual ball valves with a bypass and the second one I was going to try to make it thermostatic and automaticly controlled with a pump or zone valve.

Take a look at the Univ. of Georgia publication for hobby greenhouses. It tell you how to calculate your heating needs. Click PUBLICATIONS and follow the link.

Here is a link that might be useful: UGA Hobby Greenhouse booklet

I think you`d be better off using the hot water for underfloor heating.
With finned tube heaters the heat will tend to track up the sloped poly losing a fair amount of energy to the outside world in the process. Localised heating of the poly will reduce its lifespan.
The 120ft perimeter will drain the heat unless its well insulated.

I'm sure you would need full lenth on the heaters they are rated for btu per foot I think the slant-fin site lists that my heating guy knows it from the top of his head so might want to consult them
I have have a bunch of them probly 150ft but they are in Mn

I found a website that sells baseboard heaters. They are rated at 540 BTUs per foot and cost over $100 for 7', so that is out. But if I could just get the finned pipes and make a flange to reflect the heat out from the wall and also hang them down several inches on strap hangers, maybe that would not hurt the poly. I thot this might help as the edges and corners seem to be the cold spots. Perhaps fans to direct air down there would be more effective. I found a 100,000 BTU water to air heat exchanger that I could mount in front of a fan and then thermostat a pump and/or the fan. I can't use in floor heating because I want to use the tiller in there and plant right in the ground floor. tsmith thanks for the link but I was unable to access that info. I'll try it again.