Solar powered hydronic heated floor

[upnorthandpersonal]

Solar electricity in winter is sketch to depend on.

@blackfly ^ This is my write-up of the calculations I did to make solar work where I am. Note the numbers in the power audit. You really want to churn the numbers for your area in detail.

 

[Roswell Bob]

I am all over PV powered radiant heating, but my situation is probably quite a bit different than yours. I don't want my excitement to send you off in the wrong direction. Pay attention to everyone else and you will do well.

I am passive solar house with yards of glass facing south so I am almost warm without any heat. I also have a large fireplace with heatilator, and am considering a LP powered 30,000btu direct vent heater. I will use excess PV into the water heater. I have on most days a surplus of 15kWh, and I have a 12kW Kohler generator if I ever really need backup. I have large battery chargers that will also be running on the generator on cloudy days. Point is, if I am going to run a generator then I want it loaded up fairly well to keep efficiency reasonable. acdoctor is absolutely on point. Making electricity from LP and then heat from the electricity is a bad plan.

I might eventually install a wood fired boiler as upnorthandpersonal commented, but I may just move to Florida instead.

 

[blackfly]

I can't reply to everyone but I am again appreciative of all the comments. This has been insightful!

I think it's realistic to have a dedicated propane hot water tank for the floor, solar to run the lights and the pump (excess into the tank if possible), and a backup generator for if/when there isn't enough sun to charge the batteries. I'll focus on that for now and start putting together the data to size the system correctly, and learn about the nuts and bolts of components and how the system would work.

 

[Jordi]

Hi blackfly,

I have no experience with your question but I understand it is a very convenient system that provided of enough power it can effortless heat your place. In your off-grid situation I have seen in Spain two option; biomass burners with heat distribution and portable butane tanks to be placed on heaters. The issue of these systems is that when one becomes old and has trouble with lifting weight, it can be challenging to maintain them.

The constraint I see in your case is when you need more power, there is the least available. Better to have a fuel based support indeed.
Is there a river nearby? Is wind an option? Is geothermal something you could explore?

 

[circus]

Electric heat without grid power in a dark 50° latitude deep freeze!? Time for a reality check. At best, solar may supplement another traditional heat source. I'd find the cloud cover % during cold months and maybe hang a couple solar thermal collectors on the south facing walls.

 

[DCPower]

I suppose I haven't got an answer because there are few with experience with hydronic (glycol) heated floors.

Can someone answer me this - is a massive system required if I need to run an electric hot water tank (4500 watts) for 2-3 hours a day?

I've run the number through a calculator I downloaded, and the answer was yes I'm just hoping someone will sanity check that.

I’ve designed several solar hydronic systems - old school direct solar heat collector systems - which work well when the structure is suitably insulated and there’s enough thermal mass to store the heat energy overnight. Overcast days aren’t as big a problem with thermal collection as they are with PV, since IR penetrates cloud cover more efficiently than the spectrum slices PV uses, but you still need a heat backup source.

With todays PV prices, what you want to do makes more sense than it once did, but 80 gallons of water heated to say, 135 degrees F - which is really as hot as you want in a floor heat system else you can’t walk on it! - only will deliver 19,200 BTU after which the water is at 105 degrees F - about the minimum for in floor use. Your thermal mass will have to be a really well insulated slab - 6” of foam type insulation under and on the edges.

 

[AndyRonLI]

A problem is that the windows that allow passive heat gain get rid of it overnight and on cloudy days.

The key is to have a good thermal mass, that grabs that solar. Then we get to use... the greenhouse effect ;-). If you size the mass, window appropriately, have glazing specific to the purpose ( which may be hard to find now). You can keep at least 50% of what you gathered. The key is to get that mass warm enough to keep the house temp from dropping below some target during the night. When the house is occupied, you can use curtains, shades etc to control input, and output. When the house is unoccupied, it will simply cycle. You have to design the house from scratch to really get this right, and few folks actually know how to do this anymore.

 

[AndyRonLI]

The real issue is that the house will be unattended when solar is at its lowest. So without the human to close / open shades, throw a log on the fire, turn on the propane heater for a bit, etc etc, there is no mitigation for the shady day. I would design this house to have close out spaces when its unoccupied. Design walls that can take some frosting. Keep some core section so it stays above 40F, and have some dry wood for the stove handy. It will be brisk when you arrive for your holiday, but the human in the loop will get it warmed up.

 

[12VoltInstalls]

I would design this house to have close out spaces

Basically bedroom slab area zoned separately from main living space and bathroom on another zone. The slab will take 10-24hrs to come up to temp so some baseboard in living/bath on a third zone for one-hour heat. Might take two days for everything to temper up without a woodstove