Has Anyone Done Solar Radiant Heat?

[charles_m]

Hey guys,
Just wondering if anyone here has competed a radiant floor setup using solar panels for providing some of the hot water needs of the system. I'm in the design phase of this for someone who wants to put in radiant floor heat in their bus and I'm wondering if anyone has some great advice or practical experience in this area. Nat, if you're reading this, I know you've got a lot of input on the subject.

My floor design so far is to use 3" of poly iso board glued down to the metal floor. I will route-out channels for the pex and do 2 zones--one for the front and one for the rear. I will use aluminum reflectors either custom made by me or prefab units to radiate the heat up. The subfloor will be 3/4" formaldeheyde-free osb and whatever the customer wants on top of that.

I was thinking that a 20-30 gallon storage tank for hot water and the radiant heating would be sufficient. Should I use a working fluid for the radiant heat/solar panel lines or the tap water? Im worried too about blowing the system if the panel boils the water over and makes steam.

I'd like to use a standard 4x8' hot water panel for the roof.

This is a new area for me, but it is definitely the most comfortable and space-saving way to heat a bus imho.

[family wagon]

This is a hobby interest area of mine, however, I don't have much practical experience to bring to the table. Several years ago I built an 11x25 foot "shed" in the back yard as a passive and active solar experiment (intending to learn lessons on a small scale, then do a better job when building a full-size home). I put 1/2" PEX in the concrete floor with 1" of XPS foam under the slab. Walls are 2x4 with R-13 fiberglass, plus additional 1" polyiso fully wrapping the outside. Roof has raised-heel trusses and blown-in fiberglass for something like R-60. I experimented with my little Ecco-Temp L5 and a utility pump as a circulator; it did a fantastic job heating that space even when outdoor temperatures were in the teens (Farenheit). Radiant heated floors feel sooo good on bare feet. It makes the room feel different, too. Still haven't built the solar collectors, but at least I know the floor part works generally and learned what to do differently next time.

Your floor setup sounds reasonable to me. Personally I'm hoping to avoid the OSB sub-layer but I haven't picked a final floor finish yet.. I liked a suggestion Nat made elsewhere about laying down a sheet of metal (instead of wood) on top of the foam. I've thought about an aluminum sheet with PEX fastened to the bottom side.. Along the lines of the $2k solar DHW+space collector panels, but laid on the floor for radiating heat inside rather than outdoors collecting heat.

I'm a big fan of experiments and calculations to be sure collection, storage, and radiation are sized right. The size of the tank, for example, depends on how much heat is to be stored and what maximum temperature is allowed. Figuring the heat loss of a space is the hardest part because there are so many things to consider. Once that's nailed down, it's easy to calculate how many pounds of water are needed and at what temperature in order to coast through the night before getting more heat the following morning. More thermal mass (in the building or in the water) allows more heat to be stored, reducing large unpleasant temperature swings. But there's an upper bound; the quantity of water has to be low enough that it can be heated to a usable temperature before the sun sets.

All I can tell you about heat loss is that 5 kW of electric heat kept my bus about 40 F above ambient during an overnight experiment several winters ago (around 14 F outside and mid-50 F inside). This was before the conversion began, in which I'm removing windows and adding insulation. I'm hoping for another heat loss experiment this winter!

IMHO water vs other fluid depends on whether it needs freeze protection. Water carries heat better than antifreeze solutions do. If collectors are designed for reliable drain-back then freezing isn't a concern. Boiling may also not be a concern if the system can drain the water out and let the collectors stagnate with just air in them. Another way to avoid boiling is to have a way to shed excess heat -- a radiator outside that takes excess solar heat production and rejects it into the air.

Keep us updated on your project; I'll be interested to learn as you go.

[nat_ster]

I think harvesting heat from the sun in this way is a great untapped resource. However a few things need to be done to make it safe.

Large water storage. This is challenging in a mobile environment due to the weight. However if the heating needs are low enough (good insulation), the amount of heat needed will be low, and therefore the heat storage tank can be smaller.

I plan on having a heat sensor that detects when the tank has reached maximum heat, about 15 degrees under the boiling point. At that point it will trigger a air ram or electric ram to move a sun shade over the solar collector. This will stop any further solar heat gain.

A second sensor set at 10 degrees below boiling will trigger a water to air radiator like family Wagon mentioned as a heat dump. This will only be used as a backup because it uses electricity to run the fan and the pump to get the water to the rad. I will be completely off grid.

A pressure set blow off valve plumbed under the bus. This will be the last resort if both other safety systems fail. It needs to be able to blow near boiling hot water into a location where no one will get scalded.

The hotter you can get the fluid (water, glycol, light oil, ect) the more heat we can take back out of it before needing to reheat the storage fluid. Unfortunately pex has around the same limits as water before boiling, so we must stay with water or glycol.

As family Wagon mentioned, the choice between water or glycol will depend on climate. However, if your needing to heat, it's likely below freezing outside, you will need glycol. After all, once a line freezes and pops under the floor, fixing it becomes tearing the bus apart.
I will be running glycol in my system.

Even if you don't get all the heat you need from the sun, every bit helps. Your boiler won't need to run as much, saving you fuel.

My system will also be pulling heat from my wood / coal stove. Many of the same safety systems will also be used.

They biggest key to a good efficient system is the ability to get the heat out of the water lines in the floor, and into the room. Aluminum heat plates are 100% necessary. I learned this the hard way on the first system I ever built. Also the 14 gauge sheet steel or 3/16th aluminum make a far better subfloor than any wood. The wood will work as a insulator, where the steel or aluminum will act as a heat sink, spreading the heat into the room. The metal sub floor also permits you to use your heating fluid down to a much lower temperature before you stop getting heat transfer.

Also, the floor needs to be protected from thermal shock. Running near boiling fluid through the heat lines will be damaging to everything around them from sudden expansion. A mixing valve will fix this issue, ensuring the fluid is always a safe temperature.

I look forward to this becoming a reality. I will help in any way I can.

Nat

[Plantresilience]

Thank you all, this has been very interesting. I look forward to hearing updates. . . I was considering installing a very basic radiant floor system. Now it seems that if I am going to put in the effort, I should expand my original plans which featured a simple loop in the very center of the bus . . .unless any of you can think of a way to easily utilize some waste heat by running it through the floor. . . now I know glycol is my only option.

[charles_m]

Im trying to size the tank right now... the bus is pretty well insulated with 2.5" of spray foam in the walls and 4" in the floor and ceilings. Double pane windows.
I am thinking that 30 gallons of total fluids between the tank and panels will be on the small size, but good enough.... thoughts?
And it looks like a mixing valve is the way to go, too.

[charles_m]

Then, supplement solar heat by plumbing in a webasto in the same loop as the panel.
A thermo top C makes over 15,000 btu and should be plenty to keep the bus warm in all climates in the contiguous states.

[somewhereinusa]

I didn't do solar, but have a very nice system using an Espar boiler (kinda like a Webasto) didn't need a tank. I made it very much like an automotive system with a pressure cap and an overflow reservoir. I routed the channels for the tubing in 2 inch Styrofoam and layed the ready made aluminum panels in the grooves. I have four zones, three in the floor, bedroom, bathroom and kitchen/front room and one that goes to the original heater core in the front . There are also heaters with fan in each room, but they are really only used for quick heat up from cold. Then 3/4 plywood over everything, the plywood is free floating. There are pictures of most of the stuff in my build thread. Mine is also plumbed with heat exchangers to warm engine and domestic hot water. I get heat from the engine while going down the road. I would definitely use glycol, I found it was next to impossible to get all the water out, and equally hard to get the air out when refilling.
I personally think that OSB has no use anywhere for anything, especially in a bus.

[Plantresilience]

I'm glad you brought that up. I purchased my bus last weekend, and it is equipped with a separate supplemental heater that can be run when the bus is not running. However, it runs on diesel from the main fuel tank. . .

[family wagon]

There's a Home Heat Loss Calculator on the builtitsolar web site. I don't know how big your bus is, but I put in some round numbers just for estimation sake (in feet): floor 8x40, side walls 7x40, rear end 8x8, ceiling 9x40 (a little more length than the floor to account for its arch), front end 2.7x8 for the windshield and 5.3x8 for the parts above and below the windshield. Also guessed 5 windows at 3x3 each. I put in R16.5 (2.5"xR6.5 per inch) for the walls with R26 for the ceiling and floor. I struggled to quickly find appropriate values for the glass.. guessed 0.14 for the single-pane windshield and 2.0 for the double-pane windows. I entered 2560 cubic feet for the interior volume and 0.33 air changes/hr. With a design temperature of 0 (and I'm not sure what it assumes for the indoor temperature) it comes up with 17,777 Btu/hr heat loss. I think you're right; the 15k Btu Thermo Top C will be adequate.

For solar heat a tank is used for heat storage. Load in all the heat possible while the sun shines and draw heat back out while the sun is away.

For the Webasto heat I think the main reason for a tank is to avoid short-cycling the heater. I don't know what Webasto would consider a minimum cycle.. several minutes, at least?

Here's some guesstimating out loud.. When it's 30-40 F outside and heat loss is only maybe 7,000 Btu/hr, how long will the heater cycle be. The heater will run until the fluid reaches the high temperature limit, then it'll switch off until the fluid cools to the low temperature limit. The length of time it spends on and off depends on the mass and heat loss of the glycol. Just pulling numbers from a hat, now.. if you had 30 gallons (8.3 pounds/gal for water times 1.05 specific gravity for 50% ethylene glycol, so 261 pounds), and the heater wanted to maintain it between 165 and 185 F, that's 20 degree delta times 0.85 Btu per pound-degree.. 0.85*20*261=4437 Btu added/removed to go between those temperature limits. With all 7k Btu going to heat the bus there's 8k Btu heating the tank, and it'd take about half an hour to bring the tank from 165 to 185 F. That seems like plenty long to me. This is ignoring the thermal mass of the glycol in the rest of the system, plus the mass of the floor or whatever heat exchangers too... it was just a quick and wild guesstimate after all!

So far as the Webasto is concerned, my early guess is that 30 gallons is way more tank than you need. It doesn't surprise me somewhereinusa gets along without any tank. (Any idea how long your heating cycles are, somewhereinusa?)

For solar storage, if you allowed the tank to go 100-185 F, there's almost 19k Btu in there. For that 7k Btu/hr 30-40F day this heat would last only 3 hours. It isn't nothing, but 30 gallons probably isn't enough to get you through the night without Webasto assistance. The thermal mass in the floor etc could help, but you wouldn't be letting those elements get up to full 185 F either..

If somebody wants to take this very coarse estimate and refine it I'd love to see the results. Likewise any semi-measured experiments to get an idea whether this calculation is realistic.

[iProgramStuff]

Does anyone have anything to add to this super helpful thread? I found it whilst searching and more information and experience would be lovely. Thanks everyone!

[joeblack5]

Like familywagon this is hobby of mine also.
We have domestic hotwater and space heat with solar collectors. The storage for more then 3 days is a real problem. I would think to difficult for a mobile application.
There are some gelpacks that can be boiled and activated at will. They are not difficult to make and store a reasonable amount of heat.

I think it would make more sense to use PV panels and store in Li-Ion batteries and use a heat pump to heat and cool. The heating cycle of a good heat pump is about 4 so 1kwh delivers 4 Kwh in heat. So that part gets about the same efficiency as the how water collector. The gain is your storage size and weight. A 48 volt / 45 Ah Chevy volt battery module weighs 39-40 LBS.

An heat pump heating efficiency with radiant floor heat is very good because the lower temperatures.

In overall I think PV with lithium and heat pump is much simpler and much more universal.

A complete chevy volt battery can be had for $1500 + shipping.
Solar is less then $0.70/ watt. fater you are done with collectors , tanks, pumps, ris of freezing I think that PV is cheaper as well.

Later,

J

[somewhereinusa]

Any idea how long your heating cycles are, somewhereinusa?

I missed this post last fall. I really don't know how long the heat cycles are. It's really hard to tell when it's running when I am inside where it's warm. And of course it varies with outside temperature. I can say that even when it's down around 20° it still cycles.

I'm not familiar with how the other manufactures operate. When you light the Espar it is lit until you shut it down. The boiler cycles between high and low so there is some heat being produced even when on low. There is a timer with a 7 day clock that can be set to turn on and off at various times. For my use I have never bothered to even study how to set it.

My system has four loops of heating in the floor and each room also has a heater similar to the heater in your car with a "heater core" and fan. Each loop has it's own thermostat that turns on a water pump for that loop. The heaters have a switch on the wall but, also have a temp switch so that it won't come on unless the water is warm so that it will never blow cold air. I can say that when the loop water pump is off the water will cool enough that the heater fan will turn off. I only really use the heater fans for quicker warm up from cold, or when it's really cold out. At a temp of about -10�° it only took about an hour to get up to about 70�°.

In all of the information I could find when I was designing my system I didn't find any that had any kind of holding tank. Due to some mistakes I made when filling it's really hard to know what total capacity is but, I think it's about 10 gallons.

Dick