I am looking to install radiant floor heating in my yurt. I've been doing some reading, and have a couple of questions. This mostly stems from the unconventional building. The yurt is insulated with what is essentially bubble-wrap with tin foil on both sides. It isn't typical insulation and I am told (and through experience) that it works by reflecting the heat while actively heating, vs slowing the transmission of heat. Some numbers that I have seen is R7.5 for the walls and R5 for the ceiling. Using those numbers and that my yurt is a 24' diameter, I've calculated that I need approx. 15,000 btu/hr of heat. This is for 20 outside and 70 degree inside temp. However I calculated that number using slab on grade.

I'm not sure if our floor actually acts more like slab on grade or framed when looking at heat loss. We are building an earthen floor, and just finished the sub-floor. This consisted of 6" drainage gravel to fill up to grade. Then cinder block stem wall infilled with R10 insulation with 3/4" minus compacted gravel on top. The final coat will be 1.75" of clay, sand, and straw mixture which will contain the hydronic tubing. Is the heat loss of this floor to be more like a raised floor?

Using the calculator (http://www.borstengineeringconstruction.com/Hydronic_Radiant_Floor_Heating_Design_Calculator.html), I have a couple of questions. It recommends maximum tube spacing of 6"? this seems pretty close. It also recommends 4 circuits of 225 ft. What is the max ft I can run in a circuit with 1/2" pex? I think I read 300'.

I was originally looking at running an open loop system, but I think I have been convinced otherwise. Now I am planning on closed loop. Reading an earlier thread, someone who needed 13000 btu/hr was recommended running a standard hot water heater for the system. I am hoping to use electric as our rate in the PNW are $.08/kw, which would be cheaper to run the propane it seems. That said, how do I determine what watt heating element I need in the water heater? Does it need to equal my btu/hr needs? I would need approx. 5kw water heater? Also, what size tank do I need. Since space is limited, I was hoping a compact water heater would work (10ish gals).

Another thing to note is that this will not be our only source of heating. We will heat with a wood stove as well. My main goals for this system are to even out the temperature swings from when we go to bed to when we wake up in the morning, as the yurt cools off quickly. Especially in the long spring and fall months when the daytime highs are too warm to start a fire, but the morning lows make it chilly to get out of bed.

Thanks in advance for any help/advice.

sorry for the long winded post, all the formatting disappeared.

Posted By rodney757 on 20 Jul 2016 08:55 PM
sorry for the long winded post, all the formatting disappeared.

Yea depending on your browser, the formatting may need to be done in HTML code... If you click the "quote" option on this post, you will see the code string to perform a line break. Can't type it as the forum engine will simply act on it and add a break:)

That's an unusual floor alright, since the gravel is setting on/surrounded by earth, I would think it would perform more like a slab on grade than anything else. Because of this perimeter insulation around the outside of the stem wall would probably be in order to further isolate the under floor area from the outside earth surface

The maximum tube length has a bit to do with how much fluid you need to push thru it to deliver the required heat per hour. The faster you need to flow water to deliver heat, the greater the friction(and noise) per foot of tube, thus the need to reduce individual tube length/make more circuits...

The typical electrical water heater element is 4.5KW or good for about 15KBTU/HR The rub here is that they may not be a good choice to run at that full output for extended periods(100% duty cycle)as the elements and controls are designed for cyclic operation. A small purpose built electric boiler might be a better choice for that much sustained heat output.

24' diameter is about 425 Square Feet? Is this a permitted structure? You may have an issue attempting to use a domestic hot water heater in a space heating only application...

Yurt's are interesting, but not a lot of insulation. I don't think I would build one, other than a tiny one as a cabin or day use shelter, and then only because it would probably assemble a lot like a tent. I personally would want a lot more wall/ceiling insulation in something I was going to spend any length of time in. IF you re-run your calculations with say R20 walls and ceiling, I think you will find the energy usage numbers a lot more appealing... I am also not sure how well sealed the yurt typically is. In a leaky structure air infiltration can quickly dominate the heating equation. I am pretty sure I could make a round yurt looking structure that is significantly more energy efficient...

This of course is just my personal opinion and preference, but when it comes to energy usage, I believe less is always better than more... It surely isn't going to get any less expensive to procure energy...

Good Luck with your project.

As explained in the user instructions, our Borst hydronic radiant floor heating design software uses the design methodology published in John Siegenthaler’s “Modern Hydronic Heating”. Maximum allowed 1/2” PEX circuit length is 300’. Your hydronic radiant floor emitter is indeed very different from a heat transfer perspective than any of the conventional Siegenthaler floor assembly types and software options. I suspect your floor emitter will behave more like a below floor assembly type and perhaps having a significant amount of finished floor surface R-value too. As such, the actual required water supply temp will be significantly higher than what would be needed and reported by the software for a slab-on-grade. The fact that the software is calling for a 6” PEX spacing is indicative of how high a heat flux (BTU/hour/square feet) is required to satisfy your high building heat load. I suspect that the floor surface temp required to achieve this high heat flux may be well above the comfort level of your feet. I fully agree with Ronmar's guidance.

ronmar:
Thanks for the input. The 15kbtu/hr is for a 20 degree outside and 70 degree inside temp. However if I raise the outside temp to 40 degree, the heat loss is 9800 btu/hr. Which I think is probably when this heat source will be used the most. On colder days, I fully expect to use the wood stove. Especially as wood heat is ~2.5 times cheaper then electricity.

That being said, my main goal for the floor is too help even out the highs and lows from night until morning, especially when the daytime highs are too warm to start a fire. So maybe a standard water heater will work just fine

This is unpermitted and I totally agree on the disadvantages of the yurt. The biggest benefit for us were we don't need to acquire permits and we can sell it when we are done using it (~5 years).

sailawayrb:
So the floor temp needed for 20 degree outside and 70 degree inside was 86.5. Slightly higher then the recommended max of 85. When I use 40 and 70 temps, the spacing goes to 12" with floor temp of 80. I suspect this will be the most common use case for me. I think I will go with 9" spacing to have a bit more heat, fully knowing that I probably won't be able to use the floor as my only heat source on the coldest days.

That’s a reasonable approach. 90F is the hottest recommend temp for short duration barefoot contact (e.g., bathrooms) and 85F is the hottest recommended temp for long duration barefoot contact (e.g., living rooms). If you are using the slab-on-grade floor assembly type, the 80F and 86.5F floor surface temps output by the software are likely significantly lower than will actually be required by your actual unconventional floor as I discussed previously. The software does have an input parameter called Supplemental Heat Gain (BTU/Day) that you could use to account for your other heat source. Doing so will reduce the required heat flux and the associated recommended tube spacing.

SO where did you come up with this Clay, Sand and straw floor composition? Have you seen anything on a U factor/R value for that composition? That might help refine it's performance as a radiator. Are you covering it with anything?

this floor is used by many natural builders and in 3rd world countries. It will not be covered with anything. I haven't seen any r-factor, but from my experience there isn't much/any insulative value. It is used as thermal mass.

Well yea, it dosn't have much resistance, but it probably has more resistance than Concrete.

What is the mix ratio of the materials? I looked up some conductivities over on engineering toolbox and dry clay and sand were about the same, so if there is not a large ammount of straw in the mix, it may only be 1/3 to 1/5 as conductive as medium density concrete. The thermal conductivity will play a role in how the floor will respond as a radiator...

straw is less then 10%. So it seems it will take longer to heat up, but also hold onto heat longer then concrete?

I don't understand why you just wouldn't use a concrete slab and place a couple inches of EPS insulation below it? You want all the heat you create to go directly into the living space. Hydronic floor heat loss is directly proportional to the ratio of the R-value above/below the tube. So you want practically zero R-value above the tube and you want a lot of R-value below the tube.

If I recall his description there is an R10 inslation layer below the clay floor.

The floor being less conductive means that you will need a higher water temp and closer tube spacing to overcome this resistance to heat transfer in order to get the heat into the space.

Heat storage is proportional to mass. Clay and sand being lighter than Concrete, it's ability to store heat would be less. You are also not talking about a very thick layer of material. How much gravel goes between the foam and clay? That gravel would also store heat, but it would also serve to suck heat down away from the floor/living space as that mass "charges" with heat...

Like Sailawayrb says, you want low resistance on top and more restance below to direct the heat upwards. You can see an example of this using the Borst radiant floor calculator. Run a calculation with an above floor system(lowest R between heat and living space) and an underfloor system(highest R) and note the differences between reccomended water temp and tube spacing, Also note the heat lost downward.

Everything is a compromise, and there may be some compelling reasons to go with this type floor in this situation. Since he mentioned using it for 5 years then being able to sell it, this type of construction would be pretty easy to reverse and restore the build site to original condition. I agree CC would be a better radiator and more thermally(and perhaps structurally) stable than 1.5" of clay.

Rodney, Along the lines of how much gravel were you planning between the insulation layer and the clay layer? Too thin a layer and you may have trouble getting a good compaction on the gravel on top of the foam... Since it will draw heat away from the room when you are calling for heat in the room, I don't know that I would even use it, and just put the clay floor right on top of the foam layer...