R value of Dirt

[sschuh]

There is a company in Canada that makes under slab electric heat panels. They are designed to work with off peak storage electricity and do not use any insulation under the floor. They claim to heat a large storage sump with the cheaper electricity and the heat is released from the floor when the electricity is off. Has anybody seen or used this form of heat?
Here is a link to the web site.

http://www.thermaray.com/solutions/earth.html

My question is how much energy is lost to the ground below. At some point say ten foot of dirt must have some meaningfull R value.
Steve

[Lavrans]

Re: R value of Dirt

Well, where are you?

Attributing R-value to dirt is a little difficult. What you are talking about down there is the ambient earth temperature (I'm sure there's a term for that, but I can't dredge it out of my brain right now). Ten feet of dirt has a huge amount of thermal mass, and that's what you'll be working with or fighting against. The soil temperature at about 20' is equal to the year round ambient temperature, so that will tell you what you'll be working with/against. If you want the room warmer or cooler than that, it's easier to install insulation and create a thermal mass inside that insulated envelope, if the ambient temperature is close to what you want, well, you don't need heat.

The nice thing about going down is that you are working with a more consistent temperature and aren't working against the swings of temperature found on the surface, so it's almost always more efficient to build something below the surface than above. Earth Shelters are a good example.

[sschuh]

Re: R value of Dirt

I'm in NE Minnesota right near the Canadian border. The ground temp. is about 42 degrees. The storage electricity cost is about 1/3 the regular rate. The sales picture on the web site link shows no insulation below. The power company gives me eight hours of electricity per day so it requires a huge amount of thermal storage to carry over the rest of the day. Digging up and insulating that amount of dirt is out of the question and the heater manufacturer recommends no insulation.
Steve

[SolarPowered]

Re: R value of Dirt

A concern is possible ground water. If there is moving ground water at your location, it can carry off a huge number of BTUs.

The R-value of dirt will vary greatly depending on its composition, and especially how wet it is.

In general, if you currently have an uninsulated slab, you're already bleeding off energy into the ground. Heating it will make the rate higher, but if the current rate is acceptable, you may be OK with what you'll get.

If it were me, I would want to heat a large, insulated tank of water, which is then used to supply energy to a hydronic radiant system in the slab, with proper insulation beneath the slab. You would heat the water to a relatively high temperature so that you have a lot of delta-T available to heat the slab all day long. (Heating the water to a high temperature is not inefficient, because it takes the same one BTU to raise a pound of water from 140 to 141 degrees as is does to heat it from 70 to 71.) And by having the higher temperature to work with, you don't need anywhere near as much buffer mass as you would if you're trying to heat the slab and the ground to a temperature that will keep you at an even temperature all day long. In other words, you have a lot more control, and you're not pouring as much wasted energy into the ground.

(Edit: Or better than a slab, would be something like WarmBoard (www.warmboard.com) on top of an insulated crawlspace or a basement.)

[BigLou80]

Re: R value of Dirt

SP,
what you said is only true for electric heat not so for combustion heat. I know this thread is about electric, but your not specific in your post. i do agree 100% that heating a large tank of water for storage is a great way to provide lots of heat at the lowest cost.

Lou

[SolarPowered]

Re: R value of Dirt

Lou,

Thank you for pointing that out. A condensing boiler of course is more efficient at lower output temperatures; an electric boiler doesn't care. For the sake of readers other than the OP, who may have different situations, I should have pointed that out.

[cblock]

Re: R value of Dirt

If I were you, I'd find a system that recomends insulation. If you are not quite in Candada, then go with a not quite Canadian company.

I've seen a number of these installed with 1 1/2" or 2' of foam, then heat (elec. or hot water), 6" sand heat blanket, and last concrete. I see more water heat in this application. It is a good method, the heat will last a long time w/o much differential.

[marson]

Re: R value of Dirt

There is a company in Canada that makes under slab electric heat panels. They are designed to work with off peak storage electricity and do not use any insulation under the floor. They claim to heat a large storage sump with the cheaper electricity and the heat is released from the floor when the electricity is off. Has anybody seen or used this form of heat?
Here is a link to the web site.

http://www.thermaray.com/solutions/earth.html

My question is how much energy is lost to the ground below. At some point say ten foot of dirt must have some meaningfull R value.
Steve

I know this type of system has been around for awhile. I saw one installed about 10 years ago. It was a system promoted by Lake Country Power in northern MN (http://www.lakecountrypower.com/user...0Heat%2008.pdf). Key here is that it is based on off peak electric, so that a heat sink is required. On the project I was working on, it was not designed as the primary heat source, but was meant to temper the basement floor to increase the comfort level. Yes, I'm sure there are other heat storage systems out there, but this one was a comparatively low cost option. I never did hear how the owners liked it in the end.

My understanding of it is that the heat that you pump into the ground under the slab really doesn't go anywhere---it doesn't convect away (dirt doesn't have currents), it doesn't really radiate to the atmosphere, though I suppose you lose some to conduction to the surrounding ground. Otherwise it just radiates back into your house. Surely it isn't as effecient as conventional hydronic slab heat, but in some areas, the low cost of off peak electric offset the inefficiencies.

[Martin Holladay]

Re: R value of Dirt

SolarPowered got it right: "In general, if you currently have an uninsulated slab, you're already bleeding off energy into the ground. Heating it will make the rate higher." I don't see any advantage to spending money to increase the temperature of the one part of the building envelope without any insulation.

Since the temperature of the earth below your house is likely to be in the 40s, it makes no sense to have an uninsulated slab. It's even worse to deliberately add heat to the weak link in your building's thermal envelope. Trust me, if you are spending money to raise the temperature of several feet of dirt, that's money wasted -- because it would have been better to raise the temperature of the interior of your home.

[Jon Blakemore]

Re: R value of Dirt

I have a related question. You often do not see basement slab insulation in our area (~4,500 HDD @ 65°). I've been told by locals that it's just not worth it.

Let's assume our ambient ground temperature if 55°, would it be correct to say that not insulating under the basement slab would be akin to not insulating exterior walls in a 3,650 HDD climate? I came up with 3,650 by multiplying the 10° Delta-T by 365 days a year, because the slab is going to be constantly losing energy to the ground.

I understand that my concept is not taking solar gains, losses from convection, etc. Still, wouldn't the conductive losses be somewhere around my 3,650 number?

[sschuh]

Re: R value of Dirt

Thanks for the replies. Marson has it like the providers say. My system was purchased through a power cooperative pushing the off peak electric. It runs a little over 3 cents per KWH. I have no ground water movement as my place is high on a hilll of gravel. I have had the system for 7 years and I do like it and it seem to be fairly cheap to run. It was very inexpensive to install, takes up no room and has no moving parts except the thermostat. I'm just wondering if there is a way to visualize the amount of loss to the earth. It seem that with no currents that the loss should stabilize. Thus my question on the R value of dirt. Sort of like the R value of concrete being low for 1 foot, but if you had 20 ft. of concrete to pass then the R value would be much higher. I thank you all for you opinions.

[Lavrans]

Re: R value of Dirt

It does look to me like a system designed to generate income for that power co-operative, though. PGE (Puget General Electric) used to have a very low cost heating system they would install that was basically a big heating element they would bed into the plaster in your ceiling. The heat was extremely inefficient, and not all that comfortable (the floors were always cold), but they pretty much gave it away to guarantee electrical usage.

I'd bet your system would work better & be more efficiently if it was insulated, but would be more expensive to install, and might not provide as much return to the power company.

JMHO.

[Riversong]

Re: R value of Dirt

if you are spending money to raise the temperature of several feet of dirt, that's money wasted

Oh come now, Marson explained the value of this system: "I saw one installed about 10 years ago. It was a system promoted by Lake Country Power in northern MN".

It's a way for the electric company to sell off their off-peak power.

Interesting that the graphic on the website details heating the ground under a full basement, but the text suggests using this system under a slab where the heat will conduct immediately to the surface.

Now, if you wanted to use this system for a shallow, frost-protected foundation in lieu of insulating the slab, it would be as carefree as using electric heat tape to prevent ice dams ;-)

[Riversong]

Re: R value of Dirt

wouldn't the conductive losses be somewhere around my 3,650 number?

Not at all. For half the year the cooler ground would be replacing air-conditioning costs. So maybe you should compare it to a 1825 DD climate.

I think it's more useful to ask about the nutritive value of dirt. I hear it can be yummy.

[BigLou80]

Re: R value of Dirt

Riversong,
here is a better use for the power companies off peak electricity
http://en.wikipedia.org/wiki/Northfield_Mountain