I recently moved to New England, and after an 18 inch snowfall that I couldn’t get off my driveway, since I’m elderly, heart condition, and live out in the boonies where there are no neighborhood kids to hire or plowing services for homeowners, I’m been looking on the web for info about heated driveways.
I have a relatively small driveway, for a two car garage. It consists of a 2-3 foot lip of concrete from the garage and then 6-8 feet of asphalt.
There seem to be several types of heating systems for driveways – heated liquid circulates, electrically heated mats embedded, or electrically heated cables embedded. I would appreciate any thoughts about any of these as to suitability, problems, etc.
Thanks.
Replies
It takes a fabulous amount of energy to melt ice or snow. One BTU of heat will raise the temperature of a pound of water one degree F, but it takes 144 BTU to melt one pound of ice at 32F into one pound of water, and that's without raising the temperature, the water is still 32F after the ice is all melted. If you take 200 square feet as a nice round figure for your driveway, and figure that a foot of snow equals an inch of water and weighs 5 1/3 pounds, then 200 sf times 5 1/3 lb times 144 BTU/lb equals 153,600 BTU to melt a foot of snow. I'll have to leave it to somebody else to convert that to gallons of heating oil, and adjust for the efficiency of your boiler.
My off the cuff impression is that if you can afford the fuel to melt the snow on your driveway, you can probably afford to hire somebody to come shovel it. Look in the Yellow Pages. There are a number of commercial snow removal outfits in my area who do residential as well as commercial work.
I've thought of plumbing my driveway and walks and using warm water just to melt the very bottom layer of snow and ice so I don't have to fight with it when it freezes hard to the concrete. But that seems like a very high capital cost for a fairly small benefit, since I almost always get out and shovel it before the bottom layer freezes hard. And that approach wouldn't help you much anyway, if you can't shovel.
PS - The 144 BTU per pound figure came from this site. I don't particularly recommend it (it has annoying tripod.com popups), it's just the first one I got by Googling for
"latent heat of fusion" BTU
http://energyconcepts.tripod.com/energyconcepts/boilersteam.htm
We just had our first snowfall in the new house, with a steepish driveway. 12 to 14". As we were shovelling, my wife pointed out that "I" hadn't gotten anyone to plow yet. So I asked the neighbors who they got, and then flagged him down when he came by.
I cannot imagine any part of New England where there aren't people with plows; probably, you won't find them listed anywhere. Just ask around.
re btus, I think you're assuming that you have to supply the entire energy needed for the worse case phase transition from ice to water. I think this is unlikely in the average snowstorm. The electric cable systems I looked at on the web supply 45 to 50 watts per square foot. If my driveway is, roughly 300 sq. ft, that's 50x300=15 kilowatts, and the local cost of a kilowatthour is 12 cents, (also price does not increase with energy used, as in some states) so an hour should cost 15x.12=$1.80. Say it snows overnight, like ten hours, that's $18. Even if I had to run it twice that long, it would be $36. I imagine a plowing guy would charge that much, assuming he shows up vs. having the driveway clear basically right away.
p.s. Given the small size of the driveway, I'm not sure a plow could even get in there, unless there are such things as miniplows.
Katherine, your math is right, but you approach is flawed. Uncle Dunc is on the right track by starting with a known mass of snow and the energy needed to melt that . I'd refine his thoughts in my next post. If you don't "supply the entire energy needed for the (complete) phase transition from ice to water", what happens to the ice/snow? Only if it slid into the street, would your system not have to deal with it.
You are correct that a 15 kw electric system isn't much to operate per hour, but the numbers of hours hinges greatly on the amount of snow. And it "snowballs" so to speak. The longer it has to operate, the more losses there are to the environment. Ideally, you could hit it with a lot of heat faster.
At that watt density, it could keep up with lot snow, about an inch an hour (40-50% efficiency assumed) . It would be great to remove the thin bit of ice and snow left by a plow or snow. And that is worth a lot in a luxury house, hospital entrance or nursing home.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Well, you clearly don't need me to do any math for you. :)
>> I think you're assuming that you have to supply the entire energy needed for the worse case
>> phase transition from ice to water.
Unless you're counting on warm weather and sunshine the next day, you're going to have to melt all the snow. As long as it's cold and cloudy, snow evaporates very, very slowly. You can melt it slower and spend less on hardware, or you can melt it faster and spend more on hardware, with essentially the same energy cost either way, assuming the same heat source. If you need to be able to get the car out of the garage any time, then faster is your only option; you pretty much need to melt the snow as fast as it falls. So yes, under one possible set of constraints, I'm assuming that you'd have to supply all the necessary BTUs. But I recognize other constraints are possible.
>> The electric cable systems I looked at on the web supply 45 to 50 watts per square foot.
I don't know the conversion between watts and BTU right off the top of my head, so I don't know how much snow 50 watts per square foot will melt per hour. Will it be enough in the usage scenario you're thinking of?
Another question occurred to me, is there anyplace for the melt water to go? Things could get ugly if it just makes an ice dam at the foot of the driveway.
Edited 12/11/2003 5:12:31 PM ET by Uncle Dunc
Dunc: 3,413 BTU in a kwh. A very useful number. Blew away my UC Berkeley Eng Econ class when I spouted that off the top of my head (they don't know non-metric units anymore). But I'd done a lot of plumbing before returning to school.
Another useful number: 231.
231 cubic inches in a gallon
2.31 feet of water = 1 psi
231 mph is the world record wind speed at ground level (Mt Washington NH, 1930's)David Thomas Overlooking Cook Inlet in Kenai, Alaska
"You can melt it slower and spend less on hardware, or you can melt it faster and spend more on hardware"
Right on. This is critical to design and usability. $2/event or $20/event is all fine. But $2,000 to $8,000 for install can be the deal-breaker. Obviously you want to decide BEFORE the slab is poured. And before the electrical service / gas service and boiler is specified.
"Another question occurred to me, is there anyplace for the melt water to go? Things could get ugly if it just makes an ice dam at the foot of the driveway. "
You're nailing all the critical points here, maybe I should bow out of this discussion. I'd run a loop of PEX (or electrical heating cable) to a discharge point. That could be through a gutter or swale to a storm drain. Or to a drywall. For a drywall, I'd run the PEX/cable* down the drywell to a depth well below frost line. Gravel freezes deeper than fine-grained soils.
In my "snow-melt" sidewalk, I don't melt all the snow, I shovel it aside. The crown on the sidewalk drains water to the sides. So I don't use it as a total snow-melt system. Just for de-icing. A clean, dry sidewalk is a beautiful thing in the winter. At -20F, who cares - ice that cold is not slick. But we occassionally have 30-35F temps and then it's slicker than snot on (fill in the blank).
*Be very careful running electrical heating cable in free air. If it expects to dump a lot of heat to the surrounding concrete, its watts/linear foot may be too great to dissipate into the air / loose gravel. The resulting fire is no big deal - there's just snow and water and concrete around. But the repairs could set you back.David Thomas Overlooking Cook Inlet in Kenai, Alaska
These guys have done a great job of explaining MAINTENANCE costs. They've also touched upon the efficiency of different fuels.
But please don't forget the initial cost of install. Whatever method will require your present driveway and sidewalks to be torn out and replaced.
The cost of that installation needs to be amortized into your calculations when determining how much it costs to run.
Of course other factors can be similarly introduced that can have positive or negative effects. Resale considerations, the avoidance of healthcare costs associated with alternatives, etc.
don't forget that packed mound of snow at the end of the driveway, the one left when the plow the road.bobl Volo, non valeo
"the avoidance of healthcare costs associated with alternatives"
If a broken hip is at all a possibility, that decides it for me. Hospitals, internists (mostly old folks), nursing homes, etc.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Maybe Medicare will pay for it?
Uncle Dunc: Yes, that is the right heat of fusion (latent heat) for ice to water. And yes, you need 150,000 BTU to melt that 200 sf of a one foot snowfall.
But there are losses. One is warming up the slab. Say a 4" slab (insulated underneath) going from 20F to an average of 45F (not just to 32F because the concrete closest to the tubes/wires must get hotter to transfer heat to a 32F surface). And some areas will overshoot - exceed 33F - because you'll run the slab until ALL the snow is gone, overheating some areas. 1/3 cf of concrete, 150 lb/cf, Cp = 0.2, deltaT = 25F. BTU needed = 250 BTU/sf. Or 50,000 for that single snowfall event. Sadly, it is also going to be 50,000 BTUs even for a one inch snow fall - a much larger fraction. So much larger, I'd switch it off long before the snow is melted because the residual heat will do the job.
Another loss is to the environment. You're keeping a slab at 33-40F in a 15-25F air. At least for a while. And R-value of about 0.5 gives 8,000 BTU/hour for the 200 sf driveway. So the faster you get the job done, the better.
Youll also have eavporative losses and then it's 1,000 BTU/pound heat of vaporization. 100% won't run off. Some will evaporate and that is heat that you pump in. I'd add at least 30% for that. Not that 30% of the water will evaporate. Maybe 5% will. Times the 7-fold greater Hvap versus Hfusion.
Summary: 150,000 BTU to melt, 50,000 to warm the slab, 3 hours of stand-by losses for 24,000, 50,000 evaporative losses. Total: 274,000 BTU. Or 80 kwh ($6.60 for me). Or (80% boiler efficiency needs 343,000 BTU input) = 3.5 therms of natural gas ($1.26 for me) or 2.5 gallons of fuel oil ($2.00 ??)
The saving grace is the small assumed driveway size. 1,000 sf and multiple all the above numbers by 5. Still viable. I've got 8,400 sf and that shoots it down versus plowing. Much less the install cost!David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks for jumping in. I really don't know that much thermo, I just remembered that horrific cost for melting and the name to look it up with.
Uncle Dunc....Impressive, very impressive! and hats off to you to, Katherine! Extremely sharp minds here at 'Breaktime!! Many Many of them, too!!
Must be that "Brain Science" thing eh? (credit to Uncle Dunc, Wrecked Angle and the originator)
I used it at work today...oops, yesterday--its 1:38AM...and got a few laughs.
Edited 12/12/2003 2:19:10 AM ET by bob
It is apparently possible to retrofit the electrical cable systems into existing concrete and asphalt. They saw it, plunk in the cable, and use a sealant over the cable. I was not too thrilled by the sealant, having concerns about durability, esp. if the town snow plows go over them (there are no curbs), although the cable guys assure me they stand up to this. They say it is possible to top coat with asphalt, that it does not melt the cable, although they can't embed cable in a new installation of asphalt because it is too hot. This sounds somewhat contradictory to me, but if it worked it would both protect the cable and let the driveway slope slightly into a neighboring gravel parking area and the lawn.
My plan was actually to have the cable extending slightly into the actual road to try to avoid the giant pile of packed snow at end of driveway from snow plow effect. This really is the boonies, so there wouldn't be a problem with doing that as far as the town is concerned.
Thanks for the tip about how the water drains.
I'm in Rhode Island.
"If you take 200 square feet as a nice round figure for your driveway, and figure that a foot of snow equals an inch of water and weighs 5 1/3 pounds, then 200 sf times 5 1/3 lb times 144 BTU/lb equals 153,600 BTU to melt a foot of snow."
That's not so bad. Like an hour of continous operation for a typical residential boiler.
If she already has a boiler, it may be able to handle the additional load as a 'low-priority' zone. Modern condensing boilers thrive on these low return temps. Around here, operating costs for an electric resistance system would run around 3x more than for a fossil fuel boiler, so I'm not so sure about that route.
Snowmelt has huge potential, IMHO, particularly for walkways and stairs. Steep driveways/walkways also tip the cost/benefit scale. Many of the remaining buildable lots in the Boston area are the ones that were 'passed over' as undesirable due to very steep grades, so a lot more of these are going in with new construction.
It is rapidly gaining in popularity. Unfortunately, one of the driving forces is litigiousness. For a commercial establishment, one lawsuit settlement can buy the system. Obviously, labor costs for shoveling and the ability to be open for business are some other economic factors. In an urban environment, just finding a place to put the cleared snow can be a huge challenge. I think we will be seeing a lot more of this.
I have seen a snowmelt system heated by an exterior wood boiler that looked appealing. Heck, I would rather load wood than shovel anyday. Wood is free for the taking here in the suburbs. Perfectly good firewood is ground up in massive quantities down at the brush dump.
From an ecological and cost/benefit standpoint, the snowmelt system with the greatest potential is one that exchanges with the groundwater. Here in the northeast, you can pretty much drill anywhere and hit groundwater. Drill 2 wells, and you can get all of the 55 degree water you want for near free.
Edited 12/12/2003 12:13:15 PM ET by csnow
this doesn't solve any potential problems of runoff, street plow or the like ( and I can not get anywhere near the calculations & notions David Thomas, Uncle Dunc & Ms Katherine have ) but aren't we a bit awry on the whole premise of having to heat up a foot of snow / that heat source is going to be on before that snow begins to fall - right Katherine! - so unless it is an incredible dumping I can't imagine more than having to contend w/ a couple of inches
Our local ski lift uses this system under quite an expanse of pavers and I've never noticed the snow winning *** however we are known to have very wet snow
you've piqued my interest - I'm going to inquire at Crystal Mountain about their system stay warm John
"I have seen a snowmelt system heated by an exterior wood boiler that looked appealing. Heck, I would rather load wood than shovel anyday. Wood is free for the taking here in the suburbs. Perfectly good firewood is ground up in massive quantities down at the brush dump."
You, like me, are a tinkerer and amauter woodcutter. And I've got 13 acres and plenty of down wood. But in general, willingness paying the install costs of a convienence/safety item is inconsistent with the grunt work of dropping, bucking, splitting, hauling, and burning wood.
I ran a lot numbers to consider the viability for my driveway. 700 foot driveway, need 16,000,000 BTU, that would be 2,200 pounds of wood. A fricking metric ton! And a wood stove to hold it all or me out there loading it multiple times. For snow melt, I'd rather buy the $60 of natural gas. But again, what I actually do is plow or blow the driveway and snow-melt/de-ice the sidewalk only.
For this theoretical 200 sq ft case, it is only 50 pounds of wood (compared to the other fuels I listed in 37670.8 ), so a lot more doable than in my case.
But how about a combination system? Big hot water storage tank. BIG! With input from a wood stove if you are feeling frisky that day. And fuel gas if you're not. Then draw domestic hot water and snow melt heat and/or hydronic heat for the house space heating from that large tank. Through a separate HX in that tank for the snow-melt glycol, of course. HX also for DHW, RFH, and hydronic if required by local code.David Thomas Overlooking Cook Inlet in Kenai, Alaska
"Big hot water storage tank. BIG! With input from a wood stove if you are feeling frisky that day. And fuel gas if you're not. Then draw domestic hot water and snow melt heat and/or hydronic heat for the house space heating from that large tank. Through a separate HX in that tank for the snow-melt glycol, of course. HX also for DHW, RFH, and hydronic if required by local code."
David,
This is viable. Went to a heating seminar recently where they showed the 'big tank' method as a popular system in Germany.
Eliminates much of the complexity involved in designing conventional primary/secondary loops that behave nicely with each other. Basically, the 'big tank' was the primary loop, but the large mass meant that the secondaries had very little impact on the primary, or each other. I think we will be seeing more of this.
Your idea of using multiple heat sources to feed the 'big tank' would be a clever twist on the concept, since the heat sources would not step on each other either...
Can't remember which actor had the coils put down around his entire estate in Colorado I think....or some snowy mtn region.
Seemed to me to be extreme and beyond costly.....I just dont get that.
Buy a self propelled snow plow and a handyperson
Be in Miami
andy
My life is my practice!
http://CLIFFORDRENOVATIONS.COM
We have a heated drive in our main drive here at work and on the sidewalks. Works great! I can't say how much it costs to run, but I think I.M. Pei originally designed heated sidewalks for the building, but those installed in the 60s never did work very well. (I work in an I.M. Pei building). Last summer they put in a new system of hot water heat under the drive.
One of my internet friends, Brad, just posted about his system on a log home forum. He is in Maine. Here is the link.
http://www.lhoti.com/board/forumdisplay.php?s=&forumid=3
It is about the 4th discussion down.
Email Brad, he's very helpful and will respond to your emails. He'll tell you exactly how his system works, and what it costs to run it. Then he'll probably invite you and your family over for hot chocolate so you can see it for yourself! LOL! He's a very friendly and helpful guy.
The other option is a Bobcat or skid steer to clear your drive. Might be cheaper than a radiant heat system and who wouldn't want a thing like that around!
Katherine
what part of New England? I live in S.E. CT, Minutes from the casino's. I know a few who could help if you live around this area .... Reliable with blowers, mini loaders,etc.
Edited 12/12/2003 1:42:04 AM ET by bob