Snow melt system design

JimGPE

in Pittsburgh. I am addressing a plaza area that is a U-shape, about 10,000 sqare feet total. I will be using heated ethylene glycol solution. The plaza will be poured concrete, stamped with a brick or paver pattern.

(I talked the architect out of poured concrete with tubing in it, topped with a sand bed and true pavers. That sounded like a lot of insulation between the tubing and the snow. That's a good thing, right?)

The building is going to be hydronic heat, and we were thinking of using the hot water (200F design) to heat the building, then taking the return water (160F) through a heat exchanger to heat the glycol, then back to some high delta-T boilers.

Over the months I've been monitoring the wall, I've sorta gotten the idea that perhaps vendors are not the right place to go for this design, but where is? Who is the expert in this area?

Any "Dan-books" cover this topic?

Thanks.

Wethead7

snow melt

We need more information. Like snow fall per hour,tempature of snow,snow density{the wieght of one cubicfoot of snow},
slab thickness, how cold of a start, and where in theslab is the tubing planned. Also equiment in the building that may be used. Oh yes and what is the planned use of the area.


Mike

JimGPE

Holy cows!!!

That was fast!

Okay, here we go.

Don't know any of the data re. the snow. Where would I find this info? Never seen it in ASHRAE....

As far as the slab design, since this is primarilly a heating system, the architect would let me control slab thickness, where in the slab the tubes go, etc. for the most part, just so long as it was strong enough for the foot traffic. This is a pedestrian plaza only, nothing heavier than a Pittsburgh Steeler lineman or a four-wheeler with some stock for a retail shop - no trucks, no cars. In the winter, this area will be used for walking between the parking garage in the center of the "U" shape to the office/retail building that surrounds it. No one but desparate smokers will linger - this is not a comfort heating application by any means.

I planned to start the system at the first sign of snow (they make some pretty good temperature/moisture integrated sensors for this), so on startup the slab will be below freezing, but it is unusual to have air temperatures below 5F here, especially in the City. Slab temp might be 20, worst case, I'd guess.

I'd appreciate any help I could get in the design, but I'd also like to read up on it myself if there is any good literature out there.

Thanks!!!

Nick_3

"I will be using heated ethylene glycol solution."

Highly toxic. May not be legal. Propylene glycol is usually used.

Duncan

Yup

Yup, tubing in concrete is a good thing for faster response time.

Keep your runs short, 200 feet or less if possible, freezing concrete sucks the heat right out of tubing runs.

For the same reason, counterflow spirals (alternating supplies with returns) rather than serpentine runs is a good idea. One long run, and the tubing could be cold by the time the glycol reaches the end.

For the same reason, larger diameter tubing and high flow rate is a good idea. I like 5/8" diameter on 9" centers minimum.

Ten thousand square feet at 100 btus per square foot means you're looking at a boiler in the one million btu/hr OUTPUT range. If you need 100 btu/sq.ft., and that's not a bad number, especially for cold start. I'd look at a low mass, high fire / low fire at a minimum... condensing would be a lot better.

Does your building boiler really have a million btu/hour margin to spare for snowmelt? Consider a separate boiler!

Hey, these are just general ideas, a lot depends on your specific situation. You just need to know that snowmelt is a different animal, it REALLY cools down the return temps. Vendors can design OK if they know what they're doing. Unfortunately, some don't! Read PAH's post.

John@WattsRadiant

Snowmelt

Any vendor worth their salt will have a computer software program that can specifically model your snowmelt (based on exact thickness of specific materials, snowfall intensity, wind speed, outside design temp, etc). I don't have my computer here at the house or I'd throw some numbers at you but I'd guess the load might be closer to 125 Btu/hr/sq ft. Make sure you spend time understanding what your client wants out of their snowmelt system (ie how fast do they want to clear the slab) The thicker the slab, the slower the response. Idling the slab at a higher temp increases response time but also increase operating costs significantly. Emerson-Swan in Pittsburgh is an excellent resource - you might try calling them.
Hope this helps.
John

Art Pittaway_2

Don't forgrt the water

has to go somewhere. Either provide drains or someplace that can manage lots of water run off. Remember it melts the snow, not boil it off.
Art

JimGPE

Spare BTU's?

The building is 100,000 sqft, so we have a substantial heating load already. But the snow melt system will be a component in the boiler sizing calcs. We are designing the boiler system for this load.

Thanks!

JimGPE

Vendors

Good source of information - again if they are worth their salt. (I used to be one....)

But are there no written resources that give the background for such calcs? How do I know the vendor did it right - just trust his computer?

Thanks!

Bill Wright_2

ASHRAE ...

1999 HVAC Applications, Chapter 49, Snow Melting.

Chapter is very technical ... and hard to follow in some places.

Hydronic Institute also has the IBR S-40 Snow Melt guide -- a bit dated (looking back through my library, it doesn't appear to have changed since 1962 ... but then neither has snow!). S-40 is very simple to use and readily lends itself to homemade spreadsheets.

"Snow Melting" by Adlam is a great book ... but is long out of print (1940's to early 50's) and hard to find (I found my copy at a Goodwill). He provides a lot of experimental data w/ some pretty exotic systems (some running fluids at 600 deg F.)

Most Radiant Vendors have design guides covering Snow Melt Systems that basically break the ASHRAE information (or IBR S-40) into more digestable chunks.

Best advice is to contact the design staff of your prefered radiant vendor -- they probably have either a manual or software available for your use.

Bill W.

JimGPE

Perfect!

Thanks, Bill.

You'd think I would have caught the ASHRAE reference, but a lot of times that's the last place I look, cause the information is almost always so general it is hard to make it work for a practical application!

Terry

ONLY 100?

Must be nice to be able to use such low numbers in your area. Here is the great white loads are more like 175-225 btu's per sf.

200ft loops are pretty small considering the load. I find 40 deg delta-t's are ok with sno-melting and on that note I've used 5/8" & 3/4" pipe @ 400ft & 500ft loops respectively.

What centres do you usually use? I find 6" works best.


Regards,.
Terry

Bill Wright_2

Glad to help ...

Robert Bean @ Danfoss is another excellent resource. I think he is currently heading up the Snow Melt groups for both the RPA & ASHRAE.

As far as simple Snow Melt manuals from vendors ... I would look at Watts Radiant, Wirsbo, Rehau or RTI -- each of these companies have Snow Melt guides that are easy to follow, give reliable values and are fairly comprehensive in application details.

Also, it doesn't get much press, but the IBR S-40 guide is a very good, non-product specific manual. It also is easy to follow, generates reliable numbers (actually a little more conservative than the ASHRAE manual) and is reasonably comprehensive.

Bill W.

JimGPE

Wirsbo ADS software

Claims it has a snowmelt design module. Is this worth purchasing, or should I just have my rep run it?

John@WattsRadiant

Software

Talk to Emerson-Swan about getting some Watts Radiant software. You can also look in one of the ASHRAE Chapters, not sure which year but it will be titled Snowmelt. One of the guys that wrote that chapter helped develop the number crunching in the Watts Radiant software - Dr Birol Kilkis. Good software should let you punch in snowfall intensit, wind, outside design and snowmelt Class 1,2, o r 3.
J

Duncan_2

I'd say 100'd be a minimum

You're right, Terry. Actually, I usually START in the 150 btu/sq.ft. range, and it seems to work well. I just wanted to give Jim an idea of the size of the boiler he could be looking at.

I'm at 9,000 feet in Colorado, where it can get down to -20°F, but it rarely snows below 0°F to 10°F.

No question in my mind that six inch centers would be preferred and make a FAR better system, and I'd definitely recommend it over nine inch, and go with that if possible. I usually go nine inch for cost reasons on larger jobs (wish my customers had deeper pockets!), but I agree, it's cutting it a little close to the edge design-wise. Most customers want to leave the system off, not idling when there's no snow, and six inch centers would definitely help deliver the heat faster at turn-on time.

Speaking of delivering the heat faster, I agree with John that thicker slab needs more tubing. I've seen this with my own eyes where I unthinkingly put the same amount of tubing in a six inch thick truck driveway portion of a sidewalk, and the rest was four inch slab. Much slower initial melt in the six inch thick portion.

Also, keeping the tubing towards the top of the slab is important for fast reaction time, and bolsters, those triangle-shaped "trusses" of rewire help, but I still haven't found a way to avoid the waves that happen when walking on the mesh pushes down the parts that aren't directly supported.

JimGPE, you'd be amazed how snow melt can drag down a boiler! I'm thinking building heating could suffer when the snowmelt comes on if you sized for building heat. And if you sized for both space heat and snowmelt, the boiler could be grossly oversized (maybe around 50%?) for the space heating.

A modulating boiler with a decent turn down rate might make a double-duty boiler a decent option - but realizing that modulating boilers are not usually as efficient at lower firing rates. I'd suggest a low mass or condensing boiler dedicated to snowmelt, but it sounds like that's already been decided.

A few efficiency points might (or might not) make a substantial difference in fuel use, which becomes an important consideration in large buildings. The boiler will no doubt be used for space heating most of the time. A few percentage points in fuel use efficiency can add up to big bucks in operating costs. It might actually be cheaper for a one time snowmelt boiler cost than to pay larger ongoing fuel bills.

I like shorter loops because it means a smaller pump, but that really depends on the aspect of the snowmelt area, if it's a long stretched out driveway, or a compact square.

Edge insulation is another consideration. A couple of weekends ago, I observed a heated driveway apron that was butted against an unheated sidewalk for a good length. There was only a 3/8" asphalt fiber expansion joint between them as "insulation". Melt on one side, snow on the other - it looked like a straight edge at the expansion joint.

George Peteya

Points to Ponder

Jim -

1. Be fully aware of what the client's expectations are. This is related to ASHRAE Classes I, II, and III. Class I would be that they just don't want to plow, scrape or shovel. Class II would be a heavy duty Class I (a little snow accumulation for a while is OK). Class III: think helipad on the hospital roof. No snow, ever.

2. You can arrive at a design output direct from ASHRAE, although you would have to interpolate between two or three of their 33 cities for which they have data, if Pittsburgh isn't listed. You could also use a table that provides required output to keep a surface at 38F at varying conditions. Wirsbo has these tables in their manual, as does Flatplate, a heat exchanger vendor, in their design software. You could also use a latent heat method, multiplying the weight of 1" of snow per square foot by 144 Btu per hour, water's latent heat of fusion. I like the tables, but it's up to you.

3. I can't imagine not doing this job with fully automatic control, and maybe zoning. A control such as a tekmar 664 has two zone capability, priority control, idling, etc., and looks like the way to go on this job, but don't just take it from me. Just remember that the least expensive control is a wall switch, which is the most expensive to operate, and offers no slab or boiler protection. Spring comes and somebody remembers what that switch is for, and that it's been on all winter.

Hope this helps ...
George Peteya, PE

Tim Doran

software

Wirsbo's new Advanced Design Suites does have a very good snow melt design module or you can give me a call and I will help you design your system.(952)997-5334

Tim D

JimGPE

Thanks a LOT...

For all the advise, guys. What a great site!

I've broken open the ASHRAE guide, and it doesn't look to be any worse than what I'm used to from them, so this will be helpful. I've sent for the recommended software, and I'm going to take a poke at the design myself, using the details in the ASHRAE book.

Once I've tried my hand, I'll be back to get comments from the best and brightest, the Wallies!

Again, thanks for all the help.

JimGPE

Thank you, sir!

I'll keep that in mind for when I've got a schematic drawing I can show you.

hot_rod

Ethylene

is listed as moderate acute oral toxicity, compared to propylene based low acute oral toxicity, according to the MSD sheets.

Ethylene is generally found in large HVAC applications and has a much better heat transfer efficiency, (thiner viscousity) and better low temperature performance. Also less expensive.

Ethylene bio-degrades quicker than propylene, and is used for aircraft deicing for this reason. After 20 days or so in a biopond it goes into the public sewer system.

I agree, in some areas propylene is required by law. And systems cross connected to potable water or food processing go propylene.

hot rod