Snowmelt

TomE

Recently got a snowmelt project at a fire station. 2-12' x 100' x 9" concrete slabs at the entrance and exit of the garage. I told the General I did not include the insulation, thinking it was in the concrete package. The Mechanical engineer said there wasn't a need for insulation since we were to put the tubing 3" below the surface. I know it won't work and I need some examples to explain to them why. I'm not an engineer so of course I don't know what I'm talking about! 40+ years of experience doesn't matter.

GGross

The slab insulation helps to limit downward loss. Remind the engineer that heat does not rise, heat moves to cold. The heat from the tubing will move to cold in any direction, up, down, doesn't matter. Without the slab insulation substantially more of the heat energy will move down into the ground, instead of melting snow on the surface, because there is no thermal break between the earth and the slab you are trying to heat. Every unit of heat energy that is not melting snow, is wasted dollars, while the slab insulation does not stop this entirely, it reduces it greatly. It is not even that it necessarily won't work, if it does work it will cost them (the taxpayers?) substantially more money

GGross

Here is a visual aid, 2 examples. 1 of your slabs with no insulation, and 1 with 2" foamboard. Loads are based on Uponor load method, the default location St. Paul, location doesn't really matter for this purpose is to show the difference for design. 35 F slab temp Everything about these two slabs is identical except for the insulation. Look at how the pump spec changes, and note that the supply temp isn't valid for the un-insulated slab. In this example you will pay approximately 2x the cost to run the system as the insulated slab

STEVEusaPA

The engineer is a dope. Considering he/they are going against all best practices and conventional wisdom, ask him/them for proof it's not needed, and ask him to sign off on this with language that he/they accept all liability and agree to pay all costs to repair/fix the project when it doesn't work as expected.

Note: the dual pronouns is simply because it could be one engineer or their firm. It's not to comply with any ridiculous notion of insulting someone by using the wrong pronoun.

Mad Dog_2

Richie English of All Island Radiant @Allislandradiant and snowmelt has tons of experience. Hopefully he'll chime in here...Oh Richard...where art thou??? Mad Dog 

hot_rod

GGross said:

Here is a visual aid, 2 examples. 1 of your slabs with no insulation, and 1 with 2" foamboard. Loads are based on Uponor load method, the default location St. Paul, location doesn't really matter for this purpose is to show the difference for design. 35 F slab temp Everything about these two slabs is identical except for the insulation. Look at how the pump spec changes, and note that the supply temp isn't valid for the un-insulated slab. In this example you will pay approximately 2x the cost to run the system as the insulated slab

A 9" slab?! Yikes what is the application an airport runway?
That is a huge mass to warm up.

GGross

hot_rod said:

GGross said:

Here is a visual aid, 2 examples. 1 of your slabs with no insulation, and 1 with 2" foamboard. Loads are based on Uponor load method, the default location St. Paul, location doesn't really matter for this purpose is to show the difference for design. 35 F slab temp Everything about these two slabs is identical except for the insulation. Look at how the pump spec changes, and note that the supply temp isn't valid for the un-insulated slab. In this example you will pay approximately 2x the cost to run the system as the insulated slab

A 9" slab?! Yikes what is the application an airport runway?
That is a huge mass to warm up.

haha not my choice. 9" slab is what the OP cited for their application. Sounds like a fire hall so big trucks

Allislandradiant

Unless the engineer signs off on it and takes full responsibility I wouldn't do the job. You are going to be going back all the time making adjustments.

Mad Dog_2

Yep..or a Flat installation price and T & M for tweaking...Otherwise as The guys say...you're gonna lose your shirt...Mad Dog 

hot_rod

The edge loss is a big number also as it is exposed to ambient air temperature all the time.
At some point the earth under a non insulated slab warms and reduces the downward losses.
Not to say ignore underslab insulation, some of the early radiant design guides did not always show snowmelt slab insulation insulation.

A slab with that much mass, about 30 yard per slab at 9" = 120,000 lbs! and no insulation in a critical application like a fire station may need to be idled as the start up response will be very slow.

Hopefully the design is a high BTU/ sq ft? Appropiate for the class of melt as per ASHRAE

You would want a certified compaction test for that much slab and equipment weight also. Slabs fail due to improper site work often times.

It may require high load foam. Most foam board is 15- 25 psi.
Dow makes high load foam board up to 60 or 80 psi $$.
The insulation may cost as much as the slab

I doubt any engineer will sign off on operating costs, and that is where you will take the hit without insulation.

GGross

I wouldn't have thought about high load foam board. I see some by the pallet online, it's pretty pricey but considering how much this system will cost to run, and how much the install will cost anyway, I imagine it is a drop in the bucket for this job. Looks like about 2 pallets will cover the area

Mad Dog_2

Yeah..The Dow Polystyrene Board ain't cheap...Mad Dog 8

pecmsg

Send a carefully worded letter to the fire commissioners explaining that your withdrawing your bid due to the engineers stance on NO Insulation. 

Let them do their job. 

TomE

Thanks for all the comments and suggestions everyone. The 9” slab is at the entrance and exit of the drive through fire hall. I want to show this blog to the fire Cheif for several reasons. I know the correct way to do it but sometimes Engineers seem to carry more weight than 40+ years of experience. Also, I won’t install something I know won’t work. Too much reputation to preserve. This project was a public bid so we did not have a say in the design. Even the GC had a puzzled look on his face and questioned it. Just getting into submittal phase so we still have time to save it. Will let you know how it turns out.

Tim_D

Here is the design info using the ASHRAE design methodology. It calculates about 61 btu/sqft of downward loss with no insulation.
Snow Melt Design Sheet Project Name
Date
Location
Tag


Inputs Outputs
Length 100 Area,sqft 1200.00
Width 12 Btu/sqft 156.20
Design Temperature 0 Total Melting Load,btu/h 187439.81
Wind Speed, mph 5
Altitude Sea level Downward loss,btu/h/sqft 61.44
Snow Fall Per hour,inches 4 Edge Loss, btu/h/sqft 190.58
Area Free Ratio Snow Free=1
Total Load,btu/h 489863.82
Snow Melt Panel Assembly
Layer 1 Material Concrete
Layer 1 Thickness, inches 0 Supply Water Temperature 106.53
Layer 2 Material Concrete Flow,gpm 41.87
Layer 2 Thickness, inches 0 Loop Length 24.00
Tube Layer material Concrete Flow Per Loop 0.42
Tube Layer Thickness, inches 9 Pressure Loss,ft/hd 0.05
Tube Depth In Layer 3 Velosity,fps 0.38
Underslab Insulation None
Edge Insulation None Total Tube Required 2400
Slab Long Axis Orientation N S
Dominant Wind Direction N S

Tube Size 3/4" HePex
Tube Spacing 6 Model Conditions vs. Design
Number Of Loops 100 Outdoor Temperature 10
Wind Speed, mph 5
Glycol % 40 Snow Fall Per Hour, inches 6
Design Temperature Differential 25 Area Free Ratio Snow Free=1
Operation Mode Idle/On Btu/sqft Required 188.78
Film Temperature 33 Btu/sqft Available 156.20

Tim_D

Same design but with R-10 edge and underslab and the downward loss drops to 4 btu/sqft.

Snow Melt Design Sheet Project Name
Date
Location
Tag


Inputs Outputs
Length 100 Area,sqft 1200.00
Width 12 Btu/sqft 156.20
Design Temperature 0 Total Melting Load,btu/h 187439.81
Wind Speed, mph 5
Altitude Sea level Downward loss,btu/h/sqft 3.85
Snow Fall Per hour,inches 4 Edge Loss, btu/h/sqft 1.93
Area Free Ratio Snow Free=1
Total Load,btu/h 194378.87
Snow Melt Panel Assembly
Layer 1 Material Concrete
Layer 1 Thickness, inches 0 Supply Water Temperature 106.53
Layer 2 Material Concrete Flow,gpm 16.61
Layer 2 Thickness, inches 0 Loop Length 24.00
Tube Layer material Concrete Flow Per Loop 0.17
Tube Layer Thickness, inches 9 Pressure Loss,ft/hd 0.01
Tube Depth In Layer 3 Velosity,fps 0.15
Underslab Insulation 2" R-10
Edge Insulation 2" R-10 Total Tube Required 2400
Slab Long Axis Orientation N S
Dominant Wind Direction N S

Tube Size 3/4" HePex
Tube Spacing 6 Model Conditions vs. Design
Number Of Loops 100 Outdoor Temperature 10
Wind Speed, mph 5
Glycol % 40 Snow Fall Per Hour, inches 6
Design Temperature Differential 25 Area Free Ratio Snow Free=1
Operation Mode Idle/On Btu/sqft Required 188.78
Film Temperature 33 Btu/sqft Available 156.20

hot_rod

Load gies  from 194k to 489k

You can also use the software to predict operating costs if you know typical fuel costs
 I imagine the taxpayers pick up that tab.

Operating cost difference, the us the number to present to the board

TomE

Specified boiler is only 600,000 input for both slabs. Will cost a ton to idle that all winter long!

Mad Dog_2

Many thousands of $$$   Mad Dog 🐕 

hot_rod

Hopefully it will have  a snowmelt control for each pad. The control monitors slab and air temperature to run the system. It doesn’t idle all winter

In idle mode the boiler is not under full load it maintains a low temperature above freezing.


Does any of the slab face south? How much of the slab touches  the building floor? Typical annual snowfall? All of these effect the operating cost.

farmwi

I visit for the comments!
Tiny add. EPS foam is oft measure at 10% deflection. The thicker the foam the greater the potential deflection.
Assuming standard XPS at 20-25psi will be stressing the slab a bit. I have run across 100psi EPS in Alaska.
Even so, multiple 120psi tires will cause deflection especially where they park, ie, if a 2 inch foam slab, that would be about 1/4 of an inch with 100psi foam, so all the stresses are distribution via the concrete. I would guess a 20psi foam board would undergo permanent deformation after some time as the propagation of pressure under a dually set.

For stuff this heavy, i'd be googling for alternates under the load path. Even vapor protected CCA wood sleepers in the load path would be huge strength add compared to foam,(only R-value ~1), but use foam in low load locations. Maybe low thermal conductivity FRP I beams with foam in between. Some useful info maybe found in Permanent Wood Foundations prescriptions for underlayment gravel base and drainage.
Cheers.

Zman

The good news is that it will probably “work”. I see mountain resorts in climate zone 8 do this all the time. Unfortunately that’s also the bad news. They will probably need to idle the slab to avoid having to thaw the ground underneath every time they fire it. In the end the operating costs will be 3 to 4 times (or more) than they should be.

hot_rod

I've been told the load pattern spreads out as it transfers through a slab or footing.

So a tire patch of say 6X6" spreads that weight to the load across a wider foot print on the foam below.

Maybe @Jamie Hall has some engineering info on this?

ambercontractors

What about using reflective bubble wrap insulation, maybe even multiple layers of it will give insulation protection and the advantage is no flex.

Zman

Very little insulation value in double bubble….

hot_rod

Not great R value and I wonder about the longevity of the bubbles through or after the installation.
I suppose if you lose all the bubbles at some point the slab settles?

Sylvain

What about foamglas?
https://www.foamglas.com/en-us/advice-center/general-advice/compressive-strength-in-industrial-insulation
It seems foamglas is a TM
Otherwise, look for cellular glass.

ambercontractors

@zman & @hotrod,
Question, i always wondered what IS the R value of the reflective bubble wrap, also heat radiates, doesn't the reflective part of the bubble stuff help any???

PC7060

Reflective properties only work with an air gap.
Best practice I’ve seen is close cell sprayed directly on grade and slab poured directly over.

The slab would have to be reinforced to carry the projected loads. All of this should be detailed by the architect or engineer.   

Zman

ambercontractors said:

@zman & @hotrod,
Question, i always wondered what IS the R value of the reflective bubble wrap, also heat radiates, doesn't the reflective part of the bubble stuff help any???

The manufacture will quote an “effective R value”. This is nonsense the R-value of bubble wrap is <1 and serves little or no insulation purpose.

leonz

Referring back to Hot Rods comments on load patterns-

A modern six axle diesel locomotive can weigh as much as 400 tons with fuel.

The contact area of each steel wheel is the surface area of a "dime" so the entire 400 tons is placing this downward pressure on a total square area of 12 * .705" = 8.46 square inches
The locomotive weight of 400 tons divided by 12(the number of wheels) divided by 12 equals 47.28 tons on each wheel.

I would want to know if they are using a high strength sand mix concrete before I went further with this for sure as every truck will have a different weight per axle load.

It is no fun dealing with municipalities as it all goes to the low bidder-that is unless they being the municipality have the proviso to reject any and all bids to start all over again. This engineering firm was probably the low bidder for the design job.

Every fire station of any size in a cold climate could make good use of overhead steam and heated aprons and foil backed closed cell foam insulation placed over a 30 mil+ vapor barrier.

Hire your own licensed mechanical engineer for an hour to prove to them it won't work and maybe they will change the design.

jumper

Well I disagree. One has to do calculations and consider what happens in future years. Otherwise we're speculating. Perhaps that engineer did his work and has reasons for his recommendation.

I remember a project with A/C rejection heat going under a parking lot. Some years retained ground heat melted snow for entire winters. Some summers retained ground heat was so much that auxiliary air cooling had to be run.

leonz

If they are not willing to listen to actual facts
there is no point in even bidding on the job.

Suing the engineer after the cows break out of
the pasture fence and get hit by the train comes
to mind here.

hot_rod

I think the FTC got involved with some of the bubble foil claims

https://www.ftc.gov/sites/default/files/documents/cases/2009/03/090305meyercmpt.pdf

offdutytech

Document, Document and Document with RFI to engineer. Maintain a paper trail! No shame in withdrawing the bid for that portion of the work. 
Our engines are 48K GVW (750) gallons of water. The weights going to go up if it's a ladder truck or a water tender with 1000 gallons. Now couple that with a busy department going on multiple runs a day, that slab is going to get beat up! 

leonz

I used to build up the Hale and Waterous water pumps on the saulsbury rescue trucks in Tully, NY.
I enjoyed the work, the pay was lousy, and the management hoovered. I asked for one day off to be with my wife for her hysterectomy surgery and they said absolutely NOT. I did not miss a day for 3 solid months before this and for $8.50 an hour working the night shift and commuting 110+ miles 5 days a week in 1999-2000 really wore on me.
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That why I was thinking 8,000 PSI sand mix with expansion joints spaced every 6 feet and top fed steam heat speaking as a lay person. The top fed radiators are above the cabs of the equipment and would have full air flow and perhaps 2 very large dry steam to air garage heaters at each end.