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# Mathematicians and engineers

Member Posts: 2,651
I was looking at the recent "Snowmelt" thread and it got me thinking (too much, I think?) about glycol mixtures and below grade temps with different variables. So, imagine this scenario...

Lows of -10 degrees F for a period of 3-4 days, daytime highs of 7-8 F degrees.

1/2" pex with a concrete pour of 4".

2" of XPS with an R-10 below the tubing.

After the 3rd or 4th day, how would you approximate the temperature at the tubing level?

Is it as simple as its going to get really close to the -10 of the ambient air?

Author - Hard Knocks: My Life Inside Boiler Rooms
PHC News Columnist
Minnich Hydronic Consulting & Design, LLC
https://heatinghelp.com/find-a-contractor/detail/minnich-hydronic-consulting-and-design

• Member Posts: 6,365
edited August 2015
My off of the cuff answer:
I'm gonna figure for -10*. With R10 under the slab, it's gonna be near ambient air temp. And, you're probably only talking the difference between a 30% or a 35% antifreeze mixture.

Sorry, I know you wanted a more technical answer, but why complicate things when it's not necessary.
Bob Boan
You can choose to do what you want, but you cannot choose the consequences.
• Member Posts: 14,784
My thoughts, being neither of the above

We don't make things cold, we remove heat. So I think you would need to know a starting point temperature of the slab to get an accurate number.

Wind across a slab, like a swimming pool also changes the heat transfer.

As for glycol mix, if you will not be melting in sub zero conditions the fluid can be allowed to form ice crystals or even slush and still offer burst protection. Going overboard on % of glycol starts to take a hit on heat transfer and pumping requirements, a bit of a balancing act.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 2,651
Yeah, I've always been in the neighborhood of a 50/50 mix for snowmelt. And, I seldom ever use glycol in garages that are attached. Some may strongly disagree with that approach but I've never, ever, had any issues. I educate my customers and if someone is spending money to add radiant heat to garages then they should certainly take advantage of it...even if its to maintain 45* or so.

I was working in a single car, uninsulated, detached garage this past year when the ambient temps were near or below 0 much of the time for an entire week. The lowest I saw the thermometer in the garage was 29* and on some days I was there until early evening.

The reason I bring this up is someone is questioning (after the fact) why I didn't use glycol in two attached garages. Beforehand, I clearly stated I wouldn't be and I stick to my guns that its unnecessary.

If and when there's no heat, call.

If you go on vacation in the dead of winter, have someone check your home
Author - Hard Knocks: My Life Inside Boiler Rooms
PHC News Columnist
Minnich Hydronic Consulting & Design, LLC
https://heatinghelp.com/find-a-contractor/detail/minnich-hydronic-consulting-and-design
• Member Posts: 14,784
50-50 always seemed a bit high for most snowmelt conditions.
With Dowfrost HD, 35% gives you burst protection to -30°F. that should cover most anything your area could throw at a system.

If you want to be pumping and melting at -30F, you would want a 53% mix.

If you want to pump and melt at 0°, a 38% mix would work.

And why the heck are you working in a garage at 29°!?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 2,651
It was a customer's garage. He gave me the back wall of it for my boiler and near boiler piping for his snowmelt system.

I'm different! I'd rather work in 29* than 92* any day.
Author - Hard Knocks: My Life Inside Boiler Rooms
PHC News Columnist
Minnich Hydronic Consulting & Design, LLC
https://heatinghelp.com/find-a-contractor/detail/minnich-hydronic-consulting-and-design
• Member Posts: 5,189
I'm different! I'd rather work in 29* than 92* any day
I'm with you, 29 is fine as long as your working. I remember unloading oil drums from a CH-47 on a moutaintop in Korea when it was 20 below - now that was cold.

Bob
Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
Vaporstat with a 12oz cut-out and 4oz cut-in
3PSI gauge
• Member Posts: 1,618
I used to be paid to do such calculations.An important factor is air velocity at ground.Still air transfers one BTU per square foot per degree difference per hour.Moving air can increase that factor ten fold.At least those are the numbers rattling away in my head.There's also the question of the temperature of the fluid in the tube.
• Member Posts: 5,837
Interesting conversation. I'd like to take it the other direction. If I am "cooling" the sidewalk, how much "FREE" ambient energy can I harvest off the slab? Let's say for example I have a sidewalk that is 4 foot wide and 60 feet long. Here in Denver, the "average" temperature is probably say 40 degrees F.

Knowing the K value of concrete, and the continuous exposure to an average of 40 degrees ambient, times 1 hour, times 24 hours per day, times 365 days per year, how much FREE energy (excluding direct solar gain potentials" can I "harvest" off of this slab and use for preheating my DHW?

The free energy would be harvested using a refrigeratn based extraction system with COP's that would be no less than 3 to 1 and possibly as high as 5 to 1.

Thinking outside the normal box here...

Run the numbers.

ME
It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.