Need some really smart thinkers on this one!

Harvey Ramer

I am putting a radiant system in a shop that is 46' x 60' x 14'  The tubing is imbedded in a 5" thick concrete slab. Slab is insulated R10 bottom and perimeter. The boiler will be a slantfin electric modulating will outdoor reset. The pump that services the slab will be a Taco DeltaT. The system will be on a Load Control swicth allowing the electric company to shut it down on their peak demand. This will allow the energy to be purchased at $.05 a kw/hr. Beats LP hands down at $2.50 a gal. Now the max time they can turn it off is 12hrs in a 24hr period. Typically they would do it in 2 periods; a couple hrs in the morning and a couple hrs in the evening. It will remain on over weekends and holidays.



The slab is to not only to be the heat emitter but also the thermal storage during off times. The heat load of the building is 35,000 BTU. I oversized the boiler to 50,000 BTU thinking that along with the DeltaT circ I would be able to recover the slab more quickly after an off period. If my calculations are correct; the slab will produce just shy of 33,000 BTU/H for every 1 Deg F that it drops in temp.



Does anyone have any thoughts or pointers on this one? Keep in mind that it is a shop, not a house. Typically not occupied 7-5 on weekdays.

furnacefigher15

What is

your actual question.



Are you trying to figure out the btu output of the slab once it looses its heat source? and how long it will stay warm?

Harvey Ramer

I was hoping someone had some experience

with this type of thing and would know the ins and outs. My primary concerns are, being able to ramp up the slab efectively after an off time and before the next off time occurs and also have the slab produce sufficient heat during an off time that may stretch up to 12hrs.



Yes furnacefighter15, I guess the thermal capacities and outputs of the slab during off time is what I'm trying to get a better grip on.

Gordy

Depends

Harvey,



  What kind of shop? Is it a closed shop, or one where overhead doors may be opened on a regular basis? Will the equipment be generating heat?



  Depending on the outdoor temps could be playing catch up full tilt in the off peak. Times of power shut downs could really effect things also. Is this always a variable, or is there set times?



Gordy





 

Gordy

DP

 

Mike Kusiak_2

Energy storage

If the heat load is 33K BTU/hr and the boiler is rated at 50K BTU/hr, then the minimum duty cycle would be 33K/50K = 66%



So the electrical supply could not be shut down for more than 24 hrs x .66 - 24 = 8 hrs to maintain average temperature at setpoint. This would not include the additional losses of opening doors etc.



To determine the temperature drop of the floor you would  need to calculate the mass and specific heat of the concrete. Perhaps this is how you calculated the 1 degree drop per 33K BTU emitted? So in 8 hours off time the floor would drop 8 degrees in temp or 4 degrees if off for two equal periods?

Harvey Ramer

I like the way you think!

There is the simple math that was eluding me. 35K Btu/Load  50K Btu/H boiler output.

50K devided by 35K = 70% of 24hrs giving me 7.2 hrs of off time while maintaining an average setpoint at design conditions. So the DeltaT circ should help increase the 7.2 hr margin on any day that is warmer than design conditions. If that is not enough I could tamper with the ODR curve a little bit.



I did calculate the thermal mass of the slab based on the amount of BTU/H it takes to raise the temp of 1lb of concrete 1 deg/F in 1 hr. That's how I came up with 33K btu/h output from the slab for a 1 deg drop.



 I dove in to deep and missed the obvious.



Gordy, there is no significant heat from any equipment in the shop. There are 2 big overhead doors but being that it's a personal shop to tinker around in, they won't be opened and closed all the time.



I love it when the math clicks!

Thanks Mike!