Efficiency of Radiant floors

Wayco Wayne_2

with a job where I was asked to design a radiant floor. It is a church building being built on a concrete slab and will not be occupied consistently. My idea was to keep a certain level of heat in the floor (to keep the concrete from being a heat sucking monster that defeats comfort), and then have a secondary heat source for any fast recovery when needed. The radiant floor would also be on a timer for activities that are routine. Questions have come up as to the efficiency of the system. Some say it would not be very efficient moving the large thermal mass up and down. How would one figure this. Is efficiency lost when comfort is gained? How do you figure out operating cost of such a system. WW

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S Ebels

You're on the right track

I've recommended and done the same thing you are suggesting to them. Keep the slab at a maintenace temp and use rads/BB/hydroair/whatever to provide the second stage. It's always more efficient to maintain a slab at a constant temp than it is to drag it up and let it coast down. Along with that is the issue of overshoot when a building such as a church becomes occupied. People usually ask how long does it take to get up to temp but they never think of what happens when the heat load changes drastically downward. That building can't unload the BTU's from the slab fast enough when everything is up to temp and you suddenly add a hundred or a few hundred people at 170 btu's each. It get's real ugly, real fast.

Go with what your "gut" is telling you and two stage it somehow. Ditch the timer idea and just let the slab run at about 85% of the load. Let the second stage run on the setback and pick up the rest when needed. When the building becomes occupied, the second stage will shut down and everyone will be happy. In your localle, you need A/C too, so I'd bet hydroair with an evap coil installed besides would be the answer.

S Ebels

PS

Suggest some panel rads or something like that on an additional zone for the church offices. (if they're in the floor paln) When the pastor or secretary is there, they can turn up just one room instead of the whole building.

Weezbo

designe in hrv...

go with some means of fresh outside air and removal of over loading due to occupancy. let the warmth in the curch be bought in by the people and the comfort bought in by you.

Wayco Wayne_2

Nice line Weezbo

I just may steal that. I'll change it a little though in case you have a patent on it. (You bring the potatoe salad, I'll bring the comfort.) How do you figure the cost of operating a floor. There is a concerned member who works with people who are scientists who are saying that to warm the slab up would cost 51 times that of another system. My practical experience says otherwise but I don't know how to translate that into numbers to express that knowledge. Any suggestions? WW

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S Ebels

Remind the so-called \"scientists\"

That the slab also has 51 times the heat energy to dissipate at that point. In a constant circ, temperature reset system this occurs very few times per heating season.

Also, 51 times the energy of what?? Just what are they comparing it to?

AIR?

Well of course it does! Air is a very poor conducter of btu's. It takes very few btu's compared to a slab to heat it up but the flip side of the coin is also true. It takes the loss of very few btu's to force the system to come on again and "recharge" the air.

Another thing to point out....... Most churches have high ceilings. Radiant floor will be much more efficient than a forced air or even a BB system in that circumstance. As I'm sure you know, this is because you are not blowing 110-120* air against the area of the building that has the highest potential for heat loss.

Floyd_5

Wayne,

Yes, to load up that floor with BTU's is going to cost a bundle. Then..... after the floor is loaded the cost drops WAY down....... as to how to figure that actual costs.... I don't know either, and I think that there are many variables that will change that. Ground water content, rock slab close to heated slab, insulation, boiler selection, tubing spacing, floor covering, etc....
I do know that the 10,500 sq.ft. firehall that I started up this winter took double $$ the month that I started it, than it did the next month.....Once the slab was up to temp. that building heated for less than alot of the firefighters houses!!!!
Talk them into good insulation, good tubing, and a couple of Ultra's .... and they won't be sorry.

Floyd

Dave H_2

Dual heat dissipators

Wayco-

Done a few projects like this. To show the effiency perform a heat load calc for the radiant system but only do the design to a 60 degree indoor temp. Control wise, no thermostats but put it on outdoor reset.

Now the second part, perform your heat load calcs for the second heating system but this one only needs to heat from 60 degrees to indoor design, maybe 70 degrees. This system will be controlled by thermostats for the users and/or timers.

Add the two together and you've got your load. To show the effiency part, perform your heat load calcs based upon the entire project being heated by method two......

Dave Holdorf

Dale

Lipstick

This may sound a little off base but the church I attend is a Frank Loyd Wright design with a radiant concrete floor ( no carpet) and stone walls, lot of glass. To keep a min temp in the winter the floor gets quite warm, warm enough to melt lipstick in purses on the floor. Not that FLW would have listened to anyone even now but wall warming tubing and extra tube near the glass would have really helped, and a hydro air system to quickly add heat before occupied time. I like the idea of radiant to keep the floor at a min temp.