old slab heating system

Greg Swob

I have a customer with a 1950 slab home- 3,100 sq. ft. This home is reminiscent of a Frank Lloyd Wright design- flat roof, little insulation, lots of glass, no sub-slab or perimietr insulation, etc. Also comes with gas consumption of 500-700 therms on an average winter month. I have not completed heat loss calcs yet, but am just starting my forensics as to how to help this homeowner out. Unfortunately, no blueprint plans exist so I have no idea what size or type of tubing, it's spacing, zone length, etc. The home has had window upgrades and carpeted areas now have low R value padding. Interesting owner- older, retired doctor. He found a sub-slab leak some years ago by crawling around on his knees with his stethoscope! He loves the comfort, but not the fuel bills.

The present boiler, replaced in the 80's, has 300MBtu input. This leads to say that the slab is being handed maybe 250MBtu, (unlikely this system could operate at 80% eff.) That computes to about 81 Btu/sq. ft. of slab radiant emission. Seems high to me. The floor surface is very warm, in excess of 95*F. There is currently an outdoor reset to help out.

Once the heat loss calcs are done I will have a better handle on the situation. What would be a better Btu/sq. ft. radiation figure to end with? I know 35 is the general recommended. That means my heat loss cannot exceed 108MBtu and I doubt it will come out that low.

Any direction/advice will be appreciated to get me on track. I believe adding perimeter insulation would be a great benefit. My ultimate plan is to install a condensing boiler, reduce the water outlet temps, perhaps have constant circ. I just wish I had more data on the system design. Thanks- Greg

hr

On the right track

the perimeter insulation is critical. You NEED to be able to add this before you consider undertaking this slab heat redo, in my opinion.

In this case the construction heavily dictates the fuel consumption. Need to tighten up! It's not always simple in those FLW flat roofed, slab homes!

Cellotex was a common insulation for perimeter and under slabs in those days. Terrible when wet, and a haven for insects, from my experience.

I have done several thin slab overpours on these vintage homes. Door heights and thresholds get tricky, sometimes. Make sure you look it over carefully and discuss the pros and cons with the owner. Good luck

hot rod

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Mark Eatherton1

IR Survey...

Greg, Contact your local fire department and see if you can "borrow" one of their infrared helmets. Walk into a heated room with one of those on and you will know immediately what centers your's dealing with.

Unfortunately, my experience with the older homes like this is that they used a wider center, and a higher approach temperaturem Like 18" OC and 150 degrees F water. Your mileage may vary...

Slab edge, and any insualtion will help.

ME

Greg Swob

Geeze, Mark: right between the eyes

I never even thought of that! Sometimes the answer is right in front of you! We have a camera (Infrared) that our energy guru uses during surveys along with blower door tests. He hires out for other things, also. Ie) viewing industrial machinery hot bearings, transformer stations and transmission lines for arcing situations & hot spots from invisbily damaged insulators, etc. I'm also a volunteer firefighter and we got our IR camera about 2-1/2 years ago. I may be able to unravel some important mysteries on this job afterall, if I just get my head out of my...! Reminds me of what our old town drunk used to say (I grew up in a real small town- they had a tough time replacing him when he retired) "Greg, if ya just pays attention... ya'll do OK!"

Happy New Years to you all and thanks again. Greg