Hi

I am in the planning stage for a 30X40 shop/garage. It will be slab on grade in far northern Illinois. I will want to heat the shop part, but not the garage part. Is radiant a good solution if I will not be using the shop every day and want to keep heat costs low(i.e. keep thermostat at 50 or so)? Will heat "bleed" from the heated part of the slab to the unheated part? Can I use a foam break between them? Do they make small systems for only 400 to 600sf?

Thanks for any and all assistance.

Electric boiler or domestic hot water heater (with controls). You can keep it at 50F for very little depending on insulation, windows, etc. You will have to think ahead if you want it warmer.

agreed with morgan. definitely not gas boiler territory. foam break between slabs is a nice detail if you don't want any heat in the garage, and naturally you'll need to insulate the dividing wall as well.

Why not gas boiler territory? Too small of space? I may also be conditioning the attic space above the shop if that makes a difference.

Thanks for taking time to educate me here. I hadn't thought of an electric boiler, but that would save me having to run gas to the shop. Does an electric boiler operate cheaply? I was looking on the Burnham page--they have the "Carefree" boiler. That has some serious power supply needs! The smallest is 35MBH at 220V, with a requirement for 50Amp/12Kilowatts. Eek! Would something like their "Revolution" gas unit at 62 MBH be overkill? Wouldn't it operate much more cheaply?

Thanks

you would need to post your pricing on electricity and gas to know that. but this is a very small area so presuming you are insulating appropriately for your climate it's unlikely than anything like a 'high efficiency gas boiler" would ever pay itself back.

a tank hot water heater with external heat exchange would be a fine choice. single zone slab, you could even use an on demand but that would be pricier and use more electricity. A revolution would be overall.

If you double your area, then maybe getting a small mod/con boiler would make some sense. depends on heat loads and where you are.

A electric boiler is a good choice for this application, Have a look at the Thermolec B-6, 20,000 BTU, modulates, out door reset, 1 30 amp breaker.
You are a bit small for a mod con in my opinion, electric boilers are a fairly easy install if you have the available power.
But your electric utility may be sky high, justifying a small gas boiler, then I would look at a 50,000 btu minitherm and set a primary secondary loop up. but now you are running gas pipe, flame in a shop, floor space, exhaust......
Dan

It looks like electricity here is ~$.09-.10/kWh. Last gas bill was ~$.70/therm. The space will be extremely well sealed and insulated. This is in Northern Illinois.

Thanks for all the responses. Electric had never even crossed my mind. It may be a great alternative.

well, looks like electricity will cost 4x as much as gas for you, absent time of use metering or other special deals.

final question is what that equals. In northern IL, estimating 15 BTUs/sq ft for very well insulated, 6500 degree days, 70 differential between room and outdoor under design conditions... I'd estimate something like $150 in gas/year. not a prediction, but a decent ballpark. so you could spend an extra $300 to 500/year going electric. if my predictions are close. this is why heat load calcs are good and should always be done.

I'd say payback on a mod/con boiler at that rate would be very long. but on a tank heater with external heat exchanger... not nearly as long.

double your area and you can approximately double all of these numbers. then a mod/con might make more sense.

My little calculator says NG @80% efficiency .70 therm 1 million btu cost $8.75
My little calculator says electricity @100% efficiency .09 KW 1 million btu cost $26.38
Dan

Posted By NRT.Rob on 16 Apr 2010 10:24 AM
well, looks like electricity will cost 4x as much as gas for you, absent time of use metering or other special deals.

final question is what that equals. In northern IL, estimating 15 BTUs/sq ft for very well insulated, 6500 degree days, 70 differential between room and outdoor under design conditions... I'd estimate something like $150 in gas/year. not a prediction, but a decent ballpark. so you could spend an extra $300 to 500/year going electric. if my predictions are close. this is why heat load calcs are good and should always be done.

I'd say payback on a mod/con boiler at that rate would be very long. but on a tank heater with external heat exchanger... not nearly as long.

double your area and you can approximately double all of these numbers. then a mod/con might make more sense.

I suspect we're overestimating both the the peak load and the heating season.  If he's heating it only to 50F, it'll be a somewhat smaller delta-T than that on design day, and a smaller heating season than that.  (IIRC N. IL design day temps are in single negative digits- don't have it right in front of me.  I've personally experienced -20F there, but that was setting record lows for those calendar days.)  The seasonal low for this past winter in Rockford was only -7F. The seasonal low for Madison WI a bit north of there was -10F.  Design day deltas are probably ~55F, rather than 70F.

Rockford IL has a ~7000HDD climate using base-65F, but at base-50F it's about 3500HDD.  (Adding up the average monthly temps in months where it averages below 50F, multiplying by the number of days...)

Deep subsoil temps in northern IL are  ~55F- a lot "free" heat could be applied by insulating to at least R20 clear wall R values, minimal glazing (placed with passive solar in mind), and R10+ foam down to at least the frost line on the perimeter & footings then not insulating the slab, using earth coupling to advantage.   Potentially that would bring the heating load base for the building down to ~40F, for an even smaller active heating season.

If the goal is to use 50F as a setback temp, temporarily boosting it to 65F+ during times when occupied, using something other than the slab for radiation would be a better bet, since you'd need to heat quite a bit of thermal mass to get the room-air temp up.

A mod-con boiler/radiant slab solution would not be net-present-value positive vs. an electric boiler + slab solution during it's anticipated lifetime, but an 80-82% efficient forced-draft electronic-ignition cheap gas burner would, be it a hot water heater + radiators/baseboards, or a hot air furnace/space-heater.

good point, I forgot about the lower typical indoor temps.

Dana:

Interesting. This will be in the woods, so no passive gain(huge conifers to the south). I will keep windows to a minimum. My house is insulated with closed cell foam in the walls, and mountains of cellulose in the attic. I sealed it as best as humanly possible. I can do the same for the shop. When you say "R10+ foam down to at least the frost line on the perimeter & footings", do you mean R10 foam on the sheathing, carry that down the outside of the 3'-4' foundation wall AND the footing? Am I reading that wrong? This will be a brick clad building to match the house. I would like to use foam on the outside of the sheathing(actually was looking at http://www.cavclear.com/index.htm that product just now, drainage plane plus foam), but I never quite got how the windows would install with 2 or 4" of foam beyond the plywood sheathing. The house windows had flanges that I could seal to the plywood, then flash over that. What do you do in the case of board exterior?

...and where was this forum when I built my house? DOH! This is a great place.

Posted By Fatawan on 16 Apr 2010 04:43 PM
Dana:

Interesting. This will be in the woods, so no passive gain(huge conifers to the south). I will keep windows to a minimum. My house is insulated with closed cell foam in the walls, and mountains of cellulose in the attic. I sealed it as best as humanly possible. I can do the same for the shop. When you say "R10+ foam down to at least the frost line on the perimeter & footings", do you mean R10 foam on the sheathing, carry that down the outside of the 3'-4' foundation wall AND the footing? Am I reading that wrong? This will be a brick clad building to match the house. I would like to use foam on the outside of the sheathing(actually was looking at http://www.cavclear.com/index.htm that product just now, drainage plane plus foam), but I never quite got how the windows would install with 2 or 4" of foam beyond the plywood sheathing. The house windows had flanges that I could seal to the plywood, then flash over that. What do you do in the case of board exterior?

...and where was this forum when I built my house? DOH! This is a great place.

Yes, below grade at least down to the bottom of the footing, on outside of the foundation wall for the brick, with at least R10 foam, more if you're going higher-R on the walls.   The idea is to isolate the thermal mass of the soil that's directly beneath the building from the soil exposed to the exterior. 

The thermal mass & R-value of the soil (which varies a lot with soil type & moisture content) moderates the temps of the upper layers of soil to the daily/weekly/monthly/seasonal/annual outdoor temp averages as you go deeper & deeper.  By insulating the foundation from the seasonally-cool soil, the soil under the slab will stay between the weekly/monthly room average temp and the deep subsoil temp.  Both EPS and XPS can work- you might consider using insulating concrete forms for both the footing & foundation wall, which will automatically give you what you want. (If you go that route- leave the both sides of the EPS forms in place- the extra R is worth more than the additional thermal mass of the concrete for your purposes.)  Since the deep subsoil temp is close to, even above your minimum temperature simply earth-coupling the slab by isolating the soil below from the brunt of the seasonal swings allows you to make use of the earths stored heat.  This doesn't work nearly as well in permafrost country, as you might imagine, but it works even better in much of the southern plains.

Exactly so Dana.

I even insulate and radiate the perimeter for larger buildings. No ModCon is a bummer but I admit it isn't in the cards. If he needs domestic hot water I might still use a condensing water heater and radiate a tool room or office for kicks.

When you say you won't use it every day and you want to put the thermostat at 50 do you mean:
1. I plan to put the thermostat at 50 only when I'm using it and will use no heat when I'm not using it.

or

2. I plan on keeping the thermostat at 50 all the time whether I'm using it or not.

If #1 then a concrete radiant slab is probably not a good choice as it will take quite a while to warm up. Planning ahead and starting the heat early (and shutting it off early) may mitigate the lag problem.

Given size, temperature, and usage I'm having a hard time seeing this a a good application for slab radiant.
What am I missing?
How about a small gas radiant?

Posted By gregj on 19 Apr 2010 11:41 AM
When you say you won't use it every day and you want to put the thermostat at 50 do you mean:
1. I plan to put the thermostat at 50 only when I'm using it and will use no heat when I'm not using it.

or

2. I plan on keeping the thermostat at 50 all the time whether I'm using it or not.

If #1 then a concrete radiant slab is probably not a good choice as it will take quite a while to warm up. Planning ahead and starting the heat early (and shutting it off early) may mitigate the lag problem.

I plan to keep it at 50 and bump it up when using it.

It will take quite a while to bump up from 50. How long I'm not sure. You'd likely need to bump up the setting the night before or maybe in the morning if you are planning to use it that evening.

If you aren't painting or doing something that requires warmer temps you might find 50 to be pretty comfortable in the shop. I don't heat mine at all unless I'm working out there. I have an old electric resistance house furnace in my shop and heat it up to 50-55 when I'm working and its comfortable once those frozen handtools warm up. The electric furnace costs more to run but it cost very little to install and I don't really use it much. If I was going to keep it at 50 all the time I'd likely explore a Natural Gas heat of some sort.

Posted By Fatawan on 21 Apr 2010 12:57 PM

Posted By gregj on 19 Apr 2010 11:41 AM
When you say you won't use it every day and you want to put the thermostat at 50 do you mean:
1. I plan to put the thermostat at 50 only when I'm using it and will use no heat when I'm not using it.

or

2. I plan on keeping the thermostat at 50 all the time whether I'm using it or not.

If #1 then a concrete radiant slab is probably not a good choice as it will take quite a while to warm up. Planning ahead and starting the heat early (and shutting it off early) may mitigate the lag problem.

I plan to keep it at 50 and bump it up when using it.

Then radiant slab is absolutely NOT the sort of heating system that will be most fuel-efficient or convenient to use.

Low mass radiation is key to making quick-warmup.  Fin-tube baseboards & forced-hot-air systems have a fraction of the thermal mass of high-mass radiators, and a TINY fraction of the thermal mass of a radiant slab. If you try to pump enough heat into the slab to get the room temp up quickly you'll still end up taking a lot of time, and you'll be pretty much guaranteed to have an uncomfortable overshoot in the room temp.  And when you leave for the  day/week/whatever you'll be abandoning significantly more heat in the radiation. With very intermittent use the abandoned heat could be a significant fraction of the total.

Sizing both the burner & low-mass radiation output for ~2-3x the design load for the indoor temp you'd be using it will make the ramp-up from 50F reasonable without totally killing the efficiency of the system.   The heating system's efficiency for maintaining 50F will be lower, but earth-coupling at your subsoil temps will make it's duty cycle at 50F so low that the efficiency hit never adds up to much in the way of operating cost.  You're still talking a couple of hours, not 20 minutes to go from 50F to 68F when it's truly frigid outside, with a real heat loss out of the building envelope.  But ramp-up times at more moderate early spring/late-fall outdoor temps will be quick, even if it's been coasting at 50F, not higher.

I'm with geotek;

I have been designing and installing an increasing number of infrared gas heaters, mounted on the ceiling of garages, both commercial and a whole new line of two stage residential models. When the slab has been poured or the usage irregular, infrared is the next best thing.

Convectors are for people who can't afford to be comfortable.