need advice on garage heating strategy & calcs


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Old 12-13-09, 08:38 AM
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need advice on garage heating strategy & calcs

4 years ago I built a 3 bay garage with an additional attached 20x20 workshop (1300sqft, 2x6 walls insulated). I am now getting around to putting heat into it. My original plan was to put radiant heat in the slab: one zone for the garage, a second zone for the workshop area. My goal was to just keep the garage area at perhaps 45 or 50F degrees, and the workshop at perhaps 55-60F. Then use a natural gas air heater for the workshop to bring it up to 65-70F when I use it now and then on perhaps 2 or 3 weekends a month. Note I have equipment in the shop that must stay above the dew point in order to avoid rust.

I had a professional heat load calc done which totaled to 30,000 btu/hr assuming a 5F day outside (which is unusually cold for central CT.) The design called for 105F water in the loops. I bought ox barrier PEX and laid the loops myself; the slab was poured with a 2 inch styrene insulation all the way around the periphery Ė I believe it is well-insulated. The firm from which I bought the PEX suggested I use a low-end wall mounted tankless heater Ė a Tagaki TKjr.

I am now finally ready to buy the boiler and radiant components. I hired a contractor to hook up the gas line for me, but he took one look at my garage and suggested that I need 90,000 BTU! I canít believe the calcs are off by 3x Ė can they?? Iím having him install the gas line and a direct vent wall-mounted furnace in the workshop, and then with heat in the garage I thought I would buy and install my own radiant system. But this same contractor also suggested I should skip the radiant and just use the air heater in the workshop and a second Hot Dawg ceiling mounted heater in the garage area.

So now Iím uncertain what to do. Advice? 1) can I find heat load calculation software and revisit my assumptions and total load calcs? 2) I had originally thought that my strategy for radiant slab with periodic boost from the wall-mounted unit was an efficient solution Ė comments?

Assuming I do go with radiant, Iíll be back with some addíl questions re the right boiler. Thanks.
 
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Old 12-13-09, 09:05 AM
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I wouldn't trust a contractor that just throws a number out based on eye. The calculations are sure to be more accurate than that. Every building is different.

There is a free heat loss software available from TACO that I haven't used myself. I have used to the software from HVAC-Calc that cost $50 for 2-3 months of usage. The original heat loss, was that done with the 50/60 degree inside temperature in mind?

I agree that the radiant setup you envisioned would be more efficient. With all that radiation in the floor and the desire to have low temperatures with the radiant will allow you to run very low water temperatures. The lower the water temperature, the higher the efficiency you will get from a mod/con.
 
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Old 12-13-09, 10:43 AM
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Yeah, I wouldn't trust an off the cuff remark either, especially since you paid a pro to do the heat loss.

In a garage, I definitely would want the slab warm, versus a crap load of hot dawg air at the ceiling... talk about stratification! and COLD FEET! I hate cold feet!

You mentioned perimeter band of 2" insulation, but tell me that you also have insulation UNDER the slab as well! Also, ceiling/roof should be well insulated... R40 or so.

Echoing droop's question about the calculation... was it done with the 'normal' indoor temp you have in mind? Or did they do it with say a 70 degree indoor? Does the contractor understand that you want to keep the garage only at 45-50?

When you laid the tubing, did you 'perimeter band' the area in front of the overhead doors? And I am assuming that you used insulated doors, and they are well sealed for infiltration around the edges...

more later, spousey has honeydew's...
 
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Old 12-13-09, 12:15 PM
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Your BTUs come out to about 9 kw. I'd rent a heater and confirm the design value heat loss. Fear, Uncertainty and Doubt sells a lot of stuff.
 
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Old 12-13-09, 02:15 PM
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FYI, I played with the information you gave and fudged in the missing. Using a 70 degree inside temp I came closer to 20K, a long ways from 90K. If there is no insulation under the slab as Trooper asks, it could be an issue going with the radiant. A lot of guessing, but I do it a lot.

Bud
 
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Old 12-13-09, 03:23 PM
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Bud, what'd ya do with the garage doors? just curious...
 
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Old 12-13-09, 03:53 PM
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I just used r-19 throughout, but used 10 foot ceilings, not stated. Added a couple of windows and r-38 for the ceiling. The calc doesn't deal with slabs well so just used the 2" as perimeter insulation. It's a wild guess, but a long ways from 90K. I can add details if provided.

If I adjust for doors and thermal bridging, I will move closer to the 30K but would not expect to go much over.

Now, we need ceiling insulation, what's under the slab, what are those doors, number of windows, confirm the r-19 walls, assumed all sheetrocked, and one common wall.

Bud
 

Last edited by Bud9051; 12-13-09 at 03:54 PM. Reason: correction
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Old 12-13-09, 06:09 PM
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closer to the 30K but would not expect to go much over.
Me either... what happens if you run the same numbers at 50į indoor?

huh... weird... just ran some numbers with my program and the indoor temp didn't change a durn thing... went from 50 to 90 indoors, all else equal, and NO CHANGE in the heat loss... maybe that's one of the 'bugs' that SlantFin pulled the program for?

OK, little more foolin'... you can't make a 'global' change in the S/F program, you have to change each room individually in order to change the indoor temp on an existing dataset. Just an FYI for anyone who still has the S/F program.
 

Last edited by NJT; 12-13-09 at 06:29 PM.
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Old 12-13-09, 06:35 PM
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The program I'm using does not take house temp input, other than the HDD. If you were to reduce your HDD by 50% that would simulate the lower inside temp (50 degrees). Long explanation as to where that data comes from and it is based on 7500 HDD, my area. I had used 3800 DD for Hartford/central CT, but not sure if 1900 would be a realistic number as it reflects more total energy use and not necessarily the peak energy demand. Have to think about that.

Bud
 
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Old 12-14-09, 11:40 AM
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For some reason my reply post last night failed ... but, thank you all v much. Rather than answer all of your questions, I owe it to all who responded to do my load calc homework before I post again. I did find and download the TACO software after watching the video wizard. I need to dig out my heat transfer text book and really understand / estimate R values for walls, windows, doors, and insulation design. And air infiltration. I also need to estimate downward losses, and then use this software to do sensitivity analysis.
Once I have a believable number I'll come back to the forum with questions about size of the boiler and thoughts on which mfgs are reliable.
Thanks again for the help.
 
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Old 12-14-09, 12:47 PM
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30,000 btu/hr divided by 1300 sq ft is 23 btu/hr per square foot. That's certainly in the range of radiant, but probably an underestimate of the heat load at 5F outdoor design, even with an indoor temp of 50-55. You'd need some seriously tight and well-insulated garage doors. Doubtful.

I'll guess the load is more like 50-55k btu/hr. But that's literally a guess.
 

Last edited by xiphias; 12-14-09 at 01:17 PM.
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Old 12-14-09, 06:18 PM
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Consumer reports has a simple load calculator. You can find the link in the load calc article at iheatCool.com
 
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Old 12-27-09, 07:56 PM
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Size of boiler for garage radiant heat project

I'm back for boiler advice. Two weeks ago I posted about my garage project, and that I had embedded PEX tubing in the concrete slab for radiant heat. An analysis had been done 5 years ago suggesting that I will require 32,000 BTU/hr in (for 70F inside on a 5F outside day.) Iím now finally finishing the job, trying to find a natural gas boiler, and making sure the heat loads are right.
I have just gone through a detailed analysis of the heat loads, rather than relying on the old analysis and some off-the-cuff remarks from the guy who installed my gas line. Given the uncertainties in the analysis, and the fact that I really only want part of the structure to be at 45F or 50F, I believe that in fact the most I might need is 40,000 BTU/hr.
So now I am trying to find a good boiler that has good reliability, efficiency, and will give me domestic hot water in addition to the radiant supply. And at a reasonable price. However Iím seeing several brands at different prices and with different features. For instance, the Ambassador BMS series has a built-in circulator, mixing valve, expansion tank, heat exchanger, etc. But it is a non-condensing unit, although it is about $1,000 lower price than the condensing unit. On the other hand, I canít find anyone who has used it or has any reviews. Then there are perhaps 4 or 5 other brands, but again, I canít find any advice on which ones to pick. Can anyone on this forum recommend any brands that have both good value and reliability? Any advice about springing for the extra price to get a condensing unit?
One other thing: all these units seem to be able to put out far more BTUs than I need Ė is it pretty standard to choke off the flow of gas so that it modulates efficiently to the heat output I need?
Thanks for the help.
 
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Old 12-28-09, 05:57 AM
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Put in a mod/con and an indirect water heater.Pipe the indirect off the indirect piping in reverse flow so the tank heats the floor. That gives you hot water and heat with a buffer. You don't need fast recovery for the indirect in a garage situation unless you are going to install a shower. Throw a mixing valve on the radiant heat so the tank can maintain about 140ļ and use a well insulated tank. Thermal mass here would also help in the indirect.
 
 

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