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Old 05-05-2012, 11:43 AM   #1
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radiant heat


i am looking at putting concrete in a 20'x30'x 10 low side and 14' high side "shed roof" it will be insulated with blow in insulation 2x4 walls. the temp. needs to stay around 50 f. no gas available. i would like to use viega grid system. http://www.viega.net/xchg/en-us/hs.xsl/7413.htm
the questions are
1. how many zones needed
2. will a hot water heater work for heat source? if so what size needed

thank you

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Old 05-05-2012, 03:10 PM   #2
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might consider resistance strip heating insulating that floor then pour over that being you have no gas electric HW heater,circulator,piping....long way around. if it just a shop shed might do a tralled on floor over the strips


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Old 05-05-2012, 04:42 PM   #3
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Electric water hear is only gonna be 4500 watts. Which is 15,358 BTUs an hour. So how much heat loss does/will your shed have.
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Old 05-05-2012, 07:28 PM   #4
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it will be insulated well
i am event ok with keeping 45 f.
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Old 05-05-2012, 08:03 PM   #5
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Might want to spend the 50 bucks and use this to know how much heat it needs. That way you know how much radiant you need to install and what size heater.
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Old 05-05-2012, 10:27 PM   #6
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thanks if i need to i can talk to my wholesaler and get more details was just trying to see what others thought
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Old 05-06-2012, 09:25 AM   #7
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Quote:
Originally Posted by beenthere View Post
Might want to spend the 50 bucks and use this to know how much heat it needs. That way you know how much radiant you need to install and what size heater.
Strongly agree with recommendation to do a heat loss calculation. I have been attempting to arrive at heat load values for my residential build and have found the topic fascinating, if not a bit intimidating. Here is a good primer on the basic ideas:

http://www.greenbuildingadvisor.com/...ulation-part-1

Next comes the actual calculations for your specific project. I have discovered several tools online that I have used with varying degrees of confidence. Most are based on traditional Manual J heat load analysis. However, data out is highly dependent upon proper data in, and here's where experienced HVAC people clearly have an advantage.

For instance, I have been analyzing my garage design for in-floor radiant heating system, and have arrived at load calculations ranging from 49,000 BTU to 105,000 BTU's. One variable I have discovered that is VERY influential in final output is infiltration/exfiltration. Garages, by definition are very lossy, given big doors that open to the outside, and that are not very easy to seal and insulate well. So, does one use a value for air changes per hour of 1.5? (...typical of an "average" home 50 years ago) Or should my value be higher as it is a garage? Or maybe lower, since I will be implementing "best building" practices. I think I remember reading that a home built today to code in Seattle would have and A/C per hour of 0.5.

Plugging these ranges into the various software tools is where I derive most of my variability in my calculations.

Here's a sampler of some online tools I've found:

http://www.shophmac.com/info-center/...calculator.php
--- very simple. Results fall in line with other tools.

http://www.mrhvac.com/free-hvac-stuf...ss-calculator/
--- lacks proper instructions. Seems to generate highest load calc answers that are rich with "fudge factors"

http://www.loadcalc.net
--- appears maybe a bit more informative? Answers in range of other calc tools.

http://hvaccomputer.com/
--- downloadable software referenced here. Cost $50. Very robust. Best answers? Probably money well spent, I feel.

http://www.taco-hvac.com/en/products...t_category=370
--- another downloaded program, though free. Also very robust, but geared towards commercial buildings. Don't know if tool is reliable for residential applications, though answers nevertheless fell in line with others once I zeroed out the ventilation analysis.

On this topic, does anyone have a good reference for arriving at A/C per hour, short of doing a blower door test? See, that's hard to do when the garage (and house) not yet built!
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Old 05-06-2012, 09:36 AM   #8
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When not constructed yet. use the ACH rate for the method o construction you'll be using.
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Old 05-06-2012, 12:11 PM   #9
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I am not certain what a good value for garage construction would be... or even house, for that matter. Since ACH depends heavily upon "leakiness" of structure - and in a garage, I would think the overhead door would dominate the structure's leakiness - I wonder if anyone knows any real world overhead door thermal properties sources.

I ask as I have no real world experience. The overhead door manufacturers would have us believing they can manufacture to R17 (U factor of 0.06), with infiltration values of 0.08 CFM/ft^2, or 6.5 CFM for 9x9 door. Such values are hardly believable (and others agree
http://www.greenbuildingadvisor.com/...t-garage-doors)

Now, I suppose 6.5CFM through the door may be close, but around the edges? Really??

Anyhow, looking for answers from people with real world experience, as I said. What is a good value for garage ACH? What is a reasonable overhead door infiltration rate. Is 100 CFM close?

Thanks!

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Old 05-06-2012, 12:42 PM   #10
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Accurate ACH values for garage construction are hard to come by, and I think jaydevries would be well served to get as accurately sized a system as possible. I know I am striving for this.

The reason this is so difficult is that it is hard to rigorously calculate infiltrative/exfiltrative losses, yet leakage likely accounts for half or more of total heat load. At least it does in my case. For example:

My garage volume is 66x30. Total volume is 24,300 ft^3, and I have 3 OH doors, 2 service doors and 7 windows.

Ventilation heat loss may be calculated using this equation: Q = CFM * delta Temp * 1.08, wher CFM is exfiltration. Thus, leakage is VERY important.

If I assume 100CFM * 3 OH doors* 60min/hr, this equals 18,000 ft^3 per hour in leakage. Likewise, calculating for 7 windows and 2 service doors I wind up with 31,800 ft^/hr, which yields as ACH of 1.3 through the fenestrations when I divide 31,800 ft^3 by my garage volume. Heat loss in this scenario is 42,000 BTU/hr at my design parameters, which exceeds my transmissive heat loss of 32,000 BTU/hr by a goodly margin.

Yet, an ACH =1.3 seems pretty darn tight for a garage, and this does not include leakage through wall/floor intersections, ventilation fans, etc. An ACH=1.5 yields heat loss of 51,000 BTU/hr. ACH of 3? A whopping 100,000 BTU/hr!

So, I feel getting an accurate estimation of infiltration/exfiltration for a garage is very important, and look to those with more real world experience than I to say what a reasonable ACH for a garage is. Or, put another way, to state whether 100 CFM for overhead door leakage is reasonable.

Thoughts, anyone?
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Old 05-06-2012, 01:10 PM   #11
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Leakage around garage doors varies with how much leakage everything else has. The door can't leak as much if the service doors and windows are tight. Along with the wall plates being sealed. So if the air an't get out, it also can't leak in.

When you build your garage. make sure you seal the wall plates.

R/U value of a garage door is exaggerated as far as he entire door. But you can generally use 50% of its advertized value.

A new garage kept at 50 seldom will have more then a .5ACH.


Leakage is also effected by the difference between indoor and outdoor temp. So the warmer you keep the garage, them higher the infiltration will be.
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Old 05-06-2012, 01:59 PM   #12
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Thanks. Great feedback. At ACH 0.5, my garage total heat load works out to a mere 49,000 BTU/hr!

What, then, should a house achieve? ACH 0.3? Lower?
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Old 05-06-2012, 03:30 PM   #13
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New construction done right can be less then .2, some builders short cut an have more then.4

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