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Boontucky-girl
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Basic Member
Posts:158
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| 04/22/2009 2:09 PM |
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I am interested in doing my own heat load calcs. I have MJ8AE version. Would that be good enough? Or do I need the full MJ8?
I have ICF basement, closed cell/2x6/osb/1" R-board upstairs. Closed attic. Radiant in slab in the basement and thinking of doing forced air for the main floor. I live in Iowa.
How good is hvac-calc compared to doing MJ calcs (besides just plugging in numbers rather than reading through thousands of pages). Any help or comment will be greatly appreciated.
Thanks.
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Dana1
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Advanced Member
Posts:702
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| 04/22/2009 5:28 PM |
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These aren't exaclty precision heat loss calculations, but way better than a WAG. Whichever version you use is almost by definition "good enough", but MJ8E (version 8) is probably more than you need- Version 7 (residential) is more than adequate. But then again, cheapie & freebie software packages end up delivering pretty much the same numbers as a more thorough careful hand-calc, for a lot less time invested too, as long as the construction options come anything close to what you have.
There is significant margin built in to allow for variations in the quality installation & type of insulation, and variations in real-world air-tightness, etc., so most overestimate by 15-25% or so from "better" or "ideal" construction detail quality and to allow for some system degradation over time, which is good enough to end up with reasonably-balanced and reasonably-sized heating/ac systems. Whichever version you use it'll typically be within 20% of all other reasonably detailed calculation methods.
If your calc comes in just over one model of furnace/boiler by 15-20%, but the next larger size is more than 10% over your calc it's generally "safe" to go with the smaller unit, which will operate more efficiently than the larger one. (Some building codes now expressly dis-allow oversizing by more than 10% of Manual-J.) If you oversize it you'll pay more now, and every year during it's service life. If you undersize it, you can probably find an economical source of supplemental heat for the 16 hours/year it doesn't keep up (which usually happen at night, when you're under the electric blanket anyhow.)
I assume you'll be doing the radiant slab & hot air off the same boiler (with a hydronic coil air-handler for the air zone)? I'm curious as to forced-air choice as well- even with manual-D duct design and mastic-sealed ducts the induced pressure differentials will increase outdoor-air infiltration. (It's typically a 10-20% difference in annual operating costs, all else being equal!) It also keeps dust/mold/pollen suspended. I s'pose if you were putting in central AC and wanted to double-up on the ducts as for heat distribution it might make sense, but it creates another level of design complexity, since sizing the ducts perfectly for your AC may not be best-practices for the size of your heating coil & boiler, and conversely, which always ends up some sort of a compromise impacting efficiency &/or comfort being lost on one end or the other. If you have the option, low temp panel-radiators are far more cushy, and lower maintenance than baseboard convectors. But even low-temp baseboard convectors will be more efficient than hydro-air or hot-air furnaces since the system won't suffer induced-infiltration losses, and can run at lower temps squeaking higher efficiency out of the boiler (especially if you use a condensing boiler with outdoor reset control.) |
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Jelly
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Basic Member
Posts:496
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| 04/23/2009 6:56 AM |
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| Boontucky-girl, does your version have the option to designate exterior walls as SIP and to change the R value to whatever it needs to be? |
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James Eggert
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Veteran Member
Posts:1059
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| 04/23/2009 7:26 AM |
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hvac-calc will work for you, however, you need to understand how all the "heat" sources provide for each area.
example is you will need xyz btus for each room, so you need to know what the radiant will do and how well, these programs are not for radiant design, that is another program itself,
hvac-calc has a one time user purchase so you can do your own home. I have used it for many years for ICF houses. |
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Take Care
Jim
Design/Build/Consulting
"Not So Big" Design Proponent |
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Boontucky-girl
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Basic Member
Posts:158
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| 05/01/2009 8:56 AM |
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Thanks for the great replies.
Dana1 - wow, that's a lot of good food for thought. If I can get my DH to agree, we'll be going radiant the whole house with ductwork only for AC unit. This is one of the reasons I'm doing my calcs so that I can understand a little more about the whole process, so that when it comes time to find a contractor, I can trust what they are doing. What are panel radiators? I was thinking if we went radiant on the main floor it would be in a sandwich type of installation.
Jelly - are you asking my version of the book or my version of the software? I don't have a software, I only have the book.
James - yes, I understand hvac-calc is not for radiant, nor is MJ8AE, but I think the heat loss calculations are the starting point for both radiant and other hvac design, no? Which is my basic question - is MJ8AE good enough to get the heat loss calculations? Last time I looked at Hvac-calc demo they didn't have data for fiberglass windows. I never got around to asking the tech dept. what would I do in that case, and I don't remember if it had options for my type of wall construction. Myabe I should download the demo again and take another look.
Reading through the MJ8AE manual reminds me of my thermodynamics class in college. So much I had forgotten! I was looking at the scenarios that apply for the MJ8AE and I think I might need the MJ8 book instead. For example, I will be doing a closed attic and one of the assumptions for MJ8AE is that ventilation is done according to FHA requirements. I haven't gotten far enough into the book to see how the calculations are actually done to understand how that affects the number and decide if I do indeed need to bother with MJ8 at all.
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dmaceld
Registered Users
Advanced Member
Posts:805
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| 05/08/2009 1:49 PM |
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My experience: I used HVAC Calc to size the replacement A/C unit I needed for my house in Louisiana. Was a bit skeptical of the size it said I needed since it was somewhat smaller than the old A/C unit. I went with 3 ton while HVAC-Calc said 2 1/2 ton and the old was 3 1/2 ton. In the end the new unit performed quite well but was in fact slightly oversized. Score one for HVAC-Calc, especially since it cost only $50 for a one time use.
Now for the new ICF house in Idaho I have just moved into. Overall heating/cooling loads calculated higher by HVAC-Calc than Wrightsoft used by my nephew HVAC contractor. The single biggest advantage of having the Manual J done by with Wrightsoft was I was given an easy to read & understand room by room heat gain/ heat loss output. That in itself gave me the info I needed to decide what size of air handler(s) I needed for the living space. I couldn't make multiple indoor units sizes fit the loads of various combinations of rooms so ended up with one indoor unit for the entire house load.
I would say, for $50, buy and use HVAC-Calc and see what you can get from it. It should give you room by room numbers but it may not have all the construction material input parameters you need. Then, if you're still left wondering find someone to run it all with Wrightsoft Residential suite. Whatever you do make it so you can adjust the output of the system on a room by room basis to compensate for over/under sizing, because that will happen. Real life conditions won't be the same as what you input into the calculations!! Of course I say that because I assume you are not omniscient!! :-)
I applaud your bravery and self confidence in buying and using the Manual J book. I considered doing the same myself but when I thought about what it must mean when the developer of Wrightsoft has a big share of the market with a multi-thousand dollar program, which he has sold hundreds, if not thousands, of times, I decided against it!
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Building house - what a way to spend retirement! It's done! We're living in it! |
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Dana1
Registered Users
Advanced Member
Posts:702
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| 05/08/2009 2:59 PM |
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Posted By Boontucky-girl on 05/01/2009 8:56 AM
Thanks for the great replies.
Dana1 - wow, that's a lot of good food for thought. If I can get my DH to agree, we'll be going radiant the whole house with ductwork only for AC unit. This is one of the reasons I'm doing my calcs so that I can understand a little more about the whole process, so that when it comes time to find a contractor, I can trust what they are doing. What are panel radiators? I was thinking if we went radiant on the main floor it would be in a sandwich type of installation.
Panel radiators are thin flat low-temp heating radiators that are set a couple of inches off the walls. (These have been pretty standard in northern Europe for 25 years or so now.) Unlike fin-tube convectors (like the ubiquitous baseboards common in the US) a significant portion of the heat is radiated into the room rather than relying totally on air contact & convection, letting the room air warm the occupants. The comfort level is noticably better, since you feel the heat when sitting or standing next to the panel before the air is up to temp, even with water temps as low as 100F.
They come in any number of sizes and appearances. Search Google-images on [radiant heating panel radiator] for numerous examples & vendors.
But a couple of examples:
https://www.hygienesuppliesdirect.com/images/productpics/1331881.jpg
http://www.infolink.com.au/odin/images/230628/Hydronic-Heating-steel-panel-radiators-from-Dynamic-Heating-and-Cooling.jpg
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