You should have this done for you. It is too important to guess about.

If you get it wrong, your county building authority will likely set you straight...

Yes, you certainly need it to properly accomplish the heat loss analysis to allow best sizing your heating system requirements. However, there will be many days when you will actually be operating above and below this value. With global warming now coming relatively quickly, the published numbers should now be changing relatively frequently. So I am not sure how important it is other than allowing one to properly accomplish the analysis and perhaps avoiding getting slapped by your county building authority afterward for using the wrong value.

Before you spend too much time with your manual j calculation, is this project being inspected.Manual J will not be recognized as the energy calc as per Title 24. You will need a licensed professional engineer to perform and stamp any drawings before the building department will issue you a permit.

Yes, Mr Wizard, this project is being inspected. An of course a title 24 is being done by a PE. That said will Manual J be part of the Title 24, or will i be able to get my energy requirements to size radiant from the title 24 calcs?
thanks, STeve

title 24 calcs, and most of the people doing them, are a joke. even when done well they don't really guide a heating design by any useful degree. If a load calc is not done room by room, it is not a design aid of any significance.

Man J or equivalent calc, done room by room, is essential to the comfort success of most systems. while "design day load" may in fact be varying, they are not varying enough to change a 50 year average very quickly, and even a worst case scenario for global warming wouldn't change a peak load calculation by very much either.

what a room by room calc is REALLY important for is figuring out relative loads between rooms so you can zone and figure your water temps appropriately. It can also inform heat source class and size selection, which varies in importance by project, heat source choices, and how close to break points in that selection you are. and once you get into design the load calc drives things like pipe and pump selection as well.

as for your area, I can see a station at blue canyon which is at 5300 elevation and is 28 miles away with a design temp of 21 degrees F. however, you have an outlier 33 miles away, Reno, at 4400 elevation and an 8.8 outdoor design which is a lot colder than every other station near you. I would typically, upon seeing such a large discrepancy so close to each other, ask which scenario seems closer to your experience at this site, to make sure we don't miss an important microclimate issue.

I agree with NRTRob, what I should have said is Manual J is one of the three accepted methods for determining a heat load .The computer assisted programs used for determining heat losses have been designed around Manual J but are more complex and must be approved by the California Energy Commission.I posted another comment on your other thread about use of electric water heaters or boilers.I believe you are going to be asked to use solar with your electric option in order to comply.The other approach will be an entire performance evaluation of the home.

NRT.Bob, very helpful post and appreciate that you found Blue Canyon and Reno. Given the microclimates, these are probably a bit far away. If nothing is available in Squaw (Olympic Valley, Ca), then Tahoe City is the best proxy with Truckee as second best.

In reference to prior posts by experts on this forum, i understand that using an accurate predictor of temperature gives a better system design.

WRT a room by room analysis, please advise how i should evaluate who should do the radiant design. As i read from this forum, even presumed reputable firms don't  necessarily do quality work. How do you suggest i evaluate someone (state license? Local work? References?) to do this work? I have cad drawings so presumably this should be a relatively straight forward calculation -- this may be simple minded on my part, but i think the equations are know and are standard (not inventing new science here), the inputs are factual, and the house design is done.
Thanks again for sharing your knowledge.Steve

most inputs are factual but you want people that will use real values and not fudge for safety. also, infiltration rates are a guesstimate. you should never hear "1 ACH" from anyone.

Comparing WeatherSpark.com data on colder days to compare your location to nearby locations with listed design temps can yield reasonalbe offset to use to estimate a reasonable outside design temperature, if there isn't a better version accepted by Title 24 enforcers.

http://weatherspark.com/#!dashboard;a=USA/CA/Olympic_Valley

http://weatherspark.com/#!dashboard;a=USA/CA/Truckee

http://weatherspark.com/#!dashboard;a=USA/Lake_Tahoe_Region

The inherent error from this method of estimating design temps is low- "in the noise" compared to the actual vs. estimated U-factors for building assemblies used in a Manual-J. But if code enforcers demand a rigorous validation of the method, it's not going to be easy.

Steve, When a mechanical engineer does a Title 24 calculation it will give the limits on equipment sizing and will determine the heat loss/heat gain for the envelope of the home.To design a hydronic system a room by room load calc needs to be performed in order to properly size tubing layout,pumps etc.There are plenty of smart people here on this forum who are able to design you a system.There are plenty of engineers ,mfgs, reps that can assist you.We are a design build contractor that typically would do the bulk of a system design and once finished would take our design to a mechanical engineering firm that we have a good relationship with. They intern review the design , make suggestions or changes , then they produce a stamped set of drawings that any good contractor could build off of knowing that the system will perform properly.

Everything acwizard said…and there is a difference in what you will get from a product retailer that offers a free design if you purchase their over-priced products versus an actual design/build engineering/construction company. If you are planning to build this year in a state that has stringent design/build requirements, you may be hard-pressed to find qualified help. The construction industry is coming back very strongly. We fully booked up for the 2013 construction season last week.

Steve,

As a consequence of your feedback, I did ask our s/w guru to update our DIY hydronic floor heating calculator to handle above/below floor tube/plate and thin-slab in addition to the current slab-on-grade (good for 4-6”) only. Will also have him add an option to handle 0-50% propylene glycol which someone else requested. Good luck with your project!

Bob B.

I would give some feedback that in most cases, you should really think twice about encouraging people to do their own room by room calcs. I have guys here that, with me, have been doing room by room calcs near daily for years and we still have to sit down and hash out proper room division and infiltration estimation questions on a regular basis. especially if you care about things like localized comfort within a larger space. Heck, I write my own load calculation and hydraulic design software, and I STILL have to sit down and puzzle things like that out sometime. until we have realtime 3-D modelling with fluid dynamic routines, solar gain, mass, emissivity and internal heat gains that's just life.

it's easy to make up numbers. it's not easy to make sure those numbers reflect reality. I've seen dozens of DIY load calcs over the years with a variety of free software out there. very few have passed the sniff test. with very simple floorplans it can be done with some confidence as long as infiltration estimation is done reasonably. but that's about it.

It may sound self serving, but then again, my clients are usually more technical than most... many of them are engineers and programmers with other foci and a high degree of orientation to detail. And they still almost never really nail it. And to really check the numbers, you basically just have to redo the whole calc.

As you know NRT, we encourage folks interested in hydronic floor heating to hire a PE to do the design and hire a licensed/experienced HVAC technician to do the installation…and to carefully check out their credentials and references before hiring them. In Steve’s case, he just seems interested in running numbers to better understand his situation. There is nothing wrong about getting educated on a subject even if you eventually hire someone to do it for you. Given that Steve is building in a state that has stringent design/build requirements, he will eventually be having a PE seal/sign his design as he indicated.

I would certainly agree that working with DIYers can be very challenging. We have had both very capable and less-than-capable DIYers use our DIY calculators. We can usually tell pretty quickly by the questions they send us what flavor they are... We often pro bono review/correct the results of the more capable DIYers and we actually did quite a lot of that a couple months ago for people rebuilding from hurricane Sandy. The bottom line is that there are people who just wouldn’t be able to afford doing many of the projects available on our website without having the ability to DIY. This is especially true of people living in third-world countries. In short, we believe that being self-reliant and building energy efficient homes that use renewable energy resources in a responsible manner is a good philosophy for our planet. We developed our DIY calculators to enable everyone who shares this philosophy an opportunity to accomplish it. We always encourage and welcome feedback/suggestions for improving our DIY calculators and associated instructions.

"But if code enforcers demand a rigorous validation of the method, it's not going to be easy."

We have to provide heat loads here in Minneapolis and I have never had a Manual 'J' challenged. I doubt they are look at by anyone, anywhere. You have it done to better assure your ultimate comfort and the efficiency of the installed system.

Posted By BadgerBoilerMN on 04 Feb 2013 08:25 PM
"But if code enforcers demand a rigorous validation of the method, it's not going to be easy."

We have to provide heat loads here in Minneapolis and I have never had a Manual 'J' challenged. I doubt they are look at by anyone, anywhere. You have it done to better assure your ultimate comfort and the efficiency of the installed system.

California Title 24 has created a whole code-compliance industry out there- it's not the same as in MN or MA, and design temps can vary dramatically over short distances in those mountains- far more so than in places you or I live & work.

Even a poorly done Manual-J stamped by a P.E. probably cuts it, but it's not clear if my prescribed method of coming up with a reasonable outside design temp by using observed offsets from a few nearby locations with listed design temps would pass muster, or if the credentialed person is required to use some other method prescribed somewhere in the code.

Elevation differences and micro-climate cloud cover (= radiant cooling blankets) patterns create dramatic differences in wintertime low temps over relatively short geographical differences in the Sierra Nevada- using the outside design temp for Truckee or Tahoe for the Olympic Valley would be a mistake, but since there are weatherstations with real data history at that site logging the differences in overnight lows on the coldest nights over the past 3-4 years can yield a reasonable offset to use.  If it's fairly consistently 3-4F colder at Olympic Valley than Truckee on the coldest nights of the year, using Truckee's design temp less 3F would be a reasonable design temp, etc.  It's not the same as using 25-50 years of binned hourly temperature data from Olympic Valley weather stations to glean a 99% number but it'll be pretty close.

It's more likely to track consistent offsets with Truckee than Tahoe City (even though Tahoe is closer), due to the temperature moderating effects of the lake, but its worth looking at the data just to see.  Weatherspark.com has a HUGE amount of very local data that can be tapped into, with nice graphic output to zoom into.  Granularity isn't hourly for every weatherstation on every day but it's usually tighter than 3 hours.

We are melting snow in driveways and heating homes in the most of the mountain ranges in N.America including Truckee and Lake Tahoe. Manual 'J' seems to working for us.

Manual J is more than adequate and almost always acceptable. We do ASHRAE because that is what engineers do. It takes longer to do and one would be hard pressed to find any increased accuracy or real benefit over Manual J IMHO. However, it is always accepted if sealed/signed by PE.

We did post the new and improved DIY hydronic radiant floor heating calculator on our website today. It now handles slab-on-grade, thin-slab, above-floor, and below-floor. It works for 0-50% propylene glycol and for PEX or PEX-AL-PEX. Again, we always encourage and welcome feedback/suggestions for improving our DIY calculators and associated instructions.