Radiant floor heat sizing
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| Posted in Energy, Heating & Insulation on
How critical is the design of tubing placement for RFH? Most of the companies that deal with the DIYer use very crude rules of thumb to determine the length and spacing of the tubing. Are these adequate, or are there calculations to determine the BTUH per foot of tubing so traditional heat loss calculations can be used?
Thanks for any responses.
Replies
First, let me say that I'm not in the trade. I did however, install the radiant floor heating system in my own house, but only after reading all the books and having the system professionally designed. Yes it matters how the tubing is laid out in the floor. It matters because the layout for a home in Alaska will be diffent than say the layout for a home in say Colorado. It matters because the temps you will need to run thru the tubing for slab on grade will be different than what will need to be run for a staple up job. It matters because there will be diffent heat losses in a house if there are large windows, different floor coverings, levels of insulation and lengths of unheated exterior walls. In short, the design of the tubing layout can make or break your heating system.
If you want all of the information you would ever want about radiant floor heating, go to heatinghelp.com all of the radiant floor guru's in the country hang out at the wall section.
By the by, the contractor on the "Wall" can regale you with stories of bad design jobs that were done by some of the more infamous internet radiant heat companies that they have had to sraighten out or completely scrap. I'd recommend that you pay to have a system designed by a professional. Hope this helps.
I've got a three inch thick book describing and detailing this stuff. If rules of thumb were acceptable, somebody wasted a lot of ink and I wasted lots of time studying back ten years ago.
But the calculations have more to do with assessing the house that it will be installed into and it's insulation package than the geogrphical differences between Alaska and Colorado. Maybe that was just a bad choice for comparisson since the mtns of CO can have a higher heat load than Anchorage. For any heating system, you need to know the difference between the desired temp to be attained and the mean low temp outside and then be able to calculate the heat loss.
When I have a heating engineer out, he wants to know what the R-values will be and he calculates the percentage of window space in a wall and figures an additional loss estimate for old drafty houses.
Then, once you know the heat loss, ytou know approx. how many BTUs to deliver and then you can figure out how to deliver it.
There is a world of differnce in the flow rate between 250' of 1/2" PEX and 150' of 5/8"PEX.
The instructions from the engineer for balancing the delivery system at the manifold are critical unless you weant some areas to be extremely warm while others are not recieving heat.
FWIW, There is an outfit just down the road from here a piece that sells the stuff to distant buyers without installing it. A few folks around here bought from them to install themselves several years ago. Things may have changed now but they weren't too happy with results or support back then.
Excellence is its own reward!
I appreciate the replies. I don't think I was as specific about the question as I should have been.
I understand about the need to do a heat loss analysis, and have already done so. I know how many BTUH I need for each room. What I don't know is what length of tubing (1/2 inch PEX) I need to provide those BTU's. I plan on using ThermoFin aluminum plates over the subfloor with laminate flooring on top (no carpet). A lot of the more indepth books talk about decreasing the distance between tubing near outside walls, but products like warmboard have a fixed distance, making me wonder if the placement is truly that critical. I understand that water temperature and flow rates will impact the amount of heat delivered, but right now I am needing to put the tubing down. It seems that since the temperature and flow are adjustable in the future, I should be able to balance the system later, but what do I know?
Two things that might help then,
If you can run so that first loop coming away from the supply end is along the outside wall you are better off. Also let's say that you need 550' for a room and it comes off three loops. Try to run it something like 150' for the outside loop and 180' for the two inside loops. The two inside ones will have reduced flow, incomparison to the outer wheree you need more heat.
Second - make dang sure that you record the length of each loop and map where it runs. Then your engineer can map out the flow rates at the manifold adjustments for you later. He'll need to know what he's working with.
BYW, you don't have much room for temperature adjustments, just flow rates. The thing that determines the temp you want at the mixing valves is the kind of materials it is installed into. The range is fairly narrow. I'm remembering between 111°F and 120° off top of my head. Too low and you are wasting the system. Too high and you can crack concrete or lift wood flooring.
I'm not a heating guy or plumber. I had to work with the engineer on designing a large complex system several years ago when it was new to the area and the plumbners I used were unfamiliar with it. As GC, I had to know how to shepherd the job and to provide the perp work needed so the engineer helped me through the books.
He did the hard work, mathematically - after all he had the software to do it, but I made suggesstions about runs that made sense to me after studying the book and he confirmed them with the program. He ended up saying, "By now, you probably know more than I do about this stuff."
But I forgot it shortly after that! Ha!
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Excellence is its own reward!
Thanks for the help.
I'll add to that what I can. When Wirsbo engineers a system, in addition to running the loop first along the perimeter, they'll also use closer spacing on that first pass. For 1/2", it's usually 6" by the outside walls and 12" in the field. I've seen 7/8" tube recommended at 16", too.
It's usually recommended to keep run lengths as near to equal as possible within a zone (manifold).
Sun in a room makes balancing the system difficult. Keep the tstat away from direct sun. But that doesn't always keep the rising ambiant temperature from turning the system off too soon, or on too late. But with a low-mass system, you'll have faster recovery time than you would with a high-mass system, so it may not be an issue.
Keeping runs equall length is to make their balancing work easier on the calculations but they can still do it all. Thery must know the legths they are working with though..
Excellence is its own reward!
you could do as I was forced to do,
I had a heating contrctor lined up, but he got too busy and called me the a day before the concrete went down.. I stopped at three projects near my house that had wirsbo going in and followed their design.. It may not be perfect but the choice was put the wirsbo down or use forced air.. the floor was going in..
frenchy,
You forgot the most critical part.
Does it work?
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Excellence is its own reward!
Actually piffin, the reason I chose to compare Alaska and Colorado is because my home is in Colorado. And your right, some of the homes in the mountains of Colorado have higher heating loads than homes in Alaska. The difference though in designing a radiant heat system for Alaska and Colorado is that when it gets cold in Alaska, the temperature varies between cold and way cold. In Colorado, it may be 20 F outside but the sun is streaming in in the daytime and heating up the house and then as soon as that sun goes down, heats no longer being pumped into the house - makes a big difference in how you control the system. Additionally, the outside temps here in Colorado regularly change by as much as 50 F between night time and daytime temps in the winter. So again, get a professional from the area to design your system - even if your going to do the install yourself, it's well worth the money for the comfort of a well performing radiant floor heating system.
Good point! I forgot about those wild swings. I've seen it change eighty degrees in a day there. Only a local will be totally aware of all that. I used to live in Kremmling and then in Montrose.
I just got paid a consultant fee for advising an out of state architect on how to design the exterior package for this climate here. Award winning firm in NYC and Long Island but they knew they were out of their element for up here on this coast..
Excellence is its own reward!