Quote:
Originally Posted by Drake
In the last years as the reality of my plan to heat my new project with hydronic radiant floor heat I began seriously researching what was current radiant application and what the design variations were so that,because I am building from scratch, best design my structure to make use of radiant. I was very happy to find that high mass/low temp radiant design is one of the highest efficiency/renewable compatible heat source design directions one can go. It also compliments my wish to maximize passive solar gain design into my structure as a two for one. So my first goal in understanding HyRdt was to best design my structure for it. I am confident that I have done that to the level I am willing to pay for at the best level of working knowledge that I was able to incorporate till the time I had to start(because go/no go has to be sometime and what can't be changed accepted). During that time aside from personal information seeking I contacted four major radiant DIY companies for their input into my project. From this I got four different design(pex size, spacing layout patterns) recommendations for the same, very simple, 16x32 4" slab floor. Each company had very convincing explanations for their designs and components. Pex size ranged from 7/8" to 1/2". Any of the companies design may have been the best, each may have done the same thing a different way, I do not claim to be an expert on HR design. I can see that there is not one common practice being used and that I'd better be very sure that I am getting a design that will do what I need. Granted my floor install is modest in size and the structure being maximized against heat loss may not be adversely be affected by not the "VERY BEST" design application. So maybe I'm over thinking minor things. But if I can identify a better way to go I will go it. Such as I plan to build a second floor radiant loop into my open lofted sleeping area even if I don't connect it. Right now the existing cabin structure(built on a similar floor plan but only half the insulation of the new addtn) has never needed the baseboard heat that is in it. I will be putting a thin slab floor upstairs for solar gain so I might as well run pex in it at the same time.
Again, HR heating is not a much used heating method in my area yet so finding example to query owners on is minimal. I appreciate the input from those who are doing a lot of work with it. If you think that there is just one "best" way to do anything than I don't work in the same world of contractors and journeyman.
I was impressed with Radiant Works but it did NOT layout a design that accomplish to me the ability to supply more heat to the exterior walls than the interior of room as I have tried to discuss in length. As I have admitted the size of my heated floor may make the need of this an unnecessary design consideration but even when I increased it's size in Radiant Works by 4 it did not address it any better IMO. I tried to explain my discontinuity of this commonly accepted heating practice(which I also heard from radiant floor people) and the examples of radiant layouts I have found published. In my project I have no fear in not being able to supply my space with enough heat, it is far more in not over or unequal heating it and designing to heat it no more than I need to(because I'm frugal and a closet greenie).
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Drake,
Thank you for taking the time to compose a detailed answer.
There are some Acronyms that you are using that I don't understand. I even tried to Google them, with no luck. What do you mean by "HyRdt"? The best I could come up with is 'elevated radiant change-over-time'... Is that what you mean?
Also you used the term "HR heating"... do you mean hydronic radiant heating?
Regarding the very different advice you've gotten from different contractors, part of the reason could be that what you are trying to do, by radiantly heating your new building by primarily using solar (low temp) energy is very new. It is called "low temperature heating", or "low exergy heating" to be more precise. It is being practiced in Europe more than in North America, so the pool of practical knowledge on this side of the pond is just not so large. Even John Siegenthaler, the most up-to-date North American expert on radiant heating is still a bit behind the curve on low temperature heating.
You've gotten some great advice here, from seasoned installers, who have nothing to gain from advising you. That's a very good thing.
So, I was looking into this very same heating approach and one thing I learned (by using RadiantWorks) that hasn't been mentioned so far in this thread is that
to squeeze every last BTU out of your low-temp water, you will need to increase the flow rate (gal/min) of the water through the slab.
If you increase the flow rate through a given PEX diameter, you will increase the velocity (ft/sec) of the water through the PEX. When you increase the velocity, the fluid resistance to that flow will increase by the square of velocity increase. What this means to you is that you will need to increase your pumping power by the square of velocity increase. That means more $'s of operating cost.
The solution to this is to increase the diameter of the PEX so that it becomes possible to increase flow rate without increasing fluid velocity.
So for instance, you might need to double your pumping power, but not quadruple it.
There's another consideration regarding flow rate and velocity...
When fluid flows through pipes, it may flow gently and evenly (laminar flow) or it may flow in a haphazard, tumbling manner (turbulent flow). Turbulent flow is much better for transferring heat than laminar flow.
There is an engineering term to determine the proper flow velocity, called "
Reynolds Number".
There are charts and formulas for determining this.
What I'm suggesting is that designs you have seen that use larger PEX diameters than is conventionally used, may not be so crazy after all, if their reasoning is sound. If their reasoning is not sound, then of course, they are Coo Coo.
So, I'm suggesting going with the closer-spacings as has been advised, AND increasing the tubing diameter, so that you can get elevated fluid flow without unduly increasing pumping power requirements.
* * *
But above all this, my primary advice would be to
- make sure that your house is properly oriented and fenestrated to the sun's energy
- drastically decrease your infiltration to absolute minimum levels
- use an extremely high efficiency (=>85%) HRV
- insulate your house far beyond what you think is $ufficient and prudent
- forget about a deluxe radiant heating system, because you won't need it.
If you have done all of this properly, you will be able to turn off the lights and power, buy a plane ticket for you and your wife, fly to some sunny southern climate for a week or two, and never worry about any pipes freezing, because the house will take care of itself, by design. That's what a Passive House is all about.
Best Regards,
-AC