The following questions were posted on GreenBuildingTalk.com....
hi,i am installing a geothermal and will have x gallons of hot water. i have read and looked at a lot of different designs for moving this water around and have some questions
1) pump per zone or series pumps to do all? i have seen both?
2) joist insulating. it seems the plates are everyones recomoindation but the after that insulating seems to have multiple options. foil / bat insulation / distance from pex??? has anyone yet determined the best?
3) long loops and short loops on the same header. is this where the one common pump fails to work?
4) i never seem to find any information on pump efficiencies?
5) ready made systems? are there vendors that sell complete manifold / pump / thermo systems? seems there are but how about a reference for one
thanks
Here's our response.....
1) In a typical small to mid-size home, it's more efficient to use one single system pump -- and use 24v zone valves for zoning -- than to use a pump for each zone. Each pump can cost $30 to $50 per year to operate, depending on your local electric rates. Zone valves are a fraction of that to operate.
2) Foil-faced bats with a minimum R-13 is a good way to go (install foil face up). Radiant heat wants to do just that -- radiate -- in all directions (most people assume "heat rises" -- no, warm air rises). Furthermore, radiant heat will radiate most towards the path of least resistance. In under-floor applications, the path of least resistance is typically down -- unless you insulate properly. Avoid "Astro-Foil." It's only rated for something like an R-4, and based on hard-learned lessons, it's terrible to use under radiant floors -- unless it's used in conjunction with fiberglass or blown-cellulose. I've seen people use Astro-foil just below the tubing to create the 1" air-space recommended by most radiant manufacturers, and then go over the Astro-foil with bats. This works well.
3) As for loop-length. It will make life easier if you can make all loops fairly equal in length (within 10%) -- it will keep flow-rates through each loop fairly equal. That said, it's not always possible. Not a big deal. Most all brass, bronze or stainless-steel manifolds have balancing valves that allow you to balance flow-rates across the header. Also, if you really want to get slick, you can purchase manifolds with flow-meters to give you a visual indicator of flow through each loop -- and then use the balancing mechanism to equalize the loops. That said, NEVER install loops (or circuits, as they are most often referred to as) that exceed the recommended maximum circuit-length. For most brands of PEX, the maximum circuit-lengths for each size tubing is as followed:
5/16 = 250'
3/8 = 300'
1/2 = 400'
5/8 = 500'
4) Pump-efficiency is a complicated engineering matter. If you're doing a residential install, you're likely going to use one of four makes of pump -- Grundfos, Taco, Wilo or Bell & Gosset. There are slight differences in each pump's efficiency, but only slight. You want to be concerned less with efficiency, and more concerned with choosing the right size pump -- not too small, not too big. In a typical 2-story 2,000 to 3,000 sq. ft. home, I almost always can use a Grundfos 1558 or Taco 007 (small pumps). There is a method for sizing pumps correctly, but would take too long to explain here. That said, you'll know you have the right size pump of the Delta-T (temperature difference) between the supply and return is about 20 degrees. If it's much more than that, your pump is too small. If it's less than 20 degrees, you're too big. If by chance your pump is too big, you can always install a balancing valve (a simple gate valve will do), and close it slightly (to restrict flow), until you get the desired Delta-T.
5) There are "ready-made" systems, but I find that each has its limitations. There are some independent outfits making pre-fabbed control panels, but these are often rudimentary in design and, to be frank, "cheesy." The established radiant manufacturers often have pre-fabbed systems, but they are intended for pro's that know exactly what they need and understand how to integrate them into a larger design. What you might opt for is a custom-designed, pre-fabbed panel that is tailored exactly to your application, and come completely pre-wired, mixing and/or injection controls pre-programmed, etc. If you're interested in something like this, drop an e-mail to info@enhancedliving.net (put custom panel in the subject heading).
