Here goes the first installment of the "RADIANT FLOOR HEATING MANIFESTO"
Our heating degree days are probably a smidge higher-I am actually in Dacula, 30 miles away from Atlanta's heat island, so our lows are usually 2-5F cooler than there. I read some reports from a radiant contractor in Alaska who routinely uses 8-9" spacing with 1/2" pex, both in-slab and staple up. He says it keeps up with the load fine. I can tell you that I originally tried staple up without plates downstairs to save $-it couldn't keep up with a 32F night!! Adding u-groove transfer plates made all the difference in the world, plus they saved the cost of pex-specific staples, since the plates and the joist holes the pex passes through hold up the pex fine. Extruded or Omega plates work better, but I found a great ebay vendor for u-channel plates, Gehman Iron in Knoxville PA, whom I then purchased from directly. They offered 2' and 4' plates at less than $1/ft-the best prices, reasonable shipping and great service. You could also get extra heavy duty aluminum foil on ebay that is 0.0015” or 35 microns thick to do something like what Vlad did(or even make an HRV!) This is 2-4x thicker than standard foils. 24” by 500’ is $112 shipped from pactogo_inc.
I ran a small bead of 100% silicone caulk in the groove before stapling up, which fills the gap around the bottom half of the pex. Silicone is an excellent thermal conductor, so avoiding the tiny air gap makes a huge difference and negates most of the benefits of omega or extruded plates. My house footprint is 42' square minus a cutout for the garage, and the joist bays, which vary from 19.5-24" on-center run the whole length front to back for the first floor. I ran 2 pipes per bay and max 320ft per loop, so they are on 10-12" spacing. I left about 5" between the ends of each 2' plate for expansion, and there is max 6" between the plates. You can feel where the plates are due to the warmer temp but it isn't disconcerting-the cats love to sleep above the first few plates at the hottest end of each loop! I am also stapling up a reflective micro-perfed radiant barrier (again ebay!) just below the loops to direct the radiant heat upward-be sure the barrier doesn’t touch the pipes/plates or it will conduct heat away instead of reflecting it. This evens out the floor temp to where it isn't noticable where the plates are unless you were paying attention to it. Since only some of this is done, the first floor zones are doing double duty keeping the basement warm at 68-70F and the upstairs at a toasty 72-73F. When the basement is finished, the floor will be uninsulated, using the heat stored underneath in the off season (I'll start a new thread for that one) and I will have insulated walls with radiant loops in the bottom 2 feet of wall for any extra heat needed.
Since my system is open loop circulating potable water, I got Bell & Gossett bronze housing E3-6VBTPYZ pumps ($204 ea) manufactured by Laing from Wes at Ihrie Supply (Wilson NC, 252-291-7880). These adjustable speed electronically commutated pumps can flow up to 6 gpm max or 10 ft of head max (3gpm at 8 ft of head) while drawing only 20-27w! I also used E3-4V**** pumps for zones that had only one loop which can flow 1.5gpm at 5 ft of head using 11w! Both types have no seals to leak or cause drag. Since the impeller sits on a single spherical ceramic bearing and is aligned by its own magnetic field, they are jam-proof, as they will just wobble away from any debris that could ordinarily jam the pump (looking at pics on the web makes this easy to understand.) After studying power draw, maintenance issues and noise concerns (I read about some systems where the pumps took 800-1200w just to move the water around and were so noisy they kept the occupants awake), I decided these were the only way to go. They are nearly silent and not much more expensive than other bronze/stainless pumps for potable systems. For non-potable solar/geo circulators, Laing makes non-adjustable more powerful versions called E10 in a composite housing and various nipple/thread types/voltages. It can flow up to 6gpm against 20ft of head using 60w. At 20ft of head and 2gpm, they draw 50W. These are used for spa pumps and can be had for about $130ea on fleabay. Laing also makes 12v circulators of similar design for computers, so these could be used in a battery powered system.
Don’t listen to the naysayers telling tales of woe and safety risks of a potable open-loop system. Any so-called requirements to discourage Legionella or other problems in a radiant system would by definition also need to be applied to any DHW system since they are the same thing with different pipe lengths-when was the last time you heard a requirement to flush & sanitize your entire water heater and plumbing system every week? As long as the system is plumbed so all DHW gets pulled through the loops first to avoid stagnant water in the loops during the summer, an open system meets all national building codes. There are some great schematics for plumbing the system properly on the net. The system is simpler, and you will never have to worry about corrosion problems down the road.
My current heat source is an Eternal GU145S gas condensing hybrid water heater which I bought from Ihrie (by far the best price-$1700 shipped I think.) One of the best on the planet, it runs off a ¾” gas line, vents through 2” PVC, is 98% efficient at normal inlet water temps, and around 88% efficient when return temps are 110F. Incidentally, I will have it for sale once I have the solar/geothermal done, as I won't need any gas service once the kitchen is remodeled! I chose this heater because other tankless models weren't as efficient, had massive pressure drops due to convoluted heat exchangers, or did not have onboard hot water storage, which would cause no heat or "cold water sandwiches" during low flow or stop/start flow conditions. It comes with a built-in flowmeter as well. This unit has a max 145kbtu burner output. I have had all heating zones on, all hot fixtures on, then both together and this heater could keep up. The only disadvantage to this heater is when only 1-2 small zones are calling for heat, since the minimum burner output is 30kbtu. It cycles on for 15-30 seconds, then off again for 30-45 seconds. The manufacturer, Grand Hall, said this application is perfectly fine and won’t affect the efficiency much. They now make a smaller version, the GU100, that can throttle down to 14kbtu and run off a ½” gas line.
I am using a Bell & Gossett Z-6 controller ($208 from Ihrie) to take heating calls (and soon zoned AC calls by operating normally closed duct dampers-another thread!) by switching on 120v power for the pumps. It also exercises the pumps for 10 seconds every 3 days if not used in that amount of time to keep them limbered up. I hooked up my Kill-a-Watt for a whole heating season. The water heater, all circulators and the controller took less than $3/mo of electricity at my $0.06/kwh time of use rate. I used 805 therms of natural gas for the whole year including cooking/DHW. My peak month was Jan-150 therms, and the summer months were 7-15 therms.
I set the heater at 135-140F, steady state return temps are 105-110F while flowing 0.4-0.6gpm/loop. To maximize the heater's condensing efficiency with lower return temps, I have the pumps turned down so the system can just keep up at night. When I switch to geothermal/solar input, I expect to run at full speed with 130F supply and maybe 120F return. That will also even out the floor temps.
My main zone for the entire downstairs is 4 290-320ft loops, the master suite has 2 170-180ft loops, the others have the smaller pumps and one loop each. Doing it again I would have spent the extra $40 per pump to have the larger pump on all zones-when they all are on, the slightly lower head capacity of the smaller pumps means they flow a little less, as the bigger pumps flow proportionally more. It isn't much of an issue, though. I got Sioux 6 zone manifolds that included shutoff valves from Ebay and check valves and various fittings from pexsupply.com. I removed one of the small pex fittings & valves included on the manifolds and sweated on a 1/2NPT ball valves, check valve and fittings for my large downstairs zone which required more flow than the little valves could handle.
Construction & insulation: Builder stock R13 walls/floors, R30 attic, double pane aluminum frame windows with thermal bridging gap/standard air fill/ordinary glass. I am planning to replace these, as they and air leaks are the biggest thermal "holes" in the envelope. Since I am in a mixed climate, I am debating whether I should emphasize u-value or SHGC for my replacement windows, although some e-coated argon filled offerings now let a lot of heat in from solar gain while preventing a lot of heat loss out. We built the house new and were able to make a few improvements when the house went together, such as hot water pipes insulated in the walls, closing off a large chase that was left open to the attic, etc.
AC, you’ll love this “seal & insulate” story-The BR over the garage was the first retrofit staple up room we did. I was lazy and only ran 1 pipe per joist bay, looping each run back and forth between bays 16” on-center with only 26’ of transfer plate in the entire room. This room has ceiling, floor and 3 walls exposed to outside temps, so the floor running all night could not maintain temp. Since fiberglass batts usually aren’t best at conforming to irregularities, I squirted touch n’ foam through access holes in the power, cable & phone boxes on the exterior walls, making sure it sealed up the openings in the boxes and where the edge of the box meets the drywall. Now the floor keeps up even when it is 20F outside! After that lesson, the following easier method is what I would recommend: