@mmoreland attached was the first generation of the driveway heat system diagram that I used. After installing this, living with it for one season and learning of its faults, I made changes. Live and learn, right??
The first problem (and I kind of suspected this going into it) was the size of the heater. It shows the TK-Jr in that diagram, and that was a 130,000 BTU unit. Too small. I actually needed by calculation later, about 300,000 BTU for optimal operation, but no tankless goes above 199,999 because above that, the Feds consider it a "boiler" and you have all kinds of additional rules and regulations to follow, so they simply don't make them. There were plenty of boilers in all sizes, but I was space limited. So, the largest tankless available, and the only one commonly available was the one to get: the Takagi TK-3. As previously mentioned, at the time of this installation (2009) it was the only one that would certify in a recirculating application. BTW if you need or want more than 199,999 BTU in a tankless, you just cascade them together and some manufacturers make controllers to control a bank of them. I've seen some carwashes that had 4 or 5 of these tankless supplying all the hot water, while more conventional operations (such as the car wash I had) had a 1.2M BTU boiler and an 80 gallon storage tank. In the car wash industry in the upper midwest, the general rule of thumb is that for each car wash "bay" (just about the size of a good single car garage) you need 100,000 BTU...but that's a very generalized statement. My driveway as you can see was about the size of 3 single bays, thus calling for 300,000 BTU. But, a boiler will heat and flow properly; some of them will do 30 GPM. The tankless was limited to about 5 GPM. Hence the circled part called the primary loop/secondary loop practice.
The other fault was those very commonly available stainless steel manifolds--generally used and sold in radiant heat applications for a house--as shown in the photo on the diagram. I had chosen the ones called Everhot. The problem with those is they are not full flow, and in a snowmelt/de-icing application it's all about the flow; you need full flow. So, after the first season with these manifolds and with the smaller TK-Jr. heater, I made a big change. We went with that TK-3 and changed the piping around to custom built manifolds; you can see an example here. They were fabricated with 1.25" copper tubing, on a tee-pulling machine, and ½" stubs brazed on. Then I used full flow ball valves from Dahl Brothers.
dahl Valve Limited – Plumbing and Heating Valves Manufacturer in Canada At the time, they had exceedingly limited distribution in the USA, but I found a distributor and ordered a custom set. You choose the body and the size and the fittings and the handles.
Once those changes were made, I then designed and built a control system using a PLC. It worked mostly well, but would have been much better had I had a larger heater. By calculation there was over 40 tons of concrete in my driveway, and heating that from cold to 33-35 degrees doesn't happen fast...but as you can see most of the time the system worked well. Below 15 degrees not so much, but when it gets that cold mostly there's no snow because there has to be clear skies for it to get so cold. Mostly. Polar vortexes and other things started to affect the system and in normal circumstance you "kill it with the extra BTUs" which I did not have at home.
Hope this helps. The only way I got all this knowledge was "baptism by freezing" and owning a car wash in Michigan for many years.
Surprisingly we had the "ideal" operating scenario many times. We'd be watching the news one night, and they'd say it's snowing in Chicago and that was coming to Michigan in about 4-6 hours. So, I'd turn on the driveway system. By the time the snow came, the driveway was just about freezing, and the snow melted. If a short snowfall, we ran it until the sun came out, then turned off the burner but left the pumps running. By mid afternoon that next day, the driveway was BONE DRY. No packed down snow or ice. Then there were the sub-optimal days. A polar vortex, temperatures around 5 degrees, and it snows. May as well not even turn it on, as you'll never heat up 40 tons of concrete with that heater; just wait until it warms up a bit. Get most of the snow off the old fashioned way (by shovel or snowblower) and when the temps go into the 20s, turn the system on and the rest of the snow will disappear.
