Today, we had a record snowfall.
And it was the earliest snow ever recorded in my town. So, I am assuming we are in for one heck of a winter.
That got me dreaming about a system that would automatically melt any snow off my driveway. The system would use radiant heat. And the snow would melt into a drain.
This guy is one step ahead of me – check out this video about a snow melting driveway:
Someone on YouTube asked how Ronald Walters did it. Here is how:
The pumps and expansion tank are B&G (Bell & Gossett). The large pump PL-36 circulates the system at 8 GPM. The small pump NRF-22 pumps fluid through the Rannai 2520 on-demand heater typically around 1 GPM to 1.5 GPM. The combination flow control valves with flow meter are FlowGuard. The large copper supply and return manifolds… I just kept asking around until someone could supply what I wanted… they are expensive but made life easier and made for a clean looking installation. The individual ball valves on the supply manifold and the 1/2″ heating PEX were from Vanguard. Vanguard was bought out by some other company several years ago.
There is no controller… YOU are the controller. People will try to sell you all sorts of crap to complicate the system so it supposedly will run itself. One such item is a freezing precipitation sensor and another is an outdoor temperature anticipator. Such things (to me) are a needless complication, expensive items and are more than likely going to fail. Best to control the system manually… it will give you something to play with on those nasty winter days.
Your concrete needs to be 5 inches thick. The PEX needs to be in the EXACT center of the concrete. You MUST place insulation under the concrete or it will not work. You REALLY DO need to install the wire mesh and the rebar as I have shown in the video.
I used 1/2″ PEX and the length of each loop needs to be kept relatively short or the fluid will cool off too much before it gets back to be reheated again. You could use 3/4″ PEX but then your concrete may need to be thicker and you will need to pump faster and it may be easier to heat the concrete too fast which can very easily bust up the concrete. GREAT CARE and careful attention need to be paid to ensure that the concrete temperature is very slowly increased or the concrete will break (not if – it will break if you heat it too fast). This is a judgment decision on your part as to how fast you want to push your luck when bringing the system online. My system has 8 loops, 7 in the driveway plus one in the sidewalk. I usually set the Rannai to deliver 120 degF water and start with 1 GPM through the Rannai, to begin with. Later on, I will increase it to 1.2 GPM and then later to 1.5 GPM. All the time the large pump is circulating the system at a constant 8 GPM.
Ethylene or propylene glycol will be sold by a chemical supply and you should ask for it to be inhibited to the percentage of dilution you will be using. In my case, I am using 40% glycol. If the chemical supply does not know what you mean by asking for it to be inhibited…. find another chemical supplier. Inhibited means it won’t be as likely to react with copper or iron in your system.
You want the Slabshield (insulation) which will take the weight of the concrete.
I mixed my fluid in a clean plastic trash can. Then I fill/flush with a sump pump. Fluid at a velocity will push the air out of the PEX but you will still have some trapped air that will eventually (hopefully) work itself out… otherwise, you’re screwed. The circulating pumps are pretty gutless and can become air-locked fairly easily. You need automatic air bleeds… Hoffman Water Vent No. 77 is what I use.
We really have not experienced the type snowstorms we had while I was building the house. We have had some pretty good ice storms. The Rannai 2520 is larger than I need but the selection of on-demand heaters available at that time was not too good. And all I got from the Rannai engineers was how much trouble I was going to have and how it probably wouldn’t work. Real bunch of “pain in the asses”. The on-demand heaters are going to remind you of opening up the side of a jet engine… all sorts of sensors and flow controllers and blowers and enough stuff to cause it to quit working if any one thing gets out of whack. The largest problem is the narrow temperature range they insist these heaters operate within. This is a design criterion for the supply of domestic hot water. If you set it to deliver 120 degF fluid and the fluid coming back from the driveway is too cold, the heater is only capable of heating a certain flow rate a maximum “delta T”. If you are pumping 1 GPM and asking for 120 degF fluid and the heater can only deliver 118 degF fluid then the heater starts slowing down the flow rate (it has its own built-in variable flow control valve) and changing the firebox & blowers and stuff to deliver 120 degF fluid. Long story… a bit of an operational learning curve.
Once the snowmelt system is up and running for a while and you start to see the return fluid temperature increases, then the on-demand heater settles down a bit and is easier to control… again that learning curve thing.
An alternate operational setup would be to have a large insulated tank and the on-demand heater would heat the larger quantity of fluid in that tank where it could turn on and off and operate within its engineered design parameters and then you would draw fluid from that tank to temper the water being circulated through the driveway. Review sample installations for using on-demand heaters to supply apartment buildings (or similar) with hot water and you will find examples of using large insulated holding tanks.
The normal mode of expected operation for an on-demand heater is “year round”. Therefore it is turning on and off many times per day and nothing is likely to try to crawl inside the exhaust, as it will soon be incinerated. Since your on-demand heater used for the snowmelt system will not be operational during nice weather, you need to make sure you plug the exhaust so wasps (or similar) cannot build nests inside the heater.
I installed everything myself including the Rannai on-demand heater. Some suppliers will try to insist that the heater is installed by a licensed plumber. I think I may have taken a test to become one of their certified installers. Or possibly they just waived that part as I was asking more questions than they could answer. 🙂
Your system needs to be designed for your specific situation. Whoever is going to sell you the PEX and other materials should have an engineering department with heat loss design calculation programs to help you size your system. Parameters are temperatures in your area, snow and ice expected, PEX size, loop length, number of loops, the flow rate through the PEX, circulation pump selection, etc. I cannot help you with that.
If you live in a colder part of the country, you would probably use 3/4″ PEX and the operational cost would increase.
For my installation, I would go with a heater half this size. The Rinnai has almost as many controls on the burner as a jet engine. One limiting factor built into the unit, it is programmed to keep slowing down flow rate until the selected output temperature is satisfied. Thus, when bringing the system online initially, the heater limits the flow rate until the return temperature satisfies system requirements allowing increased flow rate. The mindset of the designers of this on-demand heater was that output temperature was KING even if it has to slow down the flow rate. The programming of this on-demand heater is too rigid. If you select 120 deg F, the heater does everything to maintain that temperature, including slowing down the flow rate… and there is no way to get around this. I would expect this to be a problem even for the domestic hot water supply in the winter when water mains temperature is quite cold. When I select 120 deg F output temperature, an operating range of 118 to 122 deg F would be acceptable but do not keep changing the flow rate. Once the return temperature from the concrete slab starts to increase, this problem goes away. I would love to get rid of the electronically controlled flow valve (or just unplug the servo controlling it) and the heaters programmed insistence on meeting output temperature. But any attempt to do so results in the heater shutting itself down (system error). This is a 180,000 BTU heater and I am pumping 1 GPM minimum and 2 GPM maximum through it, using around 1/3 of its capability. A 90,000 BTU unit would be more than adequate for the job but a unit that size was not available at the time of installation. An alternative would be to use the heater to maintain the temperature of a fluid holding tank and then use that tank of fluid to temper the water circulated through the slab. But then the system footprint becomes much larger and takes up too much space. My system installation is quite compact in the corner of the garage.