Snow melt problems

coroofwalker

I was called in on a job to complete the controls and finish the plumbing on a snowmelt system that was 90% complete. That was a few months ago when the weather was much warmer. Now that the system is at design conditions we are having issues.

1. there are 3 loops of 1/2” wirsbo HE pex running into a driveway and walkway. Maybe 1000 sqft max. Nothing is labeled and lengths aren’t obtainable.
2. Each loop is plumbed with individual Grundfos 15-58 pumps for each 1/2” loop ( 3 Pumps). These pumps are on 3/4” branches off a 1” Secondary.
3. There is a 1” x 1” pri/sec webstone injection valve, correctly placed, that leads to a Rheem tankless water heater that is set to 120 and activates with a flow switch. This loop is reduced to 3/4” soon after the injection valve. There is another dedicated 15-58 on this branch to handle the heater flow.
   

The problem is that I cannot get the Rheem heater to turn on and stay on, it keeps dropping to <.4gpm flow through this loop, the primary loop. The handle on the injection valve is open, so that the primary can flow as needed through closely spaced T’s. But even with the pump running, I get a P1 error (too little flow to activate flow switch) There is also very little flow through each of the snowmelt loops, but I’m trying to focus on the heat end of things. If I can get the heater running and the loops slowly start to heat up, the viscosity will get less with a higher fluid temp and I might get better flow rates in the snowmelt loops. But it starts with the heater. I can’t figure out why it is not flowing. The primary is clear pex and I can watch a few air bubbles bounce around but not move. I even hooked up a charging pump to purge through the heater, with the injection valve closed, and the heater turned on for 5-10sec, then lost flow.? When I open the injection valve, I get what seems to be unrestricted flow. It seems like something is a miss with the heater/primary loop. Do these heaters have to run on higher pressures?

I’m at a loss and figured I’d pose the question to the group. My next plan of attack is to plumb in some valves and drains and pressurize the primary with house water pressure. If I keep the cold supply open, I should be able to ensure a good purge of the heater and see if it operates on higher pressure. Any suggestions are appreciated. One test at a time.

I’ve included a picture of the module that was used. 1/2” pex off of the 3pumps to the left and 3/4” pex with a 15-58, and a Rheem tankless off of the injection valve. No flow on primary.

Thanks for your time - CORoofwalker

GGross

Off hand I would say you need a larger pump for the rheem, it is probably a 199 model and they usually would be hooked to a higher pressure DHW system because it likely has a very restrictive heat exchanger. I would ask whoever hired you for the complete design for the snow melt side, depending on how they ran the loops you might not get flow through them. 1/2" pex is pretty small for snowmelt, I use that size for tight spaces on stairs but only really short loops, you need to know how long those loops are.

DIY people go online and see 300' length for 1/2" as a rule of thumb and don't realize they can't do that on snowmelt, also need to know the spacing because if they did 300' loops on a wide spacing you probably won't melt correctly, but first thing is to get the heat source to activate

Jamie Hall

I'm going to be that that primary circulator simply can't provide enough flow through the tankless. You probably need around 40 psi to get through the tankless at a reasonable flow. An offhand guess is that that pump may be capable of 0.5 gpm or less through the tankless. Nowhere near enough.

coroofwalker

Update: The Rheem has a magneto impellar in the Water Control Valve within the unit. Evidentally, it is tied to a solenoid that will completely shut off water flow through the unit if it isn't operating.. I plumbed in the drains and valves to try to operate the heater with house water pressure. I started to purge the unit, maybe like 10-20 seconds, then the outlet just stopped flowing. 100% clogged or closed. Tech Support is sending a new Water Control Valve within the next couple days, then we will try this again….