Didn't expect a dirty flow switch to short cycle my heater

Started by btu_frank · · 5 replies
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#1
Gas heater's been kicking on and off every couple minutes this week, figured it was finally the exchanger going after all these years near the coast. Ran the numbers first though, 12k gallons, needed maybe an 8 degree rise, heater's sized right for that so it wasn't a BTU problem. Turned out to be the flow switch, just crudded up enough to false trigger. Pulled it, cleaned it, back to normal cycles. Guess I expected corrosion or the cupro-nickel finally pitting before I expected a sensor being dirty to cause grief. Reminder that in the trade we always check the cheap stuff before assuming the expensive stuff failed. Anybody else had a flow switch act up out of nowhere like that.

5 Replies

#2
Yes, same story here two summers back, short cycling had me bracing for a heat exchanger bill and it was just the flow switch fouled up from sediment.

Two Raypaks and 25 seasons in the binder and that's the only flow switch issue I've logged, so I believe you on the cheap stuff first rule.
#3
Had almost the same scare on my gas heater a couple years ago, short cycling every few minutes and I went straight to checking flow before anything else since mine's fed off the VS pump and I run a lower gpm stage most of the day. Turned out low flow at that RPM plus a little debris on the switch paddle was enough to false trigger, not enough margin above the cut in threshold. Bumped my heater stage up maybe 200-300 rpm on the schedule and cleaned the switch and it's been fine since. Worth checking what flow your pump's actually pushing on whatever speed the heater call kicks it to, cheap sensor or not it still needs enough gpm to read clean.
#4
No gas heater here so can't speak to that flow switch specifically, but the RPM margin thing Marcus mentioned is the same lesson I learned with my cartridge filter pressure switch, cheap sensors just need enough signal to read clean, dirt or low flow both fake you out the same way.
#5
Seen it a hundred times, flow switches are cheap plastic paddles and they get gunked up way before anything else on that heater ever thinks about failing. My own gas heater's flow switch is original, coming up on 9 years, only cleaned it once and that was because a customer's job left me with crud under my nails and I figured might as well check mine while I was at it. Marcus's point about gpm margin is real too, mine runs off a single speed pump so I never have to think about it, but on the variable speed setups I service the heater call almost always gets bumped to a dedicated high stage for exactly that reason.
#6
One thing I'd push back on a little, tampapoolguy, single speed not needing to think about it isn't really the sensor being easier, it's just that you're always dumping full gpm at the switch so you never get close to the margin problem. That's not the switch being forgiving, that's brute force flow hiding a marginal design. Put a dirty paddle in that same single speed flow and you'd probably still trip it eventually, you're just further from the threshold to start.

For anyone setting up a heater call on a VS pump, I wouldn't just eyeball a bump in RPM, worth actually knowing what gpm your plumbing needs to read clean on that switch versus what stage you're running. I keep mine on a dedicated stage that clears something like 50-60 gpm through 2 inch pipe, comfortably above the cut in, and I still had crud tip it over the edge that one time. Cheap sensor, real physics behind it.