Heating control-interruptor

   / Heating control-interruptor #1  


Veteran Member
Feb 21, 2005
Central WI
I have a heating installation using a Rheem Pronto 4.2 tankless water heater to run a radiant floor system. The setup works VERY well, with one issue. The heater has a control in it that errors out after 1 hr of continuous operation and it quits until flow stops. If the heat cycle exceeds 1 hr, no heat. The heater basically trips a code thinking a faucet is left open.

I have a note in to Rheem/Ruud to see if this is defeatable, but I doubt it. I use a Honeywell thermostat to trigger an Erie SR 100? circulator relay and engage the circulator. The flow of water triggers the burner to fire. What I need to do is set up some way of pausing the circulator momentarily after a period of time less than one hour, yet re-engage it automatically. The circ is a Grundfos 120v unit.

It really shouldn't be a big deal, but I'd like to keep it neat and simple as well as RELIABLE. This is the mother-in-law's garage and I don't want to be there off and on all winter...
   / Heating control-interruptor #2  
HMMM, Lets see. Mother-in-law, unheated garage, Wisconsin, winter.
Remind me why your fixing this??? /forums/images/graemlins/smirk.gif /forums/images/graemlins/wink.gif /forums/images/graemlins/grin.gif
   / Heating control-interruptor #3  
Here's what I would use. You could alter the cam to suit your own timing on the pump cycle and it's cheap. timer
   / Heating control-interruptor #4  
is this a new system? or has it always operated like this? if it is a new system I can tell you that the makers of some of these instant type water heaters state not to use them set up for a heating system. It should state this in your installation instructions if your water heater is designed for a home heating installation.
   / Heating control-interruptor
  • Thread Starter
Yes it is new. I did mine with a Slant Fin Prodigy wall mount boiler, but they were discontinued. I did this installation and this is what my supplier sold me. I have a good understanding of the basics and then some, but am NOT a professional.

I had the water temp at 100°f initially, but have raised it to 120°f (a bit higher than I prefer) to try and shorten the cycle time...
   / Heating control-interruptor #6  
I downloaded the water heater spec. and there's no mention of an exclusion on using it for underfloor heating. Still, in light of the 1 hour limiter you've found on continuous running, it would probably be prudent to check with the manufacturer.

If they're good with it and don't have a fix for the boiler run time, should be easy enough to patch in a timer on the Grundfos pump so it runs for no more than 59 minutes. I take it it resets soon as it stops. On the temperature, 120oF is ok for output. Rule of thumb we use is 50oC (122oF) or under. We can go higher but the heaters we use (condensing gas) lose efficiency above 50oC.
   / Heating control-interruptor
  • Thread Starter
Yes it resets as soon as flow stops and restarts. The timer would need to reset itself after a short off time while the relay is still calling for it to run.
   / Heating control-interruptor #9  
Kenny that seems to be just the ticket, I would suggest that you isolate that unit so that it switches the flow switch which SHOULD be a low amp device (control voltage sensing only and not an actual power switch) if it is a power switch then you should tie in a relay so that the timer is only activating a relay which will have a greater amp rating.

also at the surplus center MIKE posted (Pineridge) they have tons of cool stuf too /forums/images/graemlins/smile.gif
MarkM /forums/images/graemlins/tongue.gif /forums/images/graemlins/smirk.gif /forums/images/graemlins/cool.gif /forums/images/graemlins/laugh.gif
   / Heating control-interruptor #10  
Is this a nat gas or propane unit ? I have known people that have installed instantaneous type water heaters on heating systems, but they add a storage tank to it so that its not on all the time. It may be turning its self off to keep from burning the combustion chamber or water coils up. Just a thought as I have never installed an instantaneous water heater for a heating system. I have only installed apollo brand hydro heating systems. Hope your idea works . Just keep an eye on your combustion chamber (if gas)& water coils.