I had a plumber install a new Rinnai model rc98 tankless heater in place of my old 50 gallon tank. The water heater has a recirculating line that feeds my heating system, the existing circulating pump is a 1/40hp pump with a 0-11gpm flow and a 0-10 head. The system was installed using Rinnai's drawing number cwh1-c-o. My old circulating pump comes on via the thermostat but i just get cold air from the registers, is the old pump to small for this water heater? And if so how do i size for the right pump?
Hey... Yes, the old pump is too small. The Tankless units have a high head loss, usually around 25 to 45 ft of Head depending upon the flow. You are going to need a pump that will produce at least 2 to 3 GPM at 25 to 30ft of head. Usually this is a 1/3 to 1/2 HP pump.
I looked at a Grunfos model #52722347 on line it has a flow of 0-32gpm and a head of 0-34 would that be sufficient?
That looks good, I would consider it on the marginal side, but if you only have one air handler it should work.