Rinnai tankless recirc. Help for a newbie

[JessieGirl]

We just had a Rinnai tankless system installed...while the concept was a good one, I am afraid that so far it has been a disaster.

We were told that our existing circ pump would be just fine (the one that worked without issue with our 75 gallon traditional tank heater), but it turns out that nope it is not just fine. It is underpowered and doesn't work at all. It takes well over 4 minutes (yes MINUTES) to get the hot water to our shower - and the kitchen is not much better. The washing machine doesn't know about this delay so also forget about doing a load with a hot wash! Too many inconveniences to list!

The square footage of our home is about 4,000sf with the tank in the garage and various water sources spread around between 2 floors. The remedy offered by the company that we purchased from and did the installation is to purchase a new pump (2.5 gpm based on mfg specs), this makes sense to us. The bigger issue is that fact that we have 1/2 inch lines and we are being told that the pump will need to run to 3/4 inch lines.

Can anyone explain to me the pros and cons of the line size differences? Will this new circ pump with the appropriate power now solve our hot water delivery delay? Honestly, if I could push the rewind button I would just have replaced our 75 gallon tank with a new one same/similar model....ugh.

Please help!

Thanks,

Jess

 

[Dana]

Most newer water-sipping washers have some sort of "sanitize" or similarly named superhot cycle where the wash water is heated electrically by the washer itself, and may be the best solution for operating with a tankless. If that's what you end up doing, feeding cold water to both sides of the washer is easier on the tankless, which would otherwise be running extra ignition cycles with absolutely no benefit. But if the updated recirculation works you can probably go with normal cycles on the washer and leave it as-is.

The existing recirculation pump is probably right at the margins of the flow detection limit for lighting up the Rinnai. If it needs at least 0.8 gpm to start up and that's the flow it's getting with just the wimpy recirculation pump driving the flow it may not detect flow right away, or maybe not at all when it's just the recirculation pump running, no taps open. Once the burner is lit the minimum flow rate to keep going is usually quite a bit lower than the flow needed to strike the flame.

With 3/4" plumbing the friction of the line is lower, the flow is higher, and if sufficient, it will light off within a few seconds. The higher volume of the pipe is less of a problem than the low flow at the water heater. The new pump alone might have enough to do just fine even with half-inch plumbing. With the higher impedance of half inch plumbing it won't hit the rated 2.5 gpm, but it probably doesn't have to. Most tankless units need ~1 gpm or higher to fire up nearly instantly, but will stay lit even if the flow drops to 0.5 gpm or lower. If you look in the manual for your Rinnai the minimum flows for ignition and min-flow sustained are usually spelled out.