Tankless Water heater storage/buffer tank

Burnett03

Thanks for taking the time to help me. I recently added a small storage tank to my Rinnai tankless water heater. The reason i did this was because my wife and i were short cycling the Rinnai to often when we would do simple tasks such as wash our hands or do laundry. I setup the system as shown in my photo and it works well for the purpose i stated. However when my wife wants to take a bath with heavy water flow the system quickly outruns the tank and the water turns cold. My thought process is maybe i can keep the system i have but install a electronic bypass valve on the hot water outlet to bypass the tank and get full hot water to the system when needed. Will this work or is there a better way to do this? Thank you!

Burnett03

I should also add incase it matters, i am running a SS Taco 0013 pump.

Hot_water_fan

It’s likely a tankless will not be able to handle a heavy bath flow - you want a tank for that application. 

Larry Weingarten

Hi, How many gallons is the tank you have? Also a band-aid approach that might help is to slow the flow when filling the tub. That might allow the tankless heater to keep up with demand.

Yours, Larry

Burnett03

It's a 10 gallon tank. Which i believe is to small. A larger tank would most likely work better for her baths.

hot_rod

what size Rinnai do you have? If capacity has dropped it may be time to descale it. In some cases a yearly descaling is needed to keep the output sufficient.

Short cycling is a way of life for tankless and combis

Tankless should ALWAYS be installed with the flush valves below to hook up the descale kit. It is easy to do if the valves were installed?

Burnett03

It's a rinnai 98i. I usually descale it once a year with Vinegar. I'd say it's been maybe 9months since my last descale.

hot_rod

Here is the output performance graph, plug in your incoming water temperature and desired supply temperature to see what it is capable of.

Adding a tank will get you some “dump” capacity if that is what you are after.
But it also slows recovery once you drain the tank temperature down. Now the heater needs to recover the tank while also supplying any concurrent demand.

The concept behind tankless is they provided the calculated output continuously, you can confirm that with a 5 gallon buck and an immersion thermometer. If it doesn’t provide what the charts show, try a descale again. You might go with a bit more acid power than plain vinegar. And you may need to run the cleaner for 30 minutes or more if the coils have much build up. Hard to know that, try another flow test after you descale it.

So either work within it capability or turn your tankless into a tank heater, At the cost of a large expensive potable water circulation pump

Mad Dog_2

If your gas piping and meter is properly sized for full gas load, you really shouldn't be having that problem. Also, what is your pipe size to the tub?  Scale build up like HR says may be the issue, but I put in a lot of 98Is without a problem.  They will crank out hot water NON-STOP like a machine gun.. for hours if needed.  Most Tanklesses are about 180K Gas Input.  Thats alot...We love the Webstone Kits with valves and purges.  So neat and compact. Mad Dog

Burnett03

Thank you guys for all the insight and help. I'll try a better descale solution instead of vinegar to start.

kcopp

I have had super great results w/ the Oatey Haymaker. Citric acid based. I had one combi that had next to no flow. As a last resort I used this and after 40 min is was almost as good as new.

hot_rod

Hercules Sizzle was a great product to keep on a service truck. A strong drain cleaner, great for urinal blockages and hair blockages in traps.
But it is also listed for potable water use. Just pay attention to the dilution table.

And don’t spill it in the truck. If you do, take that truck and trade it in the next day😳