# Which Pump for Flushing Tankless?



## CT Anderson (Aug 27, 2018)

I have a Rinnai RUR199en tankless water heater. For maintenance I need to flush it with vinegar about once/year using a bucket of vinegar, 3/4" connection hoses, and a submersible pump. Rinnai won't recommend a pump to use for this operation. They only recommend that it push just 4GPM. The customer service rep did say she thought this would be from a 1/5 HP pump, but she wasn't sure. The problem is that all the 1/4 and 1/5 pumps I find are rated at about 25GPM. (Of course I don't know how much they would push when challenged with a heat exchanger.) I don't want to hurt my tankless heater by using a pump that's too powerful. Does anyone know if I can use a 1/4 HP pump rated at 25 GPM safely? If not, then what kind of pump should I be using? 
:glasses:


----------



## Guap0_ (Dec 2, 2017)

> The customer service rep did say she thought this would be from a 1/5 HP pump, but she wasn't sure.


I would pressure her to get you a definitive answer. I don't know what agency regulates those things but that shouldn't be.


----------



## u3b3rg33k (Jul 17, 2018)

It doesn't really matter what pump you use. any cheap utility pump will work just fine. the point of this is to circulate mild acid through the heat exchanger to clear it out. Rinnai isn't in the business of selling pumps, so they're not going to recommend one. 

if you want to pick out a pump, pick a price point, and look at pump curves. pick one you like. 

i wouldn't recommend converting a pressure washer though.


----------



## CT Anderson (Aug 27, 2018)

I did some poking around and it seems that most people either buy a $170 kit with a pump of undisclosed power or they buy a 1/6 - 1/4 HP pump + their own hoses and bucket for about $75.


----------



## Gregsoldtruck79 (Dec 21, 2017)

Seems to me a little pump like this one would do fine. Sure not going to get too many GPM out of it. JMO https://www.amazon.com/Electric-Water-California-Single-Waterbeds/dp/B003XVNUTQ/ref=sr_1_3?ie=UTF8&qid=1535484515&sr=8-3&keywords=Waterbed+Pump


----------



## CT Anderson (Aug 27, 2018)

Thanks. I bought this one:
https://www.amazon.com/gp/product/B000X05G1A/
It appears that many others have used this on their tankless heaters. I haven't tried it yet, but I tested the pump emptying my daughter's kiddie pool and it's awesome.


----------



## Oso954 (Jun 23, 2012)

The 91250 is a 30 gpm pump at 0 head, 22 gpm at 5 ft of head. That’s two much flow for the job at hand. The resistance and back pressure of the piping/heat exchanger will cut that back to a more acceptable level. However, running the pump under a lot of back pressure may cavitation the pump. The cavitation may shorten the life of your impeller.

IMO, the waterbed pump recommended by GregsOT would have been a better choice.


----------



## u3b3rg33k (Jul 17, 2018)

i wouldn't spend 2 seconds worrying about head related cavitation in a single stage centrifugal pump with a max head of 25'. in fact, even typing that sentence was more thought than it's worth.

now look what you've done. i"ve spent minutes thinking about this!









2GPM @ 25' or about 11 PSI. normal feed pressure in a home is 45PSI+. zero worries.


----------



## supers05 (May 23, 2015)

u3b3rg33k said:


> i wouldn't spend 2 seconds worrying about head related cavitation in a single stage centrifugal pump with a max head of 25'. in fact, even typing that sentence was more thought than it's worth.
> 
> now look what you've done. i"ve spent minutes thinking about this!
> ...
> ...


I agree. 

OP: After the wash, flush it for 10 min with regular water before letting it sit. You can just use your sinks for that. 

Don't forget to turn the boiler off during this whole procedure. You'll find that it's easier then it all sounds. 

Cheers!


----------



## KHouse75 (May 14, 2008)

Do you have hard water? Yearly maintenance on tankless water heaters is a myth. It all depends upon the hardness of your water. If you have really hard water, once a year may be fine. If you have soft water or a water softener, you don't have to do it for many years, if ever.


----------



## supers05 (May 23, 2015)

KHouse75 said:


> Do you have hard water? Yearly maintenance on tankless water heaters is a myth. It all depends upon the hardness of your water. If you have really hard water, once a year may be fine. If you have soft water or a water softener, you don't have to do it for many years, if ever.


I thought about mentioning that. Some people need to do it every year. , others won't need to do it for 10 years. 

Deposit buildup will decrease the temp rise. Early measurable at max flow/firing rate. 

Cheers!


----------

