# Recirc controlled by flow switch



## Asparta (Mar 12, 2010)

I am installing a recirc type system on a system that is run by a Rinnai unit. The rinnai unit puts out 9.8 GPM and the area of concern is about 50 feet away. I ran a 3/4" main, and a 1/2" return line for the hot side. I want to install a pump and a flow switch to control that pump so as to speed up the time it takes for hot water to get there. A timer would not work in this application, as it would be a waste of money to run, and the customer has no peak times. Very random schedule. I did not want to use an aquastat as I do not want the pump to run all day. Just want to speed up getting hot water to the point of use, so as to make it seem like recirculation is happeneing. Big time savings for customer, and pump only works when you crack a faucet. Rinnai unit has 16 feet of head to overcome. I am just having trouble sizing this pump. Can you help?
Thanks. I know it must be of bronze or stainless construction. 1/2" inlets, and able to overcome 16 feet of head. What GPM rate should I be looking at so as to prevent short cycling of the circuit? Thanks!


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## Tommy plumber (Feb 19, 2010)

So let me get this straight, you want to put a re-circ. line on a tankless heater without a timer? If you put a flow switch, how is this going to increase speed of hot water getting to fixtures?


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## ZL700 (Dec 8, 2009)

A flow switch can be used. It enhances flow when perhaps just a .5 GPM faucet is turned on. The recirc pump could be 2-3 GPM, thus the hot water gets there much faster by purging the line versus the standard wait.

The down side is most inexpensive paddle type flow switches are not sensitive enough, so you will be looking at spending $200 to get it to activate and work right. Also, many have added a time delay relay cube so that once flow is sensed the circ stays on a period of time. Other options is a sensitive pressure switch on the hot water outlet with time delay works well.


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## TheMaster (Jun 12, 2009)

Motion sensors to control the pump.


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## ZL700 (Dec 8, 2009)

Motions are good, but for me for instance I dont want it to activate my pump as I am just going to the walkin closet on the backside of the master bath to put away clothes.


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## BROOKLYN\PLUMB (May 21, 2010)

maybe check out the taco d'mand pump


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## TheMaster (Jun 12, 2009)

ZL700 said:


> Motions are good, but for me for instance I dont want it to activate my pump as I am just going to the walkin closet on the backside of the master bath to put away clothes.


Sure every jobs different,I was just giving options.:thumbsup:


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## Protech (Sep 22, 2008)

Maybe a high head pump coupled with motion sensors in the use areas only.

Example: Motion sensor in the shower ceiling. Custom cut a blinder for teh sensor so that it only sees inside the shower/tub enclosure.

Motion sensor above the sink with a blinder that will cut the area down to just in front / on top of the sink.


Maybe Mr. hotshot engineer ZL700 can show us the equation to size a pump to a pipe system to achieve a given gpm?


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## ZL700 (Dec 8, 2009)

Protech said:


> Maybe a high head pump coupled with motion sensors in the use areas only.
> 
> Example: Motion sensor in the shower ceiling. Custom cut a blinder for teh sensor so that it only sees inside the shower/tub enclosure.
> 
> ...


Very nice protech

As you should know, need the details first
Thats the problem most plumbers dont understand or comprehend


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## Protech (Sep 22, 2008)

Copper pipe in water that will not cause a high degree of internal roughness with aging or reduction in hydraulic diameter. Internal diameter is 3/4". 100' run each way developed length with fitting equivalent lengths factored in. 80' actual developed length each way. Water temp is 125F. Static pressure to water heater is 65psi.

We want a pump that will get us hot water to the end of that run in 5 seconds with no water hammer. Cuts out when the auquastat at the end of the run reaches 125F. Pump starts by user pressing demand button.

What pump? And more importantly, HOW THE H$(( did you get to that answer???


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## Protech (Sep 22, 2008)

While the engineer works on that, I'll slap this Taco 0013 in and call it done :laughing:


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## Asparta (Mar 12, 2010)

Thanks everyone!
The purpose is to purge the line quicker. 
A 2-3 GPM pump it is.

Cheers!

Asparta


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## Protech (Sep 22, 2008)

Actually, any on demand recirculation system with a tankless is going to cause some temperature fluctuation on startup without a 1-3 gallon tempering tank.
Also, depending on just how fast you want that water to get there (what size pump) you may out run the tankless units capacity. In that case, simply putting in the biggest pump you can find won't work. Your velocity will be limited by the tankless units output.

Example: you put something in that pushes 20 gpm but the heater can only heat 7 gpm. You will just be getting instant luke warm water. You'll only get hot water when the pump shuts off.


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## ZL700 (Dec 8, 2009)

Correct, you can't over pump the tankless unit capabilities based on unit size, temp rise and piping capabilities. 
Also the general rule of thumb for Max GPM for piping is: 8fps on cold water, 5fps for hot water up to 140 degrees and 4fps or less for above 140 degrees.

Exceed these velocities and you can create a sprinkler system in copper tube real quick. Although PEX pipe is not affected as much, the fittings however are so follow the same rules. 

For instance on a DHW system 140 degrees or less, the hot water pipe flow recommendations are as follows (based on 100'):
1/2" pipe - 3.5GPM max
3/4" pipe - 7 GPM max
1" pipe - 12.5 GPM max

Above shows why so many 1/2" DHW return lines and fittings can pin-hole so easily by over pumping.

Just throwing a 013 circ on isn't the answer without knowing all the circumstance since besides the obvious above the circ may cavitate and current draw rises and amperage rate would increase as it is operating out of its efficiency curve.


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## Tommy plumber (Feb 19, 2010)

Because I know little about re-circ line w/ tankless application, I contacted Rinnai. They said in addition to motion sensors like someone posted, you can also use push-button or voice-activated as well. They also said one option is to use a small tank-type w/h to maintain temp. in hot water loop. Only re-circ lines I've installed were with tank-type not tankless water heaters.


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## ZL700 (Dec 8, 2009)

Tommy plumber said:


> Because I know little about re-circ line w/ tankless application, I contacted Rinnai. They said in addition to motion sensors like someone posted, you can also use push-button or voice-activated as well. They also said one option is to use a small tank-type w/h to maintain temp. in hot water loop. Only re-circ lines I've installed were with tank-type not tankless water heaters.


Tommy, tankless are tricky because of pressure drop/head.

For instance a Rinnai 94 is just under 5 PSI pressure drop @ 4 GPM.

They are specific about recirc, and constant they want the extra tank, which in my view diminishes the purpose of a tankless with the electric element reheating the recirc water. Or a timed, motion, momentary activation so that recirc isn't constant or prolonged through the tankless.

If the above is not followed, their residential warranty drops from 12 years to 3.


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## TheMaster (Jun 12, 2009)

Just replace that POS with this POS........http://www.navienamerica.com/KD_eng/internal.html


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## Asparta (Mar 12, 2010)

ZL700,
Great info. 
Do I need concern myself with the head loss through the tankless unit? I figure that since the re-circ line ties back into the cold line feeding the unit, at 60psi, then I should not need to worry about it if I am flowing at 2GPM with a pump. The unit has 15 feet of head through it. Do I find a pump that can be within pump limits at 2GPM and 15 feet of head? That would make most sense to me. This is the first time I am running a line this way, and it's to improve efficiencies, not to diminish them. Home owner has no peak times, and controlling it with a temperature control will only run the pump throughout the day to keep temp up. Why aren't most systems run this way with a flow switch? 


Thanks!


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## Everflow (Feb 1, 2010)

Check this out,it seems to work.

http://www.gothotwater.com/d'mand/


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## ZL700 (Dec 8, 2009)

Asparta said:


> XL,
> Great info.
> Do I need concern myself with the head loss through the tankless unit? I figure that since the re-circ line ties back into the cold line feeding the unit, at psi, then I should not need to worry about it if I am flowing at GPM with a pump. The unit has 15 feet of head through it. Do I find a pump that can be within pump limits at GPM and 15 feet of head? That would make most sense to me. This is the first time I am running a line this way, and it's to improve efficiencies, not to diminish them. Home owner has no peak times, and controlling it with a temperature control will only run the pump throughout the day to keep temp up. Why aren't most systems run this way with a flow switch?
> 
> ...


During recirc, you can assume that there is no water flow, thus the open system is a closed system at that point, the circ must be able to pump through the tankless at at least the minimum flow rate/minimum fire requirement. 

Once you have the head or pressure drop of the tankless (1' head = 2.31 PSI) you can calculate supply and return pipe, fittings & valves you can select pump.

Pumps don't always fall perfect in line (curve vs requirement), but the multi-speed circs lately certainly help. A circuit setter can dial in exact flow rate or just a SS trimmed ball valve can get you close for adjusting GPM.

Flow switches aren't "instant hot water" which so many consumers desire, but certainly are a good compromise.


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## CAPLMBR (Oct 29, 2010)

Am I missing something here? How would a recirculating pump (taco with 1/40 horsepower) get water faster to any location. Isn't that what the municipal pump is doing by providing your house with flowing water? Plus I went to a Takagi seminar and asked the question "Wouldn't a recirculating pump lower the supposed efficiency because the unit also has a flow switch in it and therefore any water movement from recirculating pump would have all burners running all the time. I know a lot of plumbers are not fans of Watt's recirc/comfort tee circulating system, but I think that would be the closest thing to what your talking about but it will circulate water to location of comfort tee installation and stop flowing as the thermostat senses approx 72 degrees then shuts. If I understood you correctly I hope this helps if I didn't understand you correctly I will refocus.


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## Protech (Sep 22, 2008)

Depending on what kind of water we would be dealing with, that may or may not be true. Also, we are talking about a demand recirc system, not a timered or continuous system. Since the high velocities are only for short bursts, (maybe 5 min total per day) it would take 50-100 years for cavitation/impingement to eat its way through. It's a mute point though because as we both agree, the tankless output flow rate is going to be the limiting factor 99% of the time.



ZL700 said:


> Correct, you can't over pump the tankless unit capabilities based on unit size, temp rise and piping capabilities.
> Also the general rule of thumb for Max GPM for piping is: 8fps on cold water, 5fps for hot water up to 140 degrees and 4fps or less for above 140 degrees.
> 
> *Exceed these velocities and you can create a sprinkler system in copper tube real quick*. Although PEX pipe is not affected as much, the fittings however are so follow the same rules.
> ...


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## gitnerdun (Nov 5, 2008)

water is cheaper than fuel, waste it in this case. 

I have connected the feed to a recirc pump to the light in the bathroom in the past.

What is the goal you wish to achieve in this situation? Alot of times it does not do any good, other than to fatten your wallet and their elec/fuel bills, to put in a recirc system. 

Is the entire loop insulated, or will it radiate heat only to be lost?

I'd think this thru or you may have a unhappy ho in your future.

Tankless and recirc together have never made a lot of sense to me, it defeats the purpose of the tankless.


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## RealLivePlumber (Jun 22, 2008)

gitnerdun said:


> water is cheaper than fuel, waste it in this case.
> 
> I have connected the feed to a recirc pump to the light in the bathroom in the past.
> 
> ...


Thats a great statement, keeping in mind that about 1 % of the worlds water is available for consumption. :no:


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